AP3456 – 11-1 - Introduction to Radar
CHAPTER 1 - INTRODUCTION TO RADAR
Introduction
1.
The word radar (from the acronym Radio Detection and Ranging) was originally used to describe the
process of locating targets by means of reflected radio waves (primary radar) or automatically retransmitted
radio waves (secondary radar). The word has now been fully integrated into the English language and,
despite being derived from an acronym, is no longer written in capital letters. Today the meaning of radar
has been extended to include a much wider variety of techniques in which electromagnetic waves are
employed for the purpose of obtaining information relating to distant objects. It includes not only active
systems, in which the energy originates from the system itself, but also semi-active systems, in which the
energy originates from some other source; and passive systems which receive energy originating at the
target.
An Elementary System
2.
An elementary form of radar consists of a transmitting aerial emitting electromagnetic radiation
generated by a high frequency oscillator, a receiving aerial, and an energy detecting device or receiver. A
portion of the transmitted signal is intercepted by a reflecting object (target) and is re-radiated in all directions.
The receiving aerial collects the returned energy and delivers it to a receiver, where it is processed to detect
the presence of the target and to extract its location and relative velocity.
3.
The distance to a target is determined by measuring the time taken for the signal to travel to the target
and back. The direction, or angular position, of the target may be determined from the direction of arrival of
the reflected wavefront. The usual method of measuring the direction of arrival is with narrow aerial beams.
If relative motion exists between target and radar, the shift in the carrier frequency of the reflected wave
(doppler effect) is a measure of the target’s relative (radial) velocity and may be used to distinguish moving
targets from stationary objects. In radars which continuously track the movement of a target, a continuous
indication of the rate of change of the target position is also available.
4.
To summarize, the information that can be communicated by radar consists mainly of:
a.
Range - by echo timing.
b.
Relative radial velocity - by measuring Doppler shift.
c.
Angular position - by observing the direction of echo arrival.
d.
Target identity - by using secondary radar.
Classification of Radar Systems
5.
The profusion of radar systems in use today necessitates a logical means of classification. One
method, which appears to have achieved general acceptance, is to classify a radar system according
to four main characteristics, namely:
a.
Installation environment (ground, airborne, etc)
.
b.
Functional characteristics (search, track, etc).
c.
Transmission characteristics (pulse, CW, etc).
d.
Operating frequency band.
Revised Jul 10
Page 1 of 8
AP3456 – 11-1 - Introduction to Radar
6.
By this method, an early warning radar might be classified as a ground search, pulse radar operating in
D-band, and an airborne interception radar as an airborne, search and track, pulse-Doppler radar operating
in I-band. Such statements provide a useful qualitative description of a radar system.
7.
Installation Environment. The main types of radar installation are ground systems (static,
ground-transportable and air-transportable), airborne systems (aircraft, missile and satellite), and ship-
borne systems.
8.
Functional Characteristics. Radar systems may perform either a single function or, as is common in
airborne applications, one of a number of functions. Multi-mode radars can offer operational flexibility, but
some compromise is usually entailed. Some important radar functions include the following:
a.
Search and Detection. The interrogation of a given volume of space for the presence or
absence of targets is one of the most important functions of radar. This is normally achieved by a
primary search radar which scans the volume to be searched by moving a concentrated beam of
energy in a repeated pattern. The beam may either be fan-shaped and scan in a single
dimension, or it may be pencil-shaped and scan in two-dimensions. The time required to
complete each scan cycle is dependent on the ratio of the solid angle searched to that of the radar
beam, and to minimize this time it is sometimes necessary to sacrifice either coverage or the
angular precision of the beam.
b.
Identification. If the volume interrogated is likely to contain both friendly and hostile targets,
an important function of radar is the identification of friend or foe (IFF). This is normally achieved
by secondary radar, in which transponding equipment carried in the friendly aircraft transmits
replies in response to coded transmissions received from the interrogating radar. The
interrogation may be performed either by the search radar or by a separate system. Other criteria
may sometimes be used to establish the identity of a target, eg by comparing the parameters of a
computed ballistic trajectory with predetermined values.
c.
Tracking. Numerous tactical situations require continuous target information for display
purposes, e.g. airborne interception, or for calculation of relative target motion or future position.
Tracking radars which perform these functions must be capable of producing continuous outputs
of the range and angular co-ordinates of the selected target and, in some cases, the rates of
change of these parameters.
d.
Target Illumination. Target illumination is the function performed by the active element in
semi-active radar. The illuminating radar must be capable of tracking the selected target whilst
the passive receiver carried in the homing missile intercepts the radiated energy after reflection
from the target. The information communicated to the missile consists of target direction only, but
if the missile is roughly on the line between the illuminating radar and target, and can receive the
energy directly as well as by reflection, its range to the target is approximately proportional to the
difference in the times of arrival of the direct and reflected signals.
e.
Mapping. Mapping by airborne radar has numerous military applications of which navigation,
bombing and reconnaissance are perhaps the most noteworthy. Other functions employing
specialized mapping techniques are submarine detection, cloud warning and terrain avoidance.
Mapping radars may employ either circular or sector scan. Alternatively, the aerial beams may be
fixed in direction and scanned by the motion of the aircraft. Mapping is normally performed by
active pulse radar but passive systems which intercept naturally radiated infra-red energy are also
possible.
Revised Jul 10
Page 2 of 8
AP3456 – 11-1 - Introduction to Radar
f.
Navigation. Numerous navigational functions may be performed by radar:
(1) Mapping radars can provide fixing facilities and both cloud and terrain warning.
(2) Height can be measured by a radar altimeter.
(3) Secondary radar techniques are used in various forms of navigational beacon, e.g. DME
and TACAN.
(4) Ground speed and drift can be measured by means of Doppler radar.
g.
Other Radar Functions. This is by no means an exhaustive list of radar functions. Among the
less familiar secondary functions which may sometimes be incorporated in a radar system are:
(1) Passive operation for the detection and location of enemy radiation.
(2) Radiation of jamming signals.
(3) Use of the radar transmission as a carrier for communicating intelligence.
9.
Transmission Characteristics. The most fundamental basis for classifying a radar system is
provided by its transmission characteristics because on this depends the nature of the target information
which the system is inherently capable of conveying. The ability to convey target information is provided by
modulating the transmission in various ways, and by observing in the receiver the manner in which the echo
signal has been affected by the target. Directional information is achieved by the radar aerial which
modulates the transmission into a narrow beam (space modulation). Range information necessitates the
provision of timing marks in the transmitted carrier in order to facilitate the measurement of the propagation
time to and from the target. This may be achieved by modulating amplitude (pulse radar) or frequency
(FMCW radar). The measurement of relative velocity between target and radar is achieved by observing the
change of frequency in the echo signal brought about by the Doppler effect. For this to be possible, both the
frequency and phase of the transmission must be present in a reference signal at the time the echoes are
received. This condition is inherent in a continuous wave (CW) radar and in coherent, pulse and Doppler
radar; but in the majority of pulse radars the Doppler shift, although present, cannot be measured. The
fundamental division in radar types lies between pulse systems (which resolve targets in range) and
continuous wave systems (which resolve targets in velocity). Other types, such as pulse Doppler, can
perform both functions if required to do so.
10. The main features of the fundamental radar classifications are as follows:
a.
Pulse Radar. In pulse radar, the transmission is concentrated into very short pulses which
are separated by sufficiently long intervals to permit all echoes from targets within the operating
range to be received from one pulse before transmission of the next. Targets are resolved in
range by virtue of the different times of arrival of their echoes and the degree of resolution being
determined by the length of the pulses. Range measurement (R) is made by observing the
elapsed time (t (in microseconds)) between the leading edge of the transmitted pulse leaving the
aerial and the leading edge of the return echo arriving back at the aerial. Because the pulse has
travelled to the target and back, t therefore equals 2 × R. If c is the velocity of propagation
(3 × 108 metres per second), then:
2R
t = c
ct
and
R = 2
b.
Moving Target Indication (MTI) Radar. MTI radar employs a pulsed transmission but, in
addition to performing range resolution and measurement, it also discriminates between fixed and
Revised Jul 10
Page 3 of 8
AP3456 – 11-1 - Introduction to Radar
moving targets by its ability to recognize the existence or absence of Doppler shift in the echo
signals. The fixed targets are suppressed and only moving targets displayed.
c.
Continuous Wave Radar. In CW radar, the transmitted and received signals are continuous
and targets are resolved in relative velocity by virtue of the differing frequencies in their echoes.
The measurement of relative radial velocity is made by observing the magnitude of the Doppler
shift (fd) in the echo signals, ie the difference in frequency between the transmitted and received
signals. If the relative velocity is ±V, and λ is the transmitted wavelength, then:
2V
f
= ±
d
λ
d.
Frequency Modulated CW (FMCW) Radar. FMCW radar employs a continuous
transmission in which the frequency is modulated. In addition to performing velocity resolution
and measurement, the system has the ability to measure the range of a discrete target, but it
cannot resolve a number of targets at differing ranges other than by virtue of their differing
velocities or directions. The measurement of range is less precise than that of a pulse radar at
medium and long ranges but can be more accurate at short range. In addition, FMCW can
measure down to zero range, which is not possible with a pulse system.
e.
Pulse Doppler Radar. Pulse Doppler radar employs a transmission in which, unlike
conventional pulse radar, there is continuity in the phase of the carrier from pulse to pulse. This
property, called coherence, permits the Doppler shift in echoes to be measured and the system is
thus able to resolve and measure both range and relative velocity. The avoidance of ambiguity in
velocity measurement requires the pulse repetition frequency to be higher than in conventional
pulse radar and, as a result, potential ambiguity is introduced into the measurement of range and
range eclipsing may occur. Nevertheless, sophisticated processing techniques have meant that
pulse Doppler radar is the most important and widely used type in airborne applications.
11.
Operating Frequency Band. Modern radar systems operate over a wide range of frequencies,
usually between about 200 and 35,000 MHz, ie wavelengths between 1.5 metres and rather less than
one centimetre. Within this range, radar systems tend to be grouped in a number of fairly distinct
regions, partly because of frequency allocation and partly because of constructional convenience.
Fig 1 gives the system of frequency classification used by the NATO Forces and summarizes the effect
of operating frequency on the following characteristics of radar systems:
a.
Resolution. The ability of radar to resolve detail in angle, range, or velocity is directly related to
transmission frequency. With increasing frequency, the beamwidth for a given physical aerial size
can be made narrower, pulselengths may be shorter and the Doppler frequency shift for a given
target velocity becomes greater. For these reasons, the inherent resolving power of radar improves
in all dimensions with increasing frequency.
b.
Size and Weight. The physical dimensions of radar components, eg power oscillators,
waveguides, aerials, etc, are fundamentally related to the transmission wavelength. Size and
weight of equipment, therefore, reduce with increasing frequency and it is mainly for this reason
that airborne systems usually operate at I band frequencies and above.
c.
Power Handling Capacity. The power handling capacity, and hence performance, of a radar
system is mainly limited by the physical dimensions of its power oscillator and waveguides, the capacity
being reduced with increased frequency.
d.
Propagational Aspects. A number of propagational aspects are affected by the transmission
frequency. Above 10,000 MHz, 3 cm wavelength, attenuation due to both atmospheric gases and rain
begins to be significant and above 35,000 MHz it becomes prohibitively high for most purposes,
although there are some windows giving opportunity for use around 32-35 GHz and 94 GHz.
Susceptibility to unwanted clutter is another aspect which tends to get worse as frequency rises. Finally,
Revised Jul 10
Page 4 of 8
AP3456 – 11-1 - Introduction to Radar
the uniformity with which power is distributed in the vertical plane by a ground radar is strongly
dependent on frequency because the number of wavelengths in the height of the aerial determines the
number of interference lobes generated by ground reflection. At metric wavelengths, large gaps in
vertical cover may be unavoidable owing to the small number of lobes.
11-1 Fig 1 Radar Band Characteristics
Metric
C
(50-25 cms)
Resolution
Equipment
Power and
and precision
and weight
operating range
Interference
Radar
E/F
(10 cms)
increase
reduce
reduce
increases
Bands
(Wavelength)
I/J
(3 cms)
K
(1 cm)
Aerial Parameters
12. The function of a radar aerial during transmission is to concentrate the radiated energy into a
shaped beam which points in the desired direction in space. On reception, the aerial collects the
energy contained in the echo signal and delivers it to the receiver. Thus, in general, the radar aerial is
called upon to fulfil reciprocal but related roles.
13. In the radar equation derived in Volume 11, Chapter 2, Para 20 et al, these two roles are
expressed as:
a.
Transmitting Gain (G). In a transmitting antenna, gain is the ratio of the field strength
produced at a point along the line of maximum radiation by a given power radiated from the
antenna, to that produced at the same point by the same power from an omnidirectional antenna.
b.
Effective Receiving Aperture (A). The large apertures required for long-range detection
result in narrow beamwidths, one of the prime characteristics of radar. Narrow beamwidths are
important if accurate angular measurements are to be made or if targets close to one another are
to be resolved. The advantage of microwave frequencies for radar application is that with
apertures of relatively small physical size, but large in terms of wavelength, narrow beamwidths
can be obtained conveniently.
The two parameters are proportional to one another. An aerial with a large effective receiving aperture
implies a large transmitting gain.
14. The subject of microwave aerials is no longer discussed in AP 3456 and readers should research via
other sources.
Displays
15. Once the radar echo signal has been processed by the receiver the resulting information is
presented on a visual display, in a suitable form, for operator interpretation and action. When the
display is connected directly to the video output of the receiver, the information displayed is called
RAW VIDEO. This is the 'traditional' type of radar presentation. When the receiver video is first
Revised Jul 10
Page 5 of 8
AP3456 – 11-1 - Introduction to Radar
processed by an automatic detector or automatic detection and tracking processor (ADT), the output
displayed is sometimes called SYNTHETIC VIDEO.
16. The cathode-ray tube (CRT) has been almost universally used as the radar display. There are
two basic methods of indicating targets on a CRT:
a.
Deflection modulation, in which the target is indicated by the deflection of the electron beam.
An example of this is the Type A scope (Fig 2), which plots the amplitude of a received signal
against range on a horizontal line.
11-1 Fig 2 Type A Scope Display (Deflection Modulated)
Target
Signal
Amplitude
Noise
0
R A N G E
b.
Intensity modulation, in which the target is indicated by intensifying the electron beam and
presenting a luminous spot on the face of the CRT. The target thus appears brighter than the
background on the screen. An example of this is the Plan Position Indicator (PPI) (Fig 3) which
displays targets in a polar plot, providing 360º of azimuth cover, centred on the radar’s position.
The PPI can be directly correlated with the corresponding geographical map or chart.
11-1 Fig 3 PPI Display (Intensity Modulated)
0
R
A
N
G
E
270
0
090
Targets
180
Both methods are capable of indicating multiple targets.
Revised Jul 10
Page 6 of 8
AP3456 – 11-1 - Introduction to Radar
17.
Two Dimensional Displays. These displays are, of necessity, intensity modulated and may be
used to display any two of the target’s co-ordinates. For special purposes, the target co-ordinates can
be displayed in Cartesian form, directly related to the aerial location, rather than geographical situation.
Some common examples are:
a.
Sector PPI. A sector of a PPI can be displayed instead of the whole 360º (Fig 4). This gives a
relatively undistorted picture of the region which is being scanned in azimuth. The zero-azimuth
indicator is normally aligned with the aircraft’s heading or track. The sector PPI display is commonly
used for weather mapping, and in tactical aircraft for ground mapping, where the weight of the radar
system and aerial size result in fewer penalties than a full PPI display.
11-1 Fig 4 Sector PPI Display
Azimuth
0
E
G
N
A
R
b.
Type B Scope. The Type B scope (Fig 5) shows range and bearing in Cartesian form. In
this display, the zero-range point is expanded into a line along the bearing axis. The B scope
display is commonly used in fighter aircraft, where the increase in angular resolution at short
range has advantages over the PPI display.
c.
Type C Scope. The Type C scope shows target position by plotting elevation angle on the
vertical scale against azimuth indication horizontally. This is useful in fighter aircraft, where the
display corresponds to the pilot’s view through the canopy. The display can be projected onto a
head-up display. Fig 6 shows a target high and to the left of the aerial’s fore-aft axis.
Revised Jul 10
Page 7 of 8
AP3456 – 11-1 - Introduction to Radar
11-1 Fig 5 Type B Scope Display
e
g
n
a
R
Target
0
Azimuth
11-1 Fig 6 Type C Scope Display
n
tio
a
0
v
Target
le
E
0
Azimuth
18.
Technological Advances in Displays. In many radars, solid-state technology has replaced the
vacuum-tube CRT for displaying target information. Liquid crystal displays which can operate in high
ambient lighting conditions are suitable for some radar requirements. The plasma panel has
applications as a bright radar display capable of incorporating alphanumeric labels. Flat displays are
described in Volume 7, Chapter 29.
Revised Jul 10
Page 8 of 8
AP3456 – 11-2 - Pulse Radar
CHAPTER 2 - PULSE RADAR
Functional Description
1.
Pulse radar will determine the location of a target by measuring its range and bearing in the
following manner:
a.
Range is measured by pulse/time technique (see Volume 11, Chapter 1 Para 10a).
b.
Bearing is measured by indication of the aerial’s azimuth during its scanning movement.
2.
Fig 1 shows a block schematic diagram of a typical pulse radar system. The functions of the
various component stages are as follows:
a.
Master Timer. The master timer (sometimes referred to as the 'synchronizer') produces timing
pulses to control the pulse repetition frequency (PRF) of the radar. These timing pulses are supplied
synchronously to:
(1) The modulator to trigger the transmitter operation at precise and regular instants of time.
(2) The timebase generator of the indicator to synchronize the start of the CRT run-down
trace with the operation of the transmitter.
b.
Modulator. Upon receipt of each timing pulse, the modulator produces a square-formed
pulse of direct current energy to switch the transmitter on and off and so control the pulse width (τ)
of the transmitter output.
c.
Transmitter. The transmitter is a high-power oscillator, usually a magnetron. For the
duration of the input pulse from the modulator, the magnetron generates a high-power radio-
frequency wave. The wave is radiated into a waveguide, which carries it to the aerial.
d.
Transmit-Receive Switch. The Transmit-Receive (TR) Switch controls the flow of radio
waves between transmitter, aerial and receiver. The TR switch is normally a duplexer within the
waveguides. A duplexer is a passive device, sensitive to direction of flow of the radio waves. It
will allow the waves from the transmitter to pass to the aerial, while blocking their flow to the
receiver. Similarly, the duplexer allows the waves received at the aerial to pass to the receiver,
whilst blocking their way to the transmitter.
Revised Jul 10
Page 1 of 9
AP3456 – 11-2 - Pulse Radar
11-2 Fig 1 Block Schematic of a Typical Pulse Radar
Aerial
Azimuth and Elevation Angles of Aerial
Transmit er
TR Switch
Receiver
High Power
Weak Reflected
Pulses
Pulses
Switching
Target
Waveform
Indication
Modulator
Master Timer
Indicator
Trigger Pulses
Trigger Pulses
e.
Aerial. The aerial focuses the radiated energy into a beam of the required shape and
picks up the echoes reflected from the targets. Scanning can be achieved by moving the
complete aerial structure in azimuth and/or elevation. In phased arrays, scanning is done by
electronic means from a fixed aerial. In both systems, the aerial scan movement is conveyed
to and replicated in the indicator.
f.
Receiver. The receiver, which is usually a superhet, amplifies the very weak echoes and
presents them to the indicator in a suitable form for display.
g.
Indicator. The indicator is often a CRT. The actual display used will vary according to the
requirements of the system. One, two or all three target parameters (range, azimuth and
elevation) may be displayed.
Pulse Radar Parameters
3.
Pulse Width. Fig 2 shows the inter-relationship between the basic parameters of pulse radar.
The pulse of energy is transmitted when triggered by the Master Timer. The time duration of a single
pulse, termed the pulse width, is represented by the period 'τ'.
11-2 Fig 2 Pulse Radar Parameters
Interpulse Period (T)
τ
Peak Power
Mean Power
Power
τ
Time
Mean Power
Duty Cycle = =
T
Peak Power
4.
Pulse Length. Pulse width may also be expressed in terms of physical length. The pulse length
is therefore the distance between the leading and trailing edges of a pulse as it travels through space.
Pulse length (PL) can be calculated by the following formula:
PL = τ × c
Revised Jul 10
Page 2 of 9
AP3456 – 11-2 - Pulse Radar
where τ is pulse width in microseconds (μs) and c is the velocity of propagation (3 × 108 metres per
second). Thus, a pulse width of 1 μs equates to a pulse length of 300 metres.
5.
The Interpulse Period. The time period between the start of one pulse and the start of the next
pulse is the interpulse period (T), also called pulse interval or pulse repetition period.
6.
The Pulse Repetition Frequency. The pulse repetition frequency (PRF) is defined as the
number of pulses occurring in one second. The interpulse period is the reciprocal of the PRF. Thus
for a PRF of 500 pulses per second (pps), the interpulse pulse period is:
1
T =
sec ond =
,
2 000 micro sec onds ( s
µ )
500
In pulse radars, the PRF normally lies between about 200 and 6,000 pps.
7.
The Radar Duty Cycle. The ratio of the pulse width to the interpulse pulse period (τ/T) is known
as the duty cycle. This represents the fraction of time during which the transmitter operates. The duty
cycle controls the relationship between the mean power of the transmitter (upon which depends the
operating range of the radar) and the peak pulse power. As there is a limit to the peak power which
can be handled in a waveguide it is desirable that the duty cycle should be high. In some cases the duty
cycle is limited by the rating of the power source, but even when this is not the case the duty cycle cannot
be raised beyond limits without introducing either range ambiguity, due to excessive shortening of the
interpulse period T
, or loss of range resolution due to excessive lengthening of the pulse width, τ.
PRF and Range Ambiguity
8.
Range ambiguity occurs if the pulse transit time to the target and back exceeds the interpulse
period T. If the PRF is made too high, the likelihood of receiving target echoes from the wrong
pulse transmission is increased.
9.
Multiple-time-around Echoes. Echo signals received after an interval exceeding the interpulse
period (T) are called MULTIPLE-TIME-AROUND echoes. They can result in erroneous or confusing
measurements. Consider the three targets labelled A, B and C in Fig 3a. Target A is located within
the interpulse period (T). Target B is at a distance greater than T but less than twice T, while target C
is greater than twice T but less than three times T. The appearance of the three signals on a radar
display would appear as shown in Fig 3b. The multiple-time-around echoes (B and C) on the display
cannot be distinguished from the proper target echo of A, actually within the maximum unambiguous
range. Only the range measured for target A is correct; those for B and C are not.
Revised Jul 10
Page 3 of 9
AP3456 – 11-2 - Pulse Radar
11-2 Fig 3 Multiple-time-around Displays
Fig 3a Multiple-time-around Echoes
A
B
A
B C A
T
Fig 3b Radar Display
B
C
A
Fig 3c Use of Varying PRF
10. One method of distinguishing multiple-time-around echoes from unambiguous echoes is to operate
with a varying PRF. The echo from an unambiguous target will appear at the same place on the
display for each sweep of the time-base no matter whether the PRF is modulated or not. However,
echoes from multiple-time-around targets will be spread over a finite range as shown in Fig 3c.
Instead of modulating the PRF, other methods of 'marking' successive pulses so as to identify multiple-
time-around echoes could include changing the pulse amplitude, pulse width, phase, frequency or
polarization of the transmission from pulse to pulse, but these methods are rarely used.
11.
Maximum Unambiguous Range. To avoid range ambiguity, it is necessary to choose a value of
T sufficiently high to permit all possible echoes from one pulse to be received before transmission of
the next. The PRF used will therefore determine the maximum range at which targets can be
measured without ambiguity. The maximum unambiguous range (Runamb) can be calculated:
c
R
=
unamb
2 × PRF
For example, a typical long range search radar may operate at a PRF of 250 pps.
0
300,000,00
R
=
metres
unamb
2× 250
600,000
=
metres
= 600 km
= 324nm
1
A more restrictive short range target radar might operate at 1,000 pps, giving an Runamb of 81 nm.
Revised Jul 10
Page 4 of 9
AP3456 – 11-2 - Pulse Radar
Range Resolution
12. If two targets are close together in range, they may merge into one target on the display. Pulse
length is the fundamental factor determining the ability of a radar to resolve targets in range. It also
imposes a theoretical limit to the minimum range, down to which the radar can operate.
13. If two targets are separated in range by less than half the radial distance occupied by the pulse, they are
seen by the radar as a single echo. Thus a radar using a pulse width of 4 microseconds (i.e. pulse length =
1200 metres) would be able to discriminate between two targets provided they were separated in range by
more than 600 metres. Similarly, provided the receiver could begin to function at the instant the pulse
transmission ceased, the smallest range the radar could measure would be 600 metres. In radar systems
required to give high resolution, e.g. ground mapping, SAM and AI radars, pulse lengths may be in the region
of a microsecond or less, but this is only feasible if the PRF can be high.
Receiver Bandwidth
14. The radio frequency (RF) in a pulse radar transmitter is generated at a spot frequency but the effect
of pulse modulation is to cause the transmitted signal to consist of separate frequency components
spread across a wide spectrum. Most of the pulse energy is contained in the components which are
close to the basic RF, and 90% of the energy lies within a frequency band 2/τ MHz wide (where τ is the
pulse width in microseconds). It follows that the shorter the pulse the more widely spread is the pulse
energy and the greater must be the bandwidth of the receiver in order to accept the echo pulse without
distortion. The bandwidth of a pulse radar receiver is normally several MHz and in this major respect it
differs from a communications receiver which has a bandwidth measured in kHz.
Pulse Compression
15. Pulse compression allows a radar to utilize a long pulse to achieve ample radiated energy, but
simultaneously to obtain the range resolution of a short pulse. Pulse compression is a method of
achieving most of the short pulse benefits outlined in para 19 while keeping within the practical
constraints of peak-power limitations.
16. The degree to which the pulse is compressed is called pulse compression ratio. It is defined as
the ratio of uncompressed pulse width to the compressed pulse width. The pulse compression ratio
might be as small as 10 or as large as 105. Values from 100 to 300 might be considered as more
typical. There are many types of modulation used for pulse compression, but two that have wide
applications are:
a.
Linear frequency modulation.
b.
Phase-code pulse.
17.
Linear Frequency Modulation. In this version of a pulse compression radar the transmission is
frequency modulated and the receiver contains a pulse compression filter. The transmitted waveform
consists of a rectangular pulse of constant amplitude. The frequency increases linearly from f1 to f2
over the duration of the pulse. On reception, the frequency-modulated echo is passed through the
pulse compression filter, which is designed so that the velocity of propagation through the filter is
proportional to frequency. The effect is to produce a narrow pulse output from a wide pulse input.
Revised Jul 10
Page 5 of 9
AP3456 – 11-2 - Pulse Radar
18.
Phase-code Pulse. In this form of pulse compression, a long pulse is divided into a number of
sub-pulses. The phase of each sub-pulse is chosen to be either 0 or π radians. The choice of phase
for each sub-pulse may be set out as code.
Application of Short Pulses
19. The radar may require short pulse widths for the following purposes:
a.
Range Resolution. It is usually easier to separate targets in the range co-ordinate than in angle.
b.
Range Accuracy. If a radar is capable of good range resolution it is also capable of good
range accuracy.
c.
Clutter Reduction. A short pulse increases the target to clutter ratio by reducing the clutter
contained in the resolution cell with which the target competes.
d.
Glint Reduction. In a tracking radar, the angle and tracking errors introduced by a finite size
target are reduced by increased range resolution since it permits individual scattering centres to
be resolved.
e.
Multipath Resolution. Sufficient range resolution permits the separation of the desired
target echo from echoes that arrive via scattering from longer paths, or multipath.
f.
Minimum Range. A short pulse width allows a radar to operate with a short minimum range.
g.
Target Classification. The characteristic echo signal from a target when observed by a
short pulse can be used to distinguish one class of target from another.
h.
Electronic Protective Measures. A short pulse radar can negate the operation of certain
electronic counter measures (ECM) such as range gate stealers and repeater jammers, if the
response time of the ECM is greater than the radar pulse duration. The wide bandwidth of a short
pulse radar also has some advantage against noise jammers.
The Radar Equation
20. The radar equation provides the basis for analysing radar system performance, and in its
fundamental form it expresses the echo signal power (S) as a function of the system and target
parameters:
P G
σ
S =
×
× A watts
2
2
4 R
π
4 R
π
21. Fig 4 illustrates the composition of the first group of terms:
P G
2
4 R
π
which represents the power density in the transmitted wavefront as it passes over the target.
Revised Jul 10
Page 6 of 9
AP3456 – 11-2 - Pulse Radar
11-2 Fig 4 Power Density of Transmitted Wavefront
Transmit ed
Wavefront
P wat s
T
Target
R
Ae
R
P G
Power Density = 4 π R
Within the term, the radar pulse peak power P (in watts) is multiplied by the transmitting gain of the
aerial G, and then divided by the surface area of a sphere of radius R (metres).
22. When the first term is multiplied by the second term, the result is the power density in the echo
wavefront as it returns to the radar aerial (Fig 5). This assumes that the target re-radiates isotropically
the whole of the power intercepted over its cross-sectional area of σ. The power returning to the aerial
is therefore:
P G
σ
Power at Echo front =
×
2
2
4 R
π
4 R
π
11-2 Fig 5 Power Density of Returned Echo
Echo
Wavefront
Ae
T
R
Target
(o square
R
metres)
S wat s
Power Density
P G
o
of Echo wavefront = 4 π
×
R
4 π R
Finally, multiplication by the effective receiving aperture of the aerial, in square metres (A) gives the
amount of echo power intercepted by the radar. The received power (S) is therefore:
P G
σ
S =
×
× A watts
2
2
4 R
π
4 R
π
PGAσ
or, S =
watts
(4 )
π 2 R4
Revised Jul 10
Page 7 of 9
AP3456 – 11-2 - Pulse Radar
23. The maximum detection range of a radar system will correspond to the smallest signal which can
be satisfactorily recognized. Expressing this as Smin it follows that:
4
PGA σ
Rmax = (4 )2
π
m
S in
In a pulse radar the factors G and A are applicable to the same aerial and are related by the
expression:
G = 4πA/λ2
It is therefore possible to put the radar equation into two other forms:
4
2 2
PG λ σ
R max = (4 )3
π Smin
or
4
2
PA σ
R max =
2
4πλ
m
S in
24. The following observations need to be made on the parameters used in the radar equation:
a.
Power. Detection range depends on the fourth root of transmitted peak power. Doubling the
power therefore increases range by 4 2 (that is, by only 19%), while to double range it is
necessary to raise the power by 24, ie 16 times.
b.
Aerial. For long range operation the radiated energy must be concentrated into a narrow beam
for high aerial gain and the received echo must be collected with a large aerial aperture (synonymous
with high gain). However, for airborne systems, the increase in aerial size may be unacceptable.
c.
Wavelength. Wavelength does not directly influence detection range except in the sense that it
determines the aerial gain for a given area, or conversely, the area required for a given gain. Indirectly,
however, wavelength has a considerable bearing on the matter because it sets an upper limit to the
peak power which can be handled. The smaller the wavelength, the smaller the peak power.
d.
Minimum Detectable Signal. The size of the minimum detectable signal depends on a
number of factors, of which the following are the most important:
(1)
Receiver Noise. The greater the noise in the receiver, the greater must be the signal
for a given probability of detection.
(2)
Scanning Parameters. The smaller the angular volume through which the radar is
required to scan, the greater is the proportion of the radiated power which falls on the target
and the greater is the extent to which enhancement of the signal can take place due to
integration of successive pulses.
(3)
Display Parameters. The skill of the operator and the persistence of the CRT screen
both affect the minimum signal that can be detected visually.
e.
Target Echoing Area. In the derivation of the radar equation it was assumed that the target re-
radiated isotropically the whole of the energy intercepted over its cross-sectional area. This is only true
for a large spherical target; normal targets are complex in shape and do not re-radiate isotropically. For
example, a flat-sided target at right angles to the incident wave would reflect nearly all of the intercepted
Revised Jul 10
Page 8 of 9
AP3456 – 11-2 - Pulse Radar
energy back towards the radar, but if slightly inclined from the right angle most of the energy would be
reflected away from the radar. Thus for practically no change in the true cross-sectional area the echo
signal would change drastically. In order to describe the reflective properties of targets it is necessary to
adopt a fictitious quantity cal ed echoing area (σ). For a particular aspect of a target it is the cross-
sectional area it would need to have in order to account for its echo signal, assuming it to re-radiate
isotropically. The echoing area of aircraft targets may vary with aspect over very wide limits and give
differences in echo power of as much as 40 to 1 for as little as 0.3º change of aspect. Since the aspect
of a target is continually changing with respect to the radar, partly due to changing position and partly to
flight oscillations and vibrations, the echo signal may fluctuate at some characteristic period or
combination of periods.
25.
Modifications of the Radar Equation. The simple radar equation discussed above does not
predict the full range performance of actual radar equipments to a satisfactory degree of accuracy. It
assumes free space propagation and takes no account of the effects of ground reflection, atmospheric
refraction, absorption or diffraction around the Earth’s surface. It also fails to take into account the
various losses that can occur throughout the system. It is possible to develop a more complex form of
the radar equation to include these and other factors which influence radar range performance. The
simple equation, which is applicable to pulse systems, may also be modified to cover the operation of
any radar system (CW, FMCW, Pulse Doppler, Secondary, Semi-active etc)
. A more detailed study of
the radar equation is, however, outside the scope of this chapter.
Revised Jul 10
Page 9 of 9
AP3456 – 11-3 - Anti-Clutter Noise Techniques
CHAPTER 3 - ANTI-CLUTTER/NOISE TECHNIQUES
Introduction
1.
In theory the maximum range at which a target can be detected by radar could be extended
almost indefinitely by adding more and more amplifier stages to the receiver. The existence of
background noise, both man-made and natural, prevents this in practice, since a stage is reached at
which the level of signal falls below the background noise level and becomes obscured by it. If the
receiver gain is increased, both signal and noise are amplified equally.
2.
Another limiting factor on the performance of a radar receiver is the presence of clutter. Clutter
may be defined as any unwanted radar echo. Its name is descriptive of the fact that such echoes can
'clutter' the radar output and make it difficult to detect wanted targets. Examples of unwanted echoes
include the reflections from land, sea, rain, birds, insects and chaff.
3.
This chapter deals with some of the devices and techniques used in radar to limit the effect of noise
and clutter on receiver performance. These devices range from such simple circuit refinements as swept
gain to more complex systems such as Moving Target Indication, Pulse Integration and Constant False
Alarm Rate receivers.
CLUTTER
Circuit Refinements
4.
Modifications and additions that can be made to the circuits of a normal radar receiver in an
attempt to reduce clutter include the following:
a.
Swept Gain. Clutter is worst at short ranges and diminishes along the time base. The
swept gain control makes use of this fact and causes the receiver gain to be lowered at the
commencement of the time base as each pulse is fired, and thereafter increases the gain
exponentially with time. In this way, saturation of the receiver is avoided without the elimination
of weaker signals from more distant ranges. In American terminology this facility is called
Sensitivity Time Control. (Not to be confused with Fast or Short Time Constant-see para 4b.)
b.
Fast (or Short) Time Constant (FTC), (STC). A differentiating circuit which only has an
output when there are increases in the level of incoming signals. As a result, only the leading
edges of long pulses are displayed.
c.
Instantaneous Automatic Gain Control (IAGC). A fast-acting AGC circuit which lowers
the receiver gain two or three pulse lengths after receipt of an incoming signal. Short pulses are
therefore unaffected and only the leading edges of long pulses are displayed.
d.
Pulse length Discrimination. Echo signals from wanted targets are rarely of greater
duration than a pulse length or so and this provides a basis for rejecting a large proportion of clutter
signals, the majority of which are of considerable duration. Certain types of jamming may also be
attenuated in this way. A number of anti-clutter devices discriminate on the basis of pulse length,
although they may differ considerably in detail, complexity and effectiveness. The main difficulty is
that wanted signals of short duration occurring within the period of a rejected pulse must still be
displayed. By the use of storage techniques all incoming signals are delayed sufficiently before
Revised Jul 10
Page 1 of 6
AP3456 – 11-3 - Anti-Clutter Noise Techniques
display to permit their lengths to be measured. Those exceeding a few pulse lengths are totally
suppressed.
e
.
Pulse Interference Suppression. This device operates on the incoming signals in such a
way that only those occurring at the transmitter repetition frequency are passed to the display. It
is, therefore, effective against interference from other radars operating at different PRFs and to
non-synchronous pulse jamming.
f
.
Dicke Fix. This is a technique that is designed to protect the receiver from fast sweep noise
jamming. It consists of a broad-band limiting IF amplifier followed by an IF amplifier of optimum
signal bandwidth for the known transmitted frequency. The wide band amplifier amplifies both the
noise and the signal. The limiter which follows cuts the peak noise amplitude associated with the
signal, with the following narrow band amplifier limiting the bandwidth to that of the signal, thus
further excluding the unwanted noise (Fig 1). The reduction in noise/jamming that can be
achieved is in the order of 20 to 40 dB ( 1
1
th to
th ).
100
10 0
, 00
11-3 Fig 1 Dicke Fix Receiver
Wide
Noise
Narrow
Maximum
Signal + Noise
Band
Limiter
Band
Signal/Noise
Amp
Amp
Ratio
Noise
f
f
f
f
Signal
Logarithmic Amplifiers
5.
The normal radar receiver has a linear response, its output being proportional to the received
signal level. Once the limit of the output is achieved, further increases in the input can have no effect
and the receiver is said to be saturated. Saturation can be brought about by strong responses from
PEs, cloud and many types of jamming. Under these conditions wanted signals may either be
swamped, or, if the receiver gain is lowered in an attempt to accommodate the large signals, the
wanted signals may fall below the visibility threshold. An alternative receiver which may be used in
these circumstances is one having an amplifier giving a logarithmic response, the output being
proportional to the log of the input amplitude. This makes it possible to receive even the strongest
signals without saturation and it is an effective way of countering clutter and certain types of jamming.
Because the logarithmic receiver amplifies small signals more than large ones it has the effect of
reducing the signal to noise ratio and so reduces the detection range of the radar. (But this is a small
price to pay for the ability to see through clutter.)
Revised Jul 10
Page 2 of 6
AP3456 – 11-3 - Anti-Clutter Noise Techniques
Circular Polarization
6.
An interesting method of attenuating returns from rain and heavy cloud is to make use of the fact
that rain-drops, unlike other targets, are nearly perfect spheres and so return incident waves without
change of polarization whereas complex targets always depolarize signals to some degree. To exploit
this fact, it is necessary to radiate circularly-polarized waves. If the rotational sense of the waves as
seen from the aerial is right-handed, then waves reflected from rain and cloud will be wholly left-
handed when seen from the point of reflection. Returns from complex targets on the other hand, will
be partly right-handed and partly left-handed. Now, the characteristics of the aerial are such that it will
totally reject circularly-polarized waves of the opposite rotational sense to that which it radiates, thus
none of the energy returned from rain and cloud should ever enter the receiver. In practice, some
energy does enter because rain-drops are not perfect spheres and, in addition to this, returns from
other targets are weaker than would have been the case with plane polarization. The net result,
however, is a significant improvement in the ratio of the amplitudes of wanted to unwanted signals.
Circular polarization is normally brought into action, when required, by interposing a grid (known as a
quarter-wave plate) between the aerial feed and the reflector. Its action is to split the electric field
vector into two equal components at right angles to one another, and to advance (or retard) one of
these by a quarter of a wavelength. The addition of the two components is then a vector of constant
amplitude rotating at the wave frequency. In some cases, the transition from plane polarization may
be progressively through elliptical to circular.
Moving Target Indication (MTI)
7.
Any target which moves in relation to a ground radar is likely to be of interest, whereas those
which do not are normally unwanted. As the frequency of echoes from moving targets is shifted by the
Doppler effect, this provides a powerful basis for distinguishing between wanted and unwanted
targets. To recognize echoes containing a frequency shift, a coherent frequency reference must be
established in the radar. When incoming signals are mixed with this reference, the components from
moving targets vary in amplitude from pulse to pulse at the Doppler frequency corresponding to the
relative velocity of the target. The components of the incoming signals returned from fixed targets do
not vary in frequency. By feeding signals which are the algebraic difference of the outputs from
successive pulses to the display, the fixed echo components cancel, whereas the moving echo
components always leave a residual signal. This is achieved by dividing the receiver output into two
channels. In one channel the signals are delayed for an interval equal to the pulse period. The lines
re-unite at a subtraction unit, the output being the difference between two successive trains of pulses.
8.
Application of MTI. The effectiveness of MTI increases with the number of pulses-per-scan and it
is therefore more worthwhile to apply MTI to a broad beamed radar than to a narrow one. The
effectiveness may also be increased by employing double or triple cancellation, the cancellation circuits
being connected in series. Yet another method is to eliminate the possibility of mis-match between the
pulse period and delay line interval by using the latter to control the PRF. A refinement which may
sometimes be added is the means of applying Doppler compensation to selected areas of a PPI display
so as to attenuate returns from moving clouds or rain storms. (When this is done, fixed echoes within
the compensated area will re-appear.) The same device is effective against moving 'Chaff' clouds.
9.
Blind Speeds. A disadvantage of MTI radars is the existence of blind speeds. When the
component of a target’s velocity towards or away from the radar is equal to either zero, half
(wavelength × PRF) or any multiple of that speed, the signals returned from it will not vary from pulse
Revised Jul 10
Page 3 of 6
AP3456 – 11-3 - Anti-Clutter Noise Techniques
to pulse with the result that they will cancel when subtracted. The lowest blind speed in an MTI radar
is given by:
Wavelength (cm)× PRF (pulses per s
ec) (kt)
103
It is desirable that the first blind speed should be as high as possible but the presence of the PRF in the
numerator means that this condition is incompatible with a large unambiguous range. Fig 2 shows the
relationship between blind speed and unambiguous range for representative wavelengths. The
superiority of the longer wavelengths is apparent. Blind speeds can be avoided by using multiple PRFs
as discussed in Volume 11, Chapter 5.
11-3 Fig 2 First MTI Blind Speed as a Function of Unambiguous Range
10,000
λ =10
λ
0
=
cm
30
λ
c
=
m
t)
1
1,000
0
λ
(k
c
=
m
d
3
e
cm
e
p
S
d
lin
B
t
irs
100
F
10
1
10
100
1,000
Maximum Unambiguous Range (Nautical Miles)
10.
Airborne MTI. MTI has possible applications in airborne early warning, ASW radars,
reconnaissance radars and AI. The broad principles of airborne MTI do not differ from those
described but the methods of implementation may vary considerably. In order to be able to see
targets which move with respect to the Earth it is first necessary to compensate for the velocity of
the radar carrier. This requires a false Doppler shift in the reference signal proportional to the
component of aircraft velocity along the radar beam. If it is a scanning beam the compensation
must be continually changing.
NOISE
False Alarm Rates
11. Whenever the noise level envelope of a receiver exceeds the receiver threshold, a target
detection is considered to have taken place, by definition (see Fig 3).
Revised Jul 10
Page 4 of 6
AP3456 – 11-3 - Anti-Clutter Noise Techniques
11-3 Fig 3 False Alarms Due to Noise
)
lts
o
(V
e
Threshold
is
o V
N
T
f
o
e
p
lo
e
Threshold
Rms Noise
v
n
Voltage
Voltage
E
Time
12. The average time between false alarms, and therefore the rate, is a function of:
a.
The threshold level voltage.
b.
The receiver bandwidth.
c.
The root-mean-square (rms) noise voltage.
That is to say, increases in threshold voltage reduce the rate and increases in bandwidth and noise
voltage increase the rate. These relationships are depicted in the graphs shown in Fig 4.
11-3 Fig 4 Average Time Between False Alarms
100
z
3 days
s
H1
2 days
z
rm
H
la
01
1 day
A
z
e
H
12 h
ls
10
0
a
0
F
1
z
n
)
Hk
e
rs
e
1
u
o
z
tw
H
e
(H
k
B
0
e
1
1
z
1 h
im
H
T
k0
e
0
z
g
1
H
z
ra
M
H
15 min
e
1
M
v
0
A
0.1
1
8
9
10
11
12
13
14
15
Threshold-to-Noise Ratio
Constant False Alarm Rate (Receivers)
13. The false alarm rate is quite sensitive to receiver threshold voltage level. For example, a 1dB
change in threshold can result in three orders of magnitude change in false alarm probability. It does
not take much of a drift in receiver gain, a change in receiver noise, or the presence of external noise
or clutter echoes to inundate the radar display with extraneous responses.
14. If the changes in false alarm rate are gradual, an operator viewing a display can compensate with
manual gain adjustment. It is said that the maximum increase in noise level that can be tolerated with
Revised Jul 10
Page 5 of 6
AP3456 – 11-3 - Anti-Clutter Noise Techniques
a manual system using displays is between 5 dBs and 10 dBs. But with an automatic detection and
tracking system, the tolerable increase is less than l dB. Excessive false alarms in an ADT system
cause the computer to overload as it attempts to associate false alarms with established tracks or to
generate new, but false, tracks. Manual control is too slow and imprecise for automatic systems.
Some automatic, instantaneous means is required to maintain a constant false alarm rate. Devices
that accomplish this purpose are called CFAR.
Pulse Integration
15. The number of pulses returned from a point target as a radar aerial scans through its beam width
is a function of the aerial beam width, the scanning rate and PRF. Typical parameters for a ground-
based search radar might be a PRF of 300 Hz, 1.5º beam width, and a scan rate of 5 rpm. These
parameters result in 15 hits from a point target per scan. By summing these returns through a process
of integration the signal/noise ratio, and therefore the detection, can be improved. All practical
integration techniques use some sort of storage device.
16. Integration may be accomplished in a radar receiver either before the second detector (in the IF)
or after the second detector (in the video). Integration before the detector is called pre-detection, or
coherent integration, while integration after the detector is called post detection, or noncoherent
integration. If n pulses, all of the same amplitude would be integrated by an ideal pre-detection
integrator, the resultant signal/noise ratio would be exactly n times that of a single pulse. If the same n
pulses were integrated by an ideal post detection device, the resultant signal/noise ratio would be less
than n times that of a single pulse. This loss in integration efficiency is caused by the nonlinear action
of the second detector, which converts some of the signal energy into noise energy in the rectification
process. Although pre-detection integration is more efficient than post detection, the latter is easier to
implement in most applications.
Revised Jul 10
Page 6 of 6
AP3456 – 11-4 - Continuous Wave Radar
CHAPTER 4 - CONTINUOUS WAVE RADAR
Introduction
1.
A pulse transmission gives the inherent ability to measure range, permits the use of a single aerial and
eliminates the possibility that transmitter noise will interfere with the detection of weak echo signals. The
disadvantages are the breadth of the transmitted spectrum, which necessitates a large receiver bandwidth,
the finite minimum range, and the need to handle high peak powers in order to achieve the required mean
power.
2.
In continuous wave radar, the transmitted spectrum is a single frequency and the receiver
bandwidth may therefore be very narrow. Moreover, as the duty cycle is unity, the mean power can be
as high as the available transmitter will permit. The absence of timing marks in the transmission of a
pure CW radar means that the ability to measure range is lost but in place of this the coherence of the
transmission makes it possible to exploit the Doppler shift in the echo signal to resolve target velocity.
Unlike pulse radar, the transmitter is never silent, and except at very low power levels where single
aerial CW working is possible, it is necessary to employ separate aerials for transmission and
reception in order to isolate the receiver from the transmitter.
Pure CW Radar
3.
In a pure CW radar, both the transmitted and received signals consist (for practical reasons) of a
single frequency component, but if there is relative velocity between the target and radar, the echo signal
will differ in frequency from the transmitted signal due to the Doppler effect. The magnitude of the
Doppler shift, which is positive for closing velocities and negative for opening velocities, is given by:
2V
f = ±
d
λ
where:
fd = Two-way Doppler shift.
V = Radial velocity component between target and radar.
λ = Transmitted wavelength.
4.
A plot of doppler shift as a function of radar frequency and target relative velocity is shown in Fig 1.
Revised Jul 10
Page 1 of 5
AP3456 – 11-4 - Continuous Wave Radar
11-4 Fig 1 Doppler Shift
10,000
knots
5,000
z
1,000
H
2,000
y,
c
1,000
n
e
500
u
q
200
re
F
r
100
le
50
p
p
o
20
D
100
10
knots
5
10
10
100
1,000
10,000
100,000
Radar Frequency, MHz
5.
The existence of an echo is detected by mixing the incoming signal with an attenuated portion of the
transmitted signal to produce the difference frequency fd. The frequency of the output from the mixer is
therefore proportional to the radial velocity of the target, and is zero for non-moving targets. This fact
gives the CW radar the outstanding characteristic of being able to discriminate between fixed and moving
targets. A simplified block diagram of the system is shown at Fig 2.
11-4 Fig 2 Simple CW Radar
f
Transmitter
f
f + fd
Mixer
fd
6.
Velocity Resolution. In order to resolve target velocity it is necessary to determine the sense of
the Doppler shift fd and to measure its magnitude. One way of measuring fd is shown in Fig 3. The
output of the mixer is passed to a bank of Doppler filters, each of which is tuned to accept a discrete
band of frequencies within the Doppler range. The filter bank is arranged to cover the whole range of
possible Doppler frequencies and an output from a particular filter indicates the existence of a target at
the corresponding velocity. The precision with which velocity can be resolved is determined by the
bandwidths of each particular filter in the filter bank, and hence depends on the complexity which can
be tolerated in the system. For example, in order to resolve velocity to within 4 kt over a range 0 to
20,000 kt it would be necessary to employ 500 filters. The outputs of the filters are interrogated
sequentially by a fast-acting electronic switch which looks across the entire filter bank several times
during the time required for the radar beam to scan through its beamwidth. The means of determining
the sense of the Doppler shift is not shown in Fig 3.
Revised Jul 10
Page 2 of 5
AP3456 – 11-4 - Continuous Wave Radar
11-4 Fig 3 Velocity Resolution by Doppler Filters
fd (1)
fd (2)
fd (3)
fd
To Display or
fd (4)
From
Data Processing
Mixer
fd (n)
7.
Bandwidth. The overall bandwidth of a CW radar must be wide enough to encompass the range
of Doppler frequencies it is required to measure and this will amount to several kilohertz at the most.
If, however, a Doppler filter bank is used to provide the velocity resolution, the effective bandwidth is
that of the individual filters. In an I-band radar capable of resolving velocity to 4 kt, the filters would
have a bandwidth of about 125 Hz, which is at least four orders of magnitude less than is the case for a
pulse radar. An example is shown in Fig 4.
11-4 Fig 4 IF/Filter Bandwidths
Filter No 1
Det
Filter No 2
Det
IF
Filter No 3
Det
Indicator
Amplifier
Filter No 4
Det
Filter No n
Det
IF Bandwidth
f
f
f
f
f
1
2
3
4
n
Frequency
8.
Characteristics and Applications. The outstanding characteristic of CW radar is its ability to
see moving targets in the presence of large echoes from fixed targets, to which it is blind. It is a
powerful device for the detection of low flying targets and for discriminating against chaff jamming. It
will not, however, see moving targets which cross its beam at right angles. Because of its inability to
measure range, its use is confined to applications in which this can be provided by separate means, or
where range measurement is non-essential, as in ground-to-air and air-to-air guidance by active and
semi-active means. Other advantages of the CW radar are its basic simplicity, its ability to utilize high
Revised Jul 10
Page 3 of 5
AP3456 – 11-4 - Continuous Wave Radar
mean power and the fact that it is not subject to a minimum range of operation. In airborne
applications, CW is used for doppler navigation and rate-of-climb in VTOL aircraft.
Frequency Modulated CW Radar
9.
If timing marks are introduced into the transmission of a CW radar by modulating the frequency it
becomes possible to measure range, but accuracy comparable to pulse radar ranging is only possible
over very short distances. Radio altimeters employing FMCW transmissions are a familiar example of
this technique applied over distances of up to 5,000 feet or so. In many radar applications it is
sufficient to be able to obtain an approximate target range, eg in order to establish when a target
comes within the launch range of a missile. For such applications, an FMCW radar may be used.
10. A common form of transmission for an FMCW radar is shown in Fig 5. The frequency of the
transmitted signal (represented by the full line in the upper diagram) is swept linearly with time back and
forth over the band B. An echo signal (represented by the broken line) received from a stationary target at
range R will be of exactly similar form but displaced in time by the interval 2R/C. The difference frequency
fR between the transmitted and received signals can be measured and is proportional to the time interval
2R/C and hence to the target range. This is the basis of the FMCW ranging technique.
11-4 Fig 5 Range Measurement of a Stationary Target
2R
C
T
f
Frequency
R
B
Time
Difference
Frequency
fR
11. In the more general case of a moving target the echo signal still has the same form as the
transmitted signal but, in addition to being displaced in time, it is also displaced in frequency by the
Doppler shift fd. Fig 6 shows the relationship between the transmitted and received signals in the case
of a target with a closing velocity. During the first half of the modulation period the frequency
difference fR due to range is reduced by the Doppler shift fd, while during the second half of the cycle it
is increased by the same amount. If the transmitted and received signals are mixed, as in the pure
CW radar, the difference frequency will alternate between fR + fd and fR # fd as shown in the lower half
of Fig 6. In order to extract both the range and velocity of the target this signal must be processed in
such a way as to produce the sum and difference of the two components since:
(fR + fd) + (fR - fd) = 2fR is proportional to target range,
and (fR + fd) - (fR - fd) = 2fd is proportional to target velocity.
12.
Ranging Accuracy. Ranging accuracy in an FMCW radar is a function of the rate of change of
frequency in the transmitted signal which, as can be seen from Fig 5, must be high in order to produce
a large change in the difference frequency f R for a corresponding increment in range. This in turn
calls for the swept frequency B to be large or the modulation period T to be short. The former is
Revised Jul 10
Page 4 of 5
AP3456 – 11-4 - Continuous Wave Radar
undesirable beyond limits because of the spread in the transmitted spectrum which is reflected in the
receiver bandwidth, and the latter is controlled by range ambiguity considerations. Only if the ranging
is to be carried out over short distances can the modulation period be sufficiently short to permit the
accuracy to be comparable to that of a pulse radar.
11-4 Fig 6 Range Measurement of a Moving Target
10
10
GHz
KHz
10
20
30 Milli-Seconds
10
Difference
f
Frequency
5
R + fd
(KHz)
0
f
−
R fd
The FMCW Altimeter
13. The FMCW radar principle is used in the aircraft low-level radio altimeter to measure height above
the surface of the earth. The relatively short ranges required permit low transmitter power and low
aerial gain. Since the relative motion between the aircraft and ground is small, the effect of the
Doppler frequency shift may usually be neglected.
14. The absolute accuracy of radar altimeters is usually of more importance at very low altitudes than at
high altitudes. Errors of a few feet might not be significant when operating at 4,000 feet, but are important
if the altimeter is part of a blind landing system. Errors can be introduced into the system if there are
uncontrolled variations in the transmitter frequency and modulation frequency. Multipath signals also
produce errors. Fig 7 shows some of the unwanted signals that might occur in an FMCW altimeter. The
wanted signal is shown by the solid line, while the unwanted signals are shown by broken arrows.
11-4 Fig 7 Unwanted Signals in an FMCW Altimeter
Transmitter
Receiver
Aircraft Structure
Wanted
Signal
Ground
Revised Jul 10
Page 5 of 5
AP3456 – 11-5 - Pulse Doppler Radar and MTI
CHAPTER 5 - PULSE DOPPLER RADAR AND MTI
Introduction
1.
Pulse Doppler radar is an attempt to combine, in a single system, the attributes of both pulse and
CW radars. It employs a coherent, pulsed transmission of high duty cycle, sometimes described as
interrupted CW, which gives it the ability to resolve both range and velocity. Unfortunately, however,
there is an inherent incompatibility between the two processes which makes it impossible to avoid
ambiguity in both range and velocity at the same time, and the successful implementation of the
principle hinges on how effectively this incompatibility can be resolved.
2.
A Moving Target Indicator (MTI) radar, which in the broadest sense is a form of pulse Doppler
radar, normally operates at a sufficiently low PRF to avoid range ambiguity, and, as a result, suffers
from blind speeds within the velocity range of interest. If the Doppler shift could be measured, it would
be found to correspond to any one of a number of possible target speeds, separated from one another
by half the interval between the blind speeds. In other words, the velocity resolution would be
ambiguous. In the true pulse Doppler radar, the reverse situation applies. In this case, the PRF is
normally high enough to avoid blind and ambiguous velocities, but, as a consequence, it suffers from
both blind and ambiguous ranges. A blind range occurs whenever the pulse transit time is such that
the echo arrives back during the transmission of a pulse, and, under these conditions, the radar cannot
be aware of the existence of the target. There are a number of possible ways in which the blind ranges
may be alleviated and the ambiguity resolved.
3.
The pulse Doppler radar employs a single aerial and the coherent transmission is obtained from a
power- amplified master oscillator output. The pulses may be of comparable length to those used in an
equivalent pulse radar, but the PRF is many times greater and permits a high duty cycle which may well
approach ½, ie pulse length equal to separation time. This fact means that it possesses the attribute of
the CW radar and that it can utilize high mean power without the problem of handling high peak power
which occurs in pulse radar. The Doppler information is obtained by beating the echo signal with a
sample of the transmitter oscillator signal, and velocity resolution is performed by means of filters. The
resolution of range, which is fundamentally a timing process, may be performed in one of several ways
and depends mainly on the method used to sort out the ambiguities. To appreciate the extent of the latter
problem it is necessary to examine the overall question of ambiguity in greater detail.
Range and Velocity Ambiguity
4.
The maximum range which can be measured without ambiguity in a pulse radar is discussed in
Volume 11, Chapter 2. The equation to calculate it is:
c
R
=
unamb
2 × PRF
Examination of this equation shows that, for a given time interval measured between a returning radar
echo and the preceding transmitted pulse, the corresponding target range could be any one of a
number of possible target ranges separated from one another by the distance c/(2 × PRF).
5.
There is no equivalent to this situation in a pure CW radar which can, theoretically, measure
unlimited velocity. However, in a pulse Doppler radar, velocity ambiguity occurs because the
transmitted spectrum, unlike that of a pure CW transmission, consists of a number of discrete
frequency components centred on the carrier frequency and separated from one another by the PRF.
As a Doppler shift affects all components alike, the echo pulse consists of an exactly similar spectrum
in which all components have been translated to a higher or lower frequency by the extent of the
Revised Jul 10
Page 1 of 4
AP3456 – 11-5 - Pulse Doppler Radar and MTI
Doppler shift. If the Doppler frequency is detected by beating the echo spectrum with a pure CW
signal from the master oscillator, the result is the same whether the shift is positive or negative, and
the highest fundamental beat frequency which can be produced is that which occurs whenever the
reference frequency lies mid-way between two of the components of the echo spectrum. In other
words, velocity measurement will become ambiguous if the Doppler frequency shift (2V/λ) is more than
half the PRF. The avoidance of this situation requires that the operating PRF should be at least twice
the highest Doppler frequency which the system must be capable of measuring.
6.
Since the maximum unambiguous velocity occurs when the Doppler shift (2V/λ) = PRF/2, it
follows that:
PRF × λ
V
=
unamb
4
which is half the interval between blind speeds. To illustrate the mutual incompatibility of high
unambiguous velocity and range, consider the case of a three centimetre radar (λ = 0.03 m) required
to resolve velocity unambiguously up to a speed of 1,000 kt (514.8 m/s). Substituting these values in
the above equation gives:
PRF × 0.03
514.8 =
4
514.8× 4
∴ PRF =
0.03
= 68,640 pulses per s econd (pps)
Putting this value of PRF into the equation in para 4:
3×108
300,000,0 0
0
R
=
=
unamb
2 × 68,640
137,280
= 2185.3 m = (2185.3×5.399×10−4) nm
= 1.18 nm
This result shows that the range information in such a radar would be ambiguous in steps of just over
one nautical mile. Moreover, this would also be the interval between the blind ranges.
Resolution of Velocity Ambiguity
7.
The blind speeds of two independent radars operating at the same frequency will be different if
their pulse repetition frequencies are different. Therefore, if one radar was 'blind' to moving targets, it
would be unlikely that the other radar would be blind also. Instead of using two separate radars, the
same result can be obtained with one radar which 'time-shares' its pulse repetition frequency between
two or more different values (multiple PRFs). The pulse repetition frequency might be switched every
other scan or every time the aerial has scanned a half beamwidth, or the period might be alternated on
every other pulse. When the switching is pulse to pulse, it is known as 'Staggered PRF'.
8.
An example of the composite response of a radar operating with two separate pulse repetition
frequencies on a time-shared basis is shown in Fig 1. (Tl = 1/PRF1 and T2 = 1/PRF2.) The pulse repetition
frequencies are in the ratio 5:4. Note that the first blind speed of the composite response is increased
several times over that what it would be for a radar operating on only a single PRF.
Revised Jul 10
Page 2 of 4
AP3456 – 11-5 - Pulse Doppler Radar and MTI
11-5 Fig 1 Frequency Response for Time-shared PRFs
1.0
0.8
e
e
s
n
o
0.6
tiv
p
la
s
e
e
0.4
R
R
0.2
0 0
1
2
3
4
—
—
—
—
T
T
T
T
1
1
1
1
Frequency PRFI
1.0
0.8
e
e
s
n
o
0.6
tiv
p
la
s
e
e
0.4
R
R
0.2
0 0
1
2
3
4
5
—
—
—
—
—
T
T
T
T
T
2
2
2
2
2
Frequency PRF2
1.0
0.8
e
e
s
n
o
0.6
tiv
p
la
s
e
e
0.4
R
R
0.2
0 0
1
1
2
2
3
3
4
5
4
—
—
—
—
—
—
—
— —
T
T
T
T
T
2
T
2
T
2
T
2
2
T
1
1
1
1
Frequency Composite
9.
The closer the ratio Tl:T2 approaches unity, the greater will be the value of the first blind speed.
However, the first null becomes deeper. Thus the choice of Tl/T2 is a compromise between the value
of the first blind speed and the depth of the nulls. The depth of the nulls can be reduced and the first
blind speed increased by operating with more than two interpulse periods.
Resolution of Range Ambiguity
10. The recovery of target information in the range gaps is the first step which has to be taken to make a
practical pulse Doppler system. This may be done by varying the PRF continuously or between discrete
values, or by transmitting simultaneously at more than one PRF. The recovery of signal by these means is
at the expense of the signal outside the gaps. The same devices may also be used as the basis for
resolving range ambiguity.
11. Another method of resolving the ambiguity is by coding the transmission at a much lower
repetition frequency, possibly by modulating the frequency as in the FMCW radar. Over long
distances, such a system would provide only crude ranging and the fine ranging would be performed
by the pulse modulation. Yet another method, is to give the radar an alternative mode of operation
which can be brought into action periodically, or whenever it is required, to obtain the range of a target.
In essence, this would consist of reverting to a conventional pulse radar transmission.
Revised Jul 10
Page 3 of 4
AP3456 – 11-5 - Pulse Doppler Radar and MTI
Applications of Pulse Doppler Radar
12. The principle attraction of pulse Doppler radar is its ability to combine most of the advantages of
pulse and CW radars. These include the ability to resolve both range and velocity, the ability to utilize
high mean power and the fact that it requires only one aerial. The advantage that echo detection is
carried out while the transmitter is silent, is largely nullified by the loss of signals in the range gaps. On
the other hand, the use of a coherent transmission permits the adoption of techniques for the pulse-to-
pulse enhancement of weak echoes, which results in greater detection ranges.
13. The possible applications for pulse Doppler radar cover a wide field; including the long range
detection of ballistic missiles, the detection of low flying targets and all MTI applications. One of the
most promising spheres is in airborne MTI systems. Despite the complexity of pulse Doppler it is
probable that it has a greater operational potential than any other radar system yet devised.
Revised Jul 10
Page 4 of 4
AP3456 – 11-6 - Tracking Radar
CHAPTER 6 - TRACKING RADAR
Introduction
1.
Tracking radars are used in applications demanding a continuous flow of target data concerning
discrete targets. Such a requirement exists in such functions as ballistic missile and satellite tracking, and
in active and semi-active guidance from ground-to-air, air-to-air, and air-to-ground. Even when an
operator is involved, as in AI radar, it may be advantageous to employ a radar which is able to follow a
selected target automatically.
2.
The data flow from a tracking radar consists of angle and range information in the case of a pulse
radar, and angle and velocity in the case of a CW radar. Angle tracking is performed by slaving the aerial
to follow the selected target, the echo signals from which are processed so as to provide the controlling
signals for the steering servo-motors, and the angle information is derived from the aerial direction.
Range tracking is achieved by causing an electronic switching circuit, called a range gate, to operate in
synchronism with the arrival of echo pulses from the selected target. The range information is taken from
the signal controlling the time of opening of the gate in the pulse cycle. Velocity tracking is carried out by
means of a tuneable oscillator which is constrained to oscillate at a frequency bearing a fixed relationship
to that of the selected Doppler echo signal. The velocity information is obtained from the signal controlling
the oscillator frequency.
3.
Before it can track, the tracking radar must first acquire its target. The initial search may be
performed by a separate radar which determines the target’s coordinates with sufficient accuracy to put
the tracking radar on to it, or the tracking radar may perform its own search by operating in a scanning
mode. Neither arrangement is ideal, the former because of its inconvenience and complication
(particularly in an airborne system) and the latter because the narrow pencil beam required to track in
two coordinates is unsuitable for searching a large angular volume.
Angle Tracking
4.
In order to track a target in two angular coordinates, as defined by the aerial steering axes, error
signals must be generated proportional to the two components of its angular displacement from the
tracking axis. These signals may then be used to activate the corresponding steering servos so as to
drive the tracking axis into coincidence with the line to the target. The required signals may either be
generated sequentially by conical scanning or sequential lobing techniques, or they may be obtained
simultaneously by the monopulsing technique.
5.
Conical Scanning. In conical scanning, the beam axis is displaced through a small angle from
the tracking axis and is rotated in such a fashion that it describes a cone, as depicted in Fig 1. The
effect may be produced by rotating an offset feed in a concentric reflector, in which case the plane of
polarization rotates with the beam; or by wobbling the reflector behind a stationary feed, in which case
the polarization is unaffected. In the direction of the rotation axis (A in the figure) the gain of the beam
is constant irrespective of its position, but in any other direction the gain of the beam varies as the
beam rotates. Thus, a target which does not lie in the direction of the tracking axis, for example in the
directions B or C, will give rise to echo signals which vary in amplitude within the corresponding
envelopes shown in the lower part of the figure. The amplitude of the echo modulation is proportional
to the displacement of the target from the tracking axis, and the sense of the displacement, and is
determined by relating the phase of the modulation to a reference signal, shown in the centre of the
figure, generated by the beam rotating mechanism. The two signals are processed in such a way as to
produce error signals proportional to the two components of angular displacement and these cause the
appropriate steering servo-motors to drive the aerial so as to reduce the error to zero. Once the aerial
Revised Jul 10
Page 1 of 7
AP3456 – 11-6 - Tracking Radar
is tracking the target, its direction is a mechanical analogue of the required angular information and
may be converted into electrical signals for subsequent processing.
11-6 Fig 1 Conical Scanning
Beam Position at t1
Antenna
B
A
C
Beam Position at t2
Scanner
Signal
t2
Reference
t1
t
Amplitude
B
Detector
Output
A
t
C
6.
Sequential Lobing. Sequential lobing is a less common technique in which the beam is
generated sequentially in each of four directions symmetrically disposed about the tracking axis, as
shown in Fig 2. The effect may be produced by means of a single reflector and four offset feeds to
which the receiver is connected in sequence. The transmission may also take place sequentially or it
may be made in all four beams simultaneously. Fig 3 illustrates the principle applied in a single
coordinate. As in conical scanning, the echo from a target not lying on the tracking axis varies from
beam to beam and may be processed in a similar fashion to obtain the error signal needed to drive the
aerial into coincidence with the target. The reference signal in this case is taken from the beam-
switching device and each opposite pair of beams is placed in the plane of an aerial steering axis.
11-6 Fig 2 Beam Configuration for Sequential Lobing and Monopulsing
Tracking
Point
Antenna
Revised Jul 10
Page 2 of 7
AP3456 – 11-6 - Tracking Radar
11-6 Fig 3 Sequential Lobing
Switching Axis
Target
Target
Lobe 1
Lobe 2
Angle
Polar
Rectangular
Representation
Representation
e
d
litu
p
m
A
Time
Error Signal
Lobe 1 Signal
Lobe 2 Signal
7.
Both conical scanning and sequential lobing systems suffer from the limitation that any modulation
in the echo signal occurring during the scanning cycle will cause false signals to be passed to the
steering servos. This can arise because of signal amplitude changes due to changes of target aspect,
and if this modulation coincides with a harmonic of the scanning frequency, may cause the tracking
system to unlock. The same effect can be produced by a form of repeater jammer which senses the
scanning rate and sends back false echoes, amplitude modulated at the scanning rate, which may
cause the aerial to be steered away from the target. The susceptibility of conical scanning and
sequential lobing systems to this defect may be reduced by employing variable PRF in a pulse radar,
or variable scanning rate in either pulse or CW radars.
8.
Monopulse Tracking. Monopulse tracking, also called simultaneous lobing, is achieved by a
similar beam configuration to that used in sequential lobing, but with the important difference that both
transmission and reception take place in all four beams simultaneously. The error signals, proportional
to the differences in the outputs of opposite beams, are derived directly by suitable waveguide
connections at the aerial. As these signals are generated with each successive pulse (or continuously
in a CW radar) the monopulse system is not susceptible to the effects of amplitude fluctuation due to
target aspect changes or deceptive repeater jamming. Further, since all the tracking information is
gained from one pulse as compared with four pulses for a typical sequential lobe system, the data rate
for a monopulse system is higher.
Range Tracking
9.
Range tracking might be required to provide data for trajectory computation, a guidance system,
weapon release information or merely for the purpose of confining the angle tracking circuits to echoes
coming from the range of the selected target in a multiple target situation.
10. The range gates which perform the tracking are electronic switching circuits which sample the
time base once during each pulse cycle. There are usually two such gates, each about a pulselength
in duration, which operate in sequence; the late gate opening as the early gate closes. Fig 4 will serve
Revised Jul 10
Page 3 of 7
AP3456 – 11-6 - Tracking Radar
to illustrate the principle of operation. When the gates are placed in the vicinity of the selected target
echo, which may be achieved manually or by automatic search, the energy contents of the echo
components in the gates are compared in the tracking circuits. The difference is used to generate an
error signal which causes the gates to be driven in the appropriate direction to straddle the target echo.
Thereafter the gates will follow the movement of the echo on the time base and the signal controlling
the time of opening of the gates in the pulse cycle provides an electrical analogue of range. Memory
circuits are normally provided to keep the gates moving at the last target velocity in the event that the
echo should temporarily fade. Provision may also be made to prevent the tracking circuits from
responding to excessive changes in target velocity simulated by false signals returned from a form of
deceptive repeater jammer called a 'gate stealer'.
11-6 Fig 4 Range Tracking by Split Range Gate
Echo Pulse
Time
Early
Late
Gate
Gate
Time
Early
Gate
Signal
Time
Late
Gate
Signal
Velocity Tracking
11. Velocity tracking in a CW radar is performed by means of a frequency comparator. This may
consist of a voltage-tuned oscillator and a frequency discriminating device which generates an error
signal whenever the frequency of the oscillator differs from that of the selected Doppler signal, or from
some fixed relationship with it. The error signal causes the oscillator frequency to change in the sense
needed to reduce the error signal to zero, and the voltage controlling the oscillator frequency then
provides the required analogue of target velocity. The tracking oscillator must be manually adjusted to
establish the initial lock onto the selected signal, or it must be capable of searching across the Doppler
spectrum automatically. Memory circuits are required to prevent the oscillator unlocking during
temporary fading of the target signal, and the system must be made insensitive to excessive changes
in frequency falsely simulated by deceptive jamming.
Track-While-Scan
12. An alternative method of target tracking is to employ a radar which scans continuously within a
defined angular volume, and a computer which memorizes the coordinates of targets and anticipates their
positions on successive scans. This technique, called track-while-scan, is capable of providing multiple
target tracking and is therefore suitable for command guidance.
Revised Jul 10
Page 4 of 7
AP3456 – 11-6 - Tracking Radar
13. With track-while-scan techniques the tracking function is performed by the computer, and the
special characteristic of the radar which distinguishes it from a conventional search radar is its high
data rate which, in some applications, may be of the order of hundreds of scans per minute. Such
scanning rates can only be achieved by electronic or electro-mechanical means and, depending on the
angular coverage, it may also be necessary to employ a multiplicity of beams.
14.
High Speed Scanning. High speed scanning is achieved by controlling the relative phase of the
radiated signal across the aerial aperture in such a way that the wave front is inclined from the parallel.
The total angle through which the beam can be steered is less than 180º. True electronic scanning can
be achieved by use of a multiple element phased array, or planar array, in which the control of the relative
phases of the radiated signals is achieved by purely electronic means. Such techniques permit more than
one beam to be generated at any time and there is no limit to the scanning rate possible. Electronic beam
steering techniques are not confined to track-while-scan radars.
15.
Electro-mechanical Track-while-scan. The term track-while-scan is sometimes used to
describe those first-generation radars, predominantly electro-mechanical devices, which use twin radar
beams set mutually at 90º to search in azimuth and elevation simultaneously. When a target is found,
the aerials can be rotated mechanically to centre the target in the middle of each beam, and thus track
its movement (Fig 5). However, the term 'track-while-scan' is a misnomer for these systems because,
during target tracking, search capability is either inhibited totally or restricted to a small area centred on
the single discrete target already being tracked. A refinement to this elementary system was to add
extra aerials to deal with tracking only. Although limited in their effectiveness, large numbers of this
early radar type remain in service.
11-6 Fig 5 Electro-mechanical Track-while-scan Radar
Overlap Area
Tracks Target
Elevation
Beam
Azimuth
Aerials
Beam
Acquisition
16. A tracking radar must first find and acquire its target before it can operate as a tracker. Therefore, it is
usually necessary for the radar to scan an angular sector in which the presence of a target is suspected.
Most tracking radars employ a narrow pencil beam aerial. Searching a volume in space for an aircraft target
with a narrow pencil beam would be somewhat analogous to searching for a fly in a darkened auditorium
with a hand torch. It must be done with some care if the entire volume is to be covered uniformly and
efficiently. Examples of the common types of scanning patterns are illustrated in Figs 6 to 8. A Palmer scan
is a conical scan superimposed onto another pattern. Fig 9 illustrates a Palmer-Raster scan pattern.
Similarly, it is possible to have Palmer-Helical and Palmer-Spiral scan patterns.
Revised Jul 10
Page 5 of 7
AP3456 – 11-6 - Tracking Radar
11-6 Fig 6 Helical Scan Pattern
11-6 Fig 7 Spiral Scan Pattern
11-6 Fig 8 Raster Scan Pattern
11-6 Fig 9 Palmer-Raster Scan Pattern
Revised Jul 10
Page 6 of 7
AP3456 – 11-6 - Tracking Radar
Tracking Errors
17. The accuracy of a tracking radar is influenced by such factors as the mechanical properties of the
radar aerial and mounting, the method by which the angular position of the aerial is measured, the
quality of the servo system, the stability of the electronic circuits, the noise level of the receiver, the
aerial beamwidth, atmospheric fluctuations, and the reflection characteristics of the target. These
factors can degrade the tracking accuracy by causing the aerial beam to fluctuate in a random manner
about the true target path. These noise-like fluctuations are sometimes called tracking noise, or jitter.
18. A simple radar target such as smooth sphere will not cause degradation of the angular tracking
accuracy. The radar cross section of a sphere is independent of the aspect at which the sphere is viewed;
consequently, its echo will not fluctuate with time. However, most targets are of a more complex nature than
a sphere. The amplitude of an echo signal from a complex target may vary over wide limits as the aspect
changes with respect to the radar. In addition, the effective centre of the radar reflection may also change.
Both these effects - the amplitude fluctuations and the wandering of the radar centre of reflection (glint) - as
well as the limitation imposed by the receiver noise, can limit the tracking accuracy.
Revised Jul 10
Page 7 of 7
AP3456 – 11-7 - Doppler Navigation Radar
CHAPTER 7 - DOPPLER NAVIGATION RADAR
Introduction
1.
Doppler navigation radar is an airborne radar which relies on the Doppler effect to determine the
aircraft’s ground speed and drift. The values may be continuously displayed, or transmitted to other
equipment such as a navigation computer.
The Doppler Effect
2.
The Doppler effect describes the apparent change in pitch that occurs when a sound source is
moving relative to an observer. The same effect occurs with electromagnetic waves.
3.
Fig 1 shows a stationary transmitter transmitting a signal of f Hz towards a stationary receiver, the
circles representing successive wavefronts. Providing that the medium of transmission is
homogeneous, the wavefronts will be equally spaced and the receiver will detect a frequency identical
to that transmitted.
11-7 Fig 1 Stationary Transmitter and Receiver
Tx
Rx
4.
Fig 2 shows the situation where the transmitter is moving towards the receiver at a velocity of V m/s.
The first wavefront is centred on position 1, the second on position 2 and so on. The overall effect is to
decrease the wavefront spacing in front of the transmitter, which will appear to the receiver as an increase
in the received frequency. Behind the transmitter the wavefront spacing is increased and so a receiver
placed there would experience a reduced frequency.
11-7 Fig 2 Transmitter Moving Successive Wave-Fronts
Tx →
Rx
1 2 3 4
Revised Jun 10
Page 1 of 13
AP3456 – 11-7 - Doppler Navigation Radar
5.
The change in frequency, known as the Doppler shift (fd), is proportional to the transmitter’s
velocity such that:
Vf
f
=
d
c
where c is the speed of propagation of the electromagnetic waves (approx 3 × 108 m/s for radio waves
in air). Since the transmitted frequency, f, and wavelength, λ, are related by:
λ = c/f
fd may be written as V/λ.
6.
The same Doppler shift is apparent with a stationary source and a moving receiver. In the case
of an airborne radar, the transmitter and receiver are collocated and the radar energy is reflected by
the ground. When the energy reaches the ground from the moving transmitter it undergoes a
Doppler shift to produce a frequency of (f + Vf/c) and it is this frequency which is reflected back to
the aircraft. The situation is now that of a stationary transmitter (the ground) and a moving receiver
which therefore detects a further Doppler shift of Vf/c. Thus compared to the transmitted frequency,
f, the receiver detects a total frequency change of 2Vf/c. Since both f and c are known, if the
change in frequency can be measured a value for V can be determined and it is this principle which
is used in Doppler navigation radars to determine groundspeed.
Doppler Measurement of Groundspeed
7.
Fig 3 illustrates the general principle of groundspeed measurement in which a narrow radar beam
is transmitted forwards and downwards from the aircraft at an angle, θ, cal ed the depression angle. In
this situation, the difference in frequency between the transmitted signal and the echo received from
2Vf
the ground will be
cos θ , where V is the aircraft groundspeed.
c
11-7 Fig 3 Principle of Doppler Groundspeed Measurement
Depression
Angle θ
Echoes
Tx Pulses
Scattered Energy
Surface
8. The choice of depression angle for the beam is a matter of compromise. If θ is small, the beam
strikes the terrain at a shallow angle and less energy will be reflected back to the aircraft than would
be the case with a steeper beam. Conversely, if θ is made large its cosine becomes small and the
2Vf
value of
cos θ may become too small for accurate measurement. In practice the value of θ is
c
usually between 60º and 70º.
Revised Jun 10
Page 2 of 13
AP3456 – 11-7 - Doppler Navigation Radar
9.
Operating Frequency. The Doppler frequency to be measured, fd, equates to about 34 Hz per
100 MHz of transmitter frequency per 100 knots, multiplied by the cosine of the depression angle.
Since this represents a very small proportion of the transmitted frequency, it is necessary for this to
be high in order to obtain a value for fd which can be measured with sufficient accuracy. In practice,
two frequency bands have been allocated to Doppler systems, one centred on 8.8 GHz generating a fd
of about 1.5 KHz per 100 kt, and the other on 13.3 GHz giving a fd of about 2.3 KHz per 100 kt
(assuming a depression angle of 60º).
Single Beam Systems
10. So far the system described has used a single fixed beam radiating forwards and downwards
from the aircraft as shown in Fig 3. The Doppler shift would be the same if the beam were directed
rearwards and downwards, although the received frequency would in that case be less than the
transmitted frequency i.e. –fd. However, such a single beam system would have a number of
disadvantages:
a.
Transmitter Instability. High-powered I/J-band transmitters tend to suffer from some
instability of frequency and if the frequency should drift between the time of transmission and the
time of reception of the reflected signal an incorrect fd would be measured leading to an error in
measured groundspeed.
b.
Pitch Error. Unless the aerial was stabilized in the pitching plane, the depression angle
would be dependent on the attitude of the aircraft. Thus deviations from level flight would result in
changes to fd even if the horizontal velocity of the aircraft remained constant. The consequent
errors in computed groundspeed would be significant, typical values being 3% for 1º of pitch and
15% for 5º of pitch.
c.
Vertical Motion. Any vertical motion of the aerial would generate a Doppler shift not
associated with a change in groundspeed.
d.
Drift Error. In a fixed single aerial system the Doppler shift would be measured horizontally
along the direction of the beam, thus velocity would be calculated along heading, whereas
groundspeed is measured along track. This error could be eradicated by rotating the aerial until a
maximum Doppler shift was obtained, at the same time determining the drift angle by the amount
of aerial rotation. However, this technique is imprecise in practice and is not used any longer
even in multiple beam systems.
Two Beam Systems
11. Some of the errors inherent in a single beam system can be significantly reduced by employing
two beams, one directed forward and the other rearward; the multiple beam arrangement is shown
in Fig 4, in which both beams are depressed with respect to the horizontal.
Revised Jun 10
Page 3 of 13
AP3456 – 11-7 - Doppler Navigation Radar
11-7 Fig 4 Two Beam System
Depression
Depression
Angle θ
Angle θ
Surface
12. The frequency of the signal received from the forward beam, fr, is higher than the transmitted
frequency, f, by an amount equal to the Doppler shift, fd, i.e.:
fr = f + fd
The frequency received from the rearward beam has a negative Doppler shift of the same magnitude, i.e.:
fr = f − fd
In a two beam system these two received signals can be mixed together and the difference frequency
extracted as a beat frequency, fb, such that:
fb = (f + fd) − (f – fd) = 2fd
4Vf
ie
cos θ
c
13. The beat frequency produced by a two beam system has twice the value of that from a single
beam allowing greater precision in the measurement of groundspeed. Variations in transmitter
frequency become less important as such changes affect the forward and rearward echo signals
equally, and are therefore cancelled when taking the difference frequency. Similarly, any changes in
the aircraft’s vertical speed will be sensed by both beams and will be cancelled. Pitch errors will cause
an increase in depression angle for one beam and a decrease in depression angle for the other beam,
which although not providing complete compensation does reduce the errors significantly; 5º of pitch
will cause an error of around 0.38%. However, such a two beam system cannot be used to determine
drift accurately and most modern systems use four or three beam arrangements.
Four and Three Beam Systems
14. Four beam systems, known as Janus arrays, provide for the accurate measurement of both
groundspeed and drift. Consider a rotatable system of 4 beams arranged as in Fig 5a, radiating alternately
in pairs, e.g. A and A1 for a half second, B and B1 for the next, and so on. In this example there is zero drift
and the beams are disposed symmetrically about the aircraft heading which is also the track, i.e.:
fd (A + A1) = fd (B + B1) (see Fig 5b)
Revised Jun 10
Page 4 of 13
AP3456 – 11-7 - Doppler Navigation Radar
11-7 Fig 5 Four Beam System - Zero Drift
Fig 5a Plan View
Fig 5b Frequency Spectra
Aircraft Heading
and Track
Amplitude
of Received
f
B
A
Signals
d from A & A1
fd from B & B1
Aircraft with
Identical Spectra
Zero Drift
from Both Beams
A
Frequency
1
B1
15. Fig 6a illustrates the case when the aircraft has port drift. Before the drift angle has been
resolved, the two sets of beams A and A1 and B and B1 are, as before, positioned symmetrically about
the aircraft heading. Under these conditions the Doppler shift obtained from beams B and B1 is greater
than that from beams A and A1, as shown in Fig 6b.
11-7 Fig 6 Four Beam System - Port Drift
Fig 6a Plan View
Fig 6b Frequency Spectra
Aircraft
Aircraft
Track
Heading
Amplitude
fd from A & A1
B
A
of Received
Signals
fd from B & B1
Uncorrelated Spectra
due to Drift
Wind
Frequency
A1 B1
The difference in frequency is converted into an error voltage which rotates the aerial assembly to the
null position in which A and A1 and B and B1 are symmetrical about the aircraft track, and the Doppler
shifts from each set are the same (Fig 7). The angle of movement of the aerial assembly is then
reproduced as a drift indication.
Revised Jun 10
Page 5 of 13
AP3456 – 11-7 - Doppler Navigation Radar
11-7 Fig 7 Four Beam System - Port Drift but with Aerial Aligned with Track
Fig 7a Plan View
Fig 7b Frequency Spectra
Aircraft
Aircraft
Track
Heading
Amplitude
A
f
B
of Received
d from A & A1
Signals
fd from B & B1
Spectra merge after
Aerial alignment with Track
Wind
Frequency
B 1
A 1
16. Most modern lightweight systems in fact use a fixed aerial system with only 3 beams in which the
Doppler shifts are derived individually, and mixed electronically to resolve the horizontal and vertical
velocities. Such a system is illustrated in Fig 8.
11-7 Fig 8 Three Beam Fixed Aerial Arrangement
Heading
Heading
Transmission Types
17. Either pulsed or continuous wave (CW) transmissions can be used in Doppler equipments. CW
equipments have the advantage that less power is required (typically 100 mW), but they may be affected
by misleading signals reflected from vibrating parts of the aircraft structure, or more commonly from
appendages such as weapons and fuel tanks. This problem is overcome in some systems by using
frequency modulated CW (FMCW), which permits the rejection of signals which have been reflected from
nearby objects.
Revised Jun 10
Page 6 of 13
AP3456 – 11-7 - Doppler Navigation Radar
18. In pulsed systems the pulse recurrence frequency (PRF) must not be allowed to produce spurious
signals in the Doppler frequency band and is therefore made very high, usually at least twice as high as
the highest expected Doppler frequency.
Beam Shapes
19. The analysis so far has assumed that the radar energy is transmitted in pencil beams, each of
which is reflected at any instant from a single point on the ground. However, in reality the beams must
have a finite width and must illuminate a finite area of ground. The total reflected signal at each aerial
is therefore composed of the vector sum of signals from a very large number of reflecting points. The
Doppler frequency shift from any reflecting point is proportional only to the speed of the aircraft, the
angle between the line of flight and the transmitted beam, and the frequency. It is independent of the
distance of the reflecting point from the aircraft and so the same Doppler shift is produced over flat
terrain as over more mountainous country.
20. The Doppler frequency change is proportional to the cosine of the angle between the line of flight
and the beam and thus al reflecting points which lie on the surface of a cone of semi-angle θ, whose
fV
axis coincides with the direction of motion, produce a Doppler frequency of 2
cos θ. Thus if the
c
transmitted beams are so shaped as to form parts of conical surfaces, a groundspeed measurement
may be obtained as accurate and unambiguous as that from pencil beams. This arrangement is
illustrated in Fig 9 and the beams are achieved by an aerial system known as a 'squinting linear array'.
11-7 Fig 9 A Practical Doppler Beam Shape
21. The areas of ground illuminated by the beams lie on hyperbolae (Fig 10). The width of a beam
along such a hyperbola is known as the broadside beamwidth and is commonly of the order of 9º or
10º. The depression beamwidth is the nominal beamwidth of the cone measured in a vertical plane
through the aerial axis and is normally around 5º. Smaller depression beamwidths result in more
accurate Doppler shift measurement, and make the system less susceptible to sea bias error
(see para 27). The angle in the rolling plane between the vertical and the beams is known as the
broadside angle. Although a large broadside angle allows sensitive drift measurement, if it is made
too large there will be an unacceptable decrease in the power of return signals.
Revised Jun 10
Page 7 of 13
AP3456 – 11-7 - Doppler Navigation Radar
11-7 Fig 10 Beam Parameters
Aircraft
Depression
Heading
Angle
Broadside
Angle
Depression
Beamwidth
Broadside
Beamwidth
Beam C
90°
Beam B
Aircraft
Beam A
Heading
Hyperbolic Lines of Constant
Doppler Frequency
Frequency Spectrum
22. Although the beams have been described as forming part of the surface of a cone, they do in fact
have some thickness represented by the depression beamwidth. Each part of the beam therefore has
a different depression angle, and since the Doppler shift is proportional to the cosine of depression
angle, the reflected signal does not have a single frequency, but is composed of a spectrum of
frequencies. Fig 11 shows an idealized spectrum with the signal strength plotted against a range of
Doppler frequencies. Steeper depression angles result in broader spectra, and the shape depends on
the polar diagram of the beam. A frequency tracker is used to determine the mid-point of the
frequency spectrum.
11-7 Fig 11 Idealized Doppler Spectrum
Signal
Measured fd
Strength
Frequency
Spectrum
Over Land
9.4
9.6
9.8
10.0
10.2 10.4
10.6
f (kHz)
d
Revised Jun 10
Page 8 of 13
AP3456 – 11-7 - Doppler Navigation Radar
Frequency Tracking
23. The determination of fd from the spectrum of Doppler frequencies is accomplished by a frequency
tracker device, which is electro-mechanical in older equipments, and electronic in more modern
lightweight 3-beam systems.
24.
Electro-mechanical Frequency Tracking. Electro-mechanical systems employ a device known
as a phonic wheel oscillator which produces an oscillatory output voltage. The frequency can be varied
over the range of the Doppler spectrum by varying the speed of rotation of the phonic wheel shaft. In
older 'two-window' systems two oscillators are used, the output frequencies of each differing by a fixed
amount. The incoming Doppler signal is fed to two discriminator circuits, to one of which is also fed the
lower phonic wheel frequency and to the other the higher. The output of each discriminator is a measure
of the energy contained in a narrow window of the Doppler spectrum centred on the phonic wheel
frequency. If the two discriminator outputs are equal, the two windows contain equal amounts of energy,
and must therefore be symmetrically disposed around the centre frequency as in Fig 12a. If the outputs
are different, the windows are displaced from the symmetrical position as in Fig 12b, and the difference in
outputs is used as an error signal to realign the phonic wheel frequencies until parity is achieved. In
single line tracking systems a single phonic wheel oscillator is used. The Doppler spectrum is applied to
two mixer circuits, each of which is also fed with the phonic wheel frequency, but with a 90º phase
difference to each. The output of the mixers is fed to a two phase motor (integrator motor), which turns in
one direction if the phonic wheel frequency is higher than the mid Doppler frequency, and in the other
direction if it is lower. The movement of the integrator motor is used to vary the phonic wheel frequency
until there is no movement from the integrator motor. The phonic wheel shaft speed is then proportional
to the Doppler mid frequency and may be used to drive indicators and computers.
11-7 Fig 12 Two-window Frequency Tracking
Fig 12a Discriminator Outputs Equal
Output No 1
Signal
= Output No 2
Amplitude
f
No 1
No 2
d
Fig 12b Discriminator Outputs Diifer
Output No 1
Signal
Output No 2
Amplitude
No 1
No 2
fd
25.
Electronic Frequency Tracking. In a 3-beam fixed aerial system, as shown diagrammatically in
Fig 13, the Doppler shift in each beam is detected independently in a different channel. It is possible to
determine aircraft velocity along all 3 axes by subsequent Doppler mixing. In Fig 13, the aircraft has its
Revised Jun 10
Page 9 of 13
AP3456 – 11-7 - Doppler Navigation Radar
horizontal velocity split into 2 positive perpendicular components V1 and V2. FA , FB and FC equal the
Doppler shifts observed in each beam A, B and C respectively. θ is the angle between the fore and aft
axis and each beam in the horizontal plane. Then FB – FC ∝ V1 and FB – FA ∝ V2. Modern frequency
trackers are able to determine whether the Doppler shift seen by each beam is positive or negative.
The vectors are thus resolved algebraically and displayed or used in a navigation computer.
11-7 Fig 13 Transmitting/Receiving Beam Pattern
F
FB
C
B
Be
Beam
am C
V2
θ
Aircraft and Aerial
θ
V1
Fore and Aft Axis
Beam A
FA
Potential Errors
26.
Height Hole Error. A pulsed Doppler radar cannot transmit and receive signals at the same time
and it is therefore possible for a reflected signal to be lost if it arrives back at the receiver at the same time
as a pulse is being transmitted. The effect, known as height hole error, occurs at certain aircraft heights
dependent on the pulse recurrence frequency (PRF) of the radar. Over uneven ground the slant range of
the illuminated area of ground is continually varying and is unlikely to remain at a critical value long
enough for height hole error to be significant. Over the sea however the aircraft height is liable to remain
constant for a longer period and prolonged loss of signal may occur. A similar effect occurs with FMCW
transmissions when the aircraft is at such a height that the reflected signal at the instant of reception, is in
phase with the transmitter and is therefore rejected. Height hole error is usually avoided by varying the
PRF of a pulsed system, or the modulation frequency of a FMCW system.
27.
Sea Bias. The amount of energy reflected back to the aircraft depends, among other things,
upon the angle of incidence, such that more energy will be received from the rear edge of a forward
beam than from its front edge. Over land, the irregularities of the surface mask these variations in
energy level, but over a smooth sea the reflection is more specular in character. As a consequence,
not only is a larger proportion of the total energy reflected away from the aircraft, but because the
leading edge of a forward beam has a lower grazing angle, more higher frequency energy is lost than
is lower frequency energy from the trailing edge. The resulting change to the Doppler spectrum is
shown in Fig 14 and this distortion leads to the determination of a value for fd which is too low. The
consequent error in calculated groundspeed is known as sea bias error and typically results in
groundspeeds which are between 1% and 2% too low. Most systems incorporate a LAND/SEA switch
which, discriminate between Doppler frequencies over water (fdw) and over land (fdl) and when switched
to SEA, alters the calibration of the frequency tracker so as to increase the calculated groundspeed by
a nominal 1% or 2%.
Revised Jun 10
Page 10 of 13
AP3456 – 11-7 - Doppler Navigation Radar
11-7 Fig 14 Sea Bias and Spectrum Distortion
Measured f
Correct f f
or
d
d
Beam Centre Line
Sea Bias Error
Signal
Amplitude
Land
Spectrum
Water
Spectrum
f
f
f
dw
dl
d
28.
Sea Movement Error. Doppler equipments measure drift and groundspeed relative to the terrain
beneath the aircraft which, if moving, will induce an error into the results. There are two causes:
a.
Tidal Streams. The speed of tidal streams is generally greatest in narrow waterways and,
since the time during which an aircraft is likely to be affected is small, the effect is minimal.
Ocean currents occupy much larger areas but their speed is low and so cause little error.
b.
Water Transport. Wind causes movement in a body of water and, although wave motion is
quite complex, the net effect so far as Doppler systems are concerned is a down-wind movement
of the surface. The resultant error is an up-wind displacement of a Doppler derived position. An
approximate value for the error can be derived by considering an error vector in the measured drift
and groundspeed which has a direction in the up-wind direction of the surface wind, and a length
equal to about one fifth of the surface wind speed, with a maximum of approximately 8 kt.
29.
Flight Path and Pitch Error. In climb and descent a true speed over the ground will be calculated
only if the Doppler aerial is maintained horizontal. Partial compensation for pitch is inherent in multiple
beam systems (see para 13) and errors can be further reduced, if necessary, by gyro-stabilizing the aerial
to the horizontal, or by correcting the errors using attitude information in the groundspeed computer.
Without these facilities, and with the aerial slaved to the aircraft’s flight path or to the airframe, a small
error will be introduced.
30.
Roll Error. Theoretically a combination of drift and roll can cause axis cross-coupling errors, but
these are transient and too small to be of significance. However, signals may be lost in a turn if one
beam, or a pair of beams, is raised clear of the ground.
31.
Drift Error. Large drift angles will have no effect on accuracy provided that:
a.
In a moving aerial system the aerial can rotate to the same degree as the drift.
b.
In a fixed aerial system a small broadside beamwidth can be obtained to prevent adverse
widening of the frequency spectrum.
In both cases a large area in the aircraft is needed, since a moving aerial needs room in which to turn,
and a fixed aerial needs to be large to obtain a small broadside beamwidth. (Beamwidth is inversely
proportional to aerial dimension.)
Revised Jun 10
Page 11 of 13
AP3456 – 11-7 - Doppler Navigation Radar
32.
Computational Errors. Doppler systems are mechanized on the assumption than 1 nm is
equivalent to 1' of latitude. However, the actual change of latitude in minutes, equating to a distance of
one nautical mile measured on the Earths surface along a meridian, ranges from 1.0056' at the
Equator to 0.9954' at the poles, and is only correct at 47º 42' N and S. Furthermore, 1' of latitude
change equates to a greater distance at height than on the surface. Fig 15 illustrates error due to
height, by showing the comparable distance at sea level. Errors due to latitude and height are small
and are not normally corrected for in navigation. Additional small errors can be introduced into position
calculations. Since the aircraft never flies in a straight line, but rather weaves slowly from side to side
of track, the calculated distance gone will exceed the distance directly measured on a map. Doppler
drift is added to heading to deduce track and so any error in the heading input will result in an error in
any computed position.
11-7 Fig 15 Height Error
(R+h)θ
Height above
Surface (h)
Error due
to Height
Surface
Rθ
of Earth
Radius of
Earth (R)
θ
Computer Display
33. The two basic outputs from a Doppler system, as described, are groundspeed and drift angle, and
these can be combined in a computing system with aircraft heading and desired track to produce a
display which may include along and across track indications, computed position in a selected
reference frame, distance and time to go, etc. Fig 16 illustrates a typical Doppler computer chain for
the navigation function. The Doppler velocities may also be used in a mixed inertial/Doppler navigation
and weapon aiming system in which the Doppler velocities are used to dampen the Schuler oscillations
of the IN system (see Volume 7, Chapter 11).
Revised Jun 10
Page 12 of 13
AP3456 – 11-7 - Doppler Navigation Radar
11-7 Fig 16 Typical Doppler Navigation Computer Chain
Groundspeed
LEFT 0 0 3
Doppler
Inputs
Across
Drift Angle
Indicator
Track
2 3
Error
Track
Combining
Combining
Distance to go
Unit
Unit
Track Error
to Autopilot
Heading
1
6
4
Input
Desired Track
(from Compass)
Desired Track
Along/Across Display
34. For rotary wing aircraft applications, the vertical component of velocity can also be derived by
Doppler and Fig 17 illustrates a hover meter. The cross pointers show movement, in knots, forwards
and backwards on the vertical scale, and left and right on the horizontal scale. The vertical scale at the
extreme left of the display shows vertical velocity in feet/minute.
11-7 Fig 17 Hover Meter
Ft/min
F
40
U 500
30
30
100 L
Knots
D
R
B
20
Revised Jun 10
Page 13 of 13
AP3456 – 11-8 - Ground Mapping Radar
CHAPTER 8 - GROUND MAPPING RADAR
Introduction
1.
Ground Mapping Radar (GMR) carried in aircraft will present the operator with an image of terrain
features, which can then be used to aid navigation, locate targets and determine weapon aiming
parameters. In addition, the skilled operator will be able to interpret terrain relief, including those hills
whose elevation is higher than the aircraft’s altitude.
2.
From even the most basic radar mapping display, the operator will be able to define a range and
relative bearing to a recognizable ground feature, and then plot the reciprocal to obtain a 'range and
bearing' fix (Volume 9, Chapter 2).
3.
A GMR is often one component of an integrated navigation and weapon aiming system, such that
data derived from the radar can be used directly to update the aircraft’s present position. Conversely,
data from the rest of the system can be used to enhance the radar facilities (e.g. Doppler or inertial
velocities may be used to stabilize the radar image and superimpose electronic cursors).
4.
Radar can provide a means of navigation which is independent of ground beacons. The operator
can acquire information from the radar display in all but the most extreme weather conditions. Radar
also has a relatively long-range capability, only limited by the equipment parameters and Earth
curvature. It is therefore possible to find distinctive ground features, such as coastlines, at extreme
range. However, it is important to note that the information received and displayed by a radar may be
ambiguous to the unskilled, or ill-prepared operator. Hence, accurate interpretation of radar displays
requires some knowledge of basic radar principles, and the nature of reflectors.
BASIC RADAR PRINCIPLES
Basic Principles of Pulse Radar
5.
A GMR uses pulse radar techniques, which are explained fully in Volume 11, Chapter 2. A short
résumé of the pertinent aspects of pulse radar theory is repeated to assist with the understanding of
radar display interpretation skills.
6.
Determination of Range. Pulse Radar determines the range of a target by 'pulse/time'
technique. The airborne radar fires a pulse of energy, which is reflected by the target, and returns to
the radar aerial (see Fig 1). The range from the aerial to the target is measured by observing the
elapsed time (t) between the leading edge of the pulse being transmitted from the aerial, and the
leading edge of the associated return (known as an 'echo'), arriving back at the aerial. The range of a
target can therefore be expressed as:
t × c
Range =
2
where c = the velocity of propagation of the pulse (3 × 108 metres per second). The range of a target
may be measured on the radar display by eye, or by electronic means.
Revised Jun 10
Page 1 of 21
AP3456 – 11-8 - Ground Mapping Radar
11-8 Fig 1 Measuring Range by Pulse/Time Technique
Fig 1a Transmitted Pulse Outbound
Leading Edge
Target
Fig 1b Return of Reflected Pulse
Leading Edge
Target
7.
Determination of Bearing. To detect the bearing of a target, the aerial beam is moved in azimuth.
This aerial movement (known as 'scanning') is synchronized with the radar display such that when an
echo is received from a target, the radar time base is at the correct bearing on the radar display.
8.
Radar Displays. There are some airborne installations which have a 360º Plan Position Indicator (PPI)
display (see Volume 11, Chapter 1). More commonly, however, the radar is mounted in the aircraft nose and
scans a sector ahead of the aircraft, typically 60º either side of the aircraft heading or track. In this case, the
ground mapping picture will be presented on a Sector PPI display as illustrated in Fig 2.
11-8 Fig 2 Layout of a Sector PPI Display
Heading or
Track Datum
Azimuth
0
10
10
Angle
20
20
3
30
0
4
40
0
Time Bases
50
50
Return
Signal
Range
Aircraft’s Position
(point of origin)
Representation of Terrain Features
9.
A radar map of an area of the terrain is achieved by scanning the radar beam in azimuth, either
mechanically (by moving the aerial) or electronically (by using a phased array antenna). The radar
display picture is built up from a series of time bases (Fig 2), which sweep in synchronization with the
radar beam. The persistence of the CRT phosphor ensures that a continuous image of the ground is
maintained between sweeps.
Revised Jun 10
Page 2 of 21
AP3456 – 11-8 - Ground Mapping Radar
10. The time base is intensity modulated in response to the signal strength of received echoes. Those
radar targets that are highly reflective will return a greater proportion of the original transmitted energy
than poor reflectors and will thus be the brightest objects on the display. In practical terms, the
human eye can only make out 3 or 4 shades of brightness of return echoes (Fig 3) on a mono-colour
display. However, operator fatigue or unsuitable lighting conditions might diminish this capability. In
general, the ground features will be represented by:
a.
The brightest returns from highly reflective targets, typical of town centres and industrial
complexes.
b.
Medium intensity returns from targets of average reflectivity.
c.
Low intensity returns (darker than the previous two categories) from flat terrain and water
features.
d.
Areas containing no radar returns ('no show' areas).
11-8 Fig 3 Levels of Brightness
Radar Returns
Maximum
‘Bloom’
Visibility level
Brightness of radar
returns depend upon
reflective properties
of targets.
Visibility
Threshold Level
Radar ‘no show’
11. Above the maximum visibility level, signals will 'bloom' or 'flare'. The visibility threshold of the
radar display is the lower limit of brightness distinguishable by the operator’s eye. Very weak return
signals will remain below the visibility threshold, unseen.
12. A typical GMR display from medium altitude is shown at Fig 4a. Fig 4b shows a map of the same
area for comparative purposes.
11-8 Fig 4 A Typical Medium Level Radar Display
Fig 4a Medium Level Radar Picture
Fig 4b Map of the Same Area
Gt Yarmouth
Lowestoft
Spurn Head
Norwich
Skegness
The
Wash
Boston
Kings
Lynn
Peterborough
Revised Jun 10
Page 3 of 21
AP3456 – 11-8 - Ground Mapping Radar
Radar Parameters
13.
Operating Frequency. As explained in Volume 9, as with all radar parameters, the choice of
operating frequency is inevitably a compromise between conflicting requirements. High frequencies
allow narrow beamwidths to be achieved with relatively small aerials, and pulse lengths to be
relatively short - both attributes leading to improved resolution. Additionally, high frequency
equipment tends to have size and weight advantages. Conversely, high frequency radar is
restricted in the power that can be employed, and consequently in its maximum effective range.
Furthermore, the higher the frequency, the more the radar will be susceptible to interference and
atmospheric attenuation. In practice, the majority of airborne mapping radars operate in the I or J
band with frequencies around 10 GHz (wavelengths around 3 cm).
14.
Pulse Width and Pulse Length. The duration of a transmitted pulse can be measured in time
(pulse width (PW)) or distance (pulse length (PL)). As both terms are interrelated, either may be used
to describe this parameter.
a.
The energy content of a pulse is directly proportional to its length (Fig 5). Thus, a longer
pulse will give a stronger return echo from targets at long range. However, shorter pulses will
enable the radar to resolve (separate) closely spaced targets in range. This point will be
discussed in detail at a later stage within this chapter.
11-8 Fig 5 Pulse Parameters
Pulse Width
(Pulse Length
Power
if measured
in Distance)
Peak
Pulse of
Electromagnetic
Power
Energy
Time
b.
The PL will determine the minimum range that the radar can measure. The leading edge
of the echo cannot be received until the trailing edge of the pulse has left the transmitter and
must therefore travel a return distance at least equal to the distance occupied by a pulse. A
pulse occupies 300 m for every microsecond of duration and so, for a typical PW of 2 µsec, the
return distance will be 600 m, giving a minimum range of 300 m. In practice the minimum
range will be greater than this since some finite time will be necessary for the aerial to switch
from transmission to reception.
The PW for a GMR is usually between 0.5 and 5 µsec.
15.
Pulse Recurrence (or Repetition) Frequency (PRF). The PRF is defined as the number of
pulses occurring in one second and must be sufficiently high to ensure that at least one pulse of
energy strikes a target while the scanner is pointing in its direction. A very narrow radar beamwidth,
with a high rate of scanner rotation therefore needs a high PRF. In practice, the relationship between
scanner rate, beamwidth and the PRF is adjusted such that any target will receive between 5 and 25
pulses each time it is swept by the beam. However, if the PRF is too high, it will limit the radar’s
maximum unambiguous range. The PRF of a GMR is typically between 200 and 5,000 pulses per
second and is normally determined by the range scale selected.
Revised Jun 10
Page 4 of 21
AP3456 – 11-8 - Ground Mapping Radar
16.
Aerial Stabilization. The radar aerial must be stabilized, within limits, to the true horizontal both
in roll and pitch to avoid distortion of the ground image. This is normally achieved by using inputs from
the aircraft attitude system - the degree of roll and pitch for which compensation can be provided will
vary between aircraft types. In addition to the radar image, it is possible to superimpose electronically
produced symbols and cursors on to the display. In some systems a topographical map can also be
projected onto the display (see Volume 7, Chapter 30).
Beam Characteristics
17.
Vertical Beamwidth. The beam of a GMR must be broad in the vertical plane in order to
illuminate all of the ground between a point beneath the aircraft and the horizon (or effective range). A
pencil beam, as produced by a parabolic aerial (Fig 6), is not ideal for mapping purposes since it does
not illuminate sufficient area of ground. Instead, a diffuse beam known as a cosecant² (or 'spoiled')
beam is used.
11-8 Fig 6 A Pencil Beam used for Ground Mapping
Pencil Beam
Area of useable returns
18.
The Cosecant2 Beam. The main advantage of the cosecant2 beam is that greater power is
transmitted to greater ranges in order to compensate for range attenuation. In this way, similar targets
will give similar return strength regardless of range (Fig 7).
11-8 Fig 7 A Cosecant2 Beam used for Ground Mapping
Cosecant Beam
Area of useable returns
The cosecant² beam dilutes the power per unit area of ground coverage (Fig 8) and therefore is ideal
for comparison of ground returns over short and medium ranges. However, at extremely long range,
reversion to a pencil beam is necessary, in order to ensure that maximum energy is reflected from
distant targets.
Revised Jun 10
Page 5 of 21
AP3456 – 11-8 - Ground Mapping Radar
11-8 Fig 8 The Cosecant2 Beam
θ
h
Beam
R
1
At slant range R, power density received
R2
power density transmit ed must be R2
2
h
sin θ
h
2 cosec 2θ
19.
Azimuth Beamwidth. The beam of a GMR must be narrow in azimuth so that the bearing of any
echo can be defined accurately. It is impossible to produce a beam in which all of the radar energy is
distributed and confined within a finite beam. However, with a well-designed antenna, most of the
radiated power can be constrained to a given direction as illustrated by a typical polar diagram (Fig 9).
It is impossible to eradicate the side lobes completely, but it is desirable to minimize them since they
represent wasted power, and their presence makes the radar more vulnerable to interference.
11-8 Fig 9 Half Power Beam Width
Side Lobes
Main Lobe
C
A
θ
D
Half Power Points
20.
Nominal Beamwidth. The nominal beamwidth (NBW) is defined as the angle subtended at the
source by the lines joining the two points on the radiation diagram where the power has fallen to a
certain proportion (usually a half) of its maximum value. Radiation patterns are normally plotted
showing relative field strengths, and, since field strength is proportional to the square root of power,
the corresponding half power points C and D on the field strength diagram shown in Fig 9 are where
the field strength has fallen to
0.5 , ie 0.707, of the maximum value AB. Conversely, the power
radiated in the direction AC and AD = 0.7072 = .5 of the power transmitted along the centreline AB.
The angle θ is the NBW and is proportional to the wavelength (λ) of the radiation and inversely
proportional to the size of the aerial:
Kλ
NBW ∝
degrees
Dish Diameter
Revised Jun 10
Page 6 of 21
AP3456 – 11-8 - Ground Mapping Radar
where λ is the wavelength and K is a constant which varies with the side lobe level, but for a simple
parabolic aerial is typically 70.
21.
Effective Beamwidth. From an operator’s perspective, the apparent, or effective beamwidth (EBW)
is of more concern than the NBW since it is one factor which influences the accuracy with which radar
returns are displayed. The EBW is the angle through which the beam rotates whilst continuing to give a
discernible image from a point response. Fig 10 illustrates the effect of a radar beam with an EBW of 4º
scanning clockwise through a point target, due north of the aircraft. Fig 10b shows that the target will be
displayed on the CRT once the leading edge (LE) of the beam intercepts it. In this instance, it will not be
portrayed on its correct bearing of 360º, but in the direction in which the aerial centreline and time base is
pointing, ie along 358º. The target continues to be displayed until the trailing edge (TE) of the beam has
passed through it (Fig 10d). The effect is to spread the image of the point target response across the
EBW - in this case 4º. It is possible to calculate the resultant distortion. In the example used, if the point
target (treated as zero width) was at a range of 60 nm, it would appear on the displays to be 4 nm wide (1
in 60 rule), i.e. 2 nm either side of the correct bearing.
11-8 Fig 10 Effective Beamwidth
Fig 10a
Fig 10b
Fig 10c
Fig 10d
Beam CL at 356°
Beam CL at 358°
Beam CL at 360°
Beam CL at 002°
CL
TE
LE
TE
LE
TE
LE
TE
LE
Target
EBW = 4o
Clockwise
scan
Timebase
aligned with
CL of beam
Target remains
Target not yet
Target appears on
Target remains
il uminated until TE
on Display
Timebase at 358o
il uminated
of beam is clear
22.
Use of Receiver Gain Control. The EBW is largely a function of receiver gain. Both transmitted
power and receiver sensitivity are maximum along the beam centre line, decreasing towards the beam
margins. The receiver gain control determines the overall amplification of the received signal. This
amplification may be reduced, by the operator, until the signals near to the centreline of the beam are only
just above the visibility threshold. This will produce an extremely narrow EBW. Fig 11 shows a cross-section
of signal strength across the beam, in Cartesian co-ordinates. The comparison between high and low gain
settings demonstrates resulting change in EBW. Alternatively, receiver gain may be increased so that
signals at the edge of the beam are amplified sufficiently to exceed the visibility threshold.
Revised Jun 10
Page 7 of 21
AP3456 – 11-8 - Ground Mapping Radar
11-8 Fig 11 Gain Level and Effective Beam Width
EBW at
High Gain
EBW at
Low Gain
Maximum
Visibility Level
High Gain
Receiver
Amplification
Visibility
Level
Threshold
Low Gain
CL
Beam Angular Displacement
RADAR DISTORTIONS
Distortions Inherent in Radar
23. A radar reflective target will not be portrayed accurately in size and shape on the CRT due to a
combination of distortions that are inherent in radar. The size of a received signal is always
exaggerated. The distortions applicable to each radar return are:
a.
Beamwidth distortion.
b.
Pulse length distortion.
c.
Spot size distortion.
24.
Beamwidth (BW) Distortion. The cause of BW distortion has already been explained in Para
21. The effect is to add one half of the EBW to each side of the target as shown in Fig 12. The size of
BW distortion can be determined using 1 in 60 calculations.
Example: In Fig 12, if the EBW is 3º, and the target is at 60 nm range, then the BW distortion will
extend the width of the target by ½ × EBW on each side of the target.
At 60 nm range, 1º subtends 1nm (6076 ft)
EBW = 3º = 3 nm = 18,228 ft
∴ ½ EBW = 9,114 ft on each side of the target.
In this example, when at 30 nm range, ½ EBW would equal 4,557 ft.
Revised Jun 10
Page 8 of 21
AP3456 – 11-8 - Ground Mapping Radar
11-8 Fig 12 Beamwidth Distortion
½
EBW
Radar
Ta rget
Beamwidth
½
Distortion
EBW
25.
Pulse Length (PL) Distortion. The range to the near edge of a target is correctly determined by half
the time taken for the leading edge of the pulse to reach the target and return, multiplied by the propagation
speed. However, although the range to the far side of target is similarly determined by the leading edge of
the pulse, the CRT continues to 'paint' until the whole PL has completely returned from the far side of the
target. The effect is to extend the far edge of the target by an amount equivalent to ½ × PL. PL distortion is
added to the target and also to the BW distortion, as shown in Fig 13.
Example: A radar with a PW of 1 µsec (which gives a PL of approximately 300 metres) will
produce a PL distortion of 150 metres.
11-8 Fig 13 Pulse Length Distortion
Pulse Length
Distortion
Radar
Targ et
½
Pulse
Length
26.
Spot Size (SS) Distortion. The electron beam which produces the image on the CRT has a finite
size, and nothing less than a minimal 'spot size' can be displayed. The radar will try to paint the outside
edges of a response with the centre of the spot. This will draw the correct outline, but the image is blurred
by the addition of a margin with a thickness equal to the radius of the spot size (i.e. ½ × SS), as
illustrated in Fig 14. Adjustments to focus and brilliance will affect the SS. However, SS is a function
of the physical size of the radar display screen, so its real dimension does not change. The main
factor affecting the size of the distortion due to SS distortion is therefore the range scale selected. SS
distortion is relatively greater on smaller scales (i.e. on greater ranges). SS distortion is added to the
combined effects of BW and PL distortions as shown in Fig 15. On synthetic radar displays, there will
still be a minimum size for any time base illumination. This minimum size may be defined in pixels or
some other value. However, a distortion akin to SS distortion will remain present.
Revised Jun 10
Page 9 of 21
AP3456 – 11-8 - Ground Mapping Radar
11-8 Fig 14 Spot Size Distortion
Spot
Size
Spot Size
Distortion
Image Distortion
27.
Radar Distortions Combined. The total of all three radar distortions, for a given target, will
combine in the manner illustrated in Fig 15.
11-8 Fig 15Total Radar Distortions for a Reflecting Target
Pulse Length
Distortion
Beamwidth
Distortion
Target
Radar
Spot Size
Distortion
28.
Effect of Radar Distortions on 'No-show' Targets. The radar distortions increase the size of
return echoes. However, it should be noted that targets that do not reflect radar energy back to the
aircraft, e.g. lakes, will be decreased in size, as a result of distortion of the surrounding ground returns
(Fig 16). In practical terms, at longer ranges, small coastal estuaries, and small inland lakes are
generally indiscernible, and islands often appear as joined onto the mainland. However, resolution will
improve as range decreases.
11-8 Fig 16 Total Radar Distortions for a Non-reflecting Target
Resultant size of 'no-show'
Radar
Original outline of 'no-show' Target
Revised Jun 10
Page 10 of 21
AP3456 – 11-8 - Ground Mapping Radar
29.
Height Distortion. The range measured by a mapping radar is slant range, whereas for a
completely accurate display, plan range is needed. If such accuracy is necessary (e.g. for high
altitude, close range radar interpretation) the CRT time base can be made non-linear, i.e. the electron
beam producing the time base moves faster at the start of its movement from the point of origin, and
slows gradually towards maximum range. Even so, it is not possible to remove all of the distortion
close to the point of origin.
30.
Minimizing the Effect of Radar Distortions. The combined distortions produced by a radar will
be minimized if the operator obeys the following rules:
a.
Once the fix has been positively identified, reduce the gain setting in order to reduce EBW.
b.
Take the fix at close range, using the smallest range scale practicable (to minimize EBW, SS
and PL).
c.
An isolated target can be assumed to lie at the centre of the response (see Fig 15).
The Resolution Rectangle
31. As a result of radar distortions, radar targets separated in azimuth by less than EBW + SS will
merge together on the radar display (Fig 17).
11-8 Fig 17 Target Resolution in Azimuth
½ SS ½ SS
½ EBW
½ EBW
Targets
Radar Beam
Aerial
Revised Jun 10
Page 11 of 21
AP3456 – 11-8 - Ground Mapping Radar
Similarly, echoes separated in range by less than ½ PL + SS will merge (Fig 18).
11-8 Fig 18 Target Resolution in Range
½ SS
½ SS
Targets
½ PL
Radar Beam
Aerial
32. The area defined by the two dimensions (EBW + SS) and (½ PL + SS), is known as the
'Resolution Rectangle' (Fig 19). Any two reflecting objects on the ground, lying within an area the size
of the resolution rectangle will not resolve into separate images on the radar display.
11-8 Fig 19 The Resolution Rectangle
Targets
½ PL
EBW + SS
+ SS
Revised Jun 10
Page 12 of 21
AP3456 – 11-8 - Ground Mapping Radar
33. When a radar response consists of several buildings in a group, the operator can determine
whether they will remain as one response, or resolve into individual responses, by use of simple
calculation based on the Resolution Rectangle.
Example 1: Two targets are 1,500 ft apart in azimuth. If EBW = 2º and SS (of the chosen range
display scale) = 400 ft, at what range will they resolve into two?
Resolution will occur when EBW + SS is less than 1,500 ft.
1,500 – 400 = 1,100 ft
∴ EBW will need to be less than 1,100 ft
Using 1 in 60, at 60 nm range, 2º EBW subtends 2 nm (12,152 ft), so:
x
60
=
1
,
1 00
12 1
, 52
1
,
1 00
∴ x =
× 60 = 5 4
. 3nm
12 1
, 52
Therefore, resolution will occur at ranges closer than 5.43 nm.
Example 2: Two targets are 900 ft apart in range. Will they resolve on a display, assuming that
SS = 300 ft and PL = 600 ft?
Resolution in range will occur when separation is greater than SS + ½ PL.
SS + ½ PL = 300 ft + 300 ft = 600 ft.
Therefore, these two targets will resolve on the 20 nm range scale display.
RADAR REFLECTORS
34. The creation of a map-like image on a radar display relies on the relative strengths of the reflected
energy returned from the various terrain features. In turn, the amount of reflected energy depends
upon the material of the target, and, even more so, on the direction in which the radar energy is
reflected.
Reflectivity of Materials
35. All objects will simultaneously reflect and absorb electro-magnetic energy. In general, the more
electrically conductive the material, the higher the ratio of reflection to absorption. An indication of the
reflective potential for various materials is illustrated in Fig 20. The list is not exhaustive, but it can be
seen that man-made structures contain materials that are more reflective than natural substances.
Revised Jun 10
Page 13 of 21
AP3456 – 11-8 - Ground Mapping Radar
11-8 Fig 20 Reflectivity of Materials
Metal
Concrete
Masonry
Earth
Wood
Reflectivity
Specular Reflectors
36. Radar energy is reflected in the same manner as other electromagnetic waves, such as light.
Two types of reflection situations may be recognized, specular and diffuse.
37.
Specular Reflection. If the radar energy impinges on a smooth surface the reflection is known
as specular and is the same as light being reflected from a mirror, ie with the angle of reflection equal
to the angle of incidence (Fig 21). A surface may be considered smooth if is approximately planar and
contains no irregularities comparable in size with, or larger than, the wavelength of the radar. From a
horizontal surface, specular reflection causes the energy to be directed away from the receiver. Such
a surface will therefore appear dark on the display. Specular reflection is typical of smooth water and
fine sand. For a specular surface to give a good reflection back to the aircraft, it must be at, or near to
the normal (ie 90º) to the radar beam.
11-8 Fig 21 Specular Reflection
Normal
Reflected Ray
Angle of
Angle of
Incident Ray
Incidence
Reflection
Reflecting Surface
38.
Multiple Specular Reflectors. A pair of specular reflecting surfaces, mutually at right angles, will
return a signal at the same angle of elevation as the incident energy, but not necessarily at the same
angle in azimuth. This scenario is shown in Fig 22, with a concrete surface area and a factory wall.
Revised Jun 10
Page 14 of 21
AP3456 – 11-8 - Ground Mapping Radar
11-8 Fig 22 Reflection from Two Specular Surfaces
Fig 22a Elevation
Fig 22b Plan
Reflection at
Radar
Same Elevation Angle
Reflection Away
Energy
From Aircraft
From
z
Aircraft
Building
y
z
y
Building
Radar Energy
x
x
from Aircraft
Surface
39.
Corner Reflectors. Where three mutually perpendicular, specular surfaces exist, the geometry is
such that energy is reflected back to the source regardless of the angle of incidence. This
arrangement is known as a corner reflector. Although rare in natural terrain features, corner reflectors
frequently occur in built-up areas, as in Fig 23, and are largely responsible for the bright display of such
areas. Radar reflectors employing this principle are widely manufactured to enhance the radar
reflectivity of, for example, runway thresholds, small boats and targets on bombing ranges.
11-8 Fig 23 Corner Reflector in Built-up Area
Diffuse Reflectors
40.
Diffuse Reflection. When a reflecting surface is rough, i.e. when its irregularities are comparable in
size with, or larger than, the radar wavelength, then that surface acts as a mosaic of randomly orientated
specular reflecting surfaces. As a result, the reflected energy is diffused in all directions (Fig 24). The
amount of energy reflected in any direction is less than would occur in a single specular reflection. Diffuse
reflection is uncommon in man-made structures but is typical of normal undeveloped land and accounts
for the intermediate tone of such terrain on the display. The proportion of the energy which is reflected
back to the receiver depends largely on the angle of incidence.
11-8 Fig 24 Examples of Diffuse Reflection
Fig 24a Small Angle of Incidence
Fig 24b Large Angle of Incidence
Incident Energy
Reflected Energy
Reflected Energy
Incident Energy
Revised Jun 10
Page 15 of 21
AP3456 – 11-8 - Ground Mapping Radar
Change of Aspect
41.
The Cardinal Effect. The approach direction towards a target can greatly influence the strength
of returns. Even large complexes may give only poor reflection when the angle of incidence of the
transmitted energy is away from the normal. This phenomenon was first noticed with towns and cities
in the USA, which tend to be laid out on a N/S and E/W orientated grid. This is therefore known as the
'cardinal effect' (Fig 25).
11-8 Fig 25 The Cardinal Effect
Town or
Industrial
Complex
Good Radar
Response
Fair or Little
Good Radar
Radar Response
Response
42.
Aspect Change. When flying past a target complex, the availability of reflectors will change as a
result of the changing incident angle of transmitted energy. As the aircraft proceeds along its track, the
different reflectors found within the complex will produce echoes of varying strengths. In Fig 26, the
aircraft at point 1 will receive good echoes from surfaces A and B. However, by point 2, surface A can
be almost discounted, and point B has developed into a strong corner reflector. On the radar display,
the brightness level from each part of the target group will appear to change continually (known as
'glinting'), due to this series of different centres of reflection.
11-8 Fig 26 Aspect Change
Target
Complex
A
B
1
2
INTERPRETATION OF GROUND MAPPING RADAR
Introduction
43. Map reading from a radar display requires skill, and care. The normal technique requires the
operator to identify pre-selected fix points from which present position can be determined, or targets
located. In modern integrated systems it is possible to place electronic cursors over the fix point. By
knowing the fix point’s co-ordinates, and the relative range and bearing from the aircraft, the system
can then calculate the aircraft’s present position.
44.
Target Ambiguity. A radar will display all received echoes with a signal strength greater than the
visibility threshold. A small town, an industrial complex and an airfield may all look similar in
Revised Jun 10
Page 16 of 21
AP3456 – 11-8 - Ground Mapping Radar
brightness, and if close together, an element of ambiguity may exist. The skill of the operator is
required to determine which response, within a series or group, is that of the required fix point, and
which other echoes may be ignored.
High/Medium Level Radar Interpretation
45.
Selection of Fix Points. When selecting fix points for radar interpretation at high or medium
level, the following factors should be considered:
a.
Material. The fix point should be a good reflector, and therefore probably a man-made
structure.
b.
Size. The size of the reflecting object will affect the size and brightness of the response on
the radar screen. A larger target will probably be easier to identify, but due to distortions, it may
be more difficult to determine a point on it with any precision.
c.
Contrast. Contrast between the received radar return and its adjacent background will aid
identification. The table below shows the 3 main groups of reflecting ground surfaces and the
strength of their return signals.
Type of Surface
Strength of Echo
Water
Weak
Terrain
Medium
Cultural
Strong
Since water, in general, reflects little or no energy back to the receiver, the best contrast is usually
afforded by a cultural return against a water background. Examples of fix points in this category
include large bridges (such as the Severn, Forth and Humber Bridges), coastal refineries and oil
rigs. The contrast between water/terrain is better than terrain/cultural, therefore the second
choice should be coastlines or large water features. Finally, if the preceding options are not
available, then a fix can be obtained from terrain/cultural contrast. Fix points in this category
include towns/cities, power stations, large industrial complexes and airfields.
d.
Isolation. A target that is isolated should prove easier to identify than a target within a group.
e.
Ambiguity. When the target is adjacent to, or surrounded by other reflective complexes, it may
be difficult to identify. The operator can use patterns of responses to assist with this process.
f.
Transmitted Power. The higher the amount of transmitted power, the stronger the reflected
echoes will be. It must be remembered that if the transmitted power is increased by selecting a
longer PL, then the PL distortion will be greater.
g.
Range. At longer range, smaller isolated targets may not show. Also, at long range, BW
distortion is greater, and depending upon the radar parameters set, PL distortion is probably
greater also. It is therefore more accurate to fix at close range.
h.
Aspect. The angle of approach will influence the strength of the return signals (the cardinal
effect). In addition, by selecting a fix point ahead of the aircraft, and close to track, aspect change
can be avoided.
i.
Terrain Screening. The short wavelength radar energy travels in straight lines. Therefore,
any solid obstruction, such as a hill, will cast a 'radar shadow' on the far side. Any objects in the
shadow area will not receive radar energy (Fig 27a) and cannot therefore reflect any.
Revised Jun 10
Page 17 of 21
AP3456 – 11-8 - Ground Mapping Radar
11-8 Fig 27 Terrain Screening
Fig 27a Cross-section
Fig 27b Radar Display
Hill
Shadow
Radar Shadow
caused by Hill
High
Ground
Industrial
Complex
Hill Tops
On the radar display, an area of terrain screening will appear as a dark 'shadow' area containing
no radar returns (Fig 27b). If approaching a target which is behind a hill, it is possible to
determine, by trigonometry, at what range the target will appear out of hill shadow (over small
distances, Earth curvature can effectively be ignored in the calculation).
In summary, the 'ideal' radar fix point should be sufficiently large, a good reflector, unique, and exhibit
good contrast against its background.
46.
Identification of Fix Points. To identify a specific target on a radar screen, the operator should:
a.
Use larger, easily identified responses to help identify the smaller, unknown responses.
b.
Having decided which response is likely to be the target, make a positive cross-check by
confirming the relationship with surrounding responses.
47.
Pre-flight Study. Sound pre-flight study can help overcome much of the ambiguity present in
radar ground mapping displays. Most fix points are unlikely to be truly unique, and confident
identification must be achieved by relating the radar returns one to another. Coastal features are often
easily identified. However, they must be used with some caution since their appearance can vary with
tide changes, especially in shallow and estuarine waters. Precipitous and rocky coastlines (particularly
small islands) are more reliable than sandy or muddy ones. Man-made coastal features such as
harbours and piers usually show significantly, regardless of tide state.
48.
Radar Manipulation. As the range from aircraft to target changes, it is important to keep the
target illuminated with the maximum amount of radar energy. This requires the operator to make
continual adjustment to the Aerial Tilt control, to keep the aerial pointing at the best depression angle.
In addition, the operator will have to adjust the Gain control frequently, to maintain a useable amount of
radar returns above the visibility threshold. It is important not to attempt radar interpretation with too
many or too few responses on the display.
Low Level Radar Interpretation
49. The factors discussed in paras 45 to 48 are equally applicable to operation at low level. However,
the following points should also be noted:
Revised Jun 10
Page 18 of 21
AP3456 – 11-8 - Ground Mapping Radar
a.
Transmitter Power. As the aircraft is closer to ground features, more energy will be
returned, and, therefore, lower transmitter power can be selected. In addition, the radar gain
levels should be reduced to prevent the stronger signal returns from 'flooding' the radar display.
b.
Target Size. Being closer to the targets, echoes from small objects, not visible at high level,
will now be above the visibility threshold. Smaller fix points may therefore be utilized.
c.
Aspect. At very low level, the radar echo is probably reflected by the leading edge of a
structure, rather than the roof. In addition, many line features, such as railway embankments and
facing river banks will be evident as a result of the changed angle of incidence.
d.
Contrast. Level ground (eg prairie) will present a specular reflector at a higher angle of
incidence. Under this circumstance, even small cultural returns may give good contrast.
e.
Use of Terrain Features. Well-defined ridge lines will give sharp contrast between the bright
return on the nearside, and shadow on the far side. Such features might be used for navigation
check points, although they will not give the same precision fix as a discrete, man-made structure.
f.
Terrain Screening. At low level, terrain screening becomes of utmost importance, and must
be fully understood by the operator. Recognition of different stages of hill shadow can be used as
a method of ground avoidance.
g.
Radar Controls. Variations in tilt and gain settings will each make significant impact on the
radar picture displayed.
50.
Hill Shadows. Fig 27 showed a terrain cross-section, whereby a hill produces a radar shadow
area. Fig 28a shows a more complex cross-section, typical of flight over a series of undulating ridges
and hills. This will result in a radar display similar to that illustrated in Fig 28b. It should be noted that
the area of radar shadow originates at the top of the hill or ridge and extends away from the aircraft.
11-8 Fig 28 Hill Shadow from Undulating Terrain
Fig 28a Simplified Cross-section
Fig 28b Radar Display
Hill Shadow
Ridge Line 2
Hill
Shadow
Hill Shadow
Ridge Line 1
Ridge Line 2
Ridge Line 1
51.
Radar Cut-off. If the terrain elevation is equal to, or greater than the aircraft altitude, the radar will
no longer 'see' over the hill (Fig 29a). The shadow will extend to the maximum range of the radar screen
and is now termed 'cut-off'. Fig 29b illustrates radar cut-off originating from a hill directly in front of the
aircraft. In this instance, for safety, the aircraft should either climb to be higher than the hill, or turn
approximately 30º to the left, towards the low-lying terrain.
Revised Jun 10
Page 19 of 21
AP3456 – 11-8 - Ground Mapping Radar
11-8 Fig 29 Radar 'Cut-off'
Fig 29a Cross-section
Fig 29b Radar Display
Point of Radar 'Cut-off'
Low-lying
Terrain
Radar Shadow extends
to maximum range
Cut-off
Hill Top
52.
'Hill behind a hill'. Radar cut-off, as explained in para 51, gives a clear warning of the
relationship between aircraft altitude and terrain elevation. However, a potentially dangerous
situation can occur in hilly regions, when there may be some ambiguity between radar shadow and
radar cut-off. Fig 30a shows a terrain cross-section with a series of hills (A, B, and C), each
successively higher than the previous one. Although hill B is higher than the aircraft, the shading at
point 'X' will appear as normal shadow. However, if hill C was omitted from the diagram, it can be
seen that Pt 'X' would be recognized as cut-off. As the aircraft progresses at the same altitude
(Fig 30b) the relationship between the peaks and shadows changes, until the cut-off from hill B
become apparent. However, if the final hill (C) is significantly higher than hill B, the cut-off might not
become apparent until it is too late to climb over hill B. This scenario based on ascending hills
behind hills can be encountered in the foothills of major mountain ranges. The relative elevations
and distance between peaks will make each case unique. It is therefore vitally important that the
operator maintains excellent situational awareness, in order to not confuse merely undulating terrain
(as in Fig 28) with the 'hill behind a hill' scenario. In addition, good visual contact should confirm
each hill or ridge in the progression. If visual confirmation is not available, then the aircraft should
commence a climb to safety.
11-8 Fig 30 Ambiguity from the 'Hill Behind a Hill' Situation
Fig 30b Development of Cut-off as Range
Fig 30a Shadow and Potential Cut-off Mixed
Decreases
Pt 'X'
Aircraft's Flightpath
C
C
B
B
A
A
Cut-off from Hill B
Hill
Potential
Cut-off from
now apparent
Shadow
Cut-off
Furthest Hill
Radar Interpretation in Winter
53. The effects of snow and ice lying on the ground will have an effect on the quality and appearance
of the radar display.
Revised Jun 10
Page 20 of 21
AP3456 – 11-8 - Ground Mapping Radar
54.
Snow Covered Ground. A deep covering of snow will act as a specular reflector. The overall
effect will be to reduce the strength of the echoes from the ground.
55.
Ice. The effect of ice upon the radar display will depend upon its roughness. If an ice coating on
a body of water remains smooth, the return will appear approximately the same as a water return.
However, if the ice is formed from a broken and irregular surface, it will reflect echoes comparable to
terrain features (see Fig 31). Two distinct examples are worthy of mention:
a.
Offshore Ice. Offshore ice will present a diffuse reflecting surface, and may return strong
echoes, and subsequently disguise the true shape of a coastline. In the appropriate season,
therefore, the coastline may appear to extend in a seaward’s direction, sometimes for tens of miles.
b.
Picture Reversal. It is possible, for an inland lake, with a broken, irregular ice surface, to
return echoes which are stronger than those of the snow-covered land surrounding it. This extreme
scenario results in a 'picture reversal' effect. In arctic regions, a picture reversal can be obtained
from rapidly formed and irregular river ice.
11-8 Fig 31 Radar Returns from a Lake
Fig 31a Summer
Fig 31b Winter
Surface: Water - Specular Reflector
Surface: Broken Ice - Diffuse Reflector
Revised Jun 10
Page 21 of 21
AP3456 – 11-9 - Terrain Following Radar
CHAPTER 9 - TERRAIN FOLLOWING RADAR
Introduction
1.
To avoid detection by enemy radars, strike/attack aircraft need to fly at very low altitudes when in,
or approaching, defended airspace. The day/night, all-weather capability of strike/attack aircraft has
been greatly enhanced by terrain following radars.
Height/Speed Considerations
2.
Height. The optimum height to fly is a balance between being so low that collision with the ground is
a real risk and being so high that the aircraft is vulnerable to interception by the enemy. Fig 1 shows how
the risk of collision with the ground increases with decreasing height, and exposure to enemy action
increases with increasing height. This does not apply in all cases; the detection distance of a ground-
based radar over a flat desert or on a coastline is virtually independent of height and is mainly limited by
range and the curvature of the Earth. Excluding these cases, the resulting curve of total risk shows that
200 feet is the optimum operating level.
11-9 Fig 1 Optimum Height to Fly
Probability of
Hit ing Ground
Total Risk
k
is
R
g
in
s
a
re
c
In
Probability
of Enemy Hit
0
200 Ft
Increasing Height
3.
Speed. The faster an aircraft can fly, the safer it is from enemy attack. However, there is a point
where the penalty of increased fuel consumption in flying at supersonic speed at low level is hardly
compensated for by a reduction in vulnerability. Therefore, the highest sustainable subsonic cruise
speed is regarded as the optimum at low level.
Safety Factors
4.
The safety requirement for a terrain radar system is that no single failure should endanger the
safety of the aircraft. Any failure must result in a safe pull-up manoeuvre. From the fail-safe aspect,
duplication, where feasible, is adopted.
Terms
5.
The following terms describe the capabilities of particular systems:
a.
Terrain Warning (TW). A TW system warns the crew of terrain which lies directly in their
flight path. Virtually any airborne radar can be used for this purpose, provided the operator is
skilled in interpreting hilly terrain.
Revised Jun 10
Page 1 of 6
AP3456 – 11-9 - Terrain Following Radar
b.
Terrain Clearance (TC). A TC system enables the aircraft to fly 'peak-to-peak', rather than
accurately following the terrain contours.
c.
Terrain Avoidance (TA). A specialist TA radar will usually only display terrain which
penetrates higher than a pre-set clearance level.
d.
Terrain Following (TF). A TF system enables the aircraft to closely follow all ground
contours in elevation and it is the most effective of all terrain radar systems.
Monopulse Radars
6.
In a non-monopulse radar, the aerial produces a single beam with a width dependent on the size
of the aerial and the frequency used. A typical beamwidth in an airborne radar is 4º, and any ground
within this beam will give a return, the range of which can be measured. However, since the strength
of the return can have any value, depending on the reflecting properties of the particular piece of
ground, its position within the beam cannot be determined. The angular accuracy would therefore
be 4º or more which is inadequate for terrain following (see Fig 2a).
Revised Jun 10
Page 2 of 6
AP3456 – 11-9 - Terrain Following Radar
11-9 Fig 2 Comparison of Transmitted Beams
a Non-monopulse Radar
Datum
Poorly-defined Angle
Beam
Ground
Range to Nearest
Point in Beam
Range to Furthest
Point in Beam
b Monopulse Radar
Datum
Accurately
Defined Angle
Beam B
Beam A
Ground
Range Zero
Boresigh
Range to
t
nearest point
in Beam
'Sum' Signal =
Output A + Output B
Signal from
Beam A
Signal from
Beam B
'Difference' Signal =
Output A
− Output B after
Phase Detection
Exact Range
Along Boresight
7.
The single-plane monopulse aerial has two feeds (see Fig 2b), simultaneously producing two
slightly divergent and overlapping beams. The area in which the beams overlap is known as the radar
boresight. The ground returns illuminated by these beams are simultaneously processed in two
different ways. A 'sum' signal is formed by adding together the returned signals of each beam. This
gives the effect of a single beam, equal in width to that of the sum of both beams. At the same time, a
'difference' signal is formed by subtracting the returned signal of one beam from the other. The
difference signal has a phase which may be compared with that of the sum signal and it also has a
minimum amplitude where the beams are equal, ie along the boresight. The phase and amplitude
relationships between sum and difference signals are such that returns from above the boresight
produce a positive output whilst those from below give a negative output. Where the boresight
intersects the ground, the output is zero; the exact range along the boresight to the ground can
therefore be accurately determined.
Revised Jun 10
Page 3 of 6
AP3456 – 11-9 - Terrain Following Radar
SCANNING MONOPULSE RADAR
General
8.
A scanning monopulse radar system scans the terrain ahead of the aircraft and determines the
elevation profile which the aircraft must follow to clear the ground by the required height.
Principle of Operation
9.
Guidance information for manual flying is produced by the radar on a head-up display (HUD), or
the output can be coupled directly to the autopilot.
10. Fig 3 illustrates a simple HUD. The target 'dot' on the flight director symbology represents the TFR
demand, which the aircraft should be following to clear the ground ahead by the required amount. The
aircraft symbol indicates the aircraft’s present flight path. The situation illustrated in Fig 3 indicates that
the pilot needs to pitch the aircraft’s nose down, to follow the TFR demand.
11-9 Fig 3 HUD - TFR Presentation
Aircraft Symbol
Horizon Bars
Flight Director
symbology
with target dot
11. Other sources of information, which are necessary in a sophisticated TFR system, are provided by
the radar altimeter and airstream direction detector (ADD). Attitude reference is also required to
provide signals for radar roll axis stabilization.
12. The radar aerial scans in the vertical, +8º to –22º, about the radar roll axis. As the aircraft
manoeuvres, the radar roll axis changes to ensure that the beam covers the ground at all times. The
range to the ground along the aerial boresight for each pulse is measured and range to the ground at
all scanner angles is therefore known. The ground profile is compared with a computer generated
'ideal flight path', called the zero command line (ZCL), shown in Fig 4. The ZCL is made up of two
parts, the ski toe and the base line (not shown). The effect of various parameters on the ZCL is
summarized as follows:
a.
Set Clearance Height. The ZCL moves downwards with increased set clearance height to
keep the aircraft further from the terrain.
b.
Flight Vector. The ZCL is moved downward with increased climb angle but there is little
movement of the ZCL during a dive.
c.
Groundspeed. The flat portion of the ZCL is elongated and the curved portion flattened with
increased groundspeed.
d.
Ride and Weather Mode. The flat portion of the ZCL is elongated and the curved portion
flattened with the selection of soft ride or weather mode (used in normal/heavy rain conditions).
Revised Jun 10
Page 4 of 6
AP3456 – 11-9 - Terrain Following Radar
11-9 Fig 4 Flight Path Indications
Ground Returns
Ground Returns
Radar
Penetrate
On
Display
Ski Toe
Ski Toe
33 1
2
4 6
33
1
2
4 6
Pull-up Command
Command Satisfied
Head-up
Display
(HUD)
Head Down
Display
(ADI)
1
2
3
4
2
A/C Flight Paths
1
Zero Command Line
(Ski Toe)
Scan Limits
Terrain
0
1
2
3
4
5
6
Range in Nautical Miles
3
4
Ground Returns
Ground Returns
Below
Penetrate
Ski Toe
Ski Toe
33
1
2
4 6
33
1
2
4 6
Push Over Command
Pull-up Command
Revised Jun 10
Page 5 of 6
AP3456 – 11-9 - Terrain Following Radar
The 'Ski' in Action
13. The overall principle of the system is illustrated in Fig 4. Over reasonably smooth terrain, the
aircraft flies straight and level and the base line (not shown) 'rests' on the terrain. On approaching an
obstacle (position 1), the terrain profile penetrates the ski toe generating a pull-up command. The
command is satisfied at position 2. At position 3, the aircraft pushes over into a shallow dive. By
position 4, the terrain profile again penetrates the ski toe generating a pull-up command. The speed at
which this happens is controlled so that, by keeping the target dot within the circle of the aircraft
symbol, the pilot should not experience uncomfortable g forces.
14. When the elevation is zero, the base line must be parallel to the direction of aircraft flight, ie
parallel to the velocity vector. The ADD is used to measure the difference between the radar roll angle
and the velocity vector. This angle is fed to the computer to position the ZCL correctly.
15. The height measured by the radar altimeter is compared with the selected clearance height and
an up or down command is generated. This command is fed to the terrain radar computer and then
onto the head-up display or autopilot.
OPERATIONAL CONSIDERATIONS
Radar Returns
16. Radar returns from built-up areas and isolated buildings can be very much stronger than those from
sand or arid ground, therefore the strength of the return is not a measure of its importance, since the top
of a hill may be a poor reflector. If the receiver is sensitive enough to see such weak signals, strong
reflections to one side may swamp or break through the main signal and appear at the output as ground
at a higher angle than in reality. Automatic gain control (AGC) minimizes this problem.
High Speed, Low Level Scanning
17. The equipment has to provide safe steering signals in elevation, while the aircraft navigation
system demands a turn either via the autopilot or the head-up display. A bank angle of 45º at 0.9 M
has to be tolerated. To satisfy this requirement the aerial scan pattern changes, from being purely
vertical, to one which 'leans over' into the Terrain Following Radar turn. The angle of lean is a function
of aircraft bank angle and speed, thereby ensuring that the terrain inside the turn is scanned sufficiently
early to generate any necessary climb demand.
Revised Jun 10
Page 6 of 6
AP3456 – 11-10 - Sideways Looking Airborne Radar
CHAPTER 10 - SIDEWAYS-LOOKING AIRBORNE RADAR
Introduction
1.
Conventional airborne radars used for ground mapping or reconnaissance use either a
mechanically scanning, or a planar phased array aerial, to produce a PPI display. In order to achieve
good resolution it is desirable to have an azimuth beamwidth that is as narrow as possible, but since
beamwidth is inversely proportional to aerial size, this implies the use of large aerials. Unless the radar
is mounted in a large external radome with the attendant weight and aerodynamic penalties, the size of
a forward facing aerial will be restricted by the frontal area of the carrying aircraft, and in the case of a
mechanically scanning aerial, there must also be room to accommodate the aerial movement. A
further disadvantage of forward facing transmissions is that they give advance notice of the aircraft’s
approach to the enemy’s electronic scanners.
2.
Sideways-Looking Airborne Radar (SLAR) is used for reconnaissance, and overcomes these
disadvantages by looking sideways and downwards from the aircraft using a non-scanning aerial
mounted parallel to the aircraft fuselage. This arrangement allows the aerial to be made long, so
giving a narrow azimuth beamwidth and good resolution in that plane.
3.
The aerial is normally of the slotted waveguide type where one or more slotted waveguides are
physically attached parallel to the fore-and-aft axis of the aircraft, either along the side of the fuselage
or in an underslung pod. A SLAR aerial for a fighter-type aircraft would typically be 3 to 5 m long
compared to a maximum size of about 2 m for a circular scanning aerial in a large aircraft. The
equipment is normally designed for two aerials, one looking to port and the other to starboard.
Operation
4.
A beam of radar pulses is transmitted at 90º to the aircraft’s fore-and-aft axis, the number of pulses
transmitted being proportional to the ground speed of the aircraft to maintain a series of overlapping scans.
The radar transmission is switched from one aerial to the other on a pulse to pulse basis to produce two
maps of parallel strips of ground, equally spaced either side of the aircraft, as illustrated in Fig 1.
11-10 Fig 1 Radar Ground Cover
Area of Image Extends
Proportionally as
Aircraft Height Increases
Radar
Shadow
Aircraft
Flight Path
Aircraft
Track
5.
The output from the receiver can be displayed on a CRT, processed by an optical system to
produce a film record, stored on magnetic tape for post-flight display and analysis, or telemetered to a
ground receiving station.
Revised Jun 10
Page 1 of 3
AP3456 – 11-10 - Sideways Looking Airborne Radar
6.
SLAR typically operates in the K or L band and uses pulse compression techniques to retain good
range resolution while transmitting sufficient energy to achieve a satisfactory range performance. In
elevation, the beams are cosec2 in shape and are adjustable in depression angle so as to obtain the
optimum ground illumination with changing aircraft height.
7. The beamwidth of a linear array aerial may be determined approximately, in degrees, by 50λ/D
where λ is the operating wavelength and D is the aerial array length. Using this approximation for a
fighter-type aircraft installation with an aerial length of 3m, and assuming a wavelength of 1 cm, yields
a beamwidth of about 0.16º. On a large aircraft where an aerial length might be 10 m the beamwidth
would be 0.05º at the same wavelength, which at a range of 10 nm equates to a linear beamwidth of
some 16 m (50 ft). Although this may be adequate for some situations, many reconnaissance
applications require a narrower beamwidth. However, since there is clearly a practical limit to the
length of the aerial which can be accommodated on an aircraft, an alternative approach is needed - a
technique known as synthetic aperture.
8.
Synthetic Aperture. The synthetic aperture technique relies on the aircraft movement to simulate an
aerial array much longer than the physical installation. The parameters (including phase) of the radar returns
from a pulse are recorded, and then combined and processed together with returns from subsequent pulses
transmitted from different aircraft positions. The effect is to synthesize an aerial with a length equal to the
distance flown during the period in which the data is collected. Synthetic aperture techniques achieve useful
improvements, typically yielding linear resolutions an order smaller than a conventional linear array.
However, this improvement is at the expense of considerable processing complexity, which, particularly if a
near real-time performance is needed, would preclude their use on other than large aircraft. There is a
further requirement to stabilize the antenna to a high degree of accuracy.
Image Quality
9.
SLAR systems use high frequency transmissions with high pulse recurrence frequencies which,
together with pulse compression techniques and narrow beamwidths, enable high quality, good
resolution images to be obtained; Fig 2 shows an example low altitude SLAR image together with the
equivalent Ordnance Survey map for comparison.
Revised Jun 10
Page 2 of 3
AP3456 – 11-10 - Sideways Looking Airborne Radar
11-10 Fig 2 SLAR Map
10.
Distortions. Uncompensated deviations from the planned flight profile can cause distortions in
the image:
a.
Height and Ground Speed. Any variation in planned height or ground speed will cause
distorted scales across the map.
b.
Roll. If the aerial is not roll stabilized, rolling will cause uneven illumination of the ground
which will be apparent on the resulting imagery but may not produce distortion. However, large
roll angles may result in complete loss of the picture.
c.
Drift. Without drift stabilization of the aerial or drift compensation in the display system,
parallelogram distortion will result as shown in Fig 3.
11-10 Fig 3 Parallelogram Distortion
Y nm
Y nm
Planned
Target Area
Coverage
X nm
X nm
d
Echo-map
SLAR Beam
Revised Jun 10
Page 3 of 3
AP3456 – 11-11 - Weather Radar
CHAPTER 11 - WEATHER RADAR
Introduction
1.
A weather radar is an airborne pulse radar designed to locate turbulent clouds ahead of the
aircraft so that, in the interests of safety and comfort, they may be circumnavigated either laterally or
vertically, or penetrated where the turbulence is likely to be least. The radar beam is conical, and
typically scans in azimuth 75º to 90º either side of the aircraft’s heading (see Fig 1).
11-11 Fig 1 Conical Beam Scanning in Azimuth
Conical
Beam
scan
Some systems can scan vertically (typically ± 25º) to give a profile display. Cloud returns are displayed
as bright areas on a sector PPI display equipped with either fixed or electronically generated range and
bearing markers as shown in Fig 2. The scanner has limited stabilization in pitch and roll so that the
scan remains horizontal and with a steady tilt angle relative to the horizon during aircraft manoeuvre.
11-11 Fig 2 Cloud Formation on a Sector PPI Display
Azimuth
0
Markers
Range
Markers
'Bright-up'
Returns from
Clouds
Zero
Open
Range
Centre
2.
Most weather radars have a secondary ground mapping application and, in this mode, the radar
transmission is often converted to a cosecant2 beam – this type of beam is described in Volume 11,
Chapter 8.
Revised Jun 10
Page 1 of 5
AP3456 – 11-11 - Weather Radar
Principle of Operation
3.
Cumuliform clouds are associated with rising and descending currents of air leading to turbulence,
which can be severe in the case of cumulo-nimbus clouds. The turbulence tends to retain the water
droplets within the cloud which increase in size until they fall as heavy precipitation. It is this
precipitation, and in particular the large water drops, which reflect the radar energy and from which
turbulence can be inferred. Hailstones are normally covered with a film of water and tend to produce
the strongest echoes; gentle rain, snow, and dry ice produce the weakest echoes. Non-turbulent,
principally stratiform, clouds are not usually detected by the radar as the water droplet size is too small,
neither can the radar detect clear air turbulence. Normally the radar energy will penetrate the
precipitation of one cloud so as to be able to display echoes from clouds beyond. However, extremely
heavy precipitation may attenuate the radar to an extent that this penetration is not achieved.
Iso-echo Contour Display
4.
The strength of the returned radar signals varies according to the precipitation rate and, by
inference, reflects the degree of turbulence. However, a normal monochrome CRT display is unable to
discriminate between these different signal amplitudes; clouds with significantly different degrees of
turbulence would appear the same on the display.
5.
In order to overcome this shortcoming, a system known as Iso-echo Contour has been developed.
In this system an amplitude threshold level is established, and all signals which exceed this level are
switched to earth (see Fig 3).
11-11 Fig 3 Generation of Iso-echo Display
This Part of
Iso-echo Contour
Signal Switched
Switching Level
to Earth
e
l
d
a
n
litu
ig
p
Areas of
S
m
‘Bright-up’
A
Video
Threshold
Time
Normal
Display
Cloud
Return
'Black Hole'
The effect is to create a 'black hole' on the display corresponding to those parts of the cloud return with
the greatest precipitation rate (Fig 4). The outer and inner edges of the surrounding return correspond
to two contours of precipitation rate, and the width of the 'painted' return reflects the precipitation
gradient in the area; the narrower the paint, the steeper the gradient, and therefore the more severe
the turbulence.
Revised Jun 10
Page 2 of 5
AP3456 – 11-11 - Weather Radar
11-11 Fig 4 Weather Radar Colour Display
Screen Brightness
ON
Control Knob
TST
Radar Function
SBY
Selector Switch
OFF
Weather/
Weather Alert
Wx
WxA
RNG
Toggle Select Button
Range Selector
RNG
Buttons
VP Mode Select
Button
Ground Mapping
MAP
Mode Selector
Button
NAV
NAV Map Selector
(Requires Optional
Equipment)
UP
Antenna Tilt
0
TILT
DN
Control
Selected Weather
GAIN
Mode
Multi-threshold Colour Displays
6.
An extension of the Iso-echo Contour system is to have a number of threshold levels in order to
generate a series of precipitation rate bands. It is necessary to have a colour CRT to display these
gradations, with a different colour used for each precipitation rate band. Conventionally, the colours
range from black, indicating no or very light precipitation, through green and yellow to red, which
corresponds approximately to the traditional Iso-echo Contour threshold. Increasingly, new systems
add another colour, magenta, to indicate areas of most intense precipitation. One such display is
shown in Fig 4.
Sensitivity Time Control
7.
As well as the nature of the cloud target, the strength of the returning radar signal is dependent on
the range of the cloud. In order to eliminate this variable, sensitivity time control (STC), or swept gain,
techniques are used in which the receiver gain is lowered at the instant each pulse is fired, and then
progressively increased according to a predetermined law. This ensures that echoes from distant
ranges are amplified more than those from close range.
8.
At long ranges a cloud is likely to fill the radar beamwidth only partially and the echo signal will
vary inversely as the fourth power of range whereas at lesser ranges, where the beamwidth is
completely filled, the reflected signal varies inversely as the square of the range. There is,
therefore, no universal law for all ranges to which STC can be made to conform and any installation
will have a display that is compensated only over a limited, fairly short, range (e.g. 25 nm).
Display Interpretation
9.
Radar is only reflected by cloud if there are water droplets above a certain size, or hail, but
rapidly building storms will typically contain ice in their upper levels which reflects very little radar
energy. When cruising at high altitude it is therefore important to use the tilt control to scan
downwards or to use the 'profile' capability of some radars to intercept the lower portion of the storm
containing the water droplets. A 'profile' scan is shown in Fig 5.
Revised Jun 10
Page 3 of 5
AP3456 – 11-11 - Weather Radar
11-11 Fig 5 'Profile' Scan
ON
TST
SBY
Vertical PROFILE
OFF
Mode Annunciation
Plus & Minus
Wx
WxA
RNG
Thousands of Feet
from Relative
Left or Right Track
RNG
Altitude. Will vary
Annunciation
with Selected
Range
Degrees of Track
MAP
Left or Right of
Relative Altitude
Aircraft Nose
NAV
Reference Line
Vertical Scan
Angle ± 25°
UP
0
TILT
DN
GAIN
10. Having detected a cloud which is likely to be turbulent, a course of action must be determined.
The best option would be to avoid the cloud altogether, however this may not be possible in practice,
and consideration must be given as to the best part of the cloud to penetrate. Fig 6 shows a typical Iso-
echo display cloud return diagrammatically. There are two areas (marked W) where the amplitude of
the received signal has exceeded the threshold level, and these, therefore, show as 'black holes'.
Although these areas can be assumed to be areas of high precipitation and therefore of turbulence,
greater consideration should probably be given to areas where the precipitation gradient is highest.
This is indicated by the width of the 'paint'. The upper part of Fig 6 illustrates the returning signal
strength and it will be seen that the gradient is steeper to the left than to the right. On the display this
variation is shown by the band at A being narrower than at B. By implication, the particularly narrow
band at Y can be considered to be the area of greatest turbulence.
11-11 Fig 6 Diagram of Typical Cloud Return Indicating Zones of Differing Turbulence
e
d
Iso-echo
litu
p
Contour Level
m
lA
a
Video
n
ig
Threshold
S
X
Plan
A
B
Display
W
Y
11. Area Y should, therefore, be the first priority for avoidance. The two areas labelled W represent
returns above the Iso-echo threshold level and are, therefore, areas of high precipitation and
turbulence. Although area X, between these 'holes', is of a lower level, the degree to which the
amplitude has dipped below that of W is not apparent from the display; it may easily be very nearly as
turbulent. The best area for penetration is likely to be B where the paint is wide (wider than A), and the
amplitude continues to fall to below the video threshold level on the right.
Revised Jun 10
Page 4 of 5
AP3456 – 11-11 - Weather Radar
Determining Cloud Vertical Extent
12. The vertical extent of cloud is most simply determined on screen if the equipment has a profile scanning
capability (Fig 5). On azimuth only systems it is possible to make an approximate estimation of the vertical
extent of a cloud by tilting the aerial both above and below the horizontal until the echo just disappears as
shown in Fig 7.
11-11 Fig 7 Cloud Height Measurement
Fig 7a Tilt Up Until Echo Just Disappears
Radar Beam
0o
Tilt Angle
Fig 7a Tilt Down Until Echo Just Disappears
0o
Tilt Angle
Radar Beam
The solution to the trigonometrical equation involving the tilt angles and range is normally solved using a
table or a graph, such as that shown in Fig 8.
11-11 Fig 8 Cloud Height Measurement Graph
25
2.5o
3.0o
2.0o
)
20
0
0
ft
0
ra
1
1.5o
x
irc 15
A
(ft
e
p
ft
v
1.0o
o 10
U
ra
b
ilt
T
irc
A
0.5o
A
5
to
e
0o
tiv
0
la
e
R
5
0.5o
d
n
u
ft
w
o
lo
ra
D
C
10
1.0o
f
irc
ilt
o
T
t
A
h
w 15
1.5o
ig
lo
e
e
H
B 20
2.0o
3.0o
2.5o
40
50
60
70
80
90
100
Range (nm)
Revised Jun 10
Page 5 of 5
AP3456 – 11-12 - Air Intercept Radar
CHAPTER 12 - AIR INTERCEPT RADAR
Introduction
1.
The principle purposes of airborne intercept (AI) radar are to detect and identify airborne targets
(including those below the radar horizon of ground-based installations) to enable the timely use of
appropriate tactics and weapons. Ideally, AI radar should be able to track, measure flight parameters
of, and predict flight paths for multiple targets simultaneously, so as to provide the fighter crew with an
overall air picture.
2.
The radar will also be required to provide the necessary cueing and guidance signals for air
launched weapons such as radar guided missiles. At closer range, the radar should provide accurate
steering and range information so that a visual identification of a target can be accomplished. In close
combat, the radar may be required to provide range and angle rate data for gun aiming, and range
cueing for infra-red (IR) homing missiles. Radar angle data may also be used to slave IR missile
heads onto a target to facilitate IR lock before missile launch.
3.
Such diverse requirements cannot be met with one type of radar transmission, and modern AI
radars are able to operate in a number of modes, the most appropriate being selected for any
particular situation. The capabilities of these various modes can only be realized with the aid of an
airborne computer capable of processing the signals and of providing the necessary inputs to a
synthetic display.
Operating Modes
4.
Whereas a non-coherent pulse radar can readily be used to determine the range of a target, it
cannot determine velocity. Furthermore, it is not suitable for detecting airborne targets against a
terrestrial background, since the target echo would be indistinguishable from the ground returns.
Conversely, a pure CW radar cannot be used to determine range but can be used to determine the
radial velocity of a target by measuring the Doppler shift in the radar echo. Since only the Doppler shift
(ie not the absolute frequency) of the returning signal is measured, a simple CW radar cannot
discriminate between multiple targets which occur in the beam at the same time. Another
disadvantage of pure CW is that it requires separate transmitting and receiving aerials. Thus, although
pure CW on its own is not a suitable transmission type for an AI radar, an AI radar will usually have a
CW mode, which is used to illuminate a target for missile guidance. In this mode the aircraft radar only
transmits, the receiver being in the missile seeker head.
5.
Clearly some combination of the characteristics of both pulse and CW is desirable and this is
achieved in a type of transmission known as pulsed Doppler (PD) or interrupted continuous wave
(ICW). In this transmission type, the pulses are coherent with respect to each other, i.e. they have a
constant phase relationship. The coherency allows a Doppler shift (and consequently velocity) to be
measured; the pulse characteristic means that range can be determined, although, in practice, not as
easily as in a simple pulse radar.
Clutter
6.
One of the desirable features of AI radar is the ability to detect targets against a terrestrial
background. This ability is conferred by the CW element of the transmission. However, in addition to
the Doppler shift in the received echo generated by the target, there will also be a spectrum of
frequencies returned from the ground as a result of the carrier’s speed. As well as the main beam, all
radars produce a number of sidelobes which, since they intercept the terrain at a variety of angles, will
Revised Jun 10
Page 1 of 11
AP3456 – 11-12 - Air Intercept Radar
detect varying Doppler shifts (Fig 1a). The sidelobes reaching the ground immediately underneath the
aircraft generate clutter (altitude returns) centred on the transmission frequency (since the range rate is
essentially zero) but with some frequency spread due to terrain variation and aircraft climb and
descent. The Doppler frequency sensed by the main beam will vary with radar angle, terrain profile,
and, of course, with the carrier aircraft’s velocity. The result, in the case of a simple CW transmission,
is to produce a clutter spectrum similar to that shown in Fig 1b. In some systems the peaks due to
altitude and Mainlobe Clutter (MLC) can be removed using filters, to leave a band of clutter of fairly
level amplitude extending either side of the central frequency by an amount equivalent to groundspeed
Doppler shift. In the 'look-up' situation, MLC is eliminated but Sidelobe Clutter (SLC) is still present.
11-12 Fig 1 Generation of Clutter in a CW Radar
Fig 1a Radar Transmission Main and Side Lobes
Sidelobes
Altitude
Return
Sidelobes
Main Lobe
Ground
Fig 1b Received Frequency Spectrum of a CW Radar
Altitude Return
Mainlobe
& Transmit/Receive
Clutter (MLC)
e
Leakage
d
litu
p
Sidelobe
Sidelobe
m
Clutter (SLC)
Clutter (SLC)
A
–V
f
V
0
(Zero Doppler
(Aircraft’s
Doppler
Shift)
Groundspeed)
Frequency
7.
In order to detect a target readily, its Doppler shift must lie outside of the clutter region. Whether
this will be the case depends on the target’s radial velocity which, in addition to its actual speed, is a
function of the intercept geometry. In the head-on case (Fig 2a) the radial velocity will be the sum of
the target’s and the carrier’s speeds, and the Doppler shift will always be outside the clutter region. In
the beam and stern chase cases however (Fig 2b and 2c), the radial velocity is likely to be low giving a
Doppler shift inside the clutter region, thereby making detection more difficult.
Revised Jun 10
Page 2 of 11
AP3456 – 11-12 - Air Intercept Radar
11-12 Fig 2 Effect of Intercept Geometry on Target/Clutter Interference
Fig 2a Head-on Chase
Fig 2b Beam Chase
Fig 2c Stern Chase
8.
The clutter problem is complicated once a pulsed transmission is considered since the
transmission comprises several spectral lines which can be determined by Fourier Analysis to be
multiples of the Pulse Repetition Frequency (PRF), as shown in Fig 3a. The clutter spectrum is
superimposed on each of these spectral lines (Fig 3b) and this is significant when choosing the PRF,
since the higher the chosen PRF, the wider will be the clutter free regions.
11-12 Fig 3 Clutter in an ICW Transmission
Fig 3a Frequency Spectrum of ICW Transmission
Centreline
Frequency (fo)
Frequency Components
Separated by PRF
Frequency
Fig 3b ICW Received Signal – Clutter Superimposed over each PRF Line
Clutter Free
Region
fo
PRF
PRF
PRF
PRF
Clutter
Revised Jun 10
Page 3 of 11
AP3456 – 11-12 - Air Intercept Radar
Eclipsing and Blind Ranges
9.
Since a common aerial is used for transmission and reception, the receiver is only able to accept
returning echoes for a proportion of the time. As a consequence, many returning echoes will be
partially cut off, an effect known as eclipsing; at certain ranges the echoes will be completely cut off
giving rise to blind ranges (see Fig 4).
11-12 Fig 4 Eclipsing and Blind Ranges
Transmitted
Signal
This Part
of Signal
Eclipsed
Eclipsing
Target
Echoes
Signal Completely
Cut off
Blind Ranges
Target
Echoes
T
Time
0
10. The problem of blind ranges can be alleviated by jittering or staggering the PRF so that no
particular range remains blind permanently. The technique also helps to spread the eclipsing effect
more uniformly over different target ranges. In very high PRF systems the loss of received power
caused by eclipsing can be compensated for in the signal processing by integrating pulses.
Velocity and Range Ambiguities - The Influence of PRF
11. In any pulsed radar system it is essential, if ambiguities are to be avoided, for each echo to be
associated with its own transmitted pulse.
12.
Velocity Ambiguity. In the example of a received spectrum of an interrupted continuous wave
radar at Fig 3b, the pulse repetition frequency is sufficiently high to allow clutter free regions to exist,
and if the target’s Doppler shifted frequency falls into this clear region it is relatively easy to detect
since it is competing only with noise. If, however, the pulse repetition frequency is reduced, the
spectral lines become closer together until, at some point, the clutter spectra overlap (see Fig 5). In
addition to the increased problem of detecting the wanted return against the clutter background, it is
also impossible to determine to which particular spectrum the return belongs and it is therefore not
possible to measure velocity unambiguously. A sufficiently high PRF must be used to prevent the
maximum Doppler shift from appearing in the adjacent spectrum.
Revised Jun 10
Page 4 of 11
AP3456 – 11-12 - Air Intercept Radar
11-12 Fig 5 Reduction of PRF Leading to Overlap of Clutter Spectra and Frequency Ambiguity
Centreline
Mainlobe Clutter
Frequency
PRF
PRF
PRF
PRF
Tgt
Tgt
Tgt
Tgt
Sidelobe Clutter
13.
Range Ambiguity. Although the problem of velocity ambiguity can be avoided by employing a
sufficiently high PRF, this approach will lead to an increased likelihood of range ambiguity. Radar
range is determined by timing the radar echo with respect to its transmitted pulse and, in order to avoid
range ambiguities, the echo must be received before the next pulse is transmitted. Using a high PRF
reduces the time available for this to be possible, and therefore reduces the maximum range at which
the radar can be used without incurring ambiguities in range measurement.
Medium PRF Radars
14. If a target is in sidelobe clutter, as in Fig 6a, the likelihood of detection is low. The situation would
not appear to improve at a lower, medium PRF if the target remains in the SLC (see Fig 6b), but this
allows time for the use of a number of range bins, as illustrated in Fig 7.
Revised Jun 10
Page 5 of 11
AP3456 – 11-12 - Air Intercept Radar
11-12 Fig 6 High/Medium Doppler Spectra
Fig 6a High PRF
f
f
f
0
0
0
Tgt
Tgt
Tgt
f
Fig 6b Medium PRF
f
f
f
0
0
0
Tgt
Tgt
Tgt
f
11-12 Fig 7 High/Medium Pulse Spacing
Fig 7a High PRF
time
Fig 7b Medium PRF
Range Bins
1
2
3
n
time
These range bins act effectively as separate radar receivers, each dealing with a different range band,
enabling the clutter to be divided between them. It is useful to consider the derivation of SLC - in the
high PRF mode it is generated from horizon to horizon; in the medium PRF mode it is generated within
annular rings as shown in Fig 8.
Revised Jun 10
Page 6 of 11
AP3456 – 11-12 - Air Intercept Radar
11-12 Fig 8 Sidelobe Clutter Regions in Medium PRF Mode
5
Range Bins
4
3
2
1
1
2
3
4
5
15. A medium PRF mode requires processing for each range bin and is therefore more complex than
a high PRF mode. This complexity is further increased by the need to resolve both velocity and range
ambiguities. Moreover, there are no clutter free regions such as those shown in Fig 3b, and so
approaching targets are more difficult to detect than in the high PRF mode. The ideal solution is to
provide operator choice of high or medium PRF modes.
16. Many AI radars can use a medium PRF mode, typically in the range 10 kHz to 30 kHz. Although a
medium PRF system suffers from both range and velocity ambiguities, neither is particularly severe
and they can be resolved using the technique of multiple PRFs.
17.
Range Resolution. The principle involved in multiple PRF ranging is shown in Fig 9. The
transmission is first made at PRF 1 which generates a series of ambiguous range values. The PRF is
then altered to PRF 2 and a further set of ambiguous ranges obtained. A coincident range value from
each PRF indicates the true range.
11-12 Fig 9 Two-PRF Ranging Techniques
Coincident
Range
PRF 1
PRF 2
True Range
= Ambiguous Returns
= Unambiguous Return
18.
Velocity Resolution. Fig 10 shows a typical medium PRF radar frequency spectrum in the
presence of ground clutter; the PRF is selected so that the MLC is at the PRF line. The target return is
repeated for every PRF line. The area for detection is limited to the frequency range between the
centre line and the first PRF line and detection filters are arranged to cover this space. In modern
Revised Jun 10
Page 7 of 11
AP3456 – 11-12 - Air Intercept Radar
systems the task of the array of filters will be carried out by the computer software. Ambiguity arises
since a target detected by the filters may be due either to the centre line, or to one or more PRFs
before the centreline.
11-12 Fig 10 Medium PRF Frequency Spectrum with Ground Clutter - Location of Detection Filters
Centre Line
Mainlobe
Frequency
Clutter
PRF
PRF
PRF
PRF
Tgt
Tgt
Tgt
Tgt
Usable
Detection Filters
Detection
Filters
Fig 11 shows the situation when two PRFs are used (15 kHz and 12 kHz). The dots represent the
detection filters. Consider a target detected by the 7th filter using the 15 kHz PRF and by the 10th filter
using the 12 kHz PRF. The correct value for the Doppler shift is found by repeatedly adding the
corresponding PRF increment to the filter number until a matching value is found. In this example:
Filter Value +
PRF +
PRF +.........
7
+
15
+
= 22
10
+
12
+
= 22
i.e., the correct Doppler shift is 22 kHz.
11-12 Fig 11 Two-PRF Velocity Resolution
PRF = 15 kHz
PRF
Centre
PRF
Line
Line
Line
15 kHz
Tgt
Detection
5
10
15
20
Filters
PRF = 12 kHz
PRF
Centre
PRF
Line
Line
Line
12 kHz
Tgt
Revised Jun 10
Page 8 of 11
AP3456 – 11-12 - Air Intercept Radar
19. In practice three PRFs are normally used for velocity resolution in order to cater for positive and
negative relative velocities, and a further two PRFs for range resolution. Both the ranging and velocity
PRFs must be transmitted during the period of target illumination.
High PRF Radar - FM Ranging
20. Whereas a medium PRF radar is usually considered the better option in an agile close combat
situation, a high PRF radar is generally a better choice when the task is to detect and engage fast, low-
flying aircraft at maximum missile range. By using a high PRF, at least twice that of the highest
Doppler frequency shift expected, velocity ambiguity can be avoided, but the system would suffer from
multiple ambiguities in range. Unfortunately, the technique of using multiple PRFs to resolve the
ambiguities is impractical at PRFs above about 20 kHz as it is difficult to provide a sufficient number of
range bins and high PRF radars typically employ PRFs in excess of 200 kHz. An alternative ranging
technique is therefore necessary and the method employed is that of frequency modulation (FM).
Since the transmission type is interrupted continuous wave (ICW), this mode of operation is often
known as FMICW.
21. The principle is illustrated in Fig 12 which shows that the transmission frequency is first ramped
up, and then down, followed by a period of zero modulation.
11-12 Fig 12 Principle of FM Ranging
Recei
y
ve
c
d
n
Si
e
gn
u
al
q
Signal
re
f
itted
d + fr
F
f
d fr
Transm
} fd
Time
At any instant, the difference between the transmitted and received frequency is due to a combination
of range delay and Doppler shift. During the up ramp the difference between the transmitted and
received frequencies (∆f) is the Doppler shift, fd, minus the change due to the range delay, fr; during the
down ramp, ∆f = fd + fr. During the zero modulation phase the difference is solely due to the Doppler
shift. Thus three frequencies are generated, the Doppler shift and two frequencies equally spaced
either side of the Doppler frequency and separated from it by an amount which is a function of the
target’s range (Fig 13). These frequencies are detected in the computer software using fast Fourier
transform (FFT) techniques, and the target’s range and velocity is computed. A target is only
recognized if all three frequencies are present.
11-12 Fig 13 FM Ranging Frequency Triplet
fd
fd fr
fd + fr
Revised Jun 10
Page 9 of 11
AP3456 – 11-12 - Air Intercept Radar
22.
Ghosting. When more than one target is present there is a possibility of false targets, known as
ghosts, being generated as the software searches for the pattern of three equally spaced frequencies.
Consider the situation in Fig 14 where there are two real targets, X and Y, each generating a frequency
triplet. The computer will recognize a further target composed of the centre and lower frequency of X,
together with the upper frequency of Y. The more real targets there are, the greater is the potential for
ghosts.
11-12 Fig 14 Generation of Ghost Target Triplet
Y
Y X
Y
X
X
Ghost Target
Triplet
Low PRF Mode
23. Some radars have the option of using a low PRF mode which is sometimes known as Pulse mode
since, although the transmission is still ICW, the severity of velocity ambiguity is such that Doppler
detection is impractical, and the radar acts in a similar manner to a simple non-coherent pulse radar.
The low PRF mode would typically be used in a stern attack situation where it has the advantages of
providing a relatively tenacious lock-on and accurate unambiguous ranging down to close range; it is
usually the most appropriate mode for the visual identification task and for close range gun attacks.
Scanning and Tracking
24. The aerial of an AI radar may be scanned either mechanically or electronically, and the degree of
scan, together with the scan pattern, is normally variable and under the control of the crew. Some
typical scan patterns are shown in Fig 15. In some systems the PRF is switched between high and
medium on alternate bars, the pattern reversing with each scanning frame (Fig 15e). This gives the
ability to detect both nose (high PRF) and tail (medium PRF) targets at nearly maximum ranges within
a single scan. However, since the scan frame time is divided between the two modes, neither mode
provides its maximum potential detection performance.
11-12 Fig 15 Typical AI Radar Scan Patterns
d Eight Bar Scan
a Single Bar Scan
b Two Bar Scan
c Four Bar Scan
e Four Bar Scan with Interleaved PRF
High
High
Medium
Medium
High
High
Medium
Medium
Frame 1
Frame 2
Revised Jun 10
Page 10 of 11
AP3456 – 11-12 - Air Intercept Radar
25. Tracking techniques are outlined in Volume 11, Chapter 6; monopulse tracking is usually
employed for angle tracking in AI radars. Range and velocity tracking is accomplished using gating
techniques, although the problem is complicated in the FMICW mode by the necessity to have three
gates to track the triplet of frequencies.
26.
Track-While-Scan (TWS). Track-while-scan (TWS) is a facility generally available in AI radar
systems, the capability being determined more by the computing power available than by the radar
parameters. A detected target is analysed to determine its velocity and on this basis the computer
predicts its position for the time of the next radar scan. If the target is detected within a small search
area around the predicted position a track is established. If the target is not detected the search area
is enlarged for the next scan until the target is either detected or declared lost. The computer will
smooth the calculated track over a number of detected positions. Since a TWS system can track a
number of targets simultaneously it can provide the crew with a good overall 'air picture'.
Displays
27. AI radars will usually provide the operators with a selection of head-up and head-down display
formats.
28. Head-up displays for pilot use may show, in addition to some basic flight parameters (e.g. TAS,
heading, altitude), the relative position of targets, an aiming mark, and maximum and minimum
engagement ranges.
29. The head-down display may be a plot of either range or velocity against azimuth. A typical
range/azimuth TWS display is illustrated in Fig 16 and shows friendly (numeric labels) and hostile
(alphabetic labels) targets with their predicted tracks. The relative elevation of a target is shown by
means of the target label against a vertical scale on the right hand side of the display.
11-12 Fig 16 Typical Range/Azimuth TWS Display Format
Speed
Heading
Altitude
435 KT
311
28100
A
A
C
Elevation
Scale
B
C
4
B
Elevation
Scan Limits
Revised Jun 10
Page 11 of 11
AP3456 – 11-13 - Maritime Radar
CHAPTER 13- MARITIME RADAR
Introduction
1.
The principal task of a maritime radar is to search an expanse of ocean for maritime targets which
may range in size from a submarine periscope to an aircraft carrier. Ideally the system should allow
friendly and hostile targets to be distinguished, and may have the capability to identify surface vessels
broadly. The radar performance should be such that the aircraft can remain outside the engagement
envelope of any hostile vessel’s weapon systems.
2.
Having located a target, the system should be capable of tracking it and, if necessary, of
prosecuting an attack either directly or by relaying target information to other units. In anti-submarine
operations the radar transmissions may be employed to deter an enemy from surfacing, thus
frustrating attempts at using its periscope for target identification, raising communication masts,
recharging batteries, or firing surface launched missiles.
3.
In addition to its primary role, a maritime radar will typically be able to operate in weather
avoidance and ground mapping (navigation) modes, and will usually have IFF interrogation facilities.
Radar Type and Parameters
4.
Maritime radars are invariably pulsed radars and, in common with the majority of airborne radars,
usually operate in I, or occasionally J, band. Although using higher frequencies would permit smaller
aerials or better resolution, in the maritime environment there would be an unacceptable increase in
clutter returns from water droplets in the atmosphere, such as from blown spray.
5.
In order to achieve an acceptable level of resolution, while still being able to operate at long range
when necessary, the technique of pulse compression using linear frequency modulation is often used.
Pulse compression techniques are explained in Volume 11, Chapter 2. Surface Acoustic Wave (SAW)
technology is employed in many maritime radars to achieve pulse compression.
6.
Pulse compression techniques using linear frequency modulation make the radar resilient to
broadband noise jamming since the receiver tends to ignore any signals which are not appropriately
coded. In addition, most radars use frequency agility as a further EPM measure.
7.
The maritime radar environment can be very noisy and cluttered since, in addition to internal and
external noise, clutter returns may be produced, for example, from sea movement, waves, flocks of
birds, and porpoises. Such returns can easily mask the desired returns from small targets such as
periscopes and masts and thus, although much of the external noise is filtered out in the pulse
compression process, only rarely are unprocessed radar returns displayed. Instead, the majority of
systems employ one or more filtering or integrating techniques to 'clean-up' the radar picture
(see Volume 11, Chapter 3).
8.
In constant false alarm rate (CFAR) receivers a voltage threshold level is set, and returns are
only processed as targets if their signal strength exceeds this level. The threshold level is
constantly adjusted in line with a running average value of the received signal.
9.
Integration may be applied on a pulse-to-pulse or on a scan-to-scan basis. The technique relies
on the premise that whereas a target return will be fairly constant in position and strength, clutter
returns tend to be more transient. It is therefore possible to set a threshold level so that, for example,
Revised Jun 10
Page 1 of 2
AP3456 – 11-13 - Maritime Radar
a return is only processed as a target if it persists for, say, six pulses out of ten on a single scan pass,
and on a minimum number of successive scans. Unfortunately, small targets such as submarine
masts and periscopes may often fail to clear these threshold levels since they will often be physically
masked in a rough sea.
Scanning
10. The scan pattern of a maritime radar may be either a forward hemisphere sector scan, or a 360º
scan. However, in the latter case only rarely is a full 360º achieved since in virtually all installations
there will be some screening by the aircraft fuselage or components. In addition where a 360º scan is
available it is normally possible to restrict transmissions to certain sectors, either to increase the data
rate from an area of interest, or to deny an enemy any EW information from the transmissions. The
radar scanner will usually be stabilized in the horizontal and vertical planes to compensate for aircraft
manoeuvre and in some systems the scanner tilt may be controlled automatically to restrict
transmissions in accordance with the selected range scale.
Target Tracking
11. One of the facilities of a maritime radar is the ability to track a number of targets automatically. In
a typical system the radar computer divides the search area into a matrix of cells, into which targets
are allocated. Once a target has been identified by the operator a computer file is opened, and as the
target moves from one cell to another the track and speed are calculated and the file updated.
Problems can occur with rapidly manoeuvring targets and with large targets where the radar return
occupies more than one cell. In these cases it may be preferable for the operator to allocate a
manually assessed track and speed to the target. The efficiency of an auto-tracking system is highly
dependent on the suppression of unwanted returns since these can cause the computer to overload as
it attempts to associate false targets with established tracks or to generate new, but false, tracks.
Displays
12. Maritime radars usually employ 360º or sector PPI displays in their normal operating mode and in
the case of a 360º display it may be heading or north orientated. In addition to the PPI display, some
systems will have high resolution displays using an A- or B-scope which enable selected targets to be
investigated more thoroughly by scanning through a narrow angle. It may be possible to achieve some
degree of target identification using these displays but this is largely dependent on operator skill and
experience.
Revised Jun 10
Page 2 of 2
Document Outline