Program : B.

E
Subject Name: Radar Engineering
Subject Code: EC-8004
Semester: 8th
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Unit-02
MTI radar, delay line canceller, multiple or staggered pulse repetition, Doppler filter, MTI radar
processor, limitations to MTI performance.

2.1 MTI RADAR

MTI radar stands for Moving Target Indication Radar. It is one type of pulsed radar. It is
characterized by very low PRF (Pulse Repetition Frequency). Hence there is no range ambiguity
in the MTI radar. It is used to determine target velocity and to distinguish moving target from
stationary target.
The unambiguous range (Run) is expressed by following equation.
Run = Vo/fp
Where,
Vo = velocity of EM in free space
fp = PRF
The radar uses doppler effect in its functional operation. It eliminates clutter due to stationary
objects and identifies moving targets.

The transmitter generates a continuous (unmodulated) oscillation of frequency f0, which is

radiated by the antenna. A portion of the radiated energy is intercepted by the target and is
scattered. Some of it in the direction of the radar, where it is collected by the receiving
antenna. If the target is in motion with a velocity Vr relative to the radar, the received signal
frequency will be shifted from the transmitted signal frequency f0 by an amount ±fd.

The plus sign applies if the distance between the radar and the target is decreasing (that is, an
approaching target) and the minus sign applies when this distance is increasing (that is, a
receding target). The received echo signal at a frequency f0 ± fd enters the radar via the
antenna and is hetero-dyned in the detector (mixer) with a portion of the transmitted signal f0
to produce a Doppler beat note of frequency fd. However, the sign of fd is lost in this process.

The purpose of the Doppler amplifier (beat frequency amplifier) is to eliminate echoes from
stationary targets and to amplify the Doppler echo signal to a level where it can operate and
indicating device. Their frequency response characteristic is as shown in Fig. 2.1. The low-
frequency cutoff must be high enough to reject the d-c component caused by stationary
targets, and yet it must be low enough to pass the smallest Doppler frequency expected.

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Sometimes both conditions cannot be met simultaneously and a compromise is necessary. The
Doppler cutoff frequency (on the higher side) is usually selected to pass the highest Doppler
frequency expected.

Fig. 2.1 Block diagram of MTI Radar & its frequency response.

The indicator could be a pair of earphones or a frequency meter. Earphones are used when an
exact knowledge of the Doppler frequency is not required. The ear then acts as a selective
(narrow) band pass filter with a pass band of the order of 50 Hz centered about the signal
frequency. This is of use for subsonic aircraft targets when the transmitter frequency falls in the
middle range of the microwave frequency region

If audio detection is desired for those combinations of target velocity and transmitter frequency
which do not result in audible Doppler frequencies, the Doppler signal could be heterodyned to
the audible range. The Doppler frequency can be detected and measured by conventional
frequency meters, usually one that counts cycles.

For example In a MTI radar the pulse repetition frequency is 200 Hz and the carrier
transmission frequency is 100 MHz Find its first, second and third blind speeds.

The pulse repetition frequency, fp = 200 Hz

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The carrier transmission frequency, ft = 100 MHz

The carrier wavelength,

C/ft = 3 × 108 / (100 × 106) = 3m

The n-th blind speed,

V rn = λ fp / 2

So, the first blind speed = (1 × 3 × 200)/ 2

= 300m/sec

The second blind speed = (2 × 3 × 200)/ 2

= 600m/sec

The third blind speed = (3 × 3 × 200)/ 2

= 900m/sec

Benefits or advantages of MTI Radar

Following are the benefits or advantages of MTI Radar:

➨MTI radar can distinguish between moving target and stationary target.
➨It uses low PRF (Pulse Repetition Frequency) to avoid range ambiguities.
➨MTI principle is used in air surveillance radar which operates in presence of clutter.
➨It is simpler compare to pulse doppler radar.
➨Antenna bandwidth is high.
➨It is economical.
➨It does not require waveforms with multiple PRF.
➨It is preferred at UHF frequencies.

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2.2 Delay line canceller is a filter, which eliminates the DC components of echo signals received
from stationary targets. This means, it allows the AC components of echo signals received from
non-stationary targets, i.e., moving targets. The simple MTI delay-line canceller is an example of
a time-domain filter. The capability of this device depends on the quality of the medium used
as the delay line. The delay line must introduce a time delay equal to the pulse repetition
interval. For typical ground-based air-surveillance radars this might be several milliseconds.
Delay times of this magnitude cannot be· achieved with practical electromagnetic
transmission lines. By converting the electromagnetic signal to an acoustic signal it is
possible to utilize delay lines of a delay line must introduce a time delay equal.
One of the advantages of a time-domain delay-line canceller as compared to the more
conventional frequency-domain filter is that a single network operates at all ranges and does
not require a separate filter for each range resolution cell. Frequency-domain Doppler filter
banks are of interest in some forms of MTI and pulse-Doppler radar.

Types of Delay Line Cancellers

Delay line cancellers can be classified into the following two types based on the number of
delay lines that are present in it.

 Single Delay Line Canceller

 Double Delay Line Canceller
Single Delay Line Canceller
The combination of a delay line and a subtractor is known as Delay line canceller. It is also
called single Delay line canceller. The block diagram of MTI receiver with single Delay line
canceller is shown in the figure below.

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We can write the mathematical equation of the received echo signal after the Doppler effect as
−
V1=Asin[2πfdt−ϕ0]

Where,

A is the amplitude of video signal

fd is the Doppler frequency

Φo is the phase shift

The output of subtractor is applied as input to Full Wave Rectifier. Therefore, the output of Full
Wave Rectifier looks like as shown in the following figure. It is nothing but the frequency
response of the single delay line canceller.

We can conclude that the frequency response of the single delay line canceller becomes zero,
when Doppler frequency fd is equal to integer multiples of reciprocal of pulse repetition
time TP.
we can conclude that the frequency response of the single delay line canceller becomes zero,
when Doppler frequency, fd is equal to integer multiples of pulse repetition frequency fP.
Blind Speeds
From what we learnt so far, single Delay line canceller eliminates the DC components of echo
signals received from stationary targets, when nn is equal to zero. In addition to that, it also
eliminates the AC components of echo signals received from non-stationary targets, when the
Doppler frequency fd is equal to integer (other than zero) multiples of pulse repetition

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frequency fP.So, the relative velocities for which the frequency response of the single delay line
canceller becomes zero are called blind speeds.
Double Delay Line Canceller
We know that a single delay line canceller consists of a delay line and a subtractor. If two such
delay line cancellers are cascaded together, then that combination is called Double delay line
canceller. The block diagram of Double delay line canceller is shown in the following figure.

Let p(t) and q(t) be the input and output of the first delay line canceller. We will get the
following mathematical relation from first delay line canceller.
q(t)=p(t)−p(t−TP) (1)

The output of the first delay line canceller is applied as an input to the second delay line
canceller. Hence, q(t) will be the input of the second delay line canceller. Let r(t) be the output
of the second delay line canceller. We will get the following mathematical relation from
the second delay line canceller.
r(t)=q(t)−q(t−TP)

The advantage of double delay line canceller is that it rejects the clutter broadly. The output of
two delay line cancellers, which are cascaded, will be equal to the square of the output of single
delay line canceller.

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The frequency response characteristics of both double delay line canceller and the cascaded
combination of two delay line cancellers are the same. The advantage of time domain delay line
canceller is that it can be operated for all frequency ranges.
2.3 Staggered PRF
The change of repetition frequency allows the radar, on a pulse-to-pulse basis, to differentiate
between returns from its own transmissions and returns from other radar systems with the
same PRF and a similar radio frequency. Consider a radar with a constant interval between
pulses; target reflections appear at a relatively constant range related to the flight-time of the
pulse.
In today's very crowded radio spectrum, there may be many other pulses detected by the
receiver, either directly from the transmitter or as reflections from elsewhere. Because their
apparent "distance" is defined by measuring their time relative to the last pulse transmitted by
"our" radar, these "jamming" pulses could appear at any apparent distance. When the PRF of
the "jamming" radar is very similar to "our" radar, those apparent distances may be very slow-
changing, just like real targets. By using stagger, a radar designer can force the "jamming" to
jump around erratically in apparent range, inhibiting integration and reducing or even
suppressing its impact on true target detection.
Without staggered PRF, any pulses originating from another radar on the same radio frequency
might appear stable in time and could be mistaken for reflections from the radar's own
tra s issio . With staggered PRF the radar’s ow targets appear stable i ra ge i relatio to
the transmit pulse, whilst the 'jamming' echoes may move around in apparent range
(uncorrelated), causing them to be rejected by the receiver.
Staggered PRF is only one of several similar techniques used for this, including jittered PRF
(where the pulse timing is varied in a less-predictable manner), pulse-frequency modulation,
and several other similar techniques whose principal purpose is to reduce the probability of
unintentional synchronicity. These techniques are in widespread use in marine safety and
navigation radars, by far the most numerous radars on planet Earth today.

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Clutter
Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to
the radar operators. Such targets include natural objects such as ground,
sea, precipitation (such as rain, snow or hail), sand storms, animals (especially birds),
atmospheric turbulence, and other atmospheric effects, such
as ionosphere reflections, meteor trails, and three body scatter spike. Clutter may also be
returned from man-made objects such as buildings and, intentionally, by radar
countermeasures such as chaff.

2.4 Doppler-Filter

The Zero Doppler-Filter works similar to the Puls-Pair-Method: a fixed target suppression
happens by the phase comparison of the echoes received by at least two pulse periods. All
targets with a considerable Doppler-frequency can pass this filter. The Zero-Doppler-Filter is a
low-pass filter. The cutoff frequency is justable mostly. All The high expenditure of Doppler
Filters is due to reduce the influence of the blind speeds too.
Division of the Doppler frequency domain into a number of N separate bands offers a very
flexible approach towards discriminating against fixed and moving clutter. If moving clutter
(such as that from weather or birds) appears with a non-zero mean Doppler shift, the
thresholds at the outputs of the various Doppler filters may be raised accordingly.
The basic principle incorporated in all MTD systems is that of a Doppler filter bank, which
enables targets and clutter to be separated in the frequency domain. The output of these filters
have separate thresholds which optimise detection for the relevant part of the frequency band
concerned. It will also be appreciated that the Doppler filters are inherently performing the
process of coherent integration. Ground clutter is shown mainly in the Zero Doppler filter. The
Zero Doppler Filter may be carry out as a digital low pass filter. It detects targets with a Doppler
frequency corresponding with a radial speed of less than 20 knots. The next filter detects
targets with a Doppler frequency corresponding with a radial speed of less than 40 knots, like
rain clutter or chaff.
Doppler Filter bank Division of the Doppler frequency domain into N separate bands offers a
very flexible approach towards discriminating against fixed and moving clutter. If moving clutter
(such as that from weather or birds) appears with a non-zero mean Doppler shift, the
thresholds at the outputs of the various Doppler filters may be raised accordingly.

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2.5 MTI radar processor

In a pulsed radar, the radar signal consists of short pulses of a sine-wave carrier. The time delay
between the transmission of each pulse and the reception of the echo of the same pulse is
proportional to the target range.

The phase of the echo signal also depends on the target range. If the phase of the transmitted
signal is stable from pulse to pulse, the phase of the echo signal received from a fixed target will
also be stable, since the target range is constant. If the target is moving, however, the phase of
the echo signal will change at a rate that depends on the target's radial velocity. The rate of
change of the phase is the Doppler frequency fd. The Doppler frequency formula is derived at
the end of this discussion.

In radar systems designed for detecting moving targets, changes in the phase of the received
signal affect the video signal at the output of the receiver. Because of this, the video signal can
be processed to determine whether or not the target is moving relative to the radar, or to
ignore the unwanted fixed targets (clutter) and display only moving targets. In digital radars,
the Doppler frequency is extracted to determine the range rate of the target.

Coherence

A pulsed radar signal which is phase-stable from pulse to pulse is said to be coherent. The word
coherent means "in phase", or more precisely, maintaining a definite phase relationship with a
certain reference waveform. The phase of a coherent signal at any point, relative to the
reference, is completely predictable. Because these radar pulses have, in a sense, been cut
from a stable continuous wave, they are called coherent interrupted waves.

Non-coherent and coherent radar

Phase detection in a radar can be accomplished by comparing the phase of the received echo
signal with that of a reference signal which is coherent with the transmitted signal. A radar that
uses such a reference signal for signal demodulation is called a coherent radar. A coherent
radar detects the phase of the received signal whereas a noncoherent radar is insensitive to the
phase.

Figure 2.6 shows a typical noncoherent radar. The pulsed transmitter in a noncoherent radar is
usually a high-power RF oscillator, such as a magnetron, which is keyed on and off by a pulse
modulator. The phase relationship between successive pulses in the transmitted signal is
completely random. For this reason, the transmitted signal is noncoherent.

The received RF signal and the local oscillator signal are heterodyned in the mixer. This shifts
the RF signal down to the intermediate frequency (IF). The IF signal is filtered and amplified by
the IF amplifier.

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Figure 2.6. Noncoherent RADAR

The envelope detector produces an output signal whose level corresponds to the envelope of
the IF signal, as shown in Figure 2.7 Depending on the type of envelope detector used, the
output voltage may be proportional to the envelope (linear detector), to the square of the
envelope (square-law detector) or to the logarithm of the envelope (logarithmic detector). In
any case, all frequency and phase information is lost. Changes in the frequency or phase of the
IF signal have no effect whatsoever on the video signal because such changes do not alter the
envelope of the IF signal.

Figure 2.7 Envelope detection.

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Figure 2.8 shows a typical coherent radar with a superheterodyne receiver. This radar differs
from the noncoherent radar by the presence of a reference signal which is coherent with the
transmitted signal, and by the fact that the transmitted RF signal itself is coherent (phase stable
from pulse to pulse).

Figure 2.8. Coherent radar with

The coherent reference signal is generated by a stable oscillator called the COHO (coherent
oscillator). The frequency of the COHO signal is equal to the IF frequency fIF used in the
receiver. The local oscillator is also a stable oscillator called the STALO (stable local oscillator).
Its frequency, shown as fLO in the figure, is usually near the transmitted frequency.

To insure that the transmitted signal is coherent with the COHO signal, the transmitted signal is
generated by heterodyning the COHO and the STALO signals in mixer 1. This produces signal
components at both the sum and difference frequencies. The difference frequency is rejected
by a filter (not shown in the figure). The sum frequency fIF + fLO is amplified by the RF amplifier.
The RF amplifier is keyed on and off by the pulse generator. The RF pulses are coherent because
they have been generated from a stable continuous wave.

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The frequency of the received signal is fIF + fLO ± fd, where fd is the Doppler frequency due to
target motion. This signal is heterodyned with the STALO signal f LO, and the difference
frequency fIF ± fd applied to the IF amplifier.

Instead of an envelope detector, the coherent radar receiver uses a phase detector to produce
the video signal. The output of the phase detector is a bipolar pulse signal whose amplitude
depends on the phase of the phase detector input signal (in this case, the IF signal) relative to
the coherent reference signal, as shown in Figure 2-6. This output video signal is therefore
referred to as coherent video.

Figure 2-6. Phase detection.

Figure 2-7 shows coherent video signals produced by both a fixed and a moving target. The
dotted lines are the envelopes of the video signals. For the fixed target, the amplitude is
constant from pulse to pulse. For the moving target, the pulse amplitude varies with the phase
of the received signal. The pulse-repetition interval T = 1/fp, where fp is the pulse-repetition
frequency (PRF), and the period of the Doppler frequency (1/fd) are indicated in the figure.

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Figure 2-7. Video signals produced by a fixed and a moving target in a coherent radar receiver.

These video signals can be thought of as a periodically sampled dc value and a periodically
sampled sine wave. Note that if a CW radar signal were used rather than a pulsed signal, the
video signal in Figure 2-7a would be a constant dc value, and in Figure 2-7b, a continuous sine
wave.

Figure 2-8 shows A-scope displays produced by fixed and moving targets. The A-scope shows all
video pulses from a particular target superimposed at a point on the horizontal axis which
corresponds to the target's range. The varying amplitude of the moving target blip gives it an
appearance sometimes called a butterfly pattern.

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Figure 2-8. A-scope displays produced by fixed and moving targets.

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