pulse radar: mti
TRANSCRIPT
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Pulse Radar:
MTI
DR Sanjeev Kumar Mishra
Lecture 24-27
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Introduction
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PRT
PW PRT=1/PRF
Carrier Freq.
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Pulse Shape
Pulse Width
Pulse Compression
Pulse power
Pulse Effects on System Performance
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• Pulsed radar waveforms can be completely defined by the following:
• (1) carrier frequency which may vary depending on the design
requirements and radar mission.
• (2) pulse width, which is closely related to the bandwidth and
defines the range resolution;
• (3) modulation; and
• (4) the pulse repetition frequency
Pulsed Radar Parameters
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Pulsed Radar Parameters
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PRF Ambiguities
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• The purpose of moving-target indication (MTI) radar is to reject
signals from fixed or slow-moving unwanted targets, such as
buildings, hills, trees, sea, and rain, and retain for detection or display
signals from moving targets such as aircraft.
MTI Radar
Simplified Block diagram of a MTI system
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• Moving target indication (MTI) is a mode of operation of a radar to
discriminate a target against clutter.
• The MTI radar uses Low Pulse Repetition Frequency (PRF) to avoid
range ambiguities.
• Radar MTI may be specialized in terms of the type of clutter and
environment:
• Airborne MTI (AMTI),
• Ground MTI (GMTI), etc., or
• may be combined mode: stationary and moving target
indication (SMTI).
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Vdiff Vr
Vt
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Vdiff Vr
Vt
c
RftfAV t
dddiff04
2sin
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(a) echo pulse train;
(b) video pulse train for doppler frequency fd > l/;
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• Moving targets distinguished from stationary targets by
observing the video output on an A-scope (amplitude vs. time).
(a-e) Successive sweeps of an MTI radar A-scope display (echo
amplitude as a function of time); (f) superposition of many sweeps;
arrows indicate position of moving targets
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• The simple MTI delay-line canceler
is used to reject stationary clutter at
zero frequency.
Vdiff
Vr
Vt
c
RftfA t
dd04
2sin
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Superposition of many sweeps; arrows indicate position of moving targets
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Effect of delay line canceller on the signal
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Delay Line Canceler
MTI Receiver delay line canceler
• The video signal received from a particular target at a range R0 is
01 2sin tfkV d
Where 0 = phase shift and k = amplitude of video signal.
• The signal from the previous transmission, which is delayed by a time
T = pulse repetition interval, is 02 2sin TtfkV d
• The output of the subtractor is
021
22cossin2
TtfTfkVV dd
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• The magnitude of the relative frequency-response of the delay-line
canceler
Tfk
TfkfH d
d
sin2sin2
Frequency response of the single delay-line canceler; T = delay time
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• Properties of DLC
• The frequency response function has zero response when
moving targets have doppler frequencies at the prf and its
harmonics.
• The clutter spectrum at zero frequency is not a delta function of
zero width but has a finite width so that clutter will appear in the
pass band of the DLC.
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• Where, vn is the nth blind speed in knot
operating wavelength in m
fr is prf in Hz
rr
n fnfn
v
02.1
Plot of MTI radar first blind speed as a function of R un
Blind Speed
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• The blind speeds are one of the limitations of pulse MTI radar which
do not occur with CW radar.
• Blind speed can be a serious limitation in MTI radar since they cause
some desired moving targets to be cancelled along with the
undesired clutter at zero frequency.
Limitation in MTI Radar
...3,2,122
nT
nfnv r
n
• Based on the equation, there are four methods for reducing the
detrimental effects of blind speeds.
• Operate the radar at lower frequencies [or long wavelengths]
• Operate with a high prf
• Operate with more than one prf [known as staggered-prf MTI]
• Operate wit more than one RF frequency [or wavelength]
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Clutter Spectrum
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• The frequency response of a single-delay-line canceler does not always have
as broad a clutter-rejection null as might be desired in the vicinity of d-c.
• The clutter-rejection notches may be widened by passing the output of the
delay-line canceler through a second delay-line canceler as shown in Fig.
N Pulse Delay line Canceler
• The output of the adder )2()(2)()( TtsTtststsout Double-delay-line canceler
Three pulse canceler
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• The output of the two single-delay line canceler in cascade is the square of
that from a single canceler. Tfd2sin4 2SDLCDDLC
fHfH
• The double delay line canceler and three pulse canceller are same
frequency response function
• The weights of double delay line canceler or three pulse canceller are +1, -2,
+1. )2()(2)()( TtsTtststsout
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General form of a transversal (or nonrecursive) filter for MTI signal processing.
This is known as trarnsversal filter. [also sometimes known as a feed
forward filter, a nonrecursive filter, a finite memory filter or a tapped delay-
line filter.]
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Canonical-configuration comb filter
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Amplitude responses for three MTI delay-line cancelers.
(1) Classical three-pulse canceler (2) five-pulse delay-line canceler with
"optimum" weights, and (3) 15-pulse Chebyshev design
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• Echoes from trees, vegetation, sea, rain, and chaff fluctuate with time,
and these fluctuations can limit the performance of MTI radar.
• The experimentally measured power spectra of clutter signals rnay be
approximated by
Clutter Spectrum
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Where, W (f) = clutter-power spectrum as a function of frequency
g (f) = Fourier transform of input waveform (clutter echo)
f0 = radar carrier frequency
a = a parameter dependent upon clutter
Power spectra of various
clutter targets.
(1) Heavily wooded hills,
20 mi/h wind blowing (a
= 2.3 x 1017; (2) sparsely
wooded hills, calm day
(a = 3.9 1019); (3) sea
echo, windy day (a =
1.41x1016), (4) rain
clouds ( a = 2.8 x 1015);
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• The clutter spectrum can also be expressed in terms of an rms clutter
frequency spread c in hertz or by the rms velocity spread v, in m/s.
Thus,
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The clutter attenuation is
Where, H (f) is the frequency response function of the canceler
Clutter Attenuation
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The improvement factor can be written as
where S0/C0, output signal-to-clutter ratio,
Si/Ci input signal-to-clutter ratio, and
CA = clutter attenuation.
The average is taken over all target doppler frequencies of interest.
MTI Improvement Factor
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ex = xn / n! = 1 + x/1 + x2 / 2 + x3 / 6 + ...
Where, fp is the pulse repetition frequency = 1/T.
The average gain (S0/Si)avg of the single delay-line canceler can be equal
to 2. Therefore. The Improvement factor is
MTI Improvement Factor for a single DLC
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Find the improvement factor for a double delay line canceler, whose
average gain is 6.
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Non-Coherent MTI
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Pulsed Coherent
MTI Search Radar
Coherent Pulsed MTI
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