pulse radar: mti

43
A N T E N N A T H E O R Y & D E S I G N I I S T Pulse Radar: MTI DR Sanjeev Kumar Mishra Lecture 24-27

Upload: ngodat

Post on 09-Feb-2017

278 views

Category:

Documents


5 download

TRANSCRIPT

Page 1: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Pulse Radar:

MTI

DR Sanjeev Kumar Mishra

Lecture 24-27

Page 2: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Introduction

Page 3: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

PRT

PW PRT=1/PRF

Carrier Freq.

Page 4: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Page 5: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Pulse Shape

Pulse Width

Pulse Compression

Pulse power

Pulse Effects on System Performance

Page 6: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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

Page 7: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Pulsed Radar Parameters

Page 8: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

PRF Ambiguities

Page 9: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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

Page 10: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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).

Page 11: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Vdiff Vr

Vt

Page 12: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Vdiff Vr

Vt

c

RftfAV t

dddiff04

2sin

Page 13: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Page 14: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

(a) echo pulse train;

(b) video pulse train for doppler frequency fd > l/;

Page 15: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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

Page 16: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Page 17: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• The simple MTI delay-line canceler

is used to reject stationary clutter at

zero frequency.

Vdiff

Vr

Vt

c

RftfA t

dd04

2sin

Page 18: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Superposition of many sweeps; arrows indicate position of moving targets

Page 19: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Effect of delay line canceller on the signal

Page 20: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Page 21: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

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

Page 22: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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

Page 23: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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.

Page 24: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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

Page 25: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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]

Page 26: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Page 27: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Clutter Spectrum

Page 28: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Page 29: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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

Page 30: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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

Page 31: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

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.]

Page 32: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Canonical-configuration comb filter

Page 33: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

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

Page 34: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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

Page 35: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

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);

Page 36: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

• 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,

Page 37: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

The clutter attenuation is

Where, H (f) is the frequency response function of the canceler

Clutter Attenuation

Page 38: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

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

Page 39: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

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

Page 40: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Find the improvement factor for a double delay line canceler, whose

average gain is 6.

Page 41: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T Non-Coherent MTI

Non-Coherent MTI

Page 42: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T

Pulsed Coherent

MTI Search Radar

Coherent Pulsed MTI

Page 43: Pulse Radar: MTI

A N T E N N A

T H E O R Y & D E S I G N

I I S T