MTI and Pulse Doppler Radar · 2020-04-02 · MTI and Pulse Doppler Radar Doppler Effect A feasible...

Dr. Md. Mostafizur Rahman

Professor

Department of Electronics and Communication Engineering (ECE)

Khulna University of Engineering & Technology (KUET)

MTI and Pulse Doppler Radar

In the real world, radars have to deal with more than receiver noise when detecting targets

since they can also receive echoes from the natural environment such as land, sea and

weather. These echoes are called clutter since they can “clutter” the radar display.


Prof. Dr. Md. Mostafizur Rahman 2


Fig. 3.3 Simple CW Radar and Basic Pulse Radar


Fig. 3.4(2nd) Block diagram of CW Doppler Radar

with non zero IF receiver, sometimes called sideband

super heterodyne.

Fig. 3.2(2nd) Simple CW Doppler Radar



Doppler Effect

A feasible technique for separating the received signal from the transmitted signal when there is relative

motion between radar and target is based on recognizing the change in the echo-signal frequency caused by

the Doppler effect. It is well known in the fields of optics and acoustics that if either the source of

oscillation or the observer of the oscillation is in motion, an apparent shift in frequency will result. This is

the Doppler effect.

The purpose of the

Doppler amplifier is to

eliminate echoes from

stationary targets and

to amplify the Doppler

echo signal to a level

where it can operate an

indicating device.


Difference between MTI and Pulse Doppler Radar

A pulse radar that employs the Doppler shift for detecting moving targets is either

an MTI radar or a pulse Doppler radar.

The MTI radar has a pulse repetition frequency low enough to not have any range

ambiguities. It does however have many ambiguities in the Doppler domain.

The pulse Doppler radar, on the other hand, is just the opposite. It has a prf large

enough to avoid Doppler ambiguities, but it can have numerous range

ambiguities.


If R is the distance from the radar to target, the total number of wavelengths λ contained in

the two-way path between the radar and the target is 2R/λ. The distance R and the

wavelength λ are assumed to be measured in the same units.

Since one wavelength corresponds to an angular excursion of 2π radians, the total angular

excursion made by the electromagnetic wave during its transit to and from the target is

φ=2π * 2R/λ= 4πR/λ radians.

If the target is in motion relative to the Radar, R and the phase are continually changing.

The Doppler angular frequency ωd is given by

ωd= 2πfd = dφ/dt = d/dt(4πR/λ) = 4π/λ (dR/dt)= 4π vr /λ

Where fd = Doppler frequency shift and vr is relative (or radial) velocity of target with

respect to radar.

The Doppler frequency shift fd = 2vr /λ= 2vr (fo /c)

Doppler Frequency Shift


Doppler Frequency Shift

Applications of Doppler Frequency Shift

Allowing CW Radar to detect the moving target and to measure radial

velocity

Synthetic aperture radar and inverse synthetic aperture for producing

images of targets and

Metrological radars concerned with measuring wind shear


-Low Prf

- Not have any range ambiguities

-High Prf

- Have numerous range ambiguities


c

RtfSinAV trreceive

22

c

tvRtfSinAV ro

trreceive

)(22

c

tv

c

RtfSinAV ro

trreceive

222

c

Rf

c

vtfSinAV otr

trreceive

4212

The received signal is heterodyned with the reference signal Aref Sin2πftt and the

difference frequency is extracted which is given as

Vd=AdCos(2πfdt - 4πRo/λ)

Extracts the Doppler Frequency Shifted Echo Signal

)2sin( tfA tt

)](2sin[ Rtr TtfA

Rtrreceive TtfSinAV 2

If target is moving toward the radar then

Range changes to R=Ro-vrt

Transmitted Signal

Received Signal

Received Signal


Fig. 3.6 Block diagram of s single Delay line canceler


Fig. 3.7 Block diagram of an MTI radar that uses a Power amplifier as the transmitter

Stalo - Stable Local Oscillator (Need for high

stability of circuit)

Coho- Coherent Oscillator(It is the reference

signal that has the phase of the transmitter)

The transmitter frequency is the sum of Stalo

and Coho frequency. The combination of the

Stalo and Coho sometimes is called the receiver

exciter portion of the MTI Radar.

Power Amplifier – Good transmitter for MTI

Radar, it can have high stability and is capable

of high power.

Pulse Modulator – Turns the amplifier on and

off to generate the radar pulses.


Frequency Response of the Single Delay Line canceller

The delay-line canceler acts as a filter which rejects the d-c component of clutter.

Because of its periodic nature, the filter also rejects energy in the vicinity of the

pulse repetition frequency and its harmonics.

The signal from a target at range Ro= at the output of the phase detector can

be written

V1=kSin(2πfdt-φo)

V2=kSin[2πfd(t-Tp)-φo] [Tp=pulse repetition interval]

Where fd=doppler frequency shift, , a constant phase,

Ro=Range , and λ=wavelength, k=amplitude of the signal

V=V1-V2=2kSin(πfdTp)Cos[2πfd(t-Tp/2)- φo]

The frequency response function of the single delay-line canceler

H(f)= 2Sin(πfdTp)

04 R

o


The single delay line canceler is a filter that does the job asked of it: it eliminates

fixed clutter that is zero Doppler frequency. Unfortunately it has two other

properties; (i) the frequency response function also has zero response when

moving targets have Doppler frequencies at the prf and its harmonics, and

(ii) the clutter spectrum at zero frequency is not a delta function at zero width,

but has a finite width so that clutter will appear in the pass band of the delay line

canceler. The result is there will be targets speeds, called blind speed. Where the

target will not be detected and there will be an uncanceled clutter residue that can

interference with the detection of moving targets.


Blind Speed :

The response of the single delay line canceler will be zero whenever the magnitude

Sin(πfdTp) is zero. Which occurs when πfdTp=0, ±π, ±2π, ± 3π

Therefore , , n=0, 1,2, 3

Blind speeds can be serious limitation in MTI radar since they cause some desired

moving targets to be canceled along with the undesired clutter at zero frequency.

p

p

rd nf

T

nvf

2

Reducing the detrimental effect of Blind Speed : Operate the Radar at long wavelength (low frequency)

Operate with a high pulse repetition frequency.

Operate with more than one pulse repetition frequency.

Operate with more than one RF frequency (wavelength)


Limitations of CW Radar and its Overcome Technique

Relatively short range CW radar is employed for -Vibration measurement, Intruder Detection, Monitoring the respiration rate of human

and animals, Miss Distance indication, Gunfire Detector, As a sensor for vehicle

braking, and for the precision measurement of the ground speed for both Railway and

automotive applications.

When CW radar is needed for long range as for; Air Defense Space Surveillance or Ballistic Missile Detection the simple CW Radar has

serious limitations ;

Lack of isolation between the transmitter and receiver, Which can cause receiver

burnout, if the transmitter power is large enough and/or introduce transmitter noise in the

receiver which masks the detection of wanted targets.

Introduction of flicker effect noise because the receiver is a homodyne (Zero IF

frequency).

Lack of matched filter in the receiver

Lack of knowledge as to whether the target is approaching or receding

Increased clutter compared to pulse radars and

No Measurement of the range to the target.


The Limitations can be overcome by modulation the CW carrier as in the

Frequency Modulated radar in the flowing figure.

Limitations of CW Radar and its Overcome Technique

Fig. 3.13(2nd ) Block diagram of FM-CW radar using super heterodyne receiver

Timing Signal FM

Transmitter Modulator

Local

Oscillator Mixer

Sideband

Filter

Receiver

Mixer

IF

Amplifier

Balanced

Detector

Low

Frequency

amplifier

Switched

frequency

Counter

Average

Frequency

Counter

Doppler

Velocity

Range


Flicker Effect and its Overcome Technique

Flicker effect noise occurs in semiconductor devices such as diode

detectors, transistors, and cathodes of vacuum tubes with oxide cathodes

generate noise whose power is inversely proportional to frequency. It is

known as 1/f noise or flicker noise.

Flicker effect noise can be avoided by replacing the homodyne (Zero IF

frequency) receiver with a super heterodyne whose IF frequency is large

enough to make the flicker effect noise negligible.

Fig. 3.4(2nd) Block diagram of CW Doppler Radar with non zero IF receiver, sometimes

called sideband super heterodyne.


Applications of CW Radar

The greater advantages of the CW radar over other (non-radar) methods of measuring

speed is that there need not be any physical contact with the object whose speed is being

measured. In industry this has been applied to the measurement of turbine blade vibration,

the peripheral speed of grinding wheel and monitoring of vibrations in the cables of

suspension bridges.


References :

i) Introduction to Radar System – Merril I. Skolnik 2nd and 3rd Edition

ii) Introduction to Radar System- Dr. Robert M. O’Donnel (MIT Lincoln Laboratory)

iii) Electronic Communication System - J Kennedy

iv) Fundamentals of Radar Technology (Applied Technology Institute) – Instructor Robert Hill

v) Microwave Engineering – A K Das & S K Das


Fig. 3.11(2nd) Block diagram of FM-CW Radar(for producing Doppler frequency) Prof. Dr. Md. Mostafizur Rahman 22

MTI and Pulse Doppler Radar · 2020-04-02 · MTI and Pulse Doppler Radar Doppler Effect A feasible...

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