1. Dr. Md. Mostafizur Rahman
Professor
Department of Electronics and Communication Engineering (ECE)
Khulna University of Engineering & Technology (KUET)
MTI and Pulse Doppler Radar
2. 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.
MTI and Pulse Doppler Radar
4. MTI and Pulse Doppler Radar
Fig. 3.3 Simple CW Radar and Basic Pulse Radar
5. 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
6. MTI and Pulse 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.
7. 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.
8. 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
9. 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
10. -Low Prf
- Not have any range ambiguities
-High Prf
- Have numerous range ambiguities
11.
c
R
t
f
Sin
A
V t
r
receive
2
2
c
t
v
R
t
f
Sin
A
V r
o
t
r
receive
)
(
2
2
c
t
v
c
R
t
f
Sin
A
V r
o
t
r
receive
2
2
2
c
R
f
c
v
t
f
Sin
A
V o
t
r
t
r
receive
4
2
1
2
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
)
2
sin( t
f
A t
t
)]
(
2
sin[ R
t
r T
t
f
A
R
t
r
receive T
t
f
Sin
A
V
2
If target is moving toward the radar then
Range changes to R=Ro-vrt
Transmitted Signal
Received Signal
Received Signal
13. 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.
14. 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)
0
4 R
o
15. 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.
16. 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
r
d nf
T
n
v
f
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)
17. 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.
18. 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
19. 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.
20. 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.
21. 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
