Radar System Design
Chapter 14
MTI and Pulsed Doppler Radar
Radar System Design
MTI and Pulsed Doppler Radar
•Moving Target Indication (MTI) radar: A delay line
canceller filter to isolate moving targets from
nonmoving background
- Ambiguous velocity
- Unambiguous range
•Pulsed Doppler radar: Doppler data are extracted
by the use of range gates and Doppler filters.
- Unambiguous velocity
- Unambiguous or ambiguous range
Chapter 14: MTI and Pulsed Doppler Radar
14 - 1
Dr. Sheng-Chou Lin
Radar System Design
Pulsed Radar
•High-PRF: unambiguous Doppler frequency, highly
ambiguous range
- solve TX-RX coupling problem of CW system
- Improve noise-limited detection relative
to low-PRF waveform
- Minimize the number of introduced blind
zones relative to low-PRF system.
- Range blind during TX time periods
•Low-PRF: unambiguous range, highly Doppler
frequency
- circumvents the TX-RX coupling
- Introduce Doppler blind zones (ground clutter)
•Medium-PRF: ambiguous Doppler frequency,
ambiguous range
- circumvents the TX-RX coupling
Chapter 14: MTI and Pulsed Doppler Radar
Dr. Sheng-Chou Lin
14 - 2
Radar System Design
Pulsed Radar Parameters
•Range: range is obtained from transmit-to-receive
pulse delay T
2R = cT  R = ct 
2
- 1s  150m , 1ns  15cm
Target 1 Target 2
Return
Return

Transmit
pulse
•Range Resolution: Pulse width must be shorter
than the propagation time from target 1 to target 2
and back
R 1 = ct 1 
2
R 2 = ct 2 
2
2
R 2 –R 1 = ct  t = 2 
R 2 –R 1 c

Combined returned
from target 1 and 2
R = c
2
•Unambiguous range
R

R unamb = cT 
2, T: pulse repetition interval (PRI)
Transmit
pulse
- There are ways to get around this by using a
staggered PRI (Multi-PRF)
T
R = ct
------2
ct'
R = -------2
Unambiguous Range (R < cT/2)
Chapter 14: MTI and Pulsed Doppler Radar
14 - 3
Ambiguous Range (R > cT/2)
Dr. Sheng-Chou Lin
Radar System Design
Pulsed Doppler Power Spectrum
2V
f d = ---------c- cos 

: angle between the platform velocityand
the
line of sight (LOS)
Chapter 14: MTI and Pulsed Doppler Radar
14 - 4
Dr. Sheng-Chou Lin
Radar System Design
Pulsed Radar
•Noncoherent Pulsed Radar
- No reference signal
•Coherent Pulsed Radar
- TX phase is reserved
•MTI Radar
- detection of moving target by
suppressing fixed targets
Chapter 14: MTI and Pulsed Doppler Radar
14 - 5
Dr. Sheng-Chou Lin
Radar System Design
Pulsed Doppler Radar
Analog
1
2 3 4
Range gate
switch sampling
Range Information
Sampling
Digital
FFT
(filter bank)
Doppler Information
Chapter 14: MTI and Pulsed Doppler Radar
14 - 6
Dr. Sheng-Chou Lin
Radar System Design
Power Spectrum Density (Pulsed)
•As the antenna scans, the beam dwell time is finite.
•T i : interpulse period; 
p : pulse period
N+1 = 5
Chapter 14: MTI and Pulsed Doppler Radar
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Dr. Sheng-Chou Lin
Radar System Design
Power Spectrum Density (Pulsed) 2
•A five-burst waveform: the return from a scatter at a
slant range R T .
N = 5
R T R AMB contains four pulse samples
- R T + R AMB contains only one pulse sample
-
•The shape of both the spectrum and ambiguity function
for is important  determine performance of MTI
and pulsed Doppler radars.
N+1=5
N+1=4
N=4
N+1=4
N+1=1
Chapter 14: MTI and Pulsed Doppler Radar
14 - 8
Dr. Sheng-Chou Lin
Radar System Design
Power Spectrum Density (Pulsed RF)
Chapter 14: MTI and Pulsed Doppler Radar
14 - 9
Dr. Sheng-Chou Lin
Radar System Design
Radar Equation for Pulsed Radar
•Until now, we have not said a great deal about
filtering of the return signal except to say that
matched filtering is desirable and B IF = 1 
 for
most pulse radars.
•In some cases, we need a better idea about
bandwidth for estimation S/N ratio.
B
·
s
•Recall that we previously developed a radar
equation of the form
P t G 2 2 
R max = ---------------------------------------------------------------3 kTBFL 

4
So 
No 
min
1/4
·
, B = 1.5 ,
s = 5rpm 
previously we consider this to be a single pulse. •An example: If 
PRF=300Hz
•Integration of pulses: Depending on scan rate &
·
PRF, we may receive more than 1 pulse from a
s = 5rpm 
round/per min. 
target. We can use that our advantage
-
= 5 360 
1
60 = 30sec

·
nB = 
B 
s f
 P : number of pulses for
integration during dwelling time.
1.5
nB = 
---------- 300 = 15 pulses
30 
·
B : beamwidth, s : antenna scan rate
- f P : PRF
-
Chapter 14: MTI and Pulsed Doppler Radar
Dr. Sheng-Chou Lin
14 - 10
Radar System Design
Pulse Integration
•Two techniques
- Predetection Integration
IF
Envelope
detection
Video
Amp.
- Postdetection Integration
•Predetection Integration is coherent but
somewhat more difficult to implement than
postdetection
Predetection
•Postdetection is incoherent but some
improvement in (So/No) can be obtained.
Postdetection
Postdetection
Predetection
Coherent addition
Noncoherent addition
Noise
signal
coherent
integration
2
P signal 
nv 
2
n
P signal n P n 
in a pulse 
Chapter 14: MTI and Pulsed Doppler Radar
14 - 11
Dr. Sheng-Chou Lin
Radar System Design
Pulse Integration
•Recall we were developing alternate expression for the •We can express
radar system equation

S
N
So 
No 


nE i 
n 
min = 
min
R max
-
P t G 2 2 nE i 
n
= ---------------------------------------------------------------3

4kTBFL 
So 
No 
min
1/4
E i 
n = 1 ;
n –1 / 2 E i 
n 1
- I i 
n = nE i 
n : effective #
- for ideal predetection

So 
No 
: single pulse S/N required for
min
prespecification P FA .
pulses integrated
•Example: P FA = 10 –12 ,P D = 0.9 ,
E i 
n :efficiency factor; n: # of pulses integrated
find 
So 
No 
15.8dB . If 1000
- Note that for a pulse radar P t is a peak power, we can
min
-
also express in terms of average power
P avg = P t 

T = 
P t f p , where : Pulse width, T :
PRI; 
T : duty cycle.
-
P t = P avg 
f p ; E= P avg 
fp
: Energy per pulse
P av G 2 2 nE i 
n
R max = -----------------------------------------------------------------------------3

4kT 
B
FL 
So 
No 
f
p
min
EG 2 2 nEi 
n
= -----------------------------------------------------------------------3 kT 

4
B
FL 
So 
No 
min
Chapter 14: MTI and Pulsed Doppler Radar
1/4
pulses are integrated (postdetection
square law)
•

S
N
So 
No


nE i 
n 
min = 
min
= 15.8 –10 log 
130 = –5.34 dB
1/4
P t G 2 2 
R max = ---------------------------------------------------------3 kTBFL 

4
S
N
min
1/4
P t G 2 2 nE i 
n
= ---------------------------------------------------------------3 kTBFL 

4
So 
No 
min
1/4
Dr. Sheng-Chou Lin
14 - 12
Radar System Design
Noncoherent Pulsed Radar
•Noncoherent Pulsed Radar
- No reference signal used by the receiver is phase
coherent to the output phase of the transmitter.
•Problems encountered in detecting
small-RCS in expected background
clutter environments
 high noise
- A free-running pulsed transmitter
- WB Filter
- Automatic Frequency Control (AFC)  Local OSC is
made to track the transmitter frequency
- Radar designer is left with only a few
techniques to minimize the
performance limits imposed by return
from background clutter.
- IF signal is bandpass filtered and amplified by IF
amplifier
- Square law (noncoherent) detection  noncoherent
integrated signal processor  CFAR
- constrain parameters: operating
frequency, maximum permitted
antenna dimension dimensions...
Bandpass Filtered
frequency coherent
to TX frequency
Chapter 14: MTI and Pulsed Doppler Radar
14 - 13
Dr. Sheng-Chou Lin
Radar System Design
Coherent Pulsed Radar
•The phase of TX waveform is preserved is a reference signal 
the receiver for signal demodulation
•The use of STRALO and COHO reference signals to store the
phase of the later signal processing identifies the radar.
•Relative complexity between coherent
and noncoherent systems
- If it were not for performance,
noncoherent configuration would be
used extensively used in search radar
applications.
•Advantage of coherent detection:
exploitation of different Doppler shift to
isolate desired target responses from
large dominating (in amplitude)
background returns
No filter bank
- Relative motion between desired
target and its background
•Some techniques through which
noncoherent radar can be used to
accomplish Doppler shift-aided
detection of targets
Chapter 14: MTI and Pulsed Doppler Radar
Dr. Sheng-Chou Lin
14 - 14
Radar System Design
Pulsed Coherent MTI
•The detection of moving targets are improved by suppression of
fixed targets.
•This is expanded to incorporate Doppler processing as one
possible form of MTI implementation
•is deified as one uses simple band reject to reject the return
from fixed targets  Enhanced detection of the moving target
•Doppler filter
- A relative narrow bandwidth
clutter is rejected.
- A broad passband (unknown
Doppler shift)
- Post-Doppler processing stage
 Noncoherent integration
- Rejection notches in the
passband should be placed in
frequency about the response
that are to be rejected and
should be as wide as required to
achieve the desired clutter
cancellation.
Chapter 14: MTI and Pulsed Doppler Radar
Noncoherent
integration
14 - 15
Dr. Sheng-Chou Lin
Radar System Design
MTI filter (Delay line canceller)
Two techniques are available for realizing MTI filter
•Delay line canceller
- Digital filter (Multipulse canceller)
- A real-time delay is equal to the PRI
- Digital implementations can provide the desired
passband with the flexibility of passband
programmability  The preferred choice
•Range gate and filter
Chapter 14: MTI and Pulsed Doppler Radar
Dr. Sheng-Chou Lin
14 - 16
Radar System Design
Range gate and filter
•output of phase detector (I or Q) is
provided as input to N sample-andhold circuits
•Each sample-and-hold circuit
receives a sample gate  a
lumped constant (active) bandpass
filter
•f L : lower corner frequency. f H :
higher corner frequency. f R : PRF
•Noncoherent MTI
- A-scope range video
p
r
es
en
t
at
i
onsc
o
nt
ai
n“
bu
t
t
er
f
l
i
es
”
at the slant range of the moving
target.
- Each butterfly is created by the
fluctuating amplitude of the sum
of the return from both the
background and the target
Chapter 14: MTI and Pulsed Doppler Radar
14 - 17
Dr. Sheng-Chou Lin
Radar System Design
Noncoherent MTI
•Noncoherent MTI
- A-scope range video presentations contain
“
butterflies”att
hes
l
ant
r
an
geoft
hemo
v
i
ng
target.
- Each butterfly is created by the fluctuating
amplitude of the sum of the return from both
the background and the target
A-scope
Chapter 14: MTI and Pulsed Doppler Radar
Dr. Sheng-Chou Lin
14 - 18
Radar System Design
Pulse Doppler Radar (Analog)
A pulsed Doppler radar exploits Doppler
shift to obtain velocity information from a
pulsed waveform
- High-PRF pulsed Doppler airbone radar
- Medium-PRF pulsed Doppler airbone
radar
Two approaches
- Analog filtering
- Digital filtering
Analog filtering
- processing at IF
select so that overlap occurs at -3dB
BW
- Output of IF amplifier is gated
- gated output is then filtered by a bank of
contiguous narrowband (NB) filters
3dB
- Automatic detection and signal-sorting.
f IF
Chapter 14: MTI and Pulsed Doppler Radar
14 - 19
Dr. Sheng-Chou Lin
Radar System Design
Pulse Doppler Radar (Digital)
- In-phase (I) and quadrature (Q) video
•selection of digital approach for most modern pulsed
Doppler radar designs.
- Analog-to-digital conversion (A/D)
•Spectral leakage
•Digital filtering (FFT)
- N-point fast Fourier transform (FFT)
- N contiguous filter passbands (span
from zero to PRF)
- The number of range cells (m) and
Doppler cells (N) increases  the
amount of hardware grows
dramatically.

Sin 
nx 
sin 
x
- Other Doppler shift  High sidelobes in the same
- uniformly weighted set of N samples
range cell
- aperture weighting is used to minimize the spectral
leakage.
- Modern digital technology leads to
hardware efficient realizations
•N-point FFT which produces a finite
impulse response filter is the desired
matched-filter for a finite sequence
(step scan algorithms)
•NB filter (infinite impulse response) in
the analog approach can be only an
approximation of the desired matchedfilter.
Chapter 14: MTI and Pulsed Doppler Radar
14 - 20
Dr. Sheng-Chou Lin