Download - Clutter Rejection
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Radar Course_1.pptODonnell 10-26-01
MIT Lincoln Laboratory
Introduction to Radar Systems
Clutter RejectionMTI and Pulse Doppler Processing
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MIT Lincoln LaboratoryRadar Course_2.pptODonnell (2) 6-19-02
Disclaimer of Endorsement and Liability
The video courseware and accompanying viewgraphs presented onthis server were prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency thereof, nor any of their employees, northe Massachusetts Institute of Technology and its Lincoln Laboratory,nor any of their contractors , subcontractors, or their employees,makes any warranty, express or implied, or assumes any legal l iabilityor responsibil ity for the accuracy, completeness, or usefulness of anyinformation, apparatus, products, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference hereinto any specific commercial product, process, or service by trade name,trademark, manufacturer, or otherwise does not necessarily constitute
or imply its endorsement, recommendation, or favoring by the UnitedStates Government, any agency thereof, or any of their contractors orsubcontractors or the Massachusetts Institute of Technology and itsLincoln Laboratory.
The views and opinions expressed herein do not necessarily state orreflect those of the United States Government or any agency thereof or
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MIT Lincoln LaboratoryRadar Course_3.pptODonnell (2) 6-19-02
MTI and Doppler Processing
Transmitter
PulseCompression
Recording
Receiver
Tracking &Parameter Estimation
Console /Display
Antenna
PropagationMedium
TargetCross
Section Doppler Processing A / D
Waveform
Generator
Detection
Signal Processor
Main Computer
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MIT Lincoln LaboratoryRadar Course_4.pptODonnell (2) 6-19-02
How to Handle Noise and Clutter
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MIT Lincoln LaboratoryRadar Course_5.pptODonnell (2) 6-19-02
How to Handle Noise and Clutter
If he doesnttake his arm off
my shoulderIm going to hide
his stash of Hershey Bars !! Why does Stevealways talk me into doing
ridiculousstunts like this ?
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MIT Lincoln LaboratoryRadar Course_6.pptODonnell (2) 6-19-02
Ground Clutter
Sea Clutter
Rain Clutter Chaff
Naval Air Defense Scenario
MIT Lincoln LaboratoryMTI_RadSys2001-6
JW 7/31/2008
Moving Target Indicator (MTI) and Pulse-Doppler (PD)processing use Doppler to reject clutter and enhancedetection of moving targets
Smaller targets require more clutter suppression
Birds
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MIT Lincoln LaboratoryRadar Course_7.pptODonnell (2) 6-19-02
Outline
Introduction Moving Target Indicator (MTI) Techniques Pulse Doppler Processing Techniques
Summary
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MIT Lincoln LaboratoryRadar Course_8.pptODonnell (2) 6-19-02
Terminology
Just separate moving targetsfrom clutter
Use short waveforms (twoor three pulses) Do not provide target velocity
estimation
Separate targets into differentvelocity regimes in addit ionto canceling clutter
Provide good estimates oftarget velocity
Use long waveforms -- (manypulses, tens to thousands ofpulses)
Moving Target Indicator (MTI)Techniques
Pulsed Doppler (PD)Techniques
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MIT Lincoln LaboratoryRadar Course_9.pptODonnell (2) 6-19-02
Doppler Frequency
1 10 100 100010
100
1000
10000
Radial Velocity (m/s)
D o p p
l e r
F r e q u e n c y
( H z )
1 5 0 M
H z 4 5 0 M H z
3 G H z
1 0 G H z 3 5 G
H z At S-Band (2800 MHz)
f d ~ 1 kHz / 40 m/s
f dV
=
2
Doppler Frequency
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MIT Lincoln LaboratoryRadar Course_10.pptODonnell (2) 6-19-02
Example Clutter Spectra
0 50 100 150 200
-20-10
0
1020
30
4050
60
70
Radial Velocity (m/s)
R e
l a t i v e
P o w e r
( d B )
Land Clutter
Sea Clutter
Rain Clutter
Chaff Clutter
Clutter comes from same range/angle cellas a target
Clutter RCS can be much larger than targetsof interest (>50 dB)
Characteristics vary with terrain (land/sea),weather, etc.
PPI Display of Heavy Rain
Aircraft
Birds
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MIT Lincoln LaboratoryRadar Course_11.pptODonnell (2) 6-19-02
MTI and Pulse Doppler Waveforms
Tc = MT r Tr
T
Time
T = Pulse length
B = 1/T Bandwidth
Tr = Pulse repetit ion interval (PRI)f r = 1/T r Pulse repetition frequency (PRF)
= T /T r Duty Factor
Tc = MT r Coherent processing interval (CPI)M = Number of pulses in the CPI
M = 2, 3, or sometimes 4 for MTIM usually much greater for Pulse Doppler
T = Pulse length
B = 1/T Bandwidth
Tr = Pulse repetit ion interval (PRI)f r = 1/T r Pulse repetition frequency (PRF)
= T /T r Duty Factor
Tc = MT r Coherent processing interval (CPI)M = Number of pulses in the CPI
M = 2, 3, or sometimes 4 for MTIM usually much greater for Pulse Doppler
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MIT Lincoln LaboratoryRadar Course_12.pptODonnell (2) 6-19-02
Data Collection for Doppler Processing
Samples at same range gate
Sample No.
Range - >
P
u l s e
N u m
b e r
( S l o w
t i m e
)
11 L
M
TimeRange
A/DIn-phase andQuadratureSampling
ComplexI / Q samples
(the complexenvelope of
receivedwaveform)
Pulse 1Sample 12e.g. 8.3 km
Pulse 3Sample 12e.g. 8.3 km
Pulse 2Sample 12e.g. 8.3 km
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MIT Lincoln LaboratoryRadar Course_13.pptODonnell (2) 6-19-02
Outline
Introduction Moving Target Indicator (MTI) Techniques Pulse Doppler Processing Techniques
Summary
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MIT Lincoln LaboratoryRadar Course_14.pptODonnell (2) 6-19-02
Moving Target Indicator (MTI) Processing
Notch out Doppler spectrum occupied by clutter Provide broad Doppler passband everywhere else
Blind speeds occur at multiples of the pulse repetitionfrequency When sample frequency (PRF) equals a multiple of the
Doppler frequency
0 f r = 1/T 2f r
Clutter Notch Blind Speeds
ClutterSpectrum
MTI Filter
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MIT Lincoln LaboratoryRadar Course_15.pptODonnell (2) 6-19-02
Two Pulse MTI Canceller
Fixed Clutter echoes If one pulse is subtracted from the previous pulse, fixed clutter
echoes will cancel and will not be detected
Moving targets Moving targets change in amplitude from one pulse to the next
because of their Doppler frequency shift.
If one pulse is subtracted from the other, the result will be anuncancelled residue
DelayTr=1/PRF Subtract
Input Output
Voutput = V i+1 - Vi
Block Diagram
Figure by MIT OCW.
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MIT Lincoln LaboratoryRadar Course_16.pptODonnell (2) 6-19-02
MTI Improvement Factor
S in and C in - Input target and clutter power per pulse S out (f d) and C out (f d) Output target and clutter power from
processor at Doppler frequency, f d
MTI Improvement Factor = I(f d) =
0 50 100 150 200
-20
0
20
40
60
Radial Velocity (m/s)
R e
l a t i v e
P o w e r
( d B ) Land Clutter
Rain Clutter
Aircraft
(Signal / Clutter) out(Signal / Clutter) in f d
I(f d) = x
f d
C inC
outS
in
S out
Clutter Attenuation
SignalGain
MTI Improvement Factor
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MIT Lincoln LaboratoryRadar Course_17.pptODonnell (2) 6-19-02
MTI Improvement Factor Examples
Spread Clutter ( v=1 m/s, c=10 Hz )
0 200 400 600 800 10000
10
20
30
40
50
60
Doppler Frequency (Hz)
I m p r o v e m e n
t F a c
t o r
( d B )
2 Pulse MTI3 Pulse MTI2 Pulse MTI3 Pulse MTI
Voutput = V i - Vi-1
Voutput = V i - 2V i-1 + V i-2
3-Pulse MTI
2-Pulse MTI
Frequency = 2800 MHzCNR = 50 dB per pulsef d = 1000 Hz
Three-pulse canceller provides wider clutter notch andgreater clutter attenuation
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MIT Lincoln LaboratoryRadar Course_18.pptODonnell (2) 6-19-02
Staggered PRFs to Increase Blind Speed
0 100 200 300 400 500 600 700 800
-20
-10
0
Radial Velocity (m/s)
S N R R
e l a t i v e
t o S i n g
l e P u
l s e
( d B )
0 100 200 300 400 500 600 700 800
-20
-10
0
Radial Velocity (m/s)
Fixed 2 kHz PRI at S-Band
Staggered 2 kHz, 1.754 kHz PRI
321-00423HGT 03/04/98
Staggering or changing thetime between pulses willraise the blind speed
Although the staggeredPRFs remove the blind
speeds that would have beenobtained with a constantPRF, there will be a newmuch higher blind speed
MTI Frequency Response
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MIT Lincoln LaboratoryRadar Course_19.pptODonnell (2) 6-19-02
Outline
Introduction Moving Target Indicator (MTI) Techniques Pulse Doppler Processing Techniques
Pulse Doppler Filtering Concept
Basic Concepts
Example - Moving Target Detector (MTD)
Range Doppler Ambiguities
Airborne Radar
Summary
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MIT Lincoln LaboratoryRadar Course_20.pptODonnell (2) 6-19-02
Data Collection for Doppler Processing
Samples at same range gate
Sample No.
Range - >
P u
l s e
N u m
b e r
( S l o w
t i m e
)
11 L
M
Time
Range
A/DIn-phase andQuadratureSampling
ComplexI / Q samples
(the complexenvelope of
receivedwaveform)
Pulse 1Sample 12e.g. 8.3 km
Pulse 3Sample 12e.g. 8.3 km
Pulse 2Sample 12e.g. 8.3 km
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MIT Lincoln LaboratoryRadar Course_21.pptODonnell (2) 6-19-02
Pulse Doppler Processing
Coherent integration of all pulses of a CPI Clutter rejection Resolving targets into different velocity segments and allowing for fine-
grain target radial velocity estimation
Filter 1(f 1, v 1)
Filter 2(f 2, v 2)
Filter 3(f 3,v 3)
Filter M(f M,v M)
Doppler Filter Bank
M pulses in
M Doppler velocitybins out
Doppler FrequencyDoppler Velocity
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MIT Lincoln LaboratoryRadar Course_22.pptODonnell (2) 6-19-02
Moving Target Detector (MTD)
Pulse Doppler filtering on groups of 8 or greater pulses with a finegrained clutter map.
Aircraft are detected in ground clutter and / or rain with the Doppler
fil ter bank & use of 2 PRFs. Birds and ground traffic are rejected in post processing, using
Doppler velocity and a 2 nd fine grained clutter map
PostProcessing
Thresholding
8 or Greater Pulse Doppler
Filter Bank
Clutter MapFilter
AdaptiveThresholding
ZeroVelocity
Filter
Digit ized Radar Echoes From
Each Range Cell
OutputDetections
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MIT Lincoln LaboratoryRadar Course_23.pptODonnell (2) 6-19-02
ASR-9 8-Pulse Filter Bank
0 20 40 60 80 100-70
-60
-50
-40
-30
-20
-10
010
Radial Velocity (kts)
M a g
n i t u
d e
( d B )
ASR-9 Filter Bank
0 20 40 60 80 100-70
-60
-50
-40
-30
-20
-10
010
Radial Velocity (kts)
M a g n
i t u
d e
( d B )
ASR-9 One Doppler Fi lter
RainEcho
Aircraft
Courtesy of Northrop GrummanUsed with permission.
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MIT Lincoln LaboratoryRadar Course_24.pptODonnell (2) 6-19-02
Unprocessed Radar Returns
0 + 60 Kt 60 Kt
Doppler Veloci ty
Doppler Spectrum of Rain
80
60
40
20
0 R e c e
i v e
d P
o w e r
( d B )
MTD Performance in Rain
Time History of Radar Tracker Output August 1975, FAA Test Center
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MIT Lincoln LaboratoryRadar Course_26.pptODonnell (2) 6-19-02
Range Ambiguities
RcT
ur
= 2
Unambiguous range
Range ambiguit ies occur when echoesfrom one pulse are not all received beforethe next pulse
Strong close targets (clut ter) can mask farweak targets
TrueRange
Radar Range
Target 1
R 1 R R R u2 1
Target 2
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MIT Lincoln LaboratoryRadar Course_27.pptODonnell (2) 6-19-02
Unambiguous Range and Doppler Velocity
100 1000 10000 100000
10
100
1000
PRF (Hz)
U n a m
b i g u o u s
V e
l o c
i t y
( m / s
)
1500 150 15 1.5Unambiguous Range (km)
500 50 5
1 5 0 M
H z
4 5 0 M
H z
3 G H z
1 0 G H
z
3 5 G H z
V
f u
r
2
R cTur =
2c=
2 f r
l ( )
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MIT Lincoln LaboratoryRadar Course_28.pptODonnell (2) 6-19-02
Sensitivity Time Control (STC)
0 dBsm Aircraft at 200 km-64 dBsm Mosquito at 5 km
Both Targets Give Returns with Same Signal-to-Noise ratio
Attenuation of radar return by R -4 will result in constant SNR asa function of range for a constant cross section target
STC cannot be used if the radars waveform is ambiguous inrange Targets which are beyond the ambiguous range of the radar will
be attenuated, because they folded over to close ranges
Deliberately reduce radar sensitivity at short rangesWhy?
321-00418HGT 03/04/98
Cl f MTI d P l D l R d
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MIT Lincoln LaboratoryRadar Course_29.pptODonnell (2) 6-19-02
Classes of MTI and Pulse Doppler Radars
Wind blown clutter maybe a problem Range eclipsing losses
Far out targets competewith near in clutter Cant use STC Ambiguities hardest toremove
Wind blown cluttermay be a problem Can use STC
Range eclipsing losses Far out targets competewith near in clutter
Cant use STC
Low PRF Medium PRF High PRF
RangeMeasurement
VelocityMeasurement
Unambiguous
Unambiguous
Ambiguous
AmbiguousVery Ambiguous
Very Ambiguous
Low PRF Medium PRF High PRF
V l i A bi i R l i
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MIT Lincoln LaboratoryRadar Course_30.pptODonnell (2) 6-19-02
Velocity Ambiguity Resolution
Split dwell into multiple CPIs at different PRFs Scan to scan, even pulse-to-pulse changes also possible
Moves blind velocities to ensure detection of all non-zero velocity targets True target velocity is where best correlation across CPIs occurs Choose PRFs so that least common multiple occurs above desired
maximum unambiguous velocity
f 2
CPI #1PRF = f 1
CPI #2PRF = f 2
f 1
2f 2 3f 2 4f 2
2f 1 3f 1 4f 1 5f 1 Doppler (Velocity)
Doppler
(Velocity)
Unfold detections out to some maximum velocity
Individual CPI
unambiguous velocityregions
BlindZones
E l f Ai b R d
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MIT Lincoln LaboratoryRadar Course_31.pptODonnell (2) 6-19-02
Examples of Airborne Radar
E-2C APS-125
F-18 APG-65
F-16 APG-66 , 68
JOINT STARS E-8A APY-3
AWACSE-3A APY-1
F-15 APG-63 , 70
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MIT Lincoln LaboratoryRadar Course_32.pptODonnell (2) 6-19-02 MIT Lincoln Laboratory
Figure by MIT OCW.
Airborne Radar Clutter Spectrum
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MIT Lincoln LaboratoryRadar Course_33.pptODonnell (2) 6-19-02
Airborne Radar Clutter Spectrum
Illustrative example without Pulse Doppler ambiguities
Figure by MIT OCW.
Airborne Radar Clutter Spectrum
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MIT Lincoln LaboratoryRadar Course_34.pptODonnell (2) 6-19-02
Airborne Radar Clutter Spectrum
Illustrative example without Pulse Doppler ambiguities
Figure by MIT OCW .
Displaced Phase Center Antenna (DPCA)C
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MIT Lincoln LaboratoryRadar Course_35.pptODonnell (2) 6-19-02
Concept
344334_2.pptRMO 9-01-00
If the aircraft motion is exactly compensated by the movement of thephase center of the antenna beam, then there will be no clutterspread due to aircraft motion, and the clutter can be cancelled with atwo pulse canceller
T1
T2
Summary
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MIT Lincoln LaboratoryRadar Course_36.pptODonnell (2) 6-19-02
Summary
Moving Target Indicator (MTI) techniques Doppler fi ltering techniques that reject stationary clutter
No velocity measurement
Blind speeds are regions of Doppler space where targets with thatDoppler veloci ty cannot be detected
Changing the PRF between sets of pulses can alleviate the blindspeed problem
MTI techniques have a limited capabil ity to suppress rain clutter
Pulse Doppler techniques Used to optimally reject various forms of radar clutter
Measurement of target radial velocity
Moving Target Detector techniques are an example of opt imum Dopplerprocessing and associated adaptive thresholding
Ambiguities in range and Doppler velocity can be resolved bytransmitting multiple bursts of pulses with different PRFs
Airborne radars use multiple PRF waveforms to suppress clutter
References
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MIT Lincoln LaboratoryRadar Course_37.pptODonnell (2) 6-19-02
References
Skolnik, M., Introduction to Radar Systems, New York,McGraw-Hill, 3 rd Edition, 2001