no slide title - mit lincoln laboratory...title no slide title author pubs created date 7/31/2008...
TRANSCRIPT
Radar Course_1.pptODonnell 10-26-01
MIT Lincoln Laboratory
Introduction to Radar Systems
Clutter Rejection MTI and Pulse Doppler Processing
MIT Lincoln LaboratoryRadar Course_2.pptODonnell (2) 6-19-02
Disclaimer of Endorsement and Liability
• The video courseware and accompanying viewgraphs presented on this server were prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, nor the 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 liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, products, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to 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 United States Government, any agency thereof, or any of their contractors or subcontractors or the Massachusetts Institute of Technology and its Lincoln Laboratory.
• The views and opinions expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or any of their contractors or subcontractors
MIT Lincoln LaboratoryRadar Course_3.pptODonnell (2) 6-19-02
MTI and Doppler Processing
Transmitter
PulseCompression
Recording
Receiver
Tracking &ParameterEstimation
Console /Display
Antenna
PropagationMedium
TargetCross
Section DopplerProcessingA / D
WaveformGenerator
Detection
Signal Processor
Main Computer
MIT Lincoln LaboratoryRadar Course_4.pptODonnell (2) 6-19-02
How to Handle Noise and Clutter
MIT Lincoln LaboratoryRadar Course_5.pptODonnell (2) 6-19-02
How to Handle Noise and Clutter
If he doesn’t take his arm off
my shoulder I’m going to hide
his stash ofHershey Bars !! Why does Steve
always talk me into doing ridiculous
stunts like this ?
MIT Lincoln LaboratoryRadar Course_6.pptODonnell (2) 6-19-02
Ground Clutter
Sea Clutter
Rain ClutterChaff
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 enhance detection of moving targets
• Smaller targets require more clutter suppression
Birds
MIT Lincoln LaboratoryRadar Course_7.pptODonnell (2) 6-19-02
Outline
• Introduction
• Moving Target Indicator (MTI) Techniques
• Pulse Doppler Processing Techniques
• Summary
MIT Lincoln LaboratoryRadar Course_8.pptODonnell (2) 6-19-02
Terminology
• Just separate moving targets from clutter
• Use short waveforms (two or three pulses)
• Do not provide target velocity estimation
• Separate targets into different velocity regimes in addition to canceling clutter
• Provide good estimates of target velocity
• Use long waveforms -- (many pulses, tens to thousands of pulses)
Moving Target Indicator (MTI)Techniques
Pulsed Doppler (PD) Techniques
MIT Lincoln LaboratoryRadar Course_9.pptODonnell (2) 6-19-02
Doppler Frequency
1 10 100 100010
100
1000
10000
Radial Velocity (m/s)
Dop
pler
Fre
quen
cy (H
z)
150 MHz450 MHz
3 GHz10 GHz35 GHz At S-Band (2800 MHz)
fd ~ 1 kHz / 40 m/s
fdV
=2λ
DopplerFrequency
MIT Lincoln LaboratoryRadar Course_10.pptODonnell (2) 6-19-02
Example Clutter Spectra
0 50 100 150 200-20
-10
0
10
20
30
40
50
60
70
Radial Velocity (m/s)
Rel
ativ
e Po
wer
(dB
)
Land Clutter
Sea Clutter
Rain Clutter
Chaff Clutter
• Clutter comes from same range/angle cell as a target
• Clutter RCS can be much larger than targets of interest (>50 dB)
• Characteristics vary with terrain (land/sea), weather, etc.
PPI Display of Heavy Rain
Aircraft
Birds
MIT Lincoln LaboratoryRadar Course_11.pptODonnell (2) 6-19-02
MTI and Pulse Doppler Waveforms
Tc = MTrTr
T
Time
T = Pulse lengthB = 1/T Bandwidth
Tr = Pulse repetition interval (PRI)fr = 1/Tr Pulse repetition frequency (PRF)δ
= T /Tr Duty Factor
Tc = MTr 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 lengthB = 1/T Bandwidth
Tr = Pulse repetition interval (PRI)fr = 1/Tr Pulse repetition frequency (PRF)δ
= T /Tr Duty Factor
Tc = MTr 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
MIT Lincoln LaboratoryRadar Course_12.pptODonnell (2) 6-19-02
Data Collection for Doppler Processing
Samples at same ‘range gate’
Sample No.Range - >
Puls
e N
umbe
r(S
low
tim
e)
11 L
M
TimeRange
A/DIn-phase andQuadratureSampling
Complex I / 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
MIT Lincoln LaboratoryRadar Course_13.pptODonnell (2) 6-19-02
Outline
• Introduction
• Moving Target Indicator (MTI) Techniques
• Pulse Doppler Processing Techniques
• Summary
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 repetition frequency
– When sample frequency (PRF) equals a multiple of the Doppler frequency
0 fr = 1/T 2fr
Clutter Notch Blind Speeds
Clutter Spectrum
MTI Filter
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 an
uncancelled residue
DelayTr =1/PRF Subtract
Input Output
Voutput = Vi+1 - Vi
Block Diagram
Figure by MIT OCW.
MIT Lincoln LaboratoryRadar Course_16.pptODonnell (2) 6-19-02
MTI Improvement Factor
• Sin and Cin - Input target and clutter power per pulse
• Sout (fd ) and Cout (fd ) – Output target and clutter power from processor at Doppler frequency, fd
• MTI Improvement Factor = I(fd ) =
0 50 100 150 200-20
0
20
40
60
Radial Velocity (m/s)
Rel
ativ
e Po
wer
(dB
) Land ClutterRain Clutter
Aircraft
(Signal / Clutter)out
(Signal / Clutter)in fd
I(fd ) = xfd
CinCout Sin
Sout
ClutterAttenuation
SignalGain
MTI Improvement Factor
MIT Lincoln LaboratoryRadar Course_17.pptODonnell (2) 6-19-02
MTI Improvement Factor Examples
Spread Clutter (σv =1 m/s, σc =10 Hz )
0 200 400 600 800 10000
10
20
30
40
50
60
Doppler Frequency (Hz)
Impr
ovem
ent F
acto
r (dB
)
2 Pulse MTI3 Pulse MTI2 Pulse MTI3 Pulse MTI
Voutput = Vi - Vi-1
Voutput = Vi - 2Vi-1 + Vi-2
3-Pulse MTI
2-Pulse MTI
Frequency = 2800 MHzCNR = 50 dB per pulsefd = 1000 Hz
Three-pulse canceller provides wider clutter notch and greater clutter attenuation
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)
SNR
Rel
ativ
e to
Sin
gle
Puls
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 the time between pulses will raise the blind speed
• Although the staggered PRF’s remove the blind speeds that would have been obtained with a constant PRF, there will be a new much higher blind speed
MTI Frequency Response
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
MIT Lincoln LaboratoryRadar Course_20.pptODonnell (2) 6-19-02
Data Collection for Doppler Processing
Samples at same ‘range gate’
Sample No.Range - >
Puls
e N
umbe
r(S
low
tim
e)
11 L
M
TimeRange
A/DIn-phase andQuadratureSampling
Complex I / 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
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(f1 , v1 )
Filter 2(f2 , v2 )
Filter 3(f3 ,v3 )
Filter M(fM ,vM )
Doppler Filter Bank
M pulses in
M Doppler velocity‘bins’ out
Doppler FrequencyDoppler Velocity
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 fine grained clutter map.
• Aircraft are detected in ground clutter and / or rain with the Doppler filter bank & use of 2 PRFs.
• Birds and ground traffic are rejected in post processing, using Doppler velocity and a 2nd fine grained clutter map
PostProcessing
Thresholding
8 or GreaterPulse Doppler
Filter Bank
Clutter MapFilter
AdaptiveThresholding
ZeroVelocityFilter
Digitized RadarEchoes From
Each Range Cell
OutputDetections
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
-100
10
Radial Velocity (kts)
Mag
nitu
de (d
B)
ASR-9 Filter Bank
0 20 40 60 80 100-70-60
-50-40-30
-20
-100
10
Radial Velocity (kts)
Mag
nitu
de (d
B)
ASR-9 One Doppler Filter
RainEcho
Aircraft
Courtesy of Northrop GrummanUsed with permission.
MIT Lincoln LaboratoryRadar Course_24.pptODonnell (2) 6-19-02
Unprocessed Radar Returns
0 + 60 Kt– 60 KtDoppler Velocity
Doppler Spectrum of Rain
80
60
40
20
0Rec
eive
d Po
wer
(dB
)MTD Performance in Rain
Time History of Radar Tracker Output August 1975, FAA Test Center
MIT Lincoln LaboratoryRadar Course_25.pptODonnell (2) 6-19-02
Doppler Ambiguities
• Pulse Doppler waveform samples target with sampling rate = PRF
• Sampling causes aliasing at multiples of PRF
• Two targets with Doppler frequencies separated by an integer multiple of the PRF are indistinguishable
• Unambiguous velocity
Vf
ur= λ
20 1 2 3
Time / PRI
Sample Times
Moving V=Vu
Moving V=3Vu
Stationary V=0
MIT Lincoln LaboratoryRadar Course_26.pptODonnell (2) 6-19-02
Range Ambiguities
RcT
ur=
2
Unambiguous range
• Range ambiguities occur when echoes from one pulse are not all received before the next pulse
• Strong close targets (clutter) can mask far weak targets
True Range
RadarRange
Target 1
R1 R R Ru2 1= +
Target 2
MIT Lincoln LaboratoryRadar Course_27.pptODonnell (2) 6-19-02
Unambiguous Range and Doppler Velocity
100 1000 10000 100000
10
100
1000
PRF (Hz)
Una
mbi
guou
s Ve
loci
ty (m
/s)
1500 150 15 1.5Unambiguous Range (km)
500 50 5
150 MHz
450 MHz
3 GHz
10 G
Hz
35 GHz
Vf
ur= λ
2
RcT
ur=
2c=
2 fr
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 as a function of range for a constant cross section target
• STC cannot be used if the radar’s waveform is ambiguous in range
– 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
MIT Lincoln LaboratoryRadar Course_29.pptODonnell (2) 6-19-02
Classes of MTI and Pulse Doppler Radars
• Wind blown clutter may be a problem• Range eclipsing losses• Far out targets compete with near in clutter• Can’t use STC• Ambiguities hardest to remove
• Wind blown clutter may be a problem• Can use STC
• Range eclipsing losses• Far out targets compete with near in clutter• Can’t use STC
Low PRF Medium PRF High PRF
RangeMeasurement
VelocityMeasurement
Unambiguous
Unambiguous
Ambiguous
AmbiguousVeryAmbiguous
VeryAmbiguous
Low PRF Medium PRF High PRF
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
f2
CPI #1PRF = f1
CPI #2PRF = f2
f1
2f2 3f2 4f2
2f1 3f1 4f1 5f1 Doppler(Velocity)
Doppler(Velocity)
Unfold detections out to some maximum velocity
Individual CPI unambiguous velocity
regions
Blind Zones
MIT Lincoln LaboratoryRadar Course_31.pptODonnell (2) 6-19-02
Examples of Airborne Radar
E-2CAPS-125
F-18APG-65
F-16APG-66 , 68
JOINT STARS E-8AAPY-3
AWACSE-3A
APY-1
F-15APG-63 , 70
MIT Lincoln LaboratoryRadar Course_32.pptODonnell (2) 6-19-02 MIT Lincoln Laboratory
Figure by MIT OCW.
MIT Lincoln LaboratoryRadar Course_33.pptODonnell (2) 6-19-02
Airborne Radar Clutter SpectrumIllustrative example without Pulse Doppler ambiguities
Figure by MIT OCW.
MIT Lincoln LaboratoryRadar Course_34.pptODonnell (2) 6-19-02
Airborne Radar Clutter Spectrum
Illustrative example without Pulse Doppler ambiguities
Figure by MIT OCW.
MIT Lincoln LaboratoryRadar Course_35.pptODonnell (2) 6-19-02
Displaced Phase Center Antenna (DPCA) Concept
344334_2.pptRMO 9-01-00
If the aircraft motion is exactly compensated by the movement of the phase center of the antenna beam, then there will be no clutter spread due to aircraft motion, and the clutter can be cancelled with a two pulse canceller
T1
T2
MIT Lincoln LaboratoryRadar Course_36.pptODonnell (2) 6-19-02
Summary
• Moving Target Indicator (MTI) techniques– Doppler filtering techniques that reject stationary clutter
• No velocity measurement– Blind speeds are regions of Doppler space where targets with that
Doppler velocity cannot be detected• Changing the PRF between sets of pulses can alleviate the blind
speed problem– MTI techniques have a limited capability 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 optimum Doppler
processing and associated adaptive thresholding
– Ambiguities in range and Doppler velocity can be resolved by transmitting multiple bursts of pulses with different PRF’s
– Airborne radars use multiple PRF waveforms to suppress clutter
MIT Lincoln LaboratoryRadar Course_37.pptODonnell (2) 6-19-02
References
• Skolnik, M., Introduction to Radar Systems, New York, McGraw-Hill, 3rd Edition, 2001