Clutter Rejection

8/14/2019 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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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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How to Handle Noise and Clutter


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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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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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Outline

Introduction Moving Target Indicator (MTI) Techniques Pulse Doppler Processing Techniques

Summary


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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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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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Example Clutter Spectra

0 50 100 150 200

-20-10

0

1020

30

4050

60

70


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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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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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




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Outline


Summary


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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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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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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


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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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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Staggered PRFs to Increase Blind Speed

0 100 200 300 400 500 600 700 800

-20

-10

0


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


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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Outline


Pulse Doppler Filtering Concept

Basic Concepts

Example - Moving Target Detector (MTD)

Range Doppler Ambiguities

Airborne Radar

Summary


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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)





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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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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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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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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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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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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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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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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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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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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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Illustrative example without Pulse Doppler ambiguities

Figure by MIT OCW.



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Illustrative example without Pulse Doppler ambiguities

Figure by MIT OCW .

Displaced Phase Center Antenna (DPCA)C


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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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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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References

Skolnik, M., Introduction to Radar Systems, New York,McGraw-Hill, 3 rd Edition, 2001

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