ch5-radar+target+and+clutter

[email protected]

Korea Aerospace Univ. (Prof. Kwag)

Lecture 5 : Targets and Clutter

Objective

- Radar Cross Section - Target Fluctuation - Clutter - Clutter

- RCS Definition and Fundamentals - RCS Fluctuation - Target Fluctuation Models - RCS of Fundamental Shapes - RCS of Complex Objects - Interfering Signals-Noise - Radar Clutter Model - Surface Clutter Characteristics - Volume Clutter Characteristics - Moving Clutter - Doppler Spectrum - Reference

2

Korea Aerospace Univ. (Prof. Kwag) 3

Part I : Radar Target


5.1 RCS Definition and Fundamentals

4

RCS Definition - Target RCS (Radar Cross Section) is defined as the ratio of its effective isotropically reflected power to the incident power density

areaUnitpowerIncident

anglesolidUnitpowerReflected 4

- RCS measures the targets reflection of signals in the direction of the radar receiving antenna

backscatter, which is the only reflected energy from the target to the radar

24 T

TTtgt

R

GPP

)(

targetbyreflectedtgt

wattsradartheofdirectionthein

thepowereffectivetheP

222 4

1

44

1

RR

TT

R

tgtRR

GP

RPP

rBackscatte

24 T

TTtgt

R

GPP



5

Mode of Radar Cross Section

< Mode of Radar Cross Section >



6

RCS Dependence on Shape and Angle

< RCS Dependence on Shape & Orientation >



7

RCS Components

)(

:

:

:

metersquareunitGA

radartheofdirectiontheintargettheofgainG

radartheofpolizationtheattargettheoftyreflectivi

targettheofareaA

tgttgttgt

tgt

tgt

tgt

Target Field Zone

.

2 2

antradartheofaxisthetonormalextentlinerstarget'Dwhere

DR

distancefieldFar

FF


5.2 RCS Fluctuation

8

RCS Fluctuation - Amplitude fluctuation : scintillation = variations in received echo power - Phase fluctuation : glint = wave fronts echoing from the target

Fluctuation speed - slow fluctuation : time constant is greater than dwell time, vary from scan to scan - rapid fluctuation : varies from pulse to pulse

< Slow and Rapid Fluctuations >


5.2 RCS Fluctuation

9

Fluctuation Mechanism - Fluctuation of extended target (single far field target)

total RCS is the vector sum of the all small elemental scatterers

RCS depends on the targets aspect angle

different angles cause different summations

reflectionnozero

negativesumvector

< Fluctuations of a Single Extended Scatterer >


5.2 RCS Fluctuation

10

- Fluctuation of array of scatterers vs. aspect depends on the aspect angle from which a complex target of many

scatterers is viewed

(EX) aircraft target RCS

: summation of many individual scattering center on target

)(....... 2 termpowerRCSTotal n21 RCSRCSRCS

- Fluctuation of complex targets as a function of freq. depends on the viewing a target at different frequencies

Notes

spacing of the scatterers and the wave length

determines the amount of fluctuation for a given freq.

The wider the spacing, the greater the RCS fluctuation with small freq. change


5.2 RCS Fluctuation

11

- RCS Dependence Angle and Frequency of 3 GHz

< Aspect Dependence of RCS for Complex Target >

< Frequency Dependence of RCS >


5.2 RCS Fluctuation

12

- Fluctuation from multi-path

< Target Fluctuation Caused by Multipath>

from signal path : d-d, d-r, r-d, r-r

As the range changes, the sum of four signals changes

* Note : RCS varies from zero to 16 times.


5.3 Target Fluctuation Models

13

Marcum / Swerling Models [Ref]

- Marcum : A Statistical Theory of Detection by Pulsed Radar IRE Tr. Vol. IT-6, PP.59-267, Apr, 1960

- Swerling : Probability of Detection for Fluctuating Targets IRE Tr. Vol. IT-6, PP.269-308, Apr, 1960

Swerling case 0 and case 5 - non fluctuating target - modeled as single isotropic scatterers - targets size



14

Swerling case 1 - slowly fluctuating target (large fluctuation, scan to scan) - modeled as several scatterers, equal RCS - little aspect change over a look (Ex) Aircraft target without pulse-to-pulse freq. agility

Swerling case 2 - rapidly fluctuating target, large fluctuation - modeled as case 1 - aspect angle is rapidly changing - complex target for pulse-to-pulse freq. Agility



15

Swerling case 3 - slowly fluctuation target, small fluctuation - modeled as one prominent scatterer with several scatterers (ex.missile)

- aspect angle changes are small(scan to scan) over a look Swerling case 4

- same as case 3 - rapidly fluctuating target - rapid aspect angle changes or from radar freq. change agility



16

RCS Case Changes through Target Argumentation

Small Target

(case 1&2 model)

Large Single RCS Scatterer Target case 3 & 4 model


5.4 RCS of Fundamental Shapes

17

Sphere RCS vs. Frequency - If wavelength = circumference, resonance case : circumference = 0.5 10 times wavelength

RCS varies from 0.3 3.5 times optical cross section

called MIE region

- wavelength >> circumference, RCS ratio = (kr) power of 4 called Rayleigh scattering region



18

RCS of Fundamental Shapes

RCS of Fundamental Shapes

(Adapted from Barton)


(a) Sphere RCS

sigma = pi.d2 /4 (equation. 4.8)

where d = spheres diameter

Cylinder RCS

Flat plate RCS

cLdffreqoftermsinRCScylinderconductingfor

Ldcylinderfosectioncrossradarbroadsidepeak

dLAareacylinder

lengthncecircumferethe

CYL

CYL

2

2

.

&

22222 44 cAfAFP

RCS of Fundamental Shape-I


RSP Lab Hankuk Aviation Univ.

dwhenoccursofvaluepeak 2

88.0 2

aircraftanoffuselagewingaofonintersectifromRCShighaccidently

.argumentortargetforusedso,plate.thethanwidermuchlobeRCSpeak

triangularforaCR

&,

,)3(4 22

224

.

.

A

lobemainwide

arriveditwhichfromdirectiontheinpowerreflectwhichdevice

LL

principlereflectorCorner

reflection%100

RCS of Fundamental Shape-II


Luneburg Lens Principle



22

Triangular Corner Reflector RCS


5.5 RCS of Complex Objects

23

RCS of Complex Object

< T-33A Aircraft >



24

< T-33A Aircraft at 425MHz >

< T-33A Aircraft at 3.0 GHz >



25

Hypothetical RCS Data


Part II : Radar Clutter


5.6 Radar Clutter Model

27

Definition - Clutter is a term used to describe, any object that generate unwanted radar returns that may interfere within normal

radar operation

- Classified into two categories surface clutter : trees, vegetation, ground terrain,

see surface (see clutter)

volume clutter : chaff, rain, birds, insects

- Note clutter signal is much higher than the receiver noise level

SCR(Signal to Clutter Ratio)



28

Radar Clutter Type

SEA

desert

stormDustfarmland

AnglesChaffvegetated

Insectssnowwoods

Birdsrainmountains

vehiclesMovingWeatherLand



29

Radar Clutter Characteristics

Weibull),Normal-Log,us(homogeneoondistributiSpatial

Rician),(RayleighnfluctuatioAmplitude

velocity)(radilshiftdopplerAverage

Gaussian) (e.g.widthandshapeSpectrum

)mmormm(tyReflectivi 3222



30

Reflectivity

length

volume

area

Lextentlinear

sectionsscroradar

m

m

caseDcorridorchaffNarrow

Volumevcellresolution

sectionsscroradar

m

m

caseDclutterVolume

Aareacellresolution

sectionsscroradar

m

m

caseDclutterSurface

ctivityRefleClutter

11

2

1

33

2

3

0

2

20

)1(

)3(

)2(



31

Clutter Environmental Characteristics



32

Clutter Doppler Characteristics - Clutter bandwidth

< Summary of Standard Ceviations of the Clutter Spectrum >



33

Clutter Spectrum Characteristics - Response of a double canceller MTI to ground, rain and chaff clutter


5.7 Surface Clutter Characteristics

34

Surface Clutter

ondistributiAmplitude

onpolarizatiBeam

velocityMean

spreadVelocity

angleGrazing

tyreflectivideterminesstateSeaSea

spreadSpectrl

ondistributiAmplitude

angleGrazing

ctivityRefleLand

sticscharactericlutterArea

)(



35

Radar Equation for Surface Clutter

gdBC

cRA

sec

23

RcSCR

R

GPclutterfromreceivedPower

R

GPfromradarbyreceivedPower

dB

gt

AC

ct

tt

3

0

43

22

c

43

22

t

cos2)(

)4(S

)4(Starget



36

Clutter Region - low grazing angle region

< Example of Clutter Regions >

heightsurfaceRMShwherehanglecritical

heightRMShwhereh

EEC :4

sin:

:8

sin

- plateau region : the scattering is by facets smaller than the wavelength - high grazing angle region : specular scattering, with diffuse clutter components disappearing. angle90o as specular mirror, clutter coefficient is larger for

smooth terrain and seas than for rough



37

Rough Surface Definition

grms

grms

g

hphasedifferencepath

hncedisabypathsmooththanlongerispathrough

hifsmooth

sin22

.sin2tan ""

2sin

4

< Rough Surface Definition >



38

Sea clutter

velocitylengthperiodheightwave

wavehighzerostepsSSStateSea

ctn

SS

C

g

g

,,,

)~(8~0:)(

tan

)2(tanexplog10

)log(sin10log10)1(664

0

0

2

2

0

2

0

Land clutter

mountainfordesertforA

uB

ctnA Cg

g

14~29:

tan

)(tanexplog10)log(sin10log10 0

02

2

02

0



39

Rayleigh Distribution - First sea & land clutter empirically, composed of many

small scatterers with in the resolution cell (not spiky)

Log-normal Distribution - Fits

Some low angle land clutter

High resolution samples of sea clutter

)(:2

clutter.RMSthe tonormalizedvariable:

0)2exp(2

)(

22

0

2

0

2

2

0

meanxofvalueaveragex

tindependenxwhere

xforxxx

xxP

xlnofdeviationstandard:

:x

variable:

02

lnlnexp)2()(

m

2

2

5.0

xofvaluemedian

tindependenxwhere

xforxx

xxP m



40

K-distribution - For sea clutter

parametershapev

parameterscaleb

ordervthkindthirdtheoffunctionBesseleK

powerclutterxwhere

bKvbxvbxP

v

v

:

:

,modifid:

:

2)(2)( 1

Weibull distribution - first low angle clutter in 0.5~5.0 for 1~10GHz

tioncrosscluttermeanx

parametershapeb

normalizedrbackscatteclutterwhere

foraa

bP

bb

sec:

:

:

0exp)(

1

11

1

11



41

Clutter Doppler Sea & Land - Sea clutter doppler

.,3

36.0,65.0

10101.0

101.0

14.0

freqpowerhalffnwhere)ff(1

1P(f)

ondistributispectral

secondpermetersifknotsinareWandifisB

speedwindWdeviationstandard

CnC

SV

SSBV

SV

- Land clutter doppler

ionapproximatGaussian)ff(11fP

terrainvegetatedforb

terrainvegetatedforaaW

terrainvegetatedfor

ddistributeGaussianspectrumstandard

Wf

3C

bSV

SC

)(

28.1

007.0

)0272.0exp(33.1


5.8 Volume Clutter Characteristics

42

Volume Clutter (weather, chaff, insect & birds) - Weather clutter

modeled as a very simple elemental scatterers

raindrops modeled as a sphere, co-polarized

dropthiofdiameterDwheretyreflectiviDZ

cellresolutiontheofvolumeVwhereV

RCSTotal

scatterersthitheofRCSwheremm

RCStheofsummation

ii

II

ii

;

)(

6

32

- Chaff

215.0 Chaff

cluttervolumegeneratedlyartificial


5.8 Volume Clutter Characteristics

43

- Angel clutter (birds, insects, dust)

s)nsects(dBmiorbirdsindividualofRCSwhere

W

bi

bibi

:

log8.546

cRV

resolutionvolume

eaW2

8


5.11 Reference

[1] Radar Cross-Section by E. F. Long, J. F. Shaeffer, and M. T. Tuley,

Artech House, 1985

[2] Radar Reflectivity of Land and Sea by M. W. Long, Artech House, 1983

[3] Handbook of Radar Scattering Statistics for Terrain by F. T. Ulaby and

M. C Dobson, Artech House, 1989

[4] Airborne Early Warning Radar by W.C. Morchin, Artech House, 1990

[5] Radar System Analysis by D. K. Barton, Artech House, 1976

[6] The Scattering of Electromagnetic Waves from Rough Surfaces by P. Beckmann

and A. Spizzichino, Artech House 1987

[7] Introduction to Radar System by I. Katz in M. I. Skolink, McGraw-Hill, 1980

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ch5-radar+target+and+clutter

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