Introduction to Synthetic Aperture Radar(SAR) Imaging

Jeong-Hee Choi, Ph.D.

Assistant Professor

Department of Computer and Communication

Taegu University

Tel : 053-850-6632, Fax : 053-850-6619

Email : choijh@biho.taegu.ac.kr

Web: http://risl.taegu.ac.kr

OVERVIEW SAR System 개요

• SAR Imaging System 특징• SAR 기술구성• SAR/ISAR System Classification

• SAR System 구성도• Frequency Bands

• SAR System Applications

Range/Cross range Imaging Raw-data Processing 원리 Existing SAR Systems

SAR Images

Imaging Geometry of a Radar Imaging System

Synthetic Aperture Radar 의 특징

장점• 기존의 레이다와 비교하여 Cross Range 의 해상도를 얻음으로써

영상정보추출 가능 : (Range • 파장 ) / Aperture size• 카메라 영상이 단면 사진인데 반해 Topographic Image 합성 가능• Active Instrument : 카메라 영상이 태양빛에 의존하는 반면

밤에도 촬영가능 ((Day or Night Imaging Capability)• Microwave 는 구름이나 , 안개등에 무관하여 흐린 날이나

화산폭발장면도 촬영가능 (All weather Imaging Capability)• 따라서 연속적인 환경 Monitoring 가능함

단점• 단면 카메라 영상에 비해 해상도가 떨어짐 . Eg) 나무 , 차 , 건물

등이 한 두개의 pixel 로 나타남• 정확한 Coherent Measurement 가 필수 Suffer from Speckle

Synthetic Aperture Radar 기술 구성

Hardware• Radar Antenna

Phased Array • Local Oscillator • Pulse Generator• Analogue-to-Digital

Converter, etc.

Multi-dimensional Signal Processing

• Raw-data processing Radiometric Calibration Geometric Calibration Modeling & Inversion

• Image Enhancement Sharpen Image for

better Recognition• Image Transformation

Graphics/ Mapping• Image Interpretation

SAR Imaging System Classification

Relative Motion of Radar and Target area

• SAR• ISAR

Mounting Vehicle• Spaceborne SAR• Airborne SAR

Radar Flight Motion• Linear SAR• Circular SAR

Beam Illumination • Spotlight Mode SAR• Stripmap Mode SAR

Transmitter/Receiver Status • Monostatic SAR• Bistatic SAR• Multistatic SAR

Advancement • Interferometric SAR• Polarimetric SAR

System Geometry for SAR Imaging

System Geometry for ISAR Imaging

System Geometry in Circular SAR Imaging

Top View

Side View

System Geometry in Stripmap SAR Imaging

Spotlight SAR : Electronic Beam Steering

Data Acquisition System for Quasi-Monostatic SAR/ISAR Imaging

Frequency Bands

Band Frequency range

Wavelength Range

사용예

P-Band 0.4 – 1 GHz 30 – 75 cm JPL AirSAR

L-Band 1 - 2 GHz 15 – 30 cm JERS-1, JPL AirSAR SEASAT, SIR-A

S-Band 2 – 4 GHz 7.5 – 15 cm

C-Band 4 – 8 GHz 3.7 – 7.5 cm Convair-580, RADARSAT JPL AirSAR, ERS-1, ERS-2

SIR-B, SIR-C

X-Band 8 – 12 GHz 2.5 – 3.7 cm Convair-580, Star-1, Star-2 SIR-B, SIR-C

• Note : 파장 (meter) = C/ 주파수 (Hz=Cycles/sec), C = 광속

Applications of Synthetic Aperture maging

상용 시스템• 지표면 , 해표면 ,

해저면의 Topographic Image

• 농작물 분포 및 생태계 변화 , Sea Ice, Oil Spill 등 환경 Monitoring

• 지하자원 /유적 탐사• 비행기 자동착륙 및 항공 Traffic Control

• 행성의 입체영상

군용 Radar 시스템• 비행 정찰• 미사일 탐지 및 추적 (ISAR)

• FOPEN SAR

(Foliage Penetration SAR)

기타• Diagnostic medicine• 기계 내부의 Non-destructive

testing

Range Imaging 원리

n

nn

ct p t sx) ( ) (2

• System Model

◦ Measured Echoed Signal

• Frequency Domain Representation

)exp( )()( 2 n

nn c

jPS x

: Transmitted Radar Signal

: Target’s Reflectivity

: Target’s Range

: Propagation Speedc

tp

xnn

)(

System Geometry for Range Imaging

Point Spread Function

] )( [ )( psf 2Pt -1

(w)F

• Target Reconstruction

)()( tsxf M n

nn cc

xx)( psf 22

where range mapping is defined by

c

tc

x k x2

,2

• Reconstruction Via Matched Filtering

)]()([)( * PStsM-1

(w)F )( psf 2c

t xnn

n

◦ Frequency Domain Support Band of Radar signal

• Signal Processing

),(p ] , [ 00 cc

Dictates Time Domain Sampling

0

t

02c : Carrier Frequency

: Baseband Bandwidth

Range Resolution

)max(2

tx

c ,4

0fc

)2

( 0

0 f

Range Resolution 을 좋게 하기위해서 송신신호의 대역폭이 넓을수록 유리함따라서 레이다의 송신신호로 Chirp pulse 신호를 사용함 .

Dictates Time Domain Measurement Interval

] T)X X(2

,)X X(2

[]T,T[ p00

fs

c

c

c

ct

: Radar Swath’s Mean Range

: Target Area’s Size

: Radar Signal Pulse Durationp

0

c

T

X2

X

◦ Spatial Domain Support Band of Target Function

),(xf ],[00 XXXX cc

x

Measuring Time Interval

p0

sf Tc

X4TT

◦ Radar Distance from a Target at

22 )( uyx n : ),( yx n

◦ Measured Echoed Signal

nnn

n

nn

uyxkjtj

c

uyxtput

])(2exp[)exp(

])(2

[),(s

22

22

◦ After Baseband Conversion

nnn

nnn

us

uyxkj

tjutus

),(

])(2exp[

)exp(),(s),(

22

Cross-Range Imaging 의 원리 (Spotlight SAR 예 )

◦ Consider Single Sinusoidal Source Signal, )exp()( tjtp

• Frequency Domain Representation of Measured Echoed Signal

)exp() 4exp(

) 4exp(

),(),(

22

22

nunu

nuun

unnu

yjkxkkj

yjkxkkj

kSkS

),( 0 ukS FunctionTarget : )( ukF

Signature of Unit Reflector Target at )0 ,() ,( xyx nn

) 2 exp( ) , (2 2

0u x k j u s

and its Fourier Transform

)4exp(),( 220 xkkjkS uu

uuuy kkkSkSkF y0 Function,Target :),( ),()(

만일 Synthesized Aperture 크기가 무한대라면 (L = )

The support band of echoed data, in Doppler frequency domain

따라서 로 Band-limit 된 신호의 Alias-free Sampling 간격 임 . 실제로 Synthesized Aperture 는 유한하므로 n-th target 에서 echo 된신호의 Doppler support 대역은

• Finite Aperture Effect

)( , , , ])(

2 ,)(

2[

])(

)(2 ,

)(

)(2[

)](sin2 ),(sin2[

2222

n

uyuyxx

Lyk

x

Lyk

uyx

Lyk

uyx

Lyk

LkLk

nnnn

nn

n

nn

n

nn

]2 ,2[ kkku ),( ukS

k2 ku 4

2

Note :

origin wrt theangleaspect target

1 ,

range

1 ),( ,2 freqradarLn

◦ Total Doppler Support Band of ),( ukS

]2,2[

]2,2[

00

0

x

LYk

x

LYk

y x

Lyk

x

Lyk

Y

nn

nn

n

◦ Cross-range Resolution

,4)(

2

L

x

size nyn

cf)

04Y

xny

For Stripmap SAR

◦ Synthetic Aperture Domain Sampling

,)(4)(

2

0 LY

x

sizeu

k

2 : Wavelength

◦ Pulse Repetition Interval =

Pulse Repetition Frequency(PRF)=1/PRI(Pulse Repetition Interval) =

vehicle)mountedradar of Speed(R

u

v

u

Rv

Imaging System Geometry for Cross-range Imaging (Broadside Target Area)

Imaging System Geometry for Cross-range Imaging (Off-Broadside(Squint) Target Area)

Aspect Angle of a Target

Spotlight SAR Signal Spectrum Support

Slow-time Doppler Signature of a Target

Waveform approximation 에 따른 SAR Image Reconstruction 기법들

Fresnel Approximation-based Reconstruction

: Range-Doppler Imaging Plain Wave Approximation-based Reconstruct

ion : Polar Format Processing Wavefront (Spherical wave) reconstruction

* References :

J. Curlander and R. McDonough, Synthetic Aperture Radar, Wiley, 1991

M. Soumekh, Synthetic Aperture Radar Signal Processing with Matlab Algorithms, Wiley, 1999

SAR Imaging : Wavefront Reconstruction

SAR Imaging : Range-Doppler Processing

(Fresnel Approximation-Based)

SAR Imaging : Polar Format Processing

Existed/Existing/Planned SAR Systems

군용 시스템• E-3 AWACS(Airborne Warning & Control System) :

걸프지역에서 공 / 해상의 Target Surveillance• E-8C joint STARS(Surveillance Target Attack Radar System):

걸프전에서 육상의 움직이는 Target Detection & Location

(NASA 의 우주선에 탑재한 것과 유사기종 ) SEASAT

• 1978 년 발사된 최초의 SAR 탑재 민간용 원격 탐사위성• L-band, HH polarized, 25m Az. Resolution, Incidence Angle :9-

15 도 SIR-A (Shuttle Imaging Radar):National Space Science Data Cente

r/NASA • 1981 Columbia 우주선 탑재 , L-band, HH polarized• URL : http://nssdc.gsfc.nasa.gov/earth/earthfaq.html

Existed/Existing/Planned SAR Systems (Cont.)

SIR-B, SIR-C (Radar Data Center, JPL)• 1984 Challenger 호 탑재 , 1993 년 이후 여러 우주선에 탑재 , • L, C, and X-band, HH Polarized• URL : http://southport.jpl.nasa.gov/

JERS-1 : National Space Development Agency of Japan(NASDA)• 1992 년 위성탑재 SAR, L-band HH polarization, 18m Az. Resolution• Incidence Angle : 35 도 , URL : http://www.nasda.go.jp/

ERS-1, ERS-2 : European Space Agency in Italy• 1991, 1995 년에 발사된 SAR 탑재위성 , C-band, VV polarized, • 30m Az. Resolution, Incidence Angle: 20-26 도• URL : http://www.esoc.esa.de

Radarsat : Canada Center for Remote Sensing/Canadian Space Agency• 1995 년 위성탑재 SAR, C-band, HH Polarization, 10 - 100m 해상도• Variable Incidence Angles : 20 - 60 도• URL : http://radarsat.space.gc.ca/

Existed/Existing/Planned SAR Systems (Cont.)

NASA/JPL AIRSAR • DC-8 비행기에 탑재된 레이다• C, L, and P-band Inteferometric, Polarimetric SAR 시스템• 0-70 도의 큰 입사각 범위 , 12m Az. Resolution• URL : http://southport.jpl.nasa.gov/

ENVISAT• 2001 년 6 월 European Space Agency 에 의해 발사 / 운용 예정인

위성탑재 레이다 • URL : http://www.esoc.esa.de

ALOS(Advanced Land Observation System)• 2002 년 일본의 NASDA 에 의해 발사 운용예정• L-band Full Polarimetric SAR 장착• URL : http://www.nasda.go.jp/

Radar Image Distortion

Slant-range scale distortion Foreshortening

Radar shadow Layover