Signal ProcessingSynthetic Aperture Radar

Chiba University

Center for Environmental Remote Sensing, Chiba University

Josaphat Tetuko Sri Sumantyo, Ph.D

Contents

1. Introduction of Synthetic Aperture Radar (SAR)2. SAR Applications (History, Theory, Relationship with Re

mote Sensing etc)3. Basic of Electromagnetic waves (Wave, Polarization, Ab

sorption, Scattering etc)4. Radar Equation and Microwave Scattering (Antenna Pat

tern) 5. Pulse Compression Technique and Image Production in

Range Direction6. Synthetic Aperture Technique and Image Production in

Azimuth Direction7. Basic of SAR Image Analysis

References

References

Introduction

SAR and Definition

　　 SAR (Synthetic Aperture Radar)

　　　　 Satellite (sensor) itself illuminates microwave, then sensor receives

backscattered wave and processes this signal to be an image.

Benefit of SAR

　　　　 All weather

　　　　 Day and night time monitoring (Active sensor)

　　　　 High coherency → InSAR applications

　　　　 Polarization characteristics → Polarimetry

Lack of SAR

　　　　 Analysis of backscattering Microwave Image very complicated

　　　　　　 (Different to the point of view in optical image analysis)

　　　　 Image distortion (foreshortening, shadowing etc) caused by side looking

　 Microwave Sensor　 Active Sensor　 Imaging Radar

Pictures : http://southport.jpl.nasa.gov/nrc/chapter7.html

SEASAT (1978) SIR-A (1981) SIR-B (1984) SIR-C (1994)

Frequency (GHz) 1 1 1 1, 5

Polarization HH HH HH HH, HV, VH, VV

Look angle 20o 50o 20o – 60o 20o – 60o

Analog data Analog data Digital data Digital data

Central

Transmitter/

Receiver

Central

Transmitter/

Receiver

Central

Transmitter/

Receiver

Distributed

T/R modules

Fixed antenna beam

Fixed antenna beam

Mechanical

beam steering

Electronic

beam steering

Introduction

ERS-1 (1991) JERS-1 (1992) Radarsat (1995)

Frequency (GHz) 5 (C band) 1.275 (L band) 5 (C band)

Polarization VV HH HH

Look angle 20o 35o 20o – 60o

Pictures : http://southport.jpl.nasa.gov/nrc/chapter7.html

Introduction

Specification of ALOS-PALSAR

Main Observation Mode High Resolution Mode SCAN SAR

Observation Frequency L-band(1.27GHz)

Polarization HH,VV,HH&HV,VV&VH HH,VV

Ground Resolution 10m 100m

Look numbers 2 8

Swap area 70km 250 ～ 350km

Off Nadir Angle 10 ～ 51°

http://alos.nasda.go.jp/

Introduction

target

far rangenear range

range direction

slant range direction

horizontal direction

azimuth direction

sensor / antenna

platform direction

Ground range

① ： off-nadir angle

　　（ look angle)

② ： depression angle

③ ： range beam width

④ ： incidence angle

⑤ ： azimuth beam width

Satellite-onboard ＳＡＲ and flat-ground geometric system

①

②

③

④⑤

JERS-1 SAR antenna Pi-SAR (NICT/JAXA)

Optic sensor and microwave sensor 　• Optic sensor 　 ： employed wavelength is recognized by human eyes

　　　　　　　　　　　　　　　　　　　 Sun light scattering easy to recognize

•　 Microwave sensor ： wavelength is cm order difficult to recognize

　　　　　　　　　　　　　　　　　　　 Mechanism of backscattering complicated

　　　　　　　　　　　　　　　　　　　 Image distortion

Mount Fuji : JERS-1 / OPS Mount Fuji : JERS-1 / SAR

Microwave characteristicsWave expression : phase and amplitude

Time changing signal can be expressed as space function by using variable of amplitude and phase

wave expression ：　 F(t)=exp[2ift] f : frequency 　

Electromagnetic fields vibrate as the function of time when observed in one point in the space

Space distribution of electromagnetic fields is the function of space when time is fixed

electric field

electric field

amplitude wavelength

space （ｘ）

phase ：

ddt = f

time （ｔ）

0.2m 1.0m 10m 1mm 10cm 1m

wavelength

Atmospheric penetration ratio

Atm

. P

en.　

%

50

100

0

Wavelength domain of electromagnetics and definition

Wavelength Domain of Microwave

0.2m 1.0m 10m 1mm 10cm 1m

Visible 　　　 　　　　　　　　　　　　　　　　　　　　　　 Microwave

IR NIR 　　　　　　　　　　　　　　　　　　　　　　　　　　　　 KaKuX C S L P

10GHz 1GHz

Band Wavelength （ｍｍ） Frequency （ＧＨｚ）

Ka 7.5 ~ 11.0 40.0 ~ 26.5

K 11.0 ~ 26.7 26.5 ~ 18.0

Ku 16.7 ~ 24.0 18.0 ~ 12.5

X 24.0 ~ 37.5 12.5 ~ 8.0

C 37.5 ~ 75.0 8.0 ~ 4.0

S 75.0 ~ 150 4.0 ~ 2.0

L 150 ~ 300 2.0 ~ 1.0

P 300 ~ 1000 1.0 ~ 0.3

incident wave scattering wave

Reflection and Penetration of Microwave

Relationship of scattering and penetration

penetrated wave

ratio of scattered and penetrated wave : 　

effect of dielectric constant

mirror / corner reflection : 　

effect of surface roughness

`

Krakatau volcano complex, Indonesia

corner reflection

water / sea surface :

high dielectric constant

perfectly scattering / corner reflection

black color on SAR image

Reflection and Penetration of Microwave

Illustration of microwave scattering by earth’s surface

① smooth surface ③ rough surface ② slightly rough surface

Effect of earth’s surface :

　　　Ｒａｙｌｅｉｇｈ conditions ： h≦/(8 cos ) → 　 standard of smooth surface

　　　　　　　　 in case of ＪＥＲＳ－１： =0.23m, =38o

　　　　　　　　　　 Conditions to satisfy ① ：　 h 3.65 cm≦

Reflection and Penetration of Microwave

Krakatau volcano complex, Indonesia

① smooth surface

③ rough surface

② slightly rough surface

Scattering of microwave : surface scattering and volume scattering

　 Surface scattering

(a) scattering on the boundary surface (different dielectric constant )

(b) scattered wave is reflected to different direction from incident wave 　　　　　　　　

　 Volume scattering 　　 (a) Penetrated electromagnetic wave is traped in the dielectical material

(b) Scattered wave in object on the earth’s surface (i.e. forest)

Scattering Models

surface scattering

volume scattering

surface scattering

icy rivervegetation (forest)

surface scattering

surface scattering

dried sandy area

surface scattering

surface scattering

volume scattering

Scattering of microwave : surface and volume scatterings

volume scattering

Linier polarization

Horizontal polarization

Left handed circular polarization （ＬＨＣＰ）

Lini

er p

olar

izat

ion

Circ

ular

Pol

.Polarizations

SAR History

1953 Carl Wiley (Good Year Corporation) invented SAR

1960s Civil application : archeology, real aperture interferometry

1978 SEASAT (NASA) : 25m resolution, L band

1980s ALMAZ (Soviet), Shuttle Imaging Radar (SIR)(NASA)

1991 ERS-1 (ESA), Interferometry, C band

1992 JERS-1 (JAXA), 12.5m resolution, L band

1995 RADARSAT (RSI)

1999 SRTM, single pass interferometry, 80% continental coverage

2002 ENVISAT (ESA)

2006 ALOS

ERS-1 JERS-1 RADARSAT ENVISAT ALOS

Launched date April 1991 February 1992 November 1995 March 2002 January 2006

Height 785 km 568 km 793 - 821 km 799.8 km 691.65 km

Inclination angle 98.5 degrees 97.7 degrees 98.6 degrees 98.55 degrees 98.16 degrees

Frequency 5.3 GHz (C band) 1.275 GHz (L band) 5.3 GHz (C band) 5.331 GHz ( C band) 1.27 GHz (L band)

Wavelength 5.7 cm 23.5 cm 5.7 cm 5.6 cm 23.6 cm

Polarization VV HH HH HH, VV, HH+VV, VV+VH, HH+HV

HH, VV, HH+HV, VV+VH, HH+VV+ HV+VH

Off-nadir angle 20 degrees 35 degrees 9 - 48 degrees 13.5 - 39 degrees 10 - 51 degrees

Incident angle 23 degrees 38.7 degrees 10 - 60 degrees 15 - 45 degrees 8 - 60 degrees

Swap width 100 km 75 km 50 - 500 km 56.5 - 104.8 km 20 - 350 km

Azimuth resolution 30 m 18 m (3 looks) 9 - 147 m

30 - 1000 m

10 - 100 m

(2 looks)

7 - 100 m

(multi looks)

Range resolution 30 m 18 m 6 - 147 m

Peak Power 4.8 kW 325 W (1.3 kW spec) 5 kW 1.4 kW 2.3 kW

Bandwidth 19 MHz 15 MHz 11.6/17.3/30.0 MHz 8.48 - 16 MHz 14 MHz/28MHz

Antenna size 1 x 10 m 2.2 x 12 m 1.5 x 15 m 1.3 x 10 m 3.1 x 8.9 m

SARs Specification

JERS-1 SAR antenna

Basic Theory of SAR : Antenna

L=11.92 m

b=2.

2 m

wave illuminating pattern

‘half value’

b : antenna length in range direction

L : antenna length in azimuth direction

: half value in range direction (JERS-1 : 5.3o)

: half value in azimuth direction (JERS-1 : 1.0o)

Basic Theory of SAR : Antennase

nsor

/ant

enna

L

P1

P0

side lobe main lobe

0

11/2

y

x

x

yz

b

L

0

0

Definition of half value :

Po : power in the center of main lobe

P1 : power in the peripheral of main lobe

The half value is defined by ‘P1 is attenuated to 3 dB (equally 50%) of Po’.

10log10Po/P1=3 dB

or

P1=0.5P0

Basic Theory of SAR : Radar Equation

R

PtG/4R2

PtPs=PtG/4R2

Pr=PsA/4R2

antenna

Scatterer

attenuation by spreading of wave = 1/4R2

Radar equation :

To realize the relationship between radar received power and characteristic of scatterer.

A : effective surface of the receiver’s antenna

G : gain

: radar cross section or back scatterer surface

Pt : transmitted power

Ps : scattered power

Pr : received powerPr=Ps A/4R2=PtGA/(4R2)2

Pulse radarPulse power

(Transmitted power ）||

peak power×pulse width

Transmitted pulse received signal

Sig

na

l in

ten

sity

tree’s echohouse’s echo

concrete building’s echo

Pulse width τ

time

（ｂ） Time flows of transmit & received signal

（ a ） Pulse front wave

sensor / antenna

propagation of transmitted pulse

tree houseconcrete building

Rn Rf

12

34

56

78

910

111212

1314

1516

1718

19

20

21

11

1213

14

1516

1718

19

11

1213

14

15

16

17

18

2220

10

Rn : slant range length at near range

Rf : slant range length at far range

Time to receive the pulse by antenna :

Near range side (start to receive) : 2Rn/C

Far range side (end of receiving) : 2Rf/C+τ

Continuity time of received pulse :

T=(2Rf /C+τ)-2Rn/C

0

0 2 4 6 8 10 12 14 16 18 20 22 24

JERS-1 SAR antenna

Start

Parameter calculation

Corner turn

Azimuth compression

Output image

Doppler center frequency

Range compression

Flowchart of SAR Signal Processing

rotated image

North

range compressed image

azimuth compressed image

raw data

sensor illumination

Range

Azim

uth

JERS-1 satellite

No

rth

Flowchart of SAR Signal Processing

corner turn

A B A B

x

Rc

earth’s surface

frequency MHz

1282.5

1275.0

1267.5

f=15 MHz

A B C

transmitted pulse

received pulse

pulse length ()

time

time

output signal

reference signal

1/f

A B C

L

R

P

L

t=0

t=0 t=0

R R

R

Az

Az Az

Az

R

Multi scattering

sea surface

(a) bridge’s architecture figure

(b) SAR image’s signature

(c) scattering mechanism

wire

1

1

2

2

Akasikaikyo bridge (http://www.oshimastudio.com)

length 3.910m