Introduction to SAR Remote Sensing - Earth Online -...

Introduction to SAR Remote Sensing

Thuy Le Toan

September 3, 2007 Lecture D1La1

September 3, 2007 Lecture D1La1 Introduction to SAR remote sensing Thuy Le Toan

• Introduction to radar remote sensing• The Synthetic Aperture Radar• Geometric properties of SAR images • Statistic properties of SAR measurements• Physical content of SAR data

Contents


Non-Imaging (ex. Microwave radiometer, magnetic sensor)

Imaging (ex: cameras, optical mechanical scanner, spectrometer, microwave radiometer)

PASSIVE SENSORSDetect the reflected or emitted electromagnetic radiation from natural sources.

ACTIVE SENSORSDetect reflected responses from objects irradiated by artificially-generated energy sources.Non-Imaging ( ex: microwave radiometer, microwave altimeter, laser)

Imaging (Real Aperture Radar, SyntheticAperture Radar)

RADAR: Radio Detection and Ranging

SLAR: Side Looking Airborne Radar, developed during the World War II, for allweather and day and night aircraft operations over land and sea,

SAR: Synthetic Aperture Radar, airborne systems developed in 1950’s

SAR: Active microwave imaging system


Radar remote sensing


Frequency bandKaKKuXCSLP

Wavelength (cm)0.8-1.11.1-1.71.7-2.42.4-3.83.8-7.57.5-1515 -30

30 -100

Frequency (GHz)40 - 26.526.5 - 1818 - 12.512.5 -88 - 44 - 22 - 11 - 0.3

Radar frequency

f (in Hertz)=C/λ C=3.108 mλ =wavelength in m


SpaceborneSpaceborne SARsSARsSatellite Years Agency Frequency -

Polarisation Resolution

- Swath Special

ERS-1 1991-2000 ESA C - VV

25 m 100 km

Interferometry (with ERS-2)

JERS 1992-1998 NASDA L-HH 25 m 100 km

Region. mosaic available

ERS-2 1995 ESA

C - VV

25 m 100 km

Interferometry (with ERS-1)

RADARSAT-1 1995 CSA C - HH 10 -100 m 45 - 500 km

Multi-incidence

ENVISAT - ASAR

2002 ESA C - HH/VV/HV 25 - 1000 m50 - 500 km

Multi-incidence

ALOS - PALSAR

2006 JAXA L - Polarimetric

10 - 100 m 100 - 350

km

Multi-incidence

TerraSAR-X Cosmo-Skymed

2007 DLR Italy

X-Polarimetric

1 m

Interferometry (1 day)

RADARSAT 2 2008 ? CSA C - Polarimetric

< 10 m Multi-incidence


Characteristics of radar remote sensingAdvantages compared to optical remote sensing

all weather capability (small sensitivity of clouds, light rain)

day and night operation (independence of sun illumination)

no effects of atmospheric constituents (multitemporal analysis)

sensitivity to dielectric properties (water content , biomass, ice)

sensitivity to surface roughness ( ocean wind speed)

accurate measurements of distance (interferometry)

sensitivity to man made objects

sensitivity to target structure (use of polarimetry)

subsurface penetration


Characteristics of radar remote sensing

Inconvenients

complex interactions (difficulty in understanding, complex processing)

speckle effects (difficulty in visual interpretation)

topograhic effects

effect of surface roughness


All-weather system

ERS-1 SAR, 11.25 a.m. LANDSAT TM, 9.45 a.m.

Ireland, 09/08/1991

An ‘ all-weather ’ imaging systemA microwaves system: cloud penetrating capabilities


Marginal atmospheric effects


From ‘ERS-1 : 500 days in orbit ’. Published by the European Space Agency’

ERS Images (C band, 23°, VV)in false colors

Gibraltar straitImage : 90 km x 100 km

• Internal waves (l≈2 km) Origin : difference ofsalinity between Atlantic Ocean and Mediterranean sea + tide effects

Effect of surface roughness- Internal waves


From ‘ERS-1 : 500 days in orbit ’. Published by the European Space Agency’

France - Côte d ’Azur90 km x 90 km, 19/09/91

Oil sheet

• Decrease of the sea local roughness because of oil sheets:

Application : detection of oil sheets, natural or illicit.

ERS Image (C band, 23°, VV)in false colors.

Effect of surface roughness- Oil sheet


• The SAR side looking makes it extremely sensible to the relief, even under vegetation coverin tropical forests.

Sedimentary basin (Kalimantan, Indonesia)RADARSAT F4 (C band, ~ 45°, resolution : 8 m)

Tropical forest in French GuyanaERS (C band, 23°, VV, resolution : 20 m)

Topographic effects


Multitemporal analysis

Multidate ERS data

Red: October 1997Green: December 1997Blue: January 1998


Magenta=HH, Green=VV

September 6th, 2004 Hongze area

yellow=rice, red=urban, black=other

Rice mapping using HH/VV at a single dateUse of polarisation


Varzea Dry Season Varzea Wet Season

P-band image P-band image

Sub-canopy penetration

Document S.Saatchi, JPL


Subsurface penetration

L-band

Document P. Paillou


Tree height inversion using Polarimetric Interferometry (PolinSAR)

Phas

e C

entre

hei

ght(

m)

Tree height

Ground level

HV

VVHH

Phas

e C

entre

hei

ght(

m)

Tree height

Ground level

HV

VVHH

Garestier, 2006


ReliefRelief Terrain displacementTerrain displacement

isoiso--displacement curvesdisplacement curvesisoiso--altitude curvesaltitude curves

EtnaEtna LandersLanders

i

B

Digital elevation models Cartography of terrain displacements

Accurate range measurementRadar Interferometry (1/2)


• Impact of a geothermal plant on the environment. Interferogram processed from two ERS images, acquired at two years interval. The fringes characterize the ground subsidence around the plant. One observe a subsidence of about 6 cm (2 fringes) which covers 17km x 8km.

ERS intensity image Interferogramme

Mesa, USA/ Mexico border

Radar Interferometry (2/2)

ERS Interferometry



Contents


Active system: day and night operations

Principle of imaging radar


Side-Looking Airborne Radar

Linear displacement of the antenna along the track (aircraft)

Azimuth direction

Range direction

Pulses

Principle of imaging radar (3/6)

The range information comes from the time needed by the pulse to travel way and back


Why side looking ?


L

θ

The larger the antenna, the narrower the aperture (finer resolution)

'L

Antenna scattering

Resolutio

n (r=θ.R

)

R

Numerical example:L ≈ 10m, R ≈ 1000 km (spaceborne radar), λ ≈ 5 cm (C band) resolution ≈ 5 km

Angular aperture(horizontal plane)

Lλθ =

Antenna length (horizontal direction)

Wavelength

r’=θ’.R

Ra=λ R

L


Azimuth direction

Range direction

TransmittedpulsesH

=800

km

R a=5

km

L=10 m

f0 =5.3 GHz

Azimuth resolution= 5 km


Synthetic aperture technique • An array of antennas is equivalent to a single antenna moving

along the flight line LS if the received signals are coherently recordedand added, and the target assumed to be static during the period

XnX2X1

P

LS

The echoes from X1, X2, ..Xnare recorded coherently (amplitudeAnd phase as a function of time)

Ra=λ RR

Azimuth resolution

Finest resolution: Ra=L

2LS

LS


Improvement of the range resolution :

based on the frequency modulation of the transmitted pulse

5 km 10 m (for B=15 MHz)

τ Modulation bandwidth : B compτ compτ = 1/B

cτ2

c2B


The SAR image

Satellite orbit

Ground range

Azimuth

Look angleOff nadir Slant range


Azi

mut

h

Raw data Detected imageAfter range compression


• Introduction to radar remote sensing• The Synthetic Aperture Radar• Geometric properties of SAR images• Statistic properties of SAR measurements• Physical content of SAR data

Contents


Geometric Effects


• ‘shortening’ of slopes facing the radar• ‘stretching’ of slopes oppositely oriented to the radar

Geometrical artifacts related to the vision in rangeThe foreshortening effect

(From Elachi, 1989)


From: RADARSAT Geology Handbook(RADARSAT International), 1997

Geometrical artifacts related to the vision in rangeThe foreshortening, layover and shadow effects


• Introduction to radar remote sensing• Synthetic Aperture Radar• Geometric properties of SAR images • Statistic properties of SAR measurements• Physical content of SAR data

Contents


Basic measurement

The basic measurement made by a SAR is S (amplitude and phase). This is the complex image.

Main types of images:

A is the amplitude image.I = A2 is the intensity image.(the phase of a single image is not exploitable)


Exploitation of multiple images: multi temporal, polarisation, incidence, (frequency)

polarimetric images derived from S the complex image if the SAR system is polarimetric

interferometric images(coherence, phase) derived from S

multi polarisation intensity images

multitemporal intensity images

ASAR repeat pass

ASAR dual pol

ASAR

Measurements from multiple images


The radar cross-section

The radar cross-section (RCS) is defined as

R is the radar-target distancePi is the incident power, Ps is the power scattered by the target.

[ ]222m44

i

spqpq P

PRS ππσ ==


The backscattering coefficient

For distributed targets each resolution cell contains many scatterers and the phase varies rapidly with position.

The differential backscattering coefficient, σo, is

[m2/m2]

where is the area of the illuminated surface over which the phase can be considered constant.

i

so

PP

AR

Δ=

24 πσ

AΔ


What is a SAR image?

The image is seen as a picture.

Pixels are numbers.

Image is affected by speckle noise

Example of an intensity imageAPP HH image 400 x 400 pixels (of 12.5m)Gaoyou, Jiangsu province, China, 2004 05 24


Same image, after speckle filtering


HH (magenta) and VV (green) images400 x400 pixels

Gaoyou, Jiangsu province2004 09 06

Initial HH and VV images HH and VV image after filtering


Ground scene

Constructive speckle Destructive

speckle

SAR image pixels

The physical origine of speckle

Resolution cells are made up of many scatterers with different phases, leading to interference and the noise-like effect known as speckle.


Prob

abili

tyde

nsity

func

tion

of th

e am

plitu

de A

Low backscatter

High

backscatter

Statistics of speckleProbability density distribution of speckle:

Intensity image: exponential distributionAmplitude image: Rayleigh distribution


Estimating the backscattering coefficientGiven L independent measurements from a uniform distributed target, the MLE of is given by

where the I(k) are individual intensity measurements.N.B. This does not depend on the original form of the data (amplitude, intensity or complex).

L is called the number of looks.

∑==

L

k

kIL

I1

)(1

0σ


Pdf of amplitude image for a given backscatter value. The distribution is narrower with anincreasing number of looks

Pdf

of a

mpl

itude

ASpeckle: Multilooking effect

Amplitude A



Contents


What is a SAR image?

The image represents physical processes.

Pixels are measurements.

Image is interpretable based on understanding of the physical processes

Intensity image, after speckle reduction


The radar scatteringIncident electric fieldEi

Backscatteredelectric field Es

i

ikr

s SEr

eE =

ijiijij eSS φ=

⎥⎦

⎤⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡=⎥

⎦

⎤⎢⎣

⎡

ih

iv

hhhv

vhvvikr

sh

sv

EE

SSSS

re

EE

the amplitude, phase andpolarisation of Es are modifiedwith respect to Ei

The scattering matrix contains information on the nature and characteristics of the observed media


PolarisationMagnetic field Electric field Trajectory of the

electric field

Propagation direction

Transverse plane


Transverse plane

Projection of the trajectoryof the electric field

Ellipticity angle

Orientation angle

In projection on the transverse plane,he tip of the electric field describesan ellipse called polarisation ellipse

τ= 1 : circular polarisationsτ =0 : linear polarisations


Plane orthogonal to the propagation direction

Propagation direction

Electric field

Horizontal polarisation Vertical polarisation


Example de linear polarisations

Transmission H

HHHHReceive H

SHH SHH SHH SHH

SVH SVH SVH SVH

Receive V


SIR-C images, L-band 24°, Ulan-Ude, Russia, 1994


Scattering mechanisms

Surface scattering

Volume scattering if penetration

Surface scatteringwater

Volume scattering

Surface scattering

snow

soil, rock

Surface scattering

Volume scattering

Volume-surfacescattering

vegetation


Scattering MechanismsThe backscattered signal results from:

- surface scattering- volume scattering- multiple volume-surface scattering

The relative importance of these contributions depend on- surface roughness- dielectric properties of the medium

All of these factors depend on- the radar frequency- the polarisation- the incidence angle


Surface scattering

The roughness of the surface (wrt to the wavelength) governs the scattering pattern

εr2

Smooth surface Rough surface

The dielectric constant (moisture content) of the medium governsthe strength of the backscatter

εr1εr1

εr2

εr2 > εr1 medium 2 is wetter than medium 1

Wetter media


Effect of surface roughness

Quaternary lithology:Bathurst Island, Canada

RADARSAT (C band, HH, 45°)

Limestone Higher backscatterbecause of rougher surface

From : RADARSAT Geology Handbook

Mud fragments (smooth surface)low radar backscatter

Mud

Lime stone


(adapted from Le Toan, T. , 1982, "Active microwave signatures of soil and crops: Significant results of three yearsof experiments", In Proceedings of International Geoscienceand Remote Sensing Symposium (IGARSS 82)

Effect of soil moistureExperimental results using a ground based scatterometer


Irrigated fields: higher backscatter

Effect of soil moisture


The relationship betweenradar backscatter at C band 23° VV and soilmoisture is modulated by surface roughness

Volumetric soil moiture content (%)

ER

S b

acks

catte

ring

coef

ficie

nt (d

B)

Effects of roughness and moiture

Mattia et al., 2000


Smooth surface (seed bed): Fractal

Rough surface (ploughed): Single scale

The surface roughness


Polarisation in surface scattering

VV

no depolarisationno HV or VH backscatter

Fresnel Reflectivity RH > RV

V H

some depolarisationHV or VH backscatter > 0

Fresnel Reflectivity RH = RV

Smooth surface Rough surface


Volume scattering

Single and multiple scattering


Austrian pine VHFλ > 3 m

P bandλ= 70 cm

L bandλ= 27 cm

X bandλ= 3 cm

The main scatterers in a canopy are the elements havingdimension of the order of the wavelength

What are the scatterers in the volume scattering?


Volume scattering

V

V

V

H

Point scatterer

-> no depolarisation

Anisotropic scatterer

-> depolarisation

Multiple scattering

-> depolarisation

V

H

H,V


* Order 1: simple scattering

soil soilsoil(negligible)

* Order 0

soil

0.

0.

00vegsoilvegsoil −++= σσσσ

Scattering from vegetation


1) Direct Crown scattering

3) Trunk scattering

4) Multiple trunk-ground5) Attenuated ground6) Direct ground scattering

2)

2

3

54

1 6

Trunk

Primary Branches

Secondarybranches

Higher orderbranches

Leaves, Needles

Scattererscontribution

Direct trunk-ground


Scattering on leaves, ears

Attenuated ground scattering

Stem-ground interaction

Scattering from a cereal canopy


HH VV

Hongze (Jiangsu) 2004 09 06


Hongze(Jiangsu)

2004 09 06

Rice mappingusing HH/VV


Phase in SAR images (1/2)iji

ijij eSS φ=The phase difference between scatterers of the incident wavestravelling from the radar to a scatterer and back to the radar changes as:

where Δr is the difference in the travel distance

Since the SAR resolution cell containsa large number of scatterers, the phaseof pixels seems randomly distributed

Δφ = 2πΔrλ

_____

ππ−

The phase of a single SAR image isof no utility

The SAR measurement contains an amplitude and a phase


Phase in SAR images (2/2)If the scene is observed in 2 images, in which the scatterersremain unchanged in the resolution cell, the phase differencebetween pixels of the 2 images can be exploited

Polarimetry: the radar measures at the same time HH, VV, HV, VH and their phase difference

Interferometry: 2 radars observe the scene with a small shift in the look angle; or the same radar at different dates from lightlyshifted orbit

Lectures on Polarimetry and interferometry

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