introduction to sar remote sensing - europa...september 7, 2010 lecture d2l4b introduction to sar...

September 7, 2010 Lecture D2L4b Introduction to SAR remote sensing Thuy Le Toan 1

Introduction to SAR remote sensing

Thuy Le ToanCentre d’Etudes Spatiales de la Biosphère (CESBIO)

Toulouse, [email protected]


• Introduction to radar remote sensing

• The Synthetic Aperture Radar

• Physical content of SAR data

Contents


Radar remote sensing


ACTIVE MICROWAVE SENSORS

Detect reflected responses from objects irradiated by artificially-generated energy sources.

Non-Imaging( ex: microwave scatterometer, microwave altimeter)Imaging (Real Aperture Radar,Synthetic Aperture Radar)

RADAR: Radio Detection and Ranging

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

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

RADAR: Active microwave imaging system


Frequency bandKaK

KuXCSLP

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 -12.5

12.5 -88 -44 -22 -11 -0.3

Radar frequency

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


Spaceborne SARsSatellite 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 m 50 - 500 km

Multi-incidence

ALOS - PALSAR

2006 JAXA L Polarimetric

10 - 100 m 100 - 350 km

Multi-incidence

TerraSAR-X CosmoSkymed

2007 DLR Italy

X Polarimetric

1 m

Interferometry (1 day)

RADARSAT 2 2007 CSA C - Polarimetric

< 10 m Multi-incidence


Characteristicsof radar remote sensing

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

day and night operation (independence of sun illumination)

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

sensitivity to surface roughness ( ocean wind speed)

accurate measurements of distance (interferometry)

sensitivity to target structure (use of polarimetry)

subsurface and volume penetration


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


Sensitivity to surface roughnessOil Spill detection

ESA: This Envisat

radar image captures the oil that is spilling into the Gulf of Mexico after a drilling rig exploded and sank off the coasts of Louisianaand Mississippi, USA, on 22 April 2010. The oil spill is visible as a dark grey whirl in the bottom right. Envisat acquired this image from its Advanced Synthetic Aperture Radar on 26 April at 15:58 UTC.

Gulf of Mexico, 26 April 2010ENVISAT ASAR


Varzea Dry Season Varzea Wet Season

SAR image SAR image

Sub-canopy penetration

Document S.Saatchi, JPL


Relief Terrain displacement

iso-displacement curvesiso-altitude curves

Etna Landers

i

B

Digital elevation models Cartography of terrain displacements

Accurate range measurementRadar Interferometry





Contents


Active system: day and night operations

Principle of imaging radar


Why side looking ?


L

θ

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

'L

Radiation into space from an antenna

Numerical example:

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

Angular aperture(horizontal plane)

L

λθ =

Antenna length (horizontal direction)

Wavelength

Ra=λ R

L


Synthetic aperture techniqueAn array of antennas is equivalent to a single antenna moving along the fight line LS if the received signals are coherently recorded and added, and the target assumed to be static during the period

XnX2X1

P

LS

The echoes from X1, X2, ..Xn (amplitude and

phase) are recorded coherently as a functionof time

Ra=λ R

R

Azimuth resolution

Finest resolution: Ra=L

2

LS

LS


v1/PRF

IMAGE FORMATION : Azimuth1 pulse 1 echo = 1 image linePRF : Pulse RepetitionFrequency


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

cτ

2

c

2BRange resolution


The SAR image

Ground range

Azimuth

Look angleOff nadir


Azi

mu

th

Raw echoesAzimuth res= 5 kmRange res= 5 km

After range processing and SAR synthesisAzimuth res= 5 mRange res= 20 m

After range processingAzimuth res= 5 kmRange res= 20 m





Contents


What is a SAR image?

The image is seen as a picture.

Pixels are numbers.

Image is affected by specklenoise

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 fltering


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


SAR measurementsThe basic measurement made by a SAR is the scattering matrix S (amplitude and phase). Main types of images:

Ais the amplitude image.I = A2 is the intensity image.

Measurements derived from a single SAR imageIntensity at single or multiple polarisation Polarimetric measurements (if the SAR is polarimetric)

Measurements derived from multiple SAR imagesTemporal variation of intensity (and polarimetry)Interferometric coherenceInterferometric phase


The radar scatteringIncident electric feld

Ei

Backscattered electric feld Es

i

ikr

s SEr

eE =

iji

ijij eSSφ=

=

ih

iv

hhhv

vhvvikr

sh

sv

E

E

SS

SS

r

e

E

E

the amplitude, phase and polarisation of Es are modifiedwith respect to Ei

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


PolarisationMagnetic feld

Electric feld Trajectory of the electric feld

Propagation direction

Transverse plane


Transverse plane

Projection of the trajectory

of the electric feld

Ellipticity angle

Orientation angle

In projection on the transverse plane,the tip of the electric feld describes an ellipse called polarisation ellipse

τ= 1 : circular polarisationsτ =0 : linear polarisations


Plane orthogonal to the propagation direction

Propagation direction

Electric feld

Horizontal polarisation Vertical polarisation

Linear polarisations


Scattering mechanisms

Surface scattering

Volume scattering if penetration

Surface scatteringwater

Volume scattering

Surface scattering

snow

soil, rock

Surface scattering

Volume scattering

Volume-surface

scattering

vegetation


Scattering mechanismsThe backscattered signal results from:

-surfacescattering-volumescattering-multiplevolume-surface scattering

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

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


P –HH HV VV L –HH HV VV

ESAR, Remningstorp forest, Sweden


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 governs the strength of the backscatter

εr1

εr1

εr2

εr2 > ε

r1 medium 2 is wetter than medium 1

Wetter media


Irrigated felds: higher backscatter

Sensitivity to soil moisture


(adapted from Le Toan, T. , 1982, "Active microwave signatures of soil and crops: Signifcant results of three years

of experiments", In Proceedings of International Geoscience and Remote Sensing Symposium (IGARSS 82)

Sensitivity to soil moisture

Experimental results using a ground based scatterometer


hv

k

scatterers size,shapescatterers orientation

scatterers density

scatterers water content

soil roughness

soil moisture

Dielectric properties

Geometric, structural properties

Vegetation scattering


Scattering on leaves, ears

Attenuated ground scattering

Stem-ground interaction

Main scattering mechanisms from a cereal canopy

Dominant at X-band

Dominant at L-band

Dominant for fooded felds


Water

Scattering from a rice canopyAt C band, HH and VV: the dominant scattering mechanism is thedouble bounce vegetation-waterHH>VV because of the stronger attenuation of VV by verticalstems (and Fresnel refection RH > RV )

HH and VV increases with the

plant biomass.The increase is very important (up to 10 dB during the growth season) (Le Toan et al., 1997).


Biomass from Intensity

0 - 1010 - 2020 - 3030 - 4040 - 7070 - 9090 -120

120 -160160 -200200 -260> 260

0 - 1010 - 2020 - 3030 - 4040 - 7070 - 9090 -120

120 -160160 -200200 -260> 260

0 - 1010 - 2020 - 3030 - 4040 - 7070 - 9090 -120

120 -160160 -200200 -260> 260

Biomass

(ton/ha)

40

30

20

10

0m

Pol-InSAR height

Height (m)

Intensity + Pol-InSAR

RMSE= 35.56 t ha -1 RMSE= 16.26 t ha -1


ρ : complex degree of coherence

| ρ | degree of coherence (or coherence

∆ϕ phase difference

ρ= Ess

Es Es

( *)

( ²) ( ²)1 2

1 2s1, s2

= −ρ φe j∆j∆ϕ

B ⊥

H

r0

h

B

B //

i

s1

s2

R1

R2

Interferometric Coherence

decreases with scatterers movement

decreases with increasing biomass


Phase in SAR images iji

ijij eSSφ=

The phase difference between scatterersof the incident wavestravelling from the radar to a scatterer and back to the radar changes as:

∆r is the difference in the travel distance

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

∆φ= 2π∆rλ

_____

The phase of a single SAR image is of no utility

The SAR measurement contains an amplitude and a phase


Phase in SAR images

If the scene is observed in 2 images, in which the scatterers remain unchanged in the resolution cell, the phase difference between 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 lightly shifted orbit

Lectures on Polarimetry and interferometry


Summary

An introduction to SAR remote sensing has been given

Specifc characteristics of SAR data (e.g. vs optical data) have been illustrated by SAR images leading to applications

Knowledge of the physical content of SAR images is essential before their use in many land applications

introduction to sar remote sensing - europa...september 7, 2010 lecture d2l4b introduction to sar...

Documents