WHY IS A SAR IMAGE

« DIFFERENT »?

La nature crée

des différences,

la société en fait

des inégalités.

Tahar Ben Jelloun

Advanced modes of SAR images

Radiometry

Interferometry

Polarimetry

POLINSAR

Important features

Geometry

Speckle

Electrical properties

2

OUTLINE

3

Important features of the wave Carrier frequency

Propagation direction (incidence)

polarization

)(cm1.0 1 10 100

)(GHzf300 30 3 3.0

Ku Ka X L PSC

h ̂

v ̂

k ̂

h ̂

n ̂

v ̂

h ̂

k ̂ n ̂

h ̂ Horizontal polarization

(RADARSAT)

Vertical polarization

(ERS)

4

A CLASSIC SAR IMAGE

one antenna

Measurement:

One complex

Coefficient

antenna Image 1

i

j

1S

Aim : 2 D imaging

Only absolute value

is used

The basic measurement made by a SAR is a complex number

S (amplitude and phase).

SLC Single Look Complex

Main observable:

A is the amplitude image, I=A2 is the intensity image

the phase of a single image is not meaningful)

The radar Cross Section is defined as:

𝜎 = 4𝜋𝑅2𝑃𝑠𝑃𝑖

R is the radar-target distance

P i is the incident power, P s is the power scattered by the target

5

THE SAR IMAGE

The image is seen as a

picture.

Pixels are numbers

Image is affected by

speckle noise

Most commonly used:

intensity image

6

WHAT IS A SAR IMAGE ?

QUESTION

Low signal : black

or white ?

8

RADIOMETRIC IMAGE

diffusion

Bright points

Specular

Bright areas are produce by strong radar response and darker areas are from

weak radar responses.

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ASAR (ENVISAT) IMAGE

What are the white

points on the sea ?

10

Quality image parameters

- Resolution

- Pixel sampling

Associate images A, B

and C to following

processing:

1. Constrast

enhancement

2. High pass filtering

3. Segmentation

enhancement

IMAGE PROCESSING

A B

C D

0

1

2

3

4

5

6

7

8

x 104

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

0.5

1

1.5

2

x 105

0 50 100 150 200 250

0

0.5

1

1.5

2

2.5

3

3.5

4

x 105

0 50 100 150 200 250

0

1

2

3

4

5

6

7

8

x 105

0 50 100 150 200 250

FIND THE TRUE HISTOGRAM

Landes, RAMSES (ONERA)

0

1

2

3

4

5

6

7

8

x 105

0 50 100 150 200 250

0

0.5

1

1.5

2

2.5

3

3.5

4

x 104

0 50 100 150 200 250

0

0.5

1

1.5

2

2.5

3

3.5

x 105

0 50 100 150 200 250

0

0.5

1

1.5

2

2.5

3

3.5

4

x 105

0 50 100 150 200 250

FIND THE TRUE HISTOGRAM

Toulouse, SETHI (ONERA)

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INTERFEROMETRIC SAR (INSAR)

Two antennas

Measurement:

two complex

coefficients

h

Antenna 2

Antenna 1 Image 1

i

j

Image 2

i

j

2S

1S

Aim : 3 D cartography

H

h

iobs

iinc

2

2

2

1

*

212,1

2

1

2 complex signals

Interferometric coherence

(Schwarz inequality) 10 2,1

φ ||

■ h is given by φ=arg()

■ coherence level: | |

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1 polarimetric antenna

1

Image 1

i

j

11

11

1

vvvh

hvhh

SS

SSS

Mesure:

One 2x2 complex matrix

Polarimetric SAR (POLSAR)

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1

0

0° 180°

0° 180°

What does the radar “see”?

VVHH

THE GENERAL BENEFIT OF

POLARIMETRY

Polarimetry helps

classification, but are we

able to understand

polarimetric behaviour ?

BASIC POLARISATION

24

The linear basis : H and V polarizations

1

0

0

1VH

25

The circular basis : L and R polarizations

1

1 jL

jR

BASIC POLARISATION

CIRCULAR POLARIZATION:

Circular polarization : H and V with pi/2 phase

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ANY POLARIZATION

Elliptic

polarization

Jones vector (a,b)

27

28

How targets can modify polarization

General properties of a medium :

• Change in intensity (absorption)

• Change in phases (refraction)

absorption refraction

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How targets can modify polarization

When absorption and refraction depend on orientation…

Linear diattenuation

Linear retardater

SINCLAIR MATRIX

complex-valued 2x2 matrix that transforms the polarization

of a plane EM wave incident upon a target to the polarization

of the wave scattered from the target

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vV

hH

S

SS

vH

hV

S

S

0

0

1

1 S

10

01 S

10

01 S

EH

EH

EH

EV

EV

EV

THE SCATTERING VECTOR

Pauli basis

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0

0

1

k

0

1

0

k

1

0

0

k

2S

2

1

hV

vVhH

vVhH

SS

SS

k

QUIZZ

Color representation

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0

0

1

k

0

1

0

k

1

0

0

k

TWO TYPES OF POLARIMETRIC

BEHAVIOR

Deterministic

Non deterministic

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Saturation level

What are the gray

(non deterministic)

areas ?

34

2 polarimetric antennas

h

Image 1

i

j

11

11

1

vvvh

hvhh

SS

SSS

Image 2

i

j

22

22

2

vvvh

hvhh

SS

SSS

Mesurement:

two 2x2 complex

matrices

Aim: information about

rhe vertical structure

Interferometric and polarimetric SAR (POLINSAR)

Unlike aerial photographs and satellite images which are

passive remote sensing systems

in active systems such as radar, the brightness or darkness

of the image is dependent on the portion of the transmitted

energy that is returned back to the radar from targets on the

surface

TARGET INTERACTION AND IMAGE SIGNATURES

SPECKLE

RADAR SPECKLE

All radar images appear with some degree of what we call radar speckle. Speckle appears as a grainy "salt and pepper" texture in an image. This is caused by random constructive and destructive interference from the multiple scattering returns that will occur within each resolution cell.

Speckle reduction can be achieved in two ways: -multi-look processing -spatial filtering.

Degrade resolution : must be balanced with the amount of detail required.

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THE PHYSICAL ORIGIN OF SPECKLE

Resolution cells are made up of many scatterers with different phases, leading

to interference and the noise-like effect known as speckle.

Probability density distribution of speckle:

Intensity image: exponential distribution

Amplitude image: Rayleigh distribution

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STATISTICS OF SPECKLE

Radar images are formed coherently and therefore inevitably

have a “noise -like” appearance

Implies that a single pixel is not representative of the

backscattering

“Averaging” needs to be done

Averaging means we also get a decrease in spatial resolution by

the same factor (N)

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SPECKLE SUMMARY

GEOMETRIC EFFECTS

Shadow

Layover/foreshortening

Multipath

Echo superpositions

Layover : B' is seen before A'

Foreshortening : C'D' is a very short distance

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LAYOVER AND FORESHORTENING

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www.intro2radar.html

Thees animarions were created by I. Woodhouse.

SHADOW IS MORE OF A

PROBLEM AT FAR RANGE

SHADOW

Viaduc de Millau

THE DARK SNAKE…

Both scatterers are seen in the same pixel.

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ECHO SUPERPOSITION

Double bounce (left) and triple bounce (right)

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MULTIPATH

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URBAN AREAS

Tokyo River Island

Which is the range

axis ?

The azimut axis ?

How would you

evaluate the height

of the buildings ?

Will the radar see the car ?

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HIDDEN TARGET

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Targets may appear at strange positions…

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WHAT IS THIS TRIPLE LINE?

PROPERTIES OF THE

TARGET

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The response to radar energy by the target is primarily

dependent on three factors:

Radar viewing and surface geometry relationship

Surface roughness of the target

Moisture content and electrical properties of the

target

cos8h

BRIGHTNESS = ROUGHNESS

Intermediate

cos25h

cos4,4h

Smooth

Rough

θi θr

Smooth surface

Moderately rough

surface

Very rough surface

A surface is classified as smooth or

rough by comparing its surface height

deviation with wavelength.

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1

0

Gray levels for different roughness

What is dark area in this image, why is it dark?

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INTERPRETATION OF BRIGHTNESS

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PENETRATION PROPERTIES

f

P

L

C

0

90

Penetration ability: depends on wavelength

59

X-BAND AND L-BAND

Which image corresponds to the shorter wavelength ?

The longer ?

Which color

corresponds to

the shorter

wavelength ?

The longer ?

COLOR CODE WITH FREQUENCY

Different vegetation types (e.g., desert, grasslands, forests or frozen tundra) will all have different backscatter properties. dielectric constant = the amount of water that the soil contains.

Dry soil = low dielectric constant = little radar energy Saturated soil = strong reflector. Moist and partially frozen soils =intermediate values.

DIELECTRIC PROPERTIES

Material Dielectric

constant

Vacuum 1 (by definition)

Air 1.00054

Paper 3.5

Pyrex glass 4.7

Water (20°) 80.4

Most common materials have dielectric constants 1-100

Affecting the absorption and propagation of electromagnetic waves . Dielectric constant controlled by the amount of moisture content

Increasing the moisture content reduces the penetration of the radar signal beneath the soil and vegetation canopy.

DIELECTRIC PROPERTIES

GET USED TO OTHER

IMAGES

Brétigny sur Orge (91 Essonne)

OPTICS VERSUS RADAR (B&W)

• Which one is the optical, which one is the radar image?

Toulouse

OPTICS VERSUS RADAR (B&W)

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WHAT IS COLOR REPRENSENTATIVE FOR ?

TERRASAR-X, Baltimore

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FALSE COLOR COMPOSITION

Color codes:

1- Polarization

2- interferometry

(elevation)

3- Classification results

based on distribution

evaluation

4- Frequency

TERRASAR-X, Dubai

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COLOR CODE ?

San Francisco Bay