Download - SAR image interpretation
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
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OUTLINE
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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)
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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
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THE SAR IMAGE
The image is seen as a
picture.
Pixels are numbers
Image is affected by
speckle noise
Most commonly used:
intensity image
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WHAT IS A SAR IMAGE ?
QUESTION
Low signal : black
or white ?
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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 ?
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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
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The linear basis : H and V polarizations
1
0
0
1VH
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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)
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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 ?
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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
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
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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