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Sar polarimetric data analysis for identification of ships
S. Swarajya lakshmiADRIN, Dept. of Space, Govt. of India
India Geospatial Forum – 14th International ConferenceFebruary 07-09, 2012, Gurgaon
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Objectives
• Exploitation of polarimetric SAR data for detection of ships
• Understanding the scattering mechanisms of ships through decomposition
• Feasibility for deriving additional information for identification and classification of ships
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pq – p –transmit q -receive
Polarization Combinations
VV
HH
VHHV
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Polarimetry : Information Content
As compared to single-polarization SAR, polarimetric SAR provides additional information on:
•Type of scatterer: Trihedral, dihedral, dipole etc.
•Orientation of the scatterer about the radar line of
sight
•Ellipticity: degree of scatterer symmetry
•Entropy: significance of the polarimetric
information
Therefore, enables better characterization of the target
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T11= |hh+vv|2 T22 = |hh-vv|2 T33=2|hv|2
Scattering Mechanisms
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Pedestal HeightPedestal Height
Circular PolarizationCircular Polarization
Elliptical PolarizationElliptical Polarization
Linear PolarizationLinear Polarization
Vertical PolarizationVertical Polarization
Horizontal Polarization = 0º or 180ºVertical Polarization = 90º
Linear Polarization = 0ºElliptical Polarization -45º < < 0º and
0º < < +45ºCircular Polarization = -45º or +45º
Polarimetric Signature
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Radarsat 2
Acquisition Type Fine Quad Polarisation
Product Type SLC
Date 22-02-2009
Pixel Spacing 4.733m
Swath 25km
Approximate Resolution Range: 12m Azimuth: 8m
Incidence Angle 20 – 41 degrees
Materials & Methods
Data Used:
Software Used: POLSAR of ESA
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Methodology
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1. Input SLC data
2. Sinclair Matrix – Shh, Shv, Svh, Svv
3. Extracting Different Target Descriptors –
Stokes matrix, Covariance Matrix, Cherence Matrix
4. Speckle Filtering
5. Polarimetric Parameter Extraction – Total Power, Entropy, Alpha, Anisotropy, Degree of Polarisation, Eigen Analysis parameters etc.
6. Extracting Polarimetric signatures
7. Polarimetric Synthesis
8. Polarimetric Decomposition and Classification
9. Separation of Land and Water
10. Identification of anomalies in water
11. Identification of ships12. Further characterisation of ships with respect to
polarimetric parameters
Steps Involved
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ENTROPYENTROPY
Eigen Values: Three eigen values of the 3x3 Coherency matrix λi represent the intensities of the three main scattering mechanisms
Probabilities Pi of each scattering mechanism
Entropy (H)
This is a measure of the dominance of a given scattering mechanism within a resolution cell.
Entropy ranging from 0 to 1, represents the randomness of a scattering medium
from isotropic scattering (H=0) to totally random scattering (H=1)
ii
i PlogPH 3
3
1
3
1jj
iiP
Where,
Entropy
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ALPHAALPHA
If the Entropy is close to 0, the alpha angle provides the nature or type of the dominant scattering mechanism for that resolution cell.
For example it will identify if the scattering is volume, surface or double bounce.
anisotropic odd bounce
anisotropic even bounce
Isotropic even
bounce
Isotropic odd
bounce
= 45 = 0 = 90Multiple
, re whe,
0113
1
13
1312
1211
11
900
iiii
i vcosP
vcosvcosvcos
1iv = first element of the ith eigenvector
Alpha
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Anisotropy (A)This is the measure of how homogeneous a target is relative to the radar look direction. For example, the
Amazon forest is a very homogeneous target and would have a low anisotropy value.
In contrast, row crops would have a high anisotropy value.
10 ,32
32
AA
A indicates the distribution of the two less significant eigenvalues
Anisotropy becomes 0 if both scattering mechanisms are of an equal proportion;
values of A > 0 indicates increasing amount of anisotropic scattering.
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Target Decomposition
• Analysis methods whereby individual scattering components that have meaningful physical interpretation can be identified in the received signal.
• Scattering matrix is decomposed into sub-matrices so that Individual component have physical meaning => Surface scatterer, double bounce, volume scattering
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H-Alpha Scattering Plane
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Classified image depicting water and ships
Land
Ships with typical signatures
Classified ImageAnomalies in
water identified as ships
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Class description
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Scattering Mechanisms with respect to the ships identified
Turbulence of water
Boundary between water & metallic ship body
Ship structure
Objects causing strong double bounce scattering
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Polarimetric signatures
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Polarimetric signatures – ship
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Proportion (%) of pixels for each class of scattering
Class
Ship
1 2 3 4 5 6 7 8
1 3 9 5 60 21 - 4 1
2 4 5 8 50 21 - 6 7
3 2 8 5 56 18 - 6 5
4 4 12 6 56 15 - 4 4
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Proportion (%) of pixels for each class of scattering
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Derived Information on Ship Measurements
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Conclusions &Way Forward
Typical scattering mechanisms were observed to be associated with the ships, which could be used towards automated detection and characterization of ships.
Potential of the polarimetric data cold be further explored with multi-parametric decomposition schemes and tested with a wide variety of ships.
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THANKS FOR YOUR KIND
ATTENTION