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

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

pq – p –transmit q -receive

Polarization Combinations

VV

HH

VHHV

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

T11= |hh+vv|2 T22 = |hh-vv|2 T33=2|hv|2

Scattering Mechanisms

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

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

Methodology

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

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

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

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.

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

H-Alpha Scattering Plane

Classified image depicting water and ships

Land

Ships with typical signatures

Classified ImageAnomalies in

water identified as ships

Class description

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

Polarimetric signatures

Polarimetric signatures – ship

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

Proportion (%) of pixels for each class of scattering

Derived Information on Ship Measurements

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.

THANKS FOR YOUR KIND

ATTENTION