COMPARATIVE ANALYSIS OF RISAT-1 AND SIMULATED RADARSAT-2 HYBRID POLARIMETRIC SAR DATA FOR DIFFERENT LAND FEATURES

The purpose of this study is to compare the performance of first hybrid polarimetric spaceborne satellite RISAT-1 data and simulated hybrid polarimetric data from quad-pol RADARSAT-2 data for different land use land cover (LULC) classes. The present study compares Stokes (g0, g1, g2 and g3) and its decomposed parameters (m, chi, delta and CPR) for satellite data acquired from RISAT1 and RADARSAT-2 over Vijayawada, Andhra Pradesh, India. Further, backscattering coefficients are also compared for different LULC types. The results indicate that both the satellites are following approximately the same trend for different classes except for settlements in RISAT-1.


INTRODUCTION
In recent years, compact polarimetry has demonstrated its potential in the field of radar remote sensing for earth observation.Compact polarimetry SAR imaging systems have advantages over conventional single and dual polarized SAR systems.Compact pol SAR systems also have advantages over quad-pol SAR systems in terms of hardware performance (wide swath coverage and half transmitted power requirement capabilities) (Souyris et al., 2005 andRaney 2007).In compact polarimetry, only one polarization is transmitted and two orthogonal polarizations are received.Hybrid polarimetry is a special case of compact polarimetry with circular (right/left) polarization transmission and dual linear (H & V) polarization reception.India's RISAT-1 is the first spaceborne hybrid polarimetry operating satellite dedicated for earth observation, operating in right circular transmission and dual linear reception mode.In the near future hybrid polarimetric SAR data will also be available from ALOS-2 and RADARSAT constellation mission (RCM) satellites.To evaluate the potential of compact polarimetry mode, earlier investigations have been conducted by using simulated spaceborne or airborne quad-pol SAR data (Ainsworth et al., 2009 andCharbonneau et al., 2010).Stokes and its decomposed parameters were used for land cover classification, mapping and crop assessment (Brisco et al., 2012 andBallester-Berman andLopez 2012).Some initial studies have been conducted with RISAT-1 data for crop acreage estimation, classification and oceanographic applications (Chakraborty et al., 2013, De et al., 2014).In this study we try to evaluate the comparative performance of real spaceborne hybrid polarimetric data with simulated hybrid polarimetric data for various land covers using Stokes and its derived parameters.

STUDY AREA AND DATASETS
The study area is a part of Vijayawada city and its surroundings in the Andhra Pradesh state of India.It is located at 16° 31' N and 80° 39' E along the Krishna river coast.The major land use land cover of the study area comprises of agricultural fields, water body, settlement and sand in the river bed.Settlement-1 and Settlement-2 are oriented and non-oriented man-made structures respectively; orientation is with respect to the direction of line of sight of RADAR (Lee & Ainsworth, 2011).Rice, cotton, banana, sugarcane and corn are main crops cultivated in the month of March.For this study rice, cotton and banana are taken under consideration.In the study area rice crop is grown twice in a year.During the field visit it has been observed that rice crop was at late tillering stage while cotton was matured with dried leaves.The majority of rice and cotton fields size are more than 100 m 100 m.Single look complex (SLC) satellite data product of RADARSAT-2 and RISAT-1 were acquired over the study area and field work was carried out synchronously.The acquired scenes are of two days gap with approximately 4 0 difference in the scene centre incidence angle.Detailed technical configurations of datasets are given in Table1.
Table 1.Technical parameters of satellite data acquired (* indicate right looking in international terminology)

METHODOLOGY
RADARSAT-2 and RISAT-1 scenes are multilooked 2 1 and 4 3 in azimuth and range direction respectively followed by a refined Lee filter with a kernel size of 5 5.The multilook is done to bring both data sets into same resolution.RADARSAT-2 scene was used as for geocoding using ASF Map ready tool.Manual image-to-image registration of RISAT-1 scene with RADARSAT-2 scene has been performed in ENVI with RMSE error less than 0.5 pixel.Simulated hybrid pol data was generated from RADARSAT-2 quad pol product with right and left circular polarization assumptions.The simulated hybrid polarimetric data are represented in terms of Stokes vector format.From Stokes vector, specific compact polarimetric parameters; degree of polarization (DoP), ellipticity angle (chi) and relative phase (delta) along with circular polarization ratio (CPR) were also extracted from both the datasets.Further, mean and standard deviation values of Stokes parameters (g0, g1, g2 and g3) and (DoP, chi, delta and CPR) were plotted for quantitative comparison.In addition, backscattering coefficient ( ) in circular polarization mode were extracted for quantitative analysis.Here left and right circular transmission polarization sign convention for simulated RADARSAT-2 data is on the basis of PolSARpro software.

STOKES PARAMETERS AND ITS DERIVED PRODUCTS
In 1852 Gabriel Stokes introduced four parameters collectively known as Stokes vector to completely describe a polarized wave.A stokes vector can describe quantitatively a fully polarized wave as well as a partial polarized wave.For a given incident polarization (left/right circular or pi/4) with a linear polarization basis Stokes parameters are givens as (Raney 2007 andShirvaney 2012) Where <…> indicates spatial averaging, E is complex electric field of received polarization and "*" refers to complex conjugate of the number.The first Stokes parameter measures the total amount of energy in the wave while other three describes its state of polarization.From the Stokes parameters it is possible to further extract its decomposed (child) parameters such as degree of polarization (DoP), ellipticity angle (Chi), relative phase (Delta) and circular polarization ratio (CPR) which are useful in quantitative measurement of received wave.DoP expresses the polarization state of received electromagnetic wave with value ranging from 0 to 1 and is given as For completely polarized waves DoP value is 1 while for completely unpolarized wave is at 0. The chi angle (eq.3) defines amount of polarization of partially polarized wave.
(3) Delta is the relative phase between two received electric field vectors and varies between -180 0 to +180 0 . (4) CPR is the ratio of difference of Stokes first and fourth parameter to the sum of these two and gives the information about sense of circular polarization (Campbell 2012). (5)

RESULTS AND DISCUSSION
For visual interpretation of various land features, the colour composite RGB image of RISAT-1 and simulated hybrid polarimetric image of RADARSAT-2 of the study area are shown in figure 1

CONCLUSIONS
The potential of spaceborne hybrid polarimetry satellite RISAT-1 and simulated RADARSAT-2 data is explored for LULC discrimination.The relative variation of Stokes and its decomposed parameters values for different feature class in both the satellite products follows same trend except settlement-1 class.This initial study demonstrates the potential use of RISAT-1 hybrid polarimetric SAR data for LULC analysis and other applications with fewer exceptions.

Figure 1
Figure 1.m-chi decomposed colour composite RGB images of RISAT-1 and simulated Left Circular and Right Circular Polarized RADARSAT-2 data.

Figure 2 .
Figure 2. Stokes parameters for different LULC types.Figure 3. Stokes derived parameters for different LULC types.
. The red/pink colour appearance of targets shows vertically oriented features like rice crop, settlement etc.But because of difference in incidence angles and other system parameters a difference in mean of intensity values are reported.In case of water bodies where specular reflection is dominant and incident angle does not play a significant role in backscatter wave, the value of Stokes first parameter is nearly same in both cases.
For bare field, water body and sand, CPR value is less than 0.5 for RISAT-1 and RADARSAT-2 LC polarized data, while for other features CPR values are higher.Opposite trend is observed in RADARSAT-2 RC polarized data.

Table 2 .
Table2indicates the comparison of average backscattering coefficient (σ°) values in right circular transmission and linear received polarization (H, V) for different features.Settlement-1 type urban areas which are oriented towards radar look direction gives very high backscatter coefficient (around 6 dB) in RH polarization mode of RISAT-1 data while in RADARSAT-2 the values is around -1.30 dB, A relative difference of 5 to 6 dB was observed for both the satellite data configurations in all identified LULC features, which may be because of difference in noise floor levels or calibration constants of RISAT-1 and RADARSAT-2.Estimated Backscatter contributions in Right circular transmit and horizontal (RH) and vertical (RV) receive respectively.