RISAT-1 Compact Polarimetric Calibration and Decomposition

ISRO’S RISAT-1 was the first SAR satellite equipped with the compact polarimetric (CP) mode for data acquisition. The CP mode has several advantages compared to the Quad-pol mode in terms of reduced hardware complexity, pulse repetition frequency (PRF), data transmission rate and double swath coverage. To exploit the advantages offered by the CP mode, the datasets need to be Polarimetric calibrated. The Polarimetric calibration procedure estimates the polarimetric distortions in the datasets caused due to channel imbalance, crosstalk and Faraday rotation. These polarimetric distortions cause the misinterpretation of the ground targets in the polarimetric decomposition techniques. The Freeman compact-pol polarimetric calibration algorithm is the most commonly used algorithm which requires a Trihedral corner reflector, a 0^o oriented Dihedral corner reflector and a 45^o oriented Dihedral corner reflector. In this study one trihedral corner reflector and one 0^o oriented Dihedral corner reflector were deployed at the Desalpar calibration site, Gujarat, India. The RISAT-1 Circular Transmit Linear Receive (CTLR) dataset of this area was used to estimate the polarimetric distortion parameters and these distortion parameters were used to polarimetrically calibrate the RISAT-1 CTLR dataset of the Haridwar region, Uttarakhand, India. The Freeman compact-pol polarimetric calibration algorithm was used for this study and the 45^o oriented Dihedral corner reflector response required for the calibration algorithm was derived from the oriented Dihedral corner reflector response. The Cloude compact-pol decomposition algorithm was used to evaluate the ground target characterization accuracy before and after polarimetric calibration using the proposed algorithm. Before polarimetric calibration, urban targets were showing surface scattering behavior and river channels were showing increased double bounce scattering behavior. After polarimetric calibration, the urban tragets were showing dominance in double bounce scattering and river channels were showing dominance in surface scattering. The calibration methodology and the results obtained from Cloude compact-pol decomposition are discussed in detail in the manuscript.