Numerical And Experimental Evaluation Of Polarimetric Calibration Using Hybrid Corner Reflectors

Fully polarimetric synthetic aperture radar(Pol SAR)has been used in a wide range of applications,including terrain classification,geophysical parameters retrieval,target detection and so forth.However,the advantages of a fully Pol SAR can be thoroughly exploited only after a careful polarimetric calibration is performed.Thus,polarimetric calibration becomes an indispensable procedure for accurate polarimetric signatures.In measuring a target scattering matrix,the received signals are constituted of the desired target scattering matrix and transmission and reception properties of the measurement system.Polarimetric calibration corrects the polarization distortion,caused by imperfect radar system,which is primarily caused by the non-ideal channel imbalance and polarization cross-talk errors for both transmitter and receiver.In a real system,under non-ideal conditions,such as polarization cross-talk errors and channel imbalance between transmitter and receiver,the polarimetric calibration is required.The wave propagation in transmitting and receiving causes the target scattering matrix S distorted to a measured scattering matrix.The goal of the polarimetric calibration is to invert the target scattering matrix S from the measured one to obtain a minimum difference.Propagation matrices and noise matrix are acquired from the known and reference targets.In this paper,we propose and evaluate a polarimetric calibration method that relies only on the characteristics of the calibration targets and requires no assumptions regarding scene statistics and radar characteristics.The correction for polarimetric distortions of the radar system employs only three types of calibration targets,i.e.trihedral,dihedral,the 22.5°and 45°-rotated dihedral corner reflectors.The method is verified with numerically simulated data and L-band airborne Pi-SAR(polarimetric and interferometric airborne synthetic aperture radar)data.The impacts of reflector size,roll angle,and misalignment error for the 22.5°-rotated dihedral corner reflector on calibration accuracy are analyzed through simulation.An amplitude error as low as 0.3 d B and phase error within 3°,can be achieved with the Pi-SAR system,showing excellent agreement between the theoretical and calibrated polarimetric responses for the reference targets.Finally,we investigate the effects of the calibration on the Pauli decomposition,Yamaguchi four-component decomposition,and estimation of the polarimetric parameters,includingsa,saF and mt,derived from TSVM decomposition.