Polarimetric Calibration Algorithm Of Airborne SAR Using Corner Reflectors

With the development of polarimetric synthetic aperture radar(PolSAR) technique, PolSAR data has been widely used in applications such as classification, land usage and ground parameter inversion, and polarimetric calibration method is the basis of analysis and application of polarimetric data. Due to incompletely isolation between polarimetric channels, observed data, containing some distortions, can not precisely depicts backscattering characteristic of ground object. Therefore,observed data must be calibrated to exactly adjust relative amplitude and phase between polarimetric channels, to precisely extract ground object information which lies in amplitudes and phases between polarization combinations and to highlight superiority of polarimetric SAR technique.At present, the calibration methods published in paper can be categorize into three major groups: (1) methods based on point calibration targets; (2) algorithms based on distributed calibration targets;(3) hybrid calibration techniques based on both point and distributed targets. In the paper, we mainly research the method based on point targets. Procedure of this kind method can be summarized as: firstly extract point targets in SAR images with known scattering matrices; then construct matrix equation between observed and theoretical values to calculate polarimetric parameters; finally, the calculated distortion parameters are applied on other unknown region to generate calibrated SAR image. Two methods based on point targets are proposed in the paper via mending some disadvantages in present algorithms. The main research contents are as following:(1) The paper firstly introduces theoretical basis of polarimetric SAR, analyzes usual polarimetric calibration model and methods. Except that, two factors effecting calibration precision are summarized as following: unstability of airborne platform and deployment deviation of corner reflector. The former includes roll, yaw, pitch, offset in range direction, in azimuth and in elevation; the latter refers to rotation deviation of corner reflector around radar line of sight in plane perpendicular to radar line of sight.(2) By analyzing and studying present methods, such as Whitt and Ainsworth calibration algorithms, the paper proposes an unconstrained method which is validated by being applied on simulated data and actual fully polarimetric C-band SAR image acquired in southern region of Hainan province in 2014. Contrasting with typical methods, the proposed method overcomes some disadvantages such as scattering reciprocity of ground object.(3) Based on researching Whitt and Ainsworth calibration algorithm,considering some problems of typical methods such as complex process in calculation, the paper proposes a simplified method using point targets to calibrate some distortions between polarimetric bands in four polarization combinations. Contrasting with other method, the proposed one simplifies the calculation steps of polarimetric parameter and is easily understood.