A Bragg-Like Point Extraction Method for Co-polarization Channel Imbalance Calibration

Calibration by distributed targets is an important part of polarimetric calibration (PolCAL) without corner reflectors (CRs). Currently, both the correlation of HH and VV (R-hhvv) and the equivalent number of looks (ENL) constitute the common method for the extraction of Bragg-like points (BLPs) in order to calibrate the co-polarization channel imbalance k. However, strict assumptions about distributed targets and complex mathematical expressions regarding ENL limit the impact analysis of k on R-hhvv and ENL; thus, the fixed thresholds for R-hhvv and ENL can be obtained only by comparing the calibration errors with the CRs or simulated values in order to calibrate k accurately. In some Earth observation and lunar exploration without CRs, the abovementioned conditions are not satisfied. In this article, a BLP extraction method is proposed for calibrating co-polarization channel imbalance k in PolCAL. H/alpha decomposition is utilized to deduce the specific scattering impacts of BLPs by different k values first. Then, a dynamic selection method is proposed to reduce the influence of fixed thresholds and improve the extraction accuracy. Multiscene polarimetric synthetic aperture radar images from AIRSAR, ALOS, and GF-3 are utilized to verify the effectiveness of the proposed algorithm in the extraction of BLPs and the calibration results of k obtained by applying the extracted BLPs.