Calibration by distributed targets is an important part of polarimetric calibration (PolCAL) without corner reflectors (CRs). Currently, both the correlation of HH and VV ( $R_{\rm{hhvv}}$ ) 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 $R_{\rm{hhvv}}$ and ENL; thus, the fixed thresholds for $R_{\mathrm{hhvv}}$ 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 above conditions are not satisfied. In this article, a BLP extraction method is proposed for calibrating co-polarization channel imbalance $k$ in PolCAL. ${H}/{\bar{\alpha }}$ decomposition is utilised 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. Multi-scene polarimetric synthetic aperture radar (PolSAR) images from AIRSAR, ALOS, and GF-3 are utilised 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 (ext. BLPs).

The calibration of channel imbalances is currently the main concern in polarimetric calibration (POLCAL) since the crosstalk of recent polarimetric synthetic aperture radar (Pol-SAR) systems is lower than −20 dB. The existing channel imbalance calibration method without corner reflectors utilizes both volume-dominated and Bragg-like targets. However, there are two limitations to using volume-dominated targets. One is that the inaccurate selection of volume-dominated areas in the uncalibrated Pol-SAR images has a negative influence on the estimation of cross-polarization (x-pol) channel imbalance, which subsequently impacts the estimation of copolarization (copol) channel imbalance. The other is that there are minimal volume-dominated areas in some special applications of Pol-SAR, such as planetary exploration. Thus, only selecting Bragg-like targets to estimate the values of both transmitting and receiving channel imbalances, which is proposed in this paper, can avoid the uncertainty brought about by selecting other distributed targets in an uncalibrated imaginary. In addition, the reciprocity assumption and characteristics corresponding to H/$\overline{\alpha }$ decomposition are introduced to eliminate the phase ambiguity for the first time. Compared with previous methods, our method had an obvious advantage in terms of universality, since Bragg-like targets are common in the most illuminating areas. The novel method was applied to both the simulated data from the L-band Advanced Land Observing Satellite (ALOS) and C-band GaoFen-3 (GF-3), and to real data with corner reflectors on site. The results from the simulated data showed that the errors of the amplitude and phase estimation were less than 0.5 dB and 5.0° in most topographical features. Meanwhile, the $VV$/$HH$ terms from all trihedral corner reflectors were less than 0.3 dB for amplitude, and 5.5° for phase after calibration by using the estimated channel imbalances.

Polarimetric calibration is crucial to ensure the accuracy and reliability of subsequent application results. Since Gaofen-3(GF-3) has been providing polarimetric Synthetic Aperture Radar (Pol-SAR) images for 6 years, it is necessary to monitor the polarimetric performance by estimating the residual channel imbalance. In this paper, we proposed an improved algorithm based on zero helix component of Bragg- like targets. The method can provide transmitting and receiving channel imbalances results and can be applied to the large-scale polarimetric evaluation. We verified the effectiveness of the algorithm and assessed the residual channel imbalance in the multi-beam polarimetric products of GF-3 quad-polarimetric Stripmap (QPSI) from 2019 to 2023. The experiments indicates that the residual channel imbalance changes in the same strip observation and the polarimetric performance remains stable. The evaluation conclusions can be used as references for subsequent application of GF-3 polarimetric products.