Polarimetric Calibration Scheme Combining Internal and External Calibrations, and Experiment for Gaofen-3

Abstract

The parameters for polarization distortion of spaceborne polarimetric synthetic aperture radar (SAR) have range-dependence (or look-angle-dependence), which requires a polarimetric calibration to be performed at any look-angle. It is a huge endeavor to rely solely on ground experiments to obtain a polarimetric calibration at all look-angles. For SAR with phased array antennas we deduce, based on the model for the general polarimetric system, the model for fine polarization distortion described by the parameters of the radar device under the condition of high polarization isolation. We point out the mechanism that causes both variable and constant polarization distortions, and we deduce the correction algorithms for the two types of polarization distortion. Then we propose a polarimetric calibration scheme combining internal and external calibrations to calibrate the two types of polarization distortions for SAR with phased array antennas. The scheme uses the internal calibration data of the radar and the model of the antenna pattern established before satellite launch to invert the in-orbit antenna patterns to correct for the variable polarization distortion, and it needs only a small amount of calibration equipment to solve for the parameters for constant polarization distortion. The scheme no longer depends on the distributed target and improves the polarization precision of the data. It is applied to the calibration experiment for the data processing of the GaoFen-3 satellite and has achieved good results in applications.

A Set of Point-Targets-Based Polarimetric Calibration Methods Based on General Polarimetric System Model

“…The original data of the experiment has been corrected by internal calibration. The normalized measurement matrices of the equipment participating in the experiment on September 8, 2016 were given in Liang's paper [42].…”

“…The GF-3 satellite successfully carried out four polarimetric calibration experiments in Erdos on September 8 and 19 in 2016, as well as on July 11 and 16 in 2017 [28], [42]. These experiments involved five PARCs, nine TCRs, three 45 • DCRs (the rotation angle around the radar line-of-sight is 45 • ), and one 0 • DCR (the rotation angle around the radar line-of-sight is 0 • ).…”

Synthetic Aperture Radar Radiometric Cross Calibration Based on Distributed Targets

“…The variation of scattering characteristics with incidence angle is significant in the saline-alkali area. The error (standard deviation) is 1.82 dB before correction and 0.81 dB after Oh model-based correction using (13), as shown in Fig. 11.…”

“…On the other hand, according to (8), the calibrated SAR data and the SAR data to be calibrated can be expressed as σ vh1 = 0.095(0.13 + sin 1.5θ 1 ) 1.4 (1 − exp(−1.3(ks) 0.9 ))σ 0 vv1 (10) σ vh2 = 0.095(0.13 + sin 1.5θ 1 ) 1.4 (1 − exp(−1.3(ks) 0.9 ))σ 0 vv2 (11) By dividing (10) with (11) and combining (9), we can obtain Rewriting (12), the relationship between the calibrated SAR data and the SAR data to be calibrated in VV polarization can be expressed as σ vv1 = cos(θ 1 ) 2.2 (0.13 + sin 1.5θ 2 ) 1.4 cos(θ 2 ) 2.2 (0.13 + sin 1.5θ 1 ) 1.4 σ vv2 (13) The derived backscattering coefficients in ( 9) and ( 13) can then be used to calibrate the SAR satellite data to be calibrated.…”