Abstract:The GaoFen-3 (GF-3) satellite is the first fully polarimetric synthetic aperture radar (SAR) satellite designed for civil use in China. The satellite operates in the C-band and has 12 imaging modes for various applications. Three fully polarimetric SAR (PolSAR) imaging modes are provided with a resolution of up to 8 m. Although polarimetric calibration (PolCAL) of the SAR system is periodically undertaken, there is still some residual distortion in the images. In order to assess the polarimetric accuracy of this satellite and improve the image quality, we analyzed the polarimetric distortion errors and performed a PolCAL experiment based on scattering properties and corner reflectors. The experiment indicates that the GF-3 images can meet the satellite's polarimetric accuracy requirements, i.e., a channel imbalance of 0.5 dB in amplitude and ±10 degrees in phase and a crosstalk accuracy of −35 dB. However, some images still contain residual polarimetric distortion. The experiment also shows that the residual errors of the GF-3 standard images can be diminished after further PolCAL, with a channel imbalance of 0.26 dB in amplitude and ±0.2 degrees in phase and a crosstalk accuracy of −42 dB.

Proposed is a new scheme of frequency-domain dynamic polarimetric calibration by designing a polarimetric active radar calibrator (PARC) with two rotatable antennas. The proposed scheme can eliminate clutter and the error due to a set of measurements in the traditional static calibration scheme. The calibration results in the proposed scheme are not affected by the initial angles of the PARC antennas. The calibration accuracy of the proposed method is better than the static calibration methods, for example, the target spinor calibration and the full polarimetric calibration technique. This result was verified by simulations under various conditions. Introduction: The condition for polarimetric calibration in the field is not as ideal as in an anechoic chamber. Polarimetric active radar calibrators (PARCs) with a large and variable radar cross-section (RCS) and an elimination ability of clutter are widely used to calibrate polarimetric radar in the field [1]. If the transmit and receive antennas of the radar are different and the quadratic term of the cross-polarisation talk is not ignored, several calibrators are necessary to solve the measurement mathematical model [2]. The set of measurements not only increases the complexity of the calibration procedure but also brings errors caused by arrangement. The present calibration algorithms solve the measurement mathematical model in the time domain by various methods and do not consider the polarimetric uncertainty of the PARC and its suppress method [1][2][3][4]. To overcome the drawbacks of the traditional static polarimetric scheme, this Letter proposes a dynamic polarimetric calibration scheme with a new PARC system.

ABSTRACT:Polarimetric calibration (PolCAL) of Synthetic Aperture Radar (SAR) images is a significant preprocessing for further applications. Since the reflection symmetry property of distributed objects can provide stable constraints for PolCAL. It is reasonable to extract these reference samples before calibration. The helix scattering generally appears in complex urban area and disappears for a natural scatterer, making it a good measure to extract distributed objects. In this paper, a novel technique that extracts reflecting symmetry samples is proposed by using helix scattering. The helix scattering information is calculated by Yamaguchi four-component decomposition algorithm. An adaptive threshold selection algorithm based on generalized Gaussian distribution is also utilized to scale the helix scattering components automatically, getting rid of the problem of various numerical range. The extracting results will be taken as PolCAL reference samples and the Quegan method are utilized to calibrate these PolSAR images. A C-band airborne PolSAR data was taken as examples to evaluate its ability in improving calibration precision. Traditional method i.e. extracting samples with span power was also evaluated as contrast experiment. The results showed that the samples extracting method based on helix scattering can improve the Polcal precision preferably.

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