GaoFen-3, the first polarimetric SAR satellite of China, carried out polarimetric calibration experiments using C-band polarimetric active radar calibrators (PARCs), trihedral corner reflectors (TCRs), and dihedral corner reflectors (DCRs). The calibration data were firstly processed referring to the classic 2 × 2 receive R and transmit T model for radar polarimeter systems, first proposed by Zebker, Zyl, and Held, and Freeman’s method based on PARCs, but the results were not good enough. After detailed analysis about the GaoFen-3 polarimetric system, we found that the system had some nonlinearity, then a new imbalance parameter was introduced to the classic model, which is equivalent to the $\gamma$ proposed in Freeman’s paper about a general polarimetric system model. Then, we proposed the calibration data processing algorithm for GaoFen-3 based on the improved model and obtained better results. The algorithm proposed here is verified to be suitable for GaoFen-3 and can be applied to other spaceborne and airborne fully-polarimetric SAR systems.

Freeman proposed a general polarimetric system model in 1991 for the characterization and calibration of fully polarimetric synthetic aperture radar (SAR) systems. The model contains three-channel imbalance parameters and four cross-talk parameters, which together describe the polarimetric distortion in a practical imaging radar more accurately than the conventional six-parameter model by including a third imbalance parameter, namely γ. This parameter is independent of the others and represents the ratio of the product of the transfer functions of the copolarized channels to the product of the transfer functions of the cross-polarized channels. Nonnegligible errors may arise if this parameter is not estimated and corrected, but since this parameter was proposed, there has been very little research into calibration methods that account for γ. In this article, based on the general polarimetric system model in the 2 × 2 matrix form, we propose a γ-factor estimation method based on an active radar calibrator (ARC) or a dihedral corner reflector. We also propose a universal iterative polarimetric calibration algorithm based on three pieces of calibration equipment, which can be all ARCs, all corner reflectors (CRs), or a combination of ARCs and CRs. The convergence of this iterative algorithm is proven mathematically. These alternatives provide a variety of practical calibration method options for polarimetric SAR calibration under different conditions. Finally, the simple calibration method based on a single piece of calibration equipment and the iterative algorithm based on three pieces of calibration equipment are verified by the polarimetric calibration experiment data from the GaoFen-3 satellite.

The paper proposes a new method for measuring the azimuth pointing of spaceborne synthetic aperture radar (SAR) antenna beams based on the ground receiver, which can receive and record complex sampling data of the pulse signals transmitted from the spaceborne SAR. The center of the antenna pattern is extracted from the complex sampling data amplitude envelope to obtain the time when the beam main lobe center irradiates the ground receiver, and the range migration information is extracted from the complex sampling data to obtain the time when the satellite is over the top of the ground receiver. The results of Chinese civilian remote sensing GaoFen-3 SAR satellite experiment data processing show that the measurement accuracy of this method is better than 0.002°, which can be applied to the accurate measurement of azimuth pointing of various low Earth orbit (LEO) SAR antenna beams.

Although synthetic aperture radar (SAR) performance and polarimetric data quality are closely related, they represent fundamentally different concepts. This paper delineates their distinctions, investigates their interdependence, and introduces a comprehensive set of technical metrics for evaluating radar system performance and assessing polarimetric data quality. Specifically, radar performance is quantified by seven independent parameters, whereas data quality is characterized by a three-component channel imbalance vector and a twelve-element channel crosstalk matrix. The paper details the measurement methods for these parameters and outlines the associated technical requirements, including calibrator specifications and test-site conditions. To improve operational applicability, an approximate method for data quality assessment is proposed, and its associated errors are analyzed. Special attention is given to the $\gamma$ factor, which is highlighted as a critical and irreplaceable indicator of radar performance. Using field data from the GaoFen-3 (GF-3) satellite, the proposed metrics are applied to evaluate both radar performance and data quality. The results provide insights into the polarimetric characteristics of the system and offer practical guidance for the calibration and application of GF-3 polarimetric SAR data.

Cycle slip detection is an important part of quality control of RTK (real-time kinematics). The occurrence of cycle slips will cause the pseudorange or carrier phase to jump, making the positioning accuracy centimeter-level or even decimeter-level errors, which cannot meet the requirements of high-precision positioning. In this paper, an improved pseudorange-carrier phase combination cycle slip detection method based on single frequency receiver is proposed to deals with the problem of small cycle slip. The proposed method uses integral doppler observations to smooth the pseudo-range observations, and predicts the cycle slip constructed by the traditional pseudo-range phase combination method through the gray Verhulst Markov dynamic model. Numerical experiments show that the cycle slip detection value with an amplitude 2 times larger than the threshold are successfully identified when the cycle slip is as small as one week. This means that the proposed method can effectively detect small cycle slip.