# Error Source Analysis and Correction of GF-3 Polarimetric Data
## 1 Introduction  
The first multi-polarization SAR in China, GaoFen-3 (GF-3) satellite, was launched on 10 August 2016. Table 1 shows all the GF-3 work modes and the imaging capability of each mode, where the satellite has 12 work modes including Strip, Scan, Sliding-Spotlight Mode, and also Wave Imaging Mode for marine applications. All working modes of GF-3 are available in either left- or right-looking orientation. Through the combination and switching of multiple imaging modes, the GF-3 satellite can acquire multi-polarization imagery, and can achieve a spatial resolution of 1 m to 500 m and a swap width ranging from 10 km to 650 km [1; 2]. Therefore, the GF-3 satellite can meet the needs of a wide-range surveys and detailed investigation of specific areas. With a design life of 8 years, the GF-3 satellite will obtain a large amount of data and play an important role in remote sensing applications of China, including ocean monitoring [3], disaster reduction, water conservancy [4], meteorology, and so on. As the first full Polarimetric SAR (PolSAR) satellite in China, the quality of the PolSAR image is on a high degree of attention. However, the data acquisition of a PolSAR system is affected by many error sources, such as magneto-ionic propagation [5], central electronic equipment, antenna [6], satellite attitude [7], and so on. Therefore, the calibration and the correction of the polarimetric error is a very important procedure for generating the high quality PolSAR image products. The GF-3 satellite obtains full-polarization images by alternately transmitting horizontal (H) and vertical (V) polarized pulses and receiving backscattering signals in both H and V polarization [8]. It possesses two Full Polarized Strip Modes (QPSI and QPSII). The ground range resolution and the swath width of QPSIare 8 m/30 km while QPSII are 25 m/40 km. With these Full Polarized Modes, there are more than 40 beams in both right- and left-looking orientation, while the look angle ranges from 18.8\({}^{\circ}\) to 42.8\({}^{\circ}\). Additionally, all beams have multiple options of bandwidth and pulse duration. With these multiple polarization modes, multiple beams and multiple bandwidths, GF-3 PolSAR can obtain more than 250 kinds of data under different system conditions, which may cause different polarimetric errors. So, GF-3 polarimetric calibration tasks face the challenge of guaranteeing the PolSAR data quality in each observing case. The product generation software of the GF-3 ground segment was developed by the Institute of Electronics, Chinese Academy of Sciences (IECAS) (details are described in [9]). In this study, we provide the methods used for the polarimetric error source analysis and calibration, and also the quality assessment results of GF-3 polarimetric data after polarimetric correction. At present, SAR satellites with the capability of polarimetric observation such as SIR-C, RADARSAT-2, TerraSAR-X and ALOS-2 have been successfully launched and calibrated for polarization errors, which provide good references for GF-3 data processing. SIR-C utilized corner reflectors and distributed targets such as grassland to accomplish the polarimetric calibration task [10]. RADARSAT-2, based on amplitude information of rainforest from RADARSAT-1, used the scattering characteristics of the rainforest to eliminate polarimetric distortion [11; 12]. TerraSAR-X used the method proposed by [PERSON] and relied on the distributed targets that satisfy reciprocity and symmetry [13]. ALOS-2 placed corner reflectors in the rainforest region and used the extended Freeman-Van zyl calibration method to extract distortion parameters [14]. In summary, there are quite a number of reports on the external calibration methods for PolSAR satellites. However, as for the various observing cases of GF-3, doing external calibration for every observing case is so time-consuming that very difficult to complete the polarimetric calibration for all GF-3 PolSAR data during the five-month in-orbit-test phase.