# C-SAR/02 Satellite Polarimetric Calibration and Validation Based on Active Radar Calibrators
## 1 Introduction  
Synthetic aperture radar (SAR) is a microwave active remote sensing imaging radar technique, which has the advantages of all-day, all-weather operation [1], and certain penetability to the ground surface and clouds compared with optical imaging systems [2, 3]; hence, it occupies an extremely important position in the field of remote sensing [4]. Polarization is an intrinsic property of electromagnetic waves, and the target will show different scattering characteristics for electromagnetic waves with different polarization states [5, 6]. A general single-polarization SAR system only transmits and receives electromagnetic waves with one polarization property and can only observe part of the target's polarization characteristics [7]. Conversely, a quad-polarization SAR system transmits and receives electromagnetic waves with two polarizations orthogonal to each other to achieve the measurement of different polarization scattering characteristics of the target [8]; the polarization information can be processed to better explain the scattering mechanism of the target [9]. Therefore, quad-polarization SAR systems can provide richer target informationthan single-polarization SAR systems, with great potential for application in oceanography [10], glaciology [11], geology [12], forestry [13], agriculture [14], environment and disaster monitoring [15], and military research [16], among others [17]. China's first C-band quad-polarization SAR satellite, Gaofen-3 (GF-3), was successfully launched on 10 August 2016 [18]. Subsequently, two follow-up satellites, C-SAR/01 [19] and C-SAR/02, were launched on 23 November 2021 and 7 April 2022, respectively, and together with the GF-3 scientific test satellite, they form an integrated constellation of ocean surveillance and monitoring satellites. C-SAR/02 has the same basic parameters as GF-3 [20]. It mainly carries C-band multi-polarization SAR, which can acquire high-resolution, wide-coverage, multi-polarization, and multi-mode sea and land observation data [21]. The satellite has been further optimized based on the stable operation of the GF-3 satellite, and the quad-polarization mode is its most important imaging mode. Ideally, there is no energy leakage between the polarization channels of a radar antenna. They do not affect each other, but owing to a variety of factors, such as insufficient isolation [22] between the channels of the radar antenna system and the complexity of the electromagnetic wave transmission conditions, distortion will be introduced into the raw Sinclair matrix obtained from the measurement of the polarization SAR system [23], which is usually considered to be able to completely describe the ground target's physical characteristics and backscattering properties [24]. Therefore, in the application of polarimetric SAR data, to fully utilize the information contained in the radar data to infer the relevant geophysical parameters, it is necessary to eliminate the effect of polarimetric SAR data distortion [25]; this process needs to be accomplished by polarimetric calibration [26], which is a critical technical step before the application of polarimetric SAR data. In 1987, [PERSON] et al. [27] showed that a single flyover could measure the complete polarization scattering matrix of the target; they established a transmit and receive model of the radar polarization system, summarized it into a mathematical model (i.e., a simple \(2\times 2\) transmit and receive matrix), and verified the system model on an airborne SAR system of the SIR. The proposed model gives a meaningful mathematical model for the polarimetric calibration of antenna systems and indicates the physical significance of each parameter in practical applications. Since 1990, polarimetric calibration algorithms have entered a period of rapid development, and various polarization calibration algorithms have emerged.