## 1 Introduction  
Synthetic aperture radar (SAR), actively emitting electromagnetic signals to ground targets, has all-day, all-weather ground observation capabilities, which is important for various applications, such as emergency disaster monitoring, agricultural monitoring, and land resource monitoring, with strict timeliness requirements [1]. Various space-borne SAR satellites, including ERS-1/2, Envisat ASAR, and Sentinel-1 of the European space agency (ESA); COSMO-SkyMed of the Italian space agency; Radarsat-2 of the Canadian space agency; and the German radar satellite-TerraSAR-X, have been launched and utilized continuously. In 2015, the National Medium- to Long-Term Civilian Space Infrastructure Development Plan (2015-2025) issued by the Chinese government, indicated that seven SAR satellites, including C-band, L-band, and S-band sensors, would be launched during the 13 th Five-Year Plan. Among the first C-band SAR satellites, C-SAR/01, which adopts the working principle of the Gaofen-3 satellite, was launched on 23 November 2021. GaoFen-3 satellite of China is an important SAR satellite, and it has played a significant role in the application fields of marine monitoring, disaster mitigation, environmental protection, water conservancy, agriculture and meteorology, etc. C-SAR/01 is the first successor of GaoFen-3, which is designed to form C-band synthetic aperture radar (SAR) satellite constellation of China's sea and land surveillance and monitoring system. This greatly enriches the SAR satellites data sources of China and presents new opportunities for quantitative applications of radar remote sensing, which is conducive towards satisfying the data requirements in various application fields. Additionally, after the launch of C-SAR/01, there is an urgent need to evaluate its performance and image quality. Radiometric calibration, through which the relationship between perceived pixel values of SAR image data and geophysical information on the earth surface can be accurately established [2], is the prerequisite for the applications of SAR data quantification. Before quantitative application, the SAR system needs to undergo in-orbit radiometric calibration campaigns. During the five-month commissioning phase (CP), multiple radiometric calibrations of TerraSAR-X were performed at The German Aerospace Center (DLR) calibration field in south Germany and showed a relative calibration accuracy better than 0.4 dB and absolute calibration accuracy better than 0.7 dB [3]. After ESA's radiometric refinement and 1.5 years of monitoring and analysis, the relative calibration accuracy of sentinel-1A for the interferometric wide swath (IW) mode, the main mode of Sentinel-1, has reached 0.3 dB [4]. During the Sentinel-1B commissioning phase, DLR performed a SAR system calibration on Sentinel-1B, and it has demonstrated a mean relative calibration accuracy of 0.25 dB and an absolute radiometric accuracy of 0.36 dB for the IW mode. During this period, the radiometric accuracy of the two SAR systems (i.e., S-1A and S-1B) was cross-checked and Sentinel-1A showed an absolute calibration accuracy of 0.38 dB [5]. [PERSON] et al. performed an absolute calibration of Fine resolution stripmap 1 (FRS-1) and Medium resolution ScanSAR (MRS) modes of Radar Imaging SATellite (RISAT-1), which is India's first indigenously developed C-band spaceborne SAR, by deploying triangular trihedral corner reflectors at various research sites in India, and the FRS-1 mode shows a relative radiometric calibration accuracy better than 0.9 dB, and the MRS mode shows a relative radiometric accuracy of 0.53 dB [6]. Radiometric calibration is generally implemented by two methods: point targets and distributed targets. Distributed targets have many features, such as known scattering characteristics, homogeneous stability and large area, and the most commonly used ones include the globally recognized Amazon rainforest and the Canadian boreal forest belt [7].