## 1 Introduction  
The geometric positioning accuracy of spaceborne synthetic aperture radars (SAR) is one of the key factors that determine whether the benefits of SAR image applications can be fully utilized. How to ensure the high geometric positioning accuracy of SAR is an important issue that needs to be solved urgently. Studying the geometric calibration of on-orbit SAR satellites, and applying the calibration parameter compensation to the image geometric positioning processing, is of great significance for the improvement of SAR geometric positioning accuracy. The ERS-1 launched by ESA in 1991 is the world's first SAR satellite to achieve high-precision geometric calibration [1], which used the calibration field to calibrate the key system parameters of the uncontrolled geometric positioning, and the image plane positioning accuracy finally reached 13.18 m. From then on, some countries have actively planned and developed various technologically advanced spaceborne SAR. In particular, around 2007, Japan, Italy, Germany, and Canada successfully launched a number of SAR satellites, providing a wealth of high-quality images for relevant research units. The acquisition of high-quality images is inseparable from the support of geometric calibration and processing. The Japan Aerospace Exploration Agency carried out the geometric calibration for PALSAR on the ALOS-1 satellite, and its positioning accuracy of the strip mode reached 9.7 m under uncontrolled conditions [2]. Italy carried out the geometric calibration on the COSMO-SkyMed images based on five calibration fields, and the uncontrolled positioning accuracy of the strip images was up to 3 m, and for the spotlight images this was up to 1 m [3]. Germany used 30 point targets placed in the 120 km \(\times\) 40 km area to geometrically calibrate the TerraSAR-X satellites, and reached the positioning accuracy of 0.5 m in azimuth and 0.3 m in range [4; 5; 6]. The plane positioning accuracy of the Canada Radarsat-2 satellite is about 10 m [7; 8]. After that, the geometric calibration of TanDEM-X launched by Germany, and the Sentinel-1A and Sentinel-1B launched by ESA were all undertaken by the TerraSAR-X satellite calibration team, and they continued to use the related geometric technology of TerraSAR-X and achieved good results [9; 10; 11]. In 2011, China carried out a geometric calibration study on the CRS1 SAR satellite and the positioning accuracy was better than two pixels after calibration [12]. Since 2017, many Chinese scholars have carried out geometric calibration on GF-3, and the final positioning accuracy is better than 3 m [13; 14; 15; 16; 17]. However, due to the staged development of SAR payloads and satellite platforms, the previous geometric calibration and positioning research is mainly applicable to traditional side-looking or small azimuth squint imaging modes. In recent years, due to its superior detection flexibility, revisit capability, and high resolution, the high-resolution, large squint sliding-spot imaging mode has shown great application potential in military and civilian applications, and has become one of the hot spots in platform development and imaging algorithm research. However, current researches are mainly focused on high-precision squint imaging algorithms [18; 19], and there are very few researches related to high-precision geometric processing under large squint sliding-spot conditions. As well as the long synthetic aperture time of the high-resolution sliding-spot mode itself, the large squint imaging condition further aggravates the complexity and spatial variability of the target range history, and increases the design difficulty of the imaging algorithm.