# High-Resolution Spaceborne SAR Geolocation Accuracy Analysis and Error Correction
## 1 Introduction  
Spaceborne synthetic aperture radar (SAR) is an indispensable tool in Earth observation. SAR satellites produce images by actively transmitting and receiving microwave signals and are, therefore, able to work all day and in all weather. For a specific kind of study, when the quality of the high-resolution optical images is unsatisfactory, SAR images are almost the only available data source. When both types of images are available, SAR images are still helpful in providing distinctive radiation information, improving the results [1, 2, 3, 4, 5], which is a common benefit in landcover classification research [6, 7, 8]. As remote sensing technology develops, an increasing number of spaceborne satellites with excellent performance have been launched, providing a wealth of high-resolution data for research and applications. For example, the advanced TerraSAR-X and Tianhui-2 satellites offer stripmap products with a resolution better than 5 m. At this resolution level, the outlines of most ground targets are recognizable, enabling SAR images to be applied in more domains, such as target change monitoring and urban feature extraction [9]. High geolocation accuracy is crucial for utilizing the geometric information of high-resolution SAR images, enabling them to correct the geometry of optical images [10, 11] and complete surveying tasks [12]. When matching the geometric features from high-geolocation-accuracy SAR images and other data, no additional processing besides geolocating is required because images are naturally registered after geolocation. Otherwise, the SAR imaging geometry must be corrected first [13, 14], increasing the complexity of the data processing, especially when control point data are unavailable. In interferometric SAR (InSAR) research, geolocation inaccuracy can be compensated for using coregistration methodslike the amplitude correlation method, but different types of images cannot be properly registered using these methods [15; 16; 17]. The factors affecting SAR geolocation accuracy are solid Earth effects, orbit error, atmospheric error, and ground elevation error [18]. The influence of solid Earth effects is relatively minor, so we only discuss this in Section 4. The orbital accuracy of advanced SAR satellites has exceeded that of early satellites by a large margin, benefitting from improvements in SAR instruments and orbit observation accuracy, thus ensuring a high upper limit for SAR geolocation accuracy. The orbit error of TerraSAR-X is at the centimeter level [18]. For Sentinel-1, Envisat, and COSMO-SkyMed, it is less than 1 m [18; 19]. Therefore, we do not treat orbit error as a primary error source and instead focus on atmospheric error and ground elevation error. The findings in the Global Navigation Satellite System (GNSS) domain help correct atmospheric error, including tropospheric delay and ionospheric delay. For SAR signals, the properties of tropospheric delay are nearly the same as those of GNSS signals, so tropospheric correction algorithms and products can be applied to SAR images directly, including the ray-tracing (RT) method [20] and zenith path delay (ZPD) data [21]. Correcting ionospheric delay also benefits from GNSS products, mainly depending on the vertical total electron content (VTEC) data. To achieve the best possible correction effect, [PERSON] et al. proposed a relatively comprehensive model based on VTEC data [22]. Given that the ground elevation error is more closely related to the quality of the elevation data than the SAR data, researchers have conducted fewer studies on it compared with other errors. Ground elevation error is expected to introduce a geolocation error proportional to itself [23; 24], but no experiments have been conducted to verify this relationship, and no typical values are available for reference. Although the theory of SAR geolocation error correction was well developed many years ago, auxiliary data for correction have recently improved relatively quickly.