# High-Resolution Spaceborne SAR Geolocation Accuracy Analysis and Error Correction
## 5 Conclusions  
SAR satellites can acquire data all day and in all weather conditions to periodically provide images for research areas. The radiometric features of SAR images make them highly complementary to optical images and other data. With improvements in SAR image resolution, progressively finer geometric features make SAR applicable to an increasing number of domains. High-accuracy geolocation is crucial for high-resolution SAR images to ensure correct feature matching. This study analyzed the properties of SAR geolocation errors and examined correction methods, proposing an effective procedure for SAR geolocation. We employed a TerraSAR-X image and a pair of Tianhui-2 images to validate the geolocation procedure, with the results demonstrating the following:  
1. The theoretical SAR geolocation accuracy meets the requirement for common SAR applications. The TerraSAR-X images in this research achieved a geolocation accuracy better than 1 m based on the GNSS elevations. Affected by the orbital error and other factors, the geolocation accuracy values of the Tianhui-2 reference and secondary images are approximately 2 m and 4 m, respectively. Considering that most advanced SAR satellites have orbit data that are more accurate than 1 m, the TerraSAR-X result is of greater reference value.
2. The accuracy of the elevation data primarily constrains the SAR geolocation accuracy, and using high-accuracy elevation data is critical to enhancing geolocation accuracy. When geolocating the TerraSAR-X image using the SRTM, the accuracy was better than 5 m for most measurement points. Upon switching to the higher-accuracy WorldDEM, the geolocation error for most measurement points fell below 3 m.
3. The GACOS mapping method is the most optimal tropospheric correction method, because the ZDM method provides accuracy comparable to that of the RT method. At the same time, the GACOS has advantages in resolution and ease of use.  
The Tianhui-2 secondary satellite receives signals transmitted by the reference satellite. The secondary-image geolocation results validate the effectiveness of the secondary-image geolocation model proposed in this research. Possibly because of the inaccuracy of the time synchronization, the geolocation accuracy of the secondary image was inferior to that of the reference image.  
The difficulty in obtaining high-accuracy DEMs is the principal bottleneck in high-accuracy SAR geolocation. In the future, it might be possible to eliminate dependence on DEMs through 3D geolocation methods and to enhance the applicability of SAR geolocation.  
[PERSON] wrote the paper and conducted the experiments; [PERSON] guided the experiments, checked the paper, and provided suggestions. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
No new data were created or analyzed in this study. Data sharing is not applicable to this article.  
The GNSS measurement points used in this study were collected by a team led by [PERSON] at the Institute of Remote Sensing and GIS, Peking University. The measurement team also included [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON]. This survey was implemented from 23 to 25 October 2020. The instrument used was a Hi-TARGET iRTK2 GNSS receiver, receiving the differential data through the FindCM service provided by Qianxun Positioning Network Co., Ltd. This research is supported by the High-Performance Computing Platform of Peking University.  
The authors declare no conflicts of interest.