# High-Resolution Spaceborne SAR Geolocation Accuracy Analysis and Error Correction
## 4 Discussion  
### Generalizability of the Results  
Limited by the availability of data, we only conducted our experiment in one area. The properties of the tropospheric error and the ground elevation error may be different in other places. However, we can predict the performance of our procedure based on relevant results from the GACOS and SRTM.  
[PERSON] et al. calculated the ZPD in eight areas with different topographic variations using the GACOS [26]. They considered the ZPD retrieved using the InSAR method as the benchmark, obtaining a root mean square (RMS) of around 1 cm and a maximum residual of around 6 cm, consistent with our results after transforming the ZPD into tropospheric delay (Figure 12b). Therefore, we believe that the quality of the GACOS is stable in most parts of the world. The remaining issue is the stability of the ZDM method. The maximum elevation in our study area is about 700 m (Figure 11a), while the distribution of N steadies when the elevation is larger than 600 m (Figure 11d). The difference between the ZDM method and the RT method lies in the fluctuation of the radio refractivity, N. Assuming that N converges when the elevation is greater than the maximum ground elevation, the fluctuation of N remains five when the elevation is less than the maximum, the same as in our research. When the maximum ground elevation is 3000 m, a tropospheric correction residual of only 1.5 cm is introduced by the ZDM method. Therefore, we believe that the GACOS-based ZDM method applies in most areas. The residual may be larger when the tropospheric condition is more complex. Further experiments are needed.  
The degree of terrain flatness significantly affects the quality of DEMs and eventually affects the geolocation accuracy. Since the related geolocation error is proportional to the DEM error, we can predict the performances of the WorldDEM and SRTM in other areas by referring to studies on DEM quality. [PERSON] et al. studied two DEMs in four areas with different slopes [29]. The terrain of their study area in Inner Mongolia, China, is similar to our study area, where 90% of the WorldDEM errors are less than 2 m, and 90% of the SRTM errors are less than 4 m. They inspected the DEM accuracy in a flatter study area in Xinjiang, China, and a more complex area in Sichuan, China. For the WorldDEM and SRTM, the errors were 0.8 m and 5 m in Xinjiang and 12 m and 16 m in Sichuan. The performance of the WorldDEM is always better than the SRTM, but the advantage may not be significant in areas with complex terrain. There are no effective algorithms to achieve high-accuracy geolocation without high-quality elevation data.  
Theoretically, our geolocation procedure applies to other satellite images, especially high-resolution images, such as those from Sentinel-1 and ALOS-2. However, it is hard to recognize our measurement points in Sentinel-1 images, so we did not conduct experiments on them. When processing C-band and L-band images, a more highly accurate ionospheric model may be needed [22]. For medium- and low-resolution satellite images, the focus of geolocation processing differs from that of our research. In those cases, geolocation errors related to atmospheric delays and ground elevation errors are usually less than 10 m, while orbit errors can be greater than 10 m [23]. Therefore, the effectiveness of our procedure will be insignificant without orbit correction.