## 4 Experiment of Geometry Processing Accuracy Evaluation
### Influence of Different Precision Elevation Data on Positioning Accuracy  
The elevations are extracted from three data sources including the field measurement, the 1 km grid Global DEM, and the ASTER DEM [29]. Compared with the high-precision measurement elevation, the elevation errors of the other two DEM sources at the deployed nine corner reflectors are shown in Table 5. It can be seen that the results are consistent with the nominal accuracy of these DEM sources.  
Figure 14: Positioning results of the test images of three groups B1, B3, and B5 (Top: range direction; Bottom: azimuth direction).  
We choose three images of different side-looking angles (from 19.54\({}^{\circ}\) to 51.72\({}^{\circ}\)) from Table 3 to verify their positioning accuracy under different DEM data sources. As shown in Table 6, the DEM error mainly affects the positioning accuracy in the range direction, and with the improvement in the DEM accuracy, the better the geometric positioning result will be. Therefore, obtaining high-precision DEM data and performing orthorectification on the entire SAR image is one of the keys to improving its overall positioning accuracy.  
We re-sort the test images in Table 3 according to the side-looking angle from large to small, and the relationship between the range positioning error under the field measurement elevation input and the side-looking angle is shown in Figure 15. As the side-looking angle decreases, the positioning accuracy is lower. This is because the smaller the side view angle, the more serious the image deformation caused by the same elevation error, which is consistent with the change trend in Figure 15.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**ID**} & **Side** & \multirow{2}{*}{**Elevation Source**} & \multicolumn{3}{c}{**Positioning Error**} \\ \cline{3-6}  & & & **Range** & **Azimuth** & **Plane** \\ \hline \multirow{3}{*}{Test Image 14} & Left view & Field measurement & 1.721110 & 0.218848 & 1.734968034 \\  & 19.54 degree & ASTER DEM & 11.113978 & 0.594016 & 11.12984106 \\  & & Global DEM & 22.464659 & 1.344352 & 22.50484806 \\  & & Measured & & & \\ Test Image 9 & Left view & elevation & 1.118257 & \(-\)2.620450 & 2.849080013 \\  & 34.99 degree & ASTER DEM & 8.961166 & \(-\)2.861411 & 9.406921334 \\  & & Global DEM & 18.429528 & \(-\)3.102371 & 18.68882576 \\  & & Measured & & & \\ Test Image 3 & Left view & elevation & 0.396214 & \(-\)1.631593 & 1.679011987 \\  & 51.72 degree & ASTER DEM & 5.514775 & \(-\)1.750255 & 5.785856537 \\  & & Global DEM & 11.693389 & \(-\)1.898581 & 11.84651662 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Positioning accuracy of different side-looking angles under different precision DEM.  
Figure 15: The relationship between side-looking angle and range positioning accuracy under field measurement elevation input.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Corner Reflector ID** & **Field Measurement (m)** & **ASTER DEM (m)** & **Global DEM (m)** \\ \hline
1 & 1087.467 & 1098 & 1094 \\
2 & 1068.961 & 1087 & 1101 \\
3 & 1091.221 & 1108 & 1122 \\
4 & 1090.578 & 1096 & 1090 \\
5 & 1089.080 & 1097 & 1107 \\
6 & 1091.104 & 1104 & 1121 \\
7 & 1092.414 & 1095 & 1121 \\
8 & 1092.992 & 1099 & 1123 \\
9 & 1093.143 & 1104 & 1119 \\  & Medium error & \(\pm\)11.240 & \(\pm\)24.984 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Elevation errors of different elevation sources.