## 4 Experiment of Geometry Processing Accuracy Evaluation
### Geometric Positioning Results after Calibration  
Firstly, the geometric positioning results of a single scene after calibration are introduced. Take Test image 3 in Table 3 as an example, there are nine corner reflectors distributed within the scene. As shown in Figure 13, without any error correction, the original positioning average error is 18.64 m in the range direction and \(-\)1.63 m in the azimuth direction. After the atmospheric delay correction, the absolute positioning error in the range direction is improved to 13.43 m. Finally, after compensation using range and azimuth calibration results from Table 4, the absolute positioning error in the range direction is 0.39 \(\pm\) 0.78 m, and \(-\)0.63 \(\pm\) 1.31 m in the azimuth direction.  
The above is the single image positioning result. Now the geometric positioning process is carried out for all the test images included in the combinations B1, B3, and B5 of Table 3. The range and azimuth positioning error of the original, after atmospheric delay correction, and after calibration, respectively, are shown in Figure 14. The calibrations are carried out separately using the two sets of calibration results (Incalib 1 and Incalib 2) given in Table 4. In the bottom figure of Figure 14, it should be noted that since the atmospheric delay mainly affects the range deviation, the two curves of the original azimuth error and the azimuth error after atmospheric correction are coincident. From the accuracy verification results in the figure, the following conclusions can be drawn: (1) If high geometric positioning accuracy is required, all error factors must be taken into account in the geometric processing including the echo atmospheric propagation delay, the satellite platform position error, and the SAR channel delay. After all factors are compensated, the geometric positioning errors of all the images can be controlled within 1-2 m, which fully proves the effectiveness of the geometric algorithm presented in this paper. (2) There are random and occasional anomalies in the single-image calibration values, so the final calibration results must be concluded from comprehensive analysis of multiple calibration images. (3) Calibration greatly improves the geometric positioning accuracy from tens of meters to decimeter level, and the calibrated system error has a certain difference among  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**Incalib 1**} & \multicolumn{2}{c}{**Incalib 2**} \\
**Pulse-Width and Bandwidth** & **The Number of** & \multicolumn{2}{c}{**(No Image Removed)**} & \multicolumn{2}{c}{**(Abnormal Images Removed)**} \\ \cline{3-5}
**Bandwidth** & **Images Involved** & **Slant Range** & **Azimuth Time** & **Slant Range** & **Azimuth Time** \\
**Combination** & **in the Calibration** & **Calibration** & **Calibration** & **Calibration** & **Calibration** \\  & & **Result (m)** & **Result (ms)** & **Result (m)** & **Result (ms)** \\ \hline B1 & 3 & 12.046 & 3.333 & 13.035 & 1.126 \\ B3 & 7 & 12.367 & 3.708 & 12.93 & 2.160 \\ B5 & 5 & 9.946 & 2.661 & 10.662 & 1.809 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Calibration results under different pulse-width and bandwidth combinations.  
Figure 13: Positioning results of Test image 3 without any error correction, after atmospheric delay correction and after calibration compensation.  
different pulse-width and bandwidth combinations. (4) After eliminating various errors, there is still a positioning error of about 1m left, which is suspected to be caused by a small random error of the SAR channel delay.