## 6 Discussion  
After making a quick assessment of the image quality and radiometric calibration accuracy of the UFS and FSI mode of C-SAR/01 data, we have got the above experimental results and conducted analysis on them.  
According to experimental results in Table 5, the SCRs of each CR on the UFS and FSI mode of C-SAR/01 data are all greater than 32 dB. The result indicates that the background environment of the site is relatively homogeneous and the backscattering intensity is weak, which is suitable for the calibration and validation of SAR satellites. According to the results of calibration constants of the UFS and FSI mode in Tables 6 and 7, the standard deviations of the calibration constants calculated by the integral method are always better than that calculated by peak methods. Furthermore, in Figure 11a,b, it can be seen that the calibration constants of each point target calculated by the integral method (green line) fluctuate less than those calculated by the peak method (blue line). Additionally, looking at the relative calibration accuracy and absolute calibration accuracy shown in Tables 8 and 9, it can be seen that the results from the integral method are always better than that from peak methods. Thus, the stability of the integral method is better than that of the peak method, and the former method got higher calibration accuracy in high-resolution SAR images.  
Compared to the results in Tables 8 and 9 with the design indexes of C-SAR/01 relative calibration accuracy and absolute calibration accuracy, which are 1.0 dB and 1.5 dB [24], respectively, it can be seen that relative calibration accuracy and absolute calibration accuracy of both UFS and FSI mode, which are better than 0.343 dB and 0.61 dB, respectively, are better than the design indexes. In addition, in terms of radiometric applications, the relative calibration accuracy and absolute calibration accuracy of C-SAR/01 in the UFS and FSI modes are satisfied requirements of the geophysical parameter measurement for radiometric calibration accuracy, which are 0.5 dB for the relative and 1 dB for the absolute calibration, in different applications, including ice classification, ice motion, wind speed over ocean, soil motion, surface roughness, vegetation mapping/monitoring, etc. [2]. At the same time, the calibration accuracy of C-SAR/01 also meets the main application requirements of Gaofen-3 satellite in the ocean, disaster reduction, water conservancy, meteorology, etc. [24]. Therefore, C-SAR/01 data will play an important role in various fields after radiometric calibration in the future.  
However, the above experimental results of the UFS and FSI mode of C-SAR/01 are different; they include the spatial resolution, PSLR, ISLR, SCR, calibration constant, and relative and absolute calibration accuracy, etc. It may be caused by a different imaging mode, different elevation orbit and left-right view, different imaging time, etc., which requires more in-depth analysis.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**CR Number**} & \multirow{2}{*}{
\begin{tabular}{c} **Theoretical** \\ **RCS/dBsm** \\ \end{tabular} } & \multicolumn{2}{c}{**Integral Method**} & \multicolumn{2}{c}{**Peak Method**} \\ \cline{3-5}  & & **Measurements** & & **Measurements** & \\  & & **/dBsm** & & **/dBsm** & **Difference /dBsm** \\ \hline CR-1 & 31.332 & 31.45 & 0.118 & 31.56 & 0.228 \\ CR-2 & 31.332 & 31.171 & \(-\)0.161 & 31.197 & \(-\)0.135 \\ CR-3 & 31.332 & 30.999 & \(-\)0.333 & 30.866 & \(-\)0.466 \\ CR-4 & 31.332 & 31.297 & \(-\)0.035 & 30.949 & \(-\)0.383 \\ CR-5 & 31.332 & 31.633 & 0.301 & 31.75 & 0.418 \\ CR-6 & 31.332 & 31.249 & \(-\)0.083 & 31.43 & 0.098 \\ CR-7 & 31.332 & 31.495 & 0.163 & 31.5 & 0.168 \\ Relative calibration accuracy & & 0.199 & & 0.304 \\ Absolute calibration accuracy & & 0.333 & & 0.466 & \\ \hline \hline \end{tabular}
\end{table}
Table 9: Each point target RCS and accuracy analysis for FSI mode data on 15 May.