# SVM-Based Sea Ice Extent Retrieval Using Multisource Scatterometer Measurements
## 4 Sea Ice Extent Mapping and Validation
### Sea Ice Mapping and Validation
#### 4.5.2 Comparison with SAR  
Sentinel-1 SAR images have a high spatial resolution (5 m), and open seawater and ice can be easily distinguished visually. SAR images are usually used for evaluating the edge of open water and closed ice. In this study, four Sentinel-1 SAR images (horizontally polarized at C-band) were used, and the results are shown in Figure 11. To facilitate comparison, the sea ice edge lines from the multisource scatterometer data and the NSIDC SIC 15% sea ice overlapped. In the SAR images, the backscattering amplitude of the sea ice is generally stronger than that of the sea water, and, as a result, the pixels of sea ice are brighter than those of water.  
Figure 11a,b show the distribution of sea ice in the local region in the Arctic on 4 December and 24 December 2021. Figure 11c,d show the local surface types in the Antarctic region on 29 November and 18 December 2021. It should be noted that SAR images give a view of sea water and ice at a single point in time, while our results and NSIDC SIC products are determined on the basis of satellite passes over the course of a single day. From Figure 11a,b, it can be seen that the multisource scatterometer sea ice boundary fits more closely to the SAR image sea ice boundary than the NSIDC SIC 15% sea ice boundary, while Figure 11c,d show that the SAR image sea ice boundary lies between the multisource scatterometer sea ice boundary and the NSIDC SIC 15% sea ice boundary, and the vast majority of the sea ice boundaries in Figure 11 are overlapping. Despite the lower-resolution conditions, it can be seen that the sea ice edges from the multisource scatterometer data are in good agreement with the SAR images.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Region**} & \multirow{2}{*}{**Comparisons**} & \multicolumn{2}{c}{**All**} & \multicolumn{2}{c}{**JFM**} & \multicolumn{2}{c}{**AMJ**} & \multicolumn{2}{c}{**JAS**} & \multicolumn{2}{c}{**OND**} \\ \cline{3-11}  & & **MAD** & **SD** & **MAD** & **SD** & **MAD** & **SD** & **MAD** & **SD** & **MAD** & **SD** \\ \hline \multirow{3}{*}{Arctic \(\times 10^{6}\) km\({}^{2}\)} & Compared to & \multirow{3}{*}{**0.121**} & \multirow{3}{*}{**0.119**} & \multirow{3}{*}{**0.122**} & \multirow{3}{*}{**0.085**} & \multirow{3}{*}{**0.109**} & \multirow{3}{*}{**0.115**} & \multirow{3}{*}{**0.128**} & \multirow{3}{*}{**0.108**} & \multirow{3}{*}{0.126} & \multirow{3}{*}{0.097} \\  & NSIDC SIC 15\% & & & & & & & & & & \\ \cline{1-1}  & Compared to & \multirow{3}{*}{0.149} & \multirow{3}{*}{0.132} & \multirow{3}{*}{0.141} & \multirow{3}{*}{0.129} & \multirow{3}{*}{0.161} & \multirow{3}{*}{0.127} & \multirow{3}{*}{0.194} & \multirow{3}{*}{0.136} & \multirow{3}{*}{**0.109**} & \multirow{3}{*}{**0.088**} \\ \cline{1-1}  & NSIDC SIC 30\% & & & & & & & & & & \\ \hline \multirow{3}{*}{Antarctic \(\times 10^{6}\) km\({}^{2}\)} & Compared to & \multirow{3}{*}{**0.166**} & \multirow{3}{*}{**0.146**} & \multirow{3}{*}{**0.159**} & \multirow{3}{*}{**0.126**} & \multirow{3}{*}{0.153} & \multirow{3}{*}{0.066} & \multirow{3}{*}{0.217} & \multirow{3}{*}{0.096} & \multirow{3}{*}{**0.133**} & \multirow{3}{*}{**0.176**} \\ \cline{1-1}  & NSIDC SIC 30\% & & & & & & & & & & \\ \hline \hline \end{tabular}  
* Notes: MAD is short for mean absolute differences; the units of statistics are \(\times 10^{6}\) km\({}^{2}\); the smallest number in each category is in bold font.  
\end{table}
Table 5: Statistics of the comparisons between multisource scatterometer and NSIDC SIC 15% and 30% sea ice extents for different periods: All refers to all data; JFM is for the months January, February, and March; AMJ is for months April, May, and June; JAS is for months July, August, and September; and OND is for months October, November, and December.