# SVM-Based Sea Ice Extent Retrieval Using Multisource Scatterometer Measurements
## 4 Sea Ice Extent Mapping and Validation
### Sea Ice Mapping and Validation
#### 4.5.1 Comparison with NSIDC SIC  
In this study, sea ice extent data retrieved using our SVM-based algorithm are compared with the NSIDC SIC operational products. As described in Section 2, we are working on the same polar projection and spatial resolution as the NSIDC SIC products. The daily polar sea ice extents from the SVM model and the NSIDC CDR ice extents (with SIC thresholds of 15% and 30%) from January 2019 to December 2022 are shown in Figures 9 and 10, respectively. The seasonal variations in ice extent can be clearly observed in the multisource scatterometer data, which show synchronous changes with NSIDC CDR ice extent. Furthermore, we calculated the mean absolute differences (MAD) and standard deviation (SD) of the sea ice extent for this comparison, and some quantitative details are given in Table 5. In Table 5, JFM represents January, February, and March; AMJ represents April, May, and June; JAS represents July, August, and September; OND represents October, November, and December. Sea ice extents from the multisource scatterometer data are closer to the NSIDC SIC 30% sea ice extent during OND in the Arctic, and closer to the NSIDC SIC 15% ice extent during the other phases. In the Antarctic, the sea ice extents determined on the basis of the multisource scatterometer data are closer to the NSIDC SIC 30% sea ice extent during the AMJ and JAS periods, while the MAD and SD results during the JFM and OND periods show good agreement with the NSIDC SIC 15% sea ice extent. We find that in the Antarctic or Arctic, the sea ice extent obtained by the multisource scatterometer is overestimate the NSIDC SIC 15% and 30% sea ice extent from the late  
Figure 8: Example daily polar sea ice extent obtained on the basis of multisource scatterometer data. Gray pixels represent land; white pixels represent water; blue pixels represent sea ice; and the black hole in the Arctic is due to there being no coverage by scatterometers.  
stages of sea-ice ablation to the beginning of the sea-ice growth phase. Our results may detect more low-concentration sea ice.  
Figure 10: The validation results for the Antarctic from 2019 to 2021: (**a**) daily sea ice area for NSIDC SIC 15% ice extent (blue line), NSIDC SIC 30% ice extent (red line), and multisource scatterometer (yellow line), respectively, and (**b**) sea ice area difference between multisource scatterometer and NSIDC SIC 15% (blue line), and between multisource scatterometer and NSIDC SIC 30% (red line).  
Figure 9: The validation results of the Arctic from 2019 to 2021; (**a**) daily sea ice area for NSIDC SIC 15% ice extent (blue line), NSIDC SIC 30% ice extent (red line), and multisource scatterometer (yellow line), respectively, and (**b**) sea ice area difference between multisource scatterometer and NSIDC SIC 15% (blue line), and between multisource scatterometer and NSIDC SIC 30% (red line).  
As listed in Table 5, our results are closer to the NSIDC SIC data with a threshold of 15% in most seasons. Overall, the differences in sea ice extent between our results and the NSIDC SIC 15% are smaller in both the Arctic and Antarctic. The MAD and SD are 0.121 and 0.119 million km\({}^{2}\), respectively, in the Arctic, while these are 0.166 and 0.146 million km\({}^{2}\), respectively, in the Antarctic.