# SVM-Based Sea Ice Extent Retrieval Using Multisource Scatterometer Measurements
## 3 Characteristic Parameters for Sea Ice Monitoring  
In this study, we aim to explore the merits of combing C-band and Ku-band scatterometer measurements in the application of sea water and ice classification. However, the HSCAT, CSCAT, and ASCAT scatterometers operate at different viewing geometries, in terms of incidence angle and antenna looking direction. Thus, we could not extract the same characteristic parameters from every scatterometer. Nevertheless, we propose using seven characteristic parameters, including horizontal and vertical polarization backscatter measurements (\(\sigma^{0}_{HH}\) and \(\sigma^{0}_{VV}\)), standard deviations of horizontal and vertical polarization measurements (\(\Delta\sigma^{0}_{HH}\) and \(\Delta\sigma^{0}_{VV}\)), polarization ratio (\(PR\)), the dependence of \(\sigma^{0}\) on incidence angle (\(K\), as defined in Equation (1)), and the ratio of backscatter measurements between C-band and Ku-band (named band ratio (\(BR\))). The availability of these parameters from the scatterometers is listed in Table 2. Figure 2 shows the normalized distribution of sea ice and open seawater in the Antarctic region for each parameter. These distributions are counted from December 2019 to February 2020 based on multisource scatterometer measurements. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Scatterometers**} & \multicolumn{8}{c}{**Characteristic Parameters**} \\ \cline{2-9}  & \(\sigma^{0}_{HH}\) & \(\sigma^{0}_{VV}\) & \(\Delta\sigma^{0}_{HH}\) & \(\Delta\sigma^{0}_{VV}\) & \(PR\) & \(K_{VV}\) & \(BR\) \\ \hline HSCAT & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & — & — \\ \hline CSCAT & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \\ \hline ASCAT & — & \(\surd\) & — & \(\surd\) & — & \(\surd\) & \(\surd\) \\ \hline \hline \end{tabular} Note: \(PR\) is the polarization ratio of VV and HH; \(BR\) is the band ratio of C-band and Ku-band; \(K_{VV}\) is the slope defined in Equation (1) for VV polarization.  
\end{table} 
Table 2: Information on the characteristic parameters of multisource scatterometer. Figure 1: A number of ASCAT measurements in (**a**) Antarctic and (**b**) Arctic areas on 4 January 2020. The coastlines are overlaid in black, while the color bar represents the count of measurements in each pixel. The image sizes of the Arctic and Antarctic ice maps are 448 \(\times\) 304 and 332 \(\times\) 316, respectively. Figure 2: _Cont_. Comparing the horizontal and vertical polarized backscatter measurements of sea ice and sea water, different scattering mechanisms play a role [3; 7]. Over sea water, backscatter coefficients are primarily dependent on wind speed. The backscatter coefficients are usually quite different between sea water under low wind speed (<5 m/s) conditions and sea ice. For a given measuring geometry (e.g., HSCAT), the backscatter coefficients of sea ice are usually stronger than those of water; see the yellow and purple curves shown in Figure 2a. In contrast to other active radar systems (e.g., SAR), scatterometers can measure the same ground area at multiple azimuth angles; this was originally required by the sea surface wind retrieval mission. It has been demonstrated that the azimuthal modulation is much smaller over polar sea ice than over open seawater [31; 32]. The daily standard deviations of horizontal and vertical polarization backscatter (\(\Delta\sigma^{0}_{HH}\) and \(\Delta\sigma^{0}_{VV}\)) can represent the azimuthal and temporal dependences of surface backscatter, as shown in Figure 2c.d. In previous studies, polarization ratios obtained on the basis of dual-polarization radar measurements from the SEASAT scatterometer, the QuikSCAT/SeaWinds scatterometer, and the HY-2A scatterometer have been used for the identification of sea ice and open seawater [3; 11; 12; 13; 14]. In this study, the \(PR\) is defined as the ratio of \(\sigma^{0}_{VV}\) and \(\sigma^{0}_{HH}\), i.e., \(PR=\sigma^{0}_{VV}/\sigma^{0}_{HH}\). \(PR\) is sensitive to surface-scattering mechanisms.