# Mapping Radar Glacier Zones and Dry Snow Line in the Antarctic Peninsula Using Sentinel-1 Images
## 2 Study Area and Data Sets
### Data Sets
#### 2.2.2 Sentinel-1 Images  
With the purpose of imaging all global landmasses, oceans, coastal zones, and shipping routes at high resolution, Sentinel-1A and Sentinel-1B were launched in 2014 and 2016, providing timely and easily accessible information in a six-day exact repeat [43]. The two polar-orbiting satellites operating in C-band (5.4 GHz) are able to acquire imagery regardless of the weather. The Sentinel-1 SAR instrument operates in four exclusive imaging modes, namely, the Wave, Strip Map, Extra Wide (EW) Swath, and Interferometric Wide Swath modes, with different resolutions and coverage. Sentinel-1 has single polarization (VV or HH) for the Wave mode, and dual polarization (VV + VH or HH + HV) for other modes [44]. This study used the HH polarized Ground Range Detected (GRD) imagery product in EW mode to derive RGZs and DSL in the AP (see the image frames in Figure 1b). The GRD products were multilooked and projected to ground range using an Earth ellipsoid model. Detailed information of the 22 images obtained from the European Space Agency (ESA) is listed in Table 1.  
These horizontally polarized images observed the AP at a spatial resolution of 40 m. The radiometric calibration, terrain correction, Lee filter with a window size of 7 \(\times\) 7, and the mosaicking of the images were conducted with the ESA's Sentinel Application Platform. The incidence angles of the EW mode images range from 18\({}^{\circ}\) to 46\({}^{\circ}\), resulting in the reduction of backscatter energy from near to far range. The wide range of incidence angles affects the detection and classification of RGZ. A cosine-based normalization method was applied to limit the \(\sigma^{0}\) variations [45; 46]:  
\[\sigma^{0}_{ref}=\frac{\sigma^{0}_{\theta}\,cos^{2}\Big{(}\theta_{ref}\Big{)}} {cos^{2}(\theta)} \tag{1}\]  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{3}{c}{**Images for the Winter Mosaic of the AP**} & \multicolumn{3}{c}{**Images for the RGZs Mapping**} \\ \hline
**Relative Orbit Number** & **Slice Number** & **Date** & **Relative Orbit Number** & **Slice Number** & **Date** \\ \hline
48 & 3 & 1 May 2015 & 48 & 4 & 21 October 2014 \\
48 & 4 & 1 May 2015 & 48 & 4 & 2 November 2014 \\
48 & 5 & 1 May 2015 & 48 & 4 & 8 December 2014 \\
62 & 5 & 1 May 2015 & 48 & 4 & 20 December 2014 \\
106 & 4 & 5 May 2015 & 48 & 4 & 1 January 2015 \\
106 & 5 & 5 May 2015 & 48 & 4 & 13 January 2015 \\
111 & 1 & 5 May 2015 & 48 & 4 & 25 January 2015 \\
18 & 5 & 10 May 2015 & 48 & 4 & 18 February 2015 \\
34 & 3 & 12 May 2015 & 48 & 4 & 2 March 2015 \\
92 & 3 & 16 May 2015 & & & \\
96 & 0 & 16 May 2015 & & & \\
19 & 4 & 4 June 2015 & & & \\
165 & 1 & 20 July 2015 & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sentinel-1 images used in this study. RGZ: radar glacier zone.  
where \(\sigma^{0}{}_{n\!f}\) means the normalized backscattering coefficient; \(\theta\) is the local incidence angle; \(\sigma^{0}{}_{\theta}\) is the measured backscatter; and \(\theta_{n\!f}\) is the reference angle, which was set as \(30^{\circ}\) in this study.