# Insights into Segmentation Methods Applied to Remote Sensing SAR Images for Wet Snow Detection
## 2 Data and Method
### Satellite Data  
Satellite observations used in this study were the C-band backscatter coefficients of the Sentinel-1 missions launched by the European Space Agency (ESA) in the framework of the Copernicus program. The Sentinel-1 mission comprises two satellites, Sentinel-1A and Sentinel-1B, which provide continuous observations over France every six days (every twelve days since the end of 2021, with the loss of Sentinel-1B). Sentinel-1A and Sentinel-1B were launched in 2014 and 2016, respectively. Sentinel-1 SAR images, due to the side-looking imaging geometry of the instruments, are subject to geometry-induced radiometric distortions (shadow, layover and foreshortening effects) and were excluded from our study. Digital elevation models and sensor acquisition parameters are necessary to simulate layover, foreshortening, and shadow regions. More information can be found in the studies of [31; 32]).  
We used Level-1 Ground Range Detected (GRD) products from the PEPS platform (peps.cnes.fr accessed on 1 January 2021). Sentinel-1 data are also available from the ESA web site ([[https://sciubub.copernicus.eu/dhus/](https://sciubub.copernicus.eu/dhus/)]([https://sciubub.copernicus.eu/dhus/](https://sciubub.copernicus.eu/dhus/)) accessed on 1 January 2021). Sentinel-1 data have a spatial resolution of 20 m in both VV and VH polarizations. We used Sentinel-1 relative orbit A161 (ascending, late afternoon) data relevant to our test site. We used the Centre National d'Etudes Spatiales (CNES) computing facilities to preprocess the Sentinel-1 data. The preprocessing included thermal noise removal, speckle filtering using the multi-temporal filtering of [33] (optional), radiometric calibration, and terrain correction using the 30 m SRTM digital elevation model. As a result of this phase, we obtained a stack of images on MGRS tiles of approximately 100 \(\times\) 100 km\({}^{2}\).  
Figure 2 shows a flowchart of this preprocessing. The Sentinel-1 images used in this study were from the relative orbit A161 (ascending orbit) and covered the period from mid-December to the end of June 2018 (an image every six days). We also used the Theia snow products derived from Sentinel-2 data when available over our test zone (in cloud-free situations) [34]. The Sentinel-2 mission, developed by the European Space Agency (ESA), is composed of two satellites (Sentinel-2A and Sentinel-2B) operating in the same orbit (786 km) amd were launched in 2015 and 2017, respectively. The CNES and CESBIO develop these snow retrieval products, which are freely available from the Theia website  
Figure 1: Location of the study area, French alpine massif delimitation and MGRS Tiles.  
([[https://theia.cnes.fr](https://theia.cnes.fr)]([https://theia.cnes.fr](https://theia.cnes.fr)) accessed on 1 January 2021). The Sentinel-2 snow retrieval products used in this study were those of January (05, 15, 25, 30), February (04, 09, 14, 19, 24), March (21, 26, 31), April (05, 15, 20, 25), May (05, 15, 25) and June (09, 14, 19) 2018. The Sentinel-2 total snow products were used as target references to evaluate Sentinel-1-derived wet snow maps.