# Advancements for Snowmelt Monitoring by Means of Sentinel-1 SAR
## 6 Conclusions  
The Sentinel satellite missions are dedicated to long-term routine observations for operational services in a wide range of applications. The data are also of great benefit to the Earth science community. Each Sentinel mission is based on a constellation of two identical satellites. By now, the first satellites of the Sentinel-1, -2, and -3 missions are in orbit. In this paper, we report on the features and performance of an algorithm for generating maps of snowmelt area based on radar images of the Sentinel-1 mission. The nominal operation mode of the S1 SAR sensor for land areas is the Interferometric Wide swath (IW) mode, which offers high radiometric accuracy and high spatial resolution and covers a swath of 250 km width. These are important attributes for snow retrievals that are based on backscatter signals. Of benefit for snowmelt retrievals, in particular in regions with steep topography, is also the acquisition of cross-polarized data, helping to compensate for the reduced sensitivity of co-polarized data for detection of wet snow on fore-slopes. The comparison with snow maps derived from high resolution optical sensors confirms the high quality of the S1 snow cover maps. In particular, during the snowmelt period, when the snow extent is subject to rapid changes, the high temporal revisit of the S1 mission is of great value for hydrology, water management, flood forecasting and support of land surface process modeling. The revisit time of a single S1 satellite operating in IW mode is about four days in mid-latitudes and about two days in high latitudes. After deployment of the Sentinel-1B satellite, scheduled for launch on 22 April 2016, the revisit time will improve by a factor of two. Whereas a main advantage of C-band SAR for snow mapping is the feasibility to penetrate clouds, the sensitivity to dry snow cover is rather limited. Therefore, the synergy with optical sensors is needed for a comprehensive observing system on snow extent. The Sentinel-2 and Sentinel-3 missions of the Copernicus program offer excellent capabilities for this task. Sentinel-2 is a polar-orbiting, multispectral imaging mission with 13 spectral bands (from 443 nm to 2190 nm wavelength) and spatial resolution of 10 m, 20 m and 60 m in the various channels, covering a swath of 290 km width [40]. With two satellites, the coverage by repeat orbits will be five days, disregarding cloud cover. The Sentinel-3 mission is equipped with a dual frequency synthetic aperture radar altimeter and two medium-resolution optical sensors: an imaging spectrometer, the Ocean Land Color Instrument (OLCI), and a dual-view multispectral radiometer, the Sea Land Surface Temperature Radiometer (SLSTR) [41]. After full deployment, the Sentinel-3 mission offers daily complete coverage in mid and high latitudes. Whereas the optical observation sequences for dynamic phenomena, such as seasonal snow, are strongly impaired by cloud cover, Sentinel-1 provides regular revisits at high temporal sequence, well-matched for capturing rapid changes due to snowmelt. The investigations reported here highlight the great potential of S1 for operational monitoring of snowmelt areas and for supporting multi-sensor snow cover mapping. **Acknowledgments:** The work was supported by the Austrian Research Promotion Agency (FFG) Austrian Space Application Programme (Contract Nr. 847998), and the EU FP7 Project SEN3 APP (Grant Agreement 607052). Sentinel-1 data were made available through ESA Sentinel Scientific Data Hub. Landsat data were obtained through GLOVIS USGS. **Author Contributions:** [PERSON] and [PERSON] performed the scientific analysis and prepared the manuscript. [PERSON] developed SAR snow mapping tools and carried out SAR data processing. [PERSON] carried out the processing and analysis of optical snow maps and SAR data. [PERSON] contributed to processing and interpretation of optical snow maps. [PERSON] contributed to the development of SAR processing tools. All authors contributed to editing the manuscript.