# Advancements for Snowmelt Monitoring by Means of Sentinel-1 SAR
## 1 Introduction  
The Sentinel satellite constellation series represents the dedicated space component of the European Earth observation program Copernicus, previously known as GMES (Global Monitoring for Environment and Security) [1]. Copernicus, coordinated and managed by the European Commission, provides global, timely and easily accessible information and services in application domains such as land, marine, atmosphere, emergency management, climate change and security [2]. In the final deployment stage, each of the six Sentinel missions that were developed and operated by the European Space Agency (ESA), comprises two satellites in order to fulfill the revisit and coverage requirements for a wide range of operational applications.  
Sentinel-1A, the first dedicated satellite of the Copernicus program, was launched on 3 April 2014. The Sentinel-1 (S1) satellites are equipped with a C-band synthetic aperture radar (SAR) [3]. The Sentinel-1 mission objectives include land monitoring of forests, water, soil and agriculture, marine monitoring (sea state, sea ice, icebergs, oils spills, vessel detection), monitoring of ice sheets,glaciers, snow cover, lake and river ice, emergency response applications (floods, geohazards, terrain deformation), and climate change monitoring [3; 4]. In order to meet the observation requirements for this breadth of applications, the Sentinel-1 SAR instrument supports four operational imaging modes providing different spatial resolution and coverage: the Interferometric Wide swath (IW) mode, the Extra Wide swath (EW) mode, the Strip-Map (SM) mode, and the Wave (WV) mode [3; 4]. In the field of cryosphere applications, the capabilities of the Sentinel-1 SAR EW data for operational sea-ice and iceberg monitoring have already been demonstrated [5], and the feasibility and performance of ice sheet wide surface velocity mapping with IW mode data has been shown for Greenland [6].  
In the cryosphere domain, C-band SAR is also a main tool for mapping and monitoring the extent of wet snow areas. We developed, tested and validated an algorithm for retrieving snowmelt areas from S1 IW mode data, which is the nominal operation mode for land areas [7]. Monitoring of the snowmelt area has been identified as an important S1 application over land areas [4; 8]. Previously, C-band SAR data of the European satellites ERS-1, ERS-2 and Envisat, and of the Canadian RADARSAT missions have been applied for mapping and monitoring snowmelt area for scientific studies and for hydrological applications at regional scale, e.g., [9; 10; 11; 12; 13; 14; 15]. The Sentinel-1 mission, with its systematic and long-term acquisition strategy and high duty cycle of the SAR sensor, enables high resolution operational snow cover monitoring over large areas at short revisit intervals. Here, we describe the method and workflow for generating maps of snowmelt area from S1 SAR data and present examples for the European Alps and Iceland. This work paves the way for an operational snow cover monitoring service within the Copernicus program, a precursor of which has been implemented within the project CryoLand of the 7 th Framework Programme for Research and Technological Development of the European Union [16].