## 1 Introduction  
The Sentinel satellite constellation series, developed by the European Space Agency (ESA), represents the dedicated space component of the European Copernicus program (formerly called Global Monitoring for Environmental Security, GMES) [1]. Copernicus, led by the European Union, is an ambitious operational Earth Observation program providing global, timely and easily accessible information in application domains such as land, marine, atmosphere, emergency response, climate change, and security. The Copernicus program comprises three components, namely the space component, the _in-situ_ component, and the service component. Though the main mandate of the Copernicus space component is the support of operational services, the improved observational capabilities and the long-term operational commitment of the Sentinel missions will also provide the science community with unique and comprehensive observations on the various components of the Earth System [2, 3].  
Here, we address the potential of the Sentinel-1 mission for mapping and monitoring the surface velocity of glaciers and ice sheets, presenting the first ice sheet wide velocity map for Greenland derived from Sentinel-1 synthetic aperture radar (SAR) data acquired between January and March 2015. The Sentinel-1 mission is based on a constellation of two satellites equipped with a C-band SAR sensor. Sentinel-1A, launched on 3 April 2014, is the first dedicated satellite of the Copernicus program in orbit [4]. The launch of the second Sentinel-1 satellite, Sentinel-1B, is scheduled for early 2016. The repeat cycle of a single satellite is 12 days, dropping to 6-day repeat orbit for the two-satellite constellation. The Sentinel-1 SAR instrument, with its active phased array antenna, supports four different operational imaging modes that are providing different resolution and coverage: Interferometric Wide swath (IW) mode, Extra Wide swath (EW) mode, Strip-Map (SM) mode, and Wave (WV) mode [5, 6]. The mode is either optimized for spatial resolution or for swath width. The Greenland ice velocity map is based on data of the IW mode, which is the nominal mode for land areas, including ice sheets [4]. In order to check the performance of the velocity maps derived from IW data, we compare the Sentinel-1 products with velocity maps derived from very high-resolution SAR data of the TerraSAR-X mission acquired over several outlet glaciers.  
Satellite observations and _in situ_ measurements reveal a high temporal variability of ice flow for outlet glaciers of ice sheets over time scales ranging from days to years [7, 8, 9, 10]. Therefore comprehensive, detailed and long-term observations on the temporal and spatial patterns of ice flow are needed to better understand and quantify the processes that are governing the response of ice sheets to climate forcing and to improve estimates on future ice sheet contributions to sea level rise. [PERSON] _et al._[11] used SAR data to produce velocity maps of the main outlet glaciers of the Greenland Ice Sheet for winter 2000 to 2001 and for each winter from 2005 to 2006 through 2010 to 2011, revealing complex spatial and temporal velocity patterns. For the 3 rd International Polar Year (IPY) 2007-2009 ice sheet wide maps on surface velocity were generated for Antarctica [12] and Greenland [13]. These maps were assembled from SAR data of several satellite missions: for the Antarctic Ice Sheet from data of PALSAR on the Japanese Advanced Land Observing Satellite (ALOS), of the Canadian RADARSAT-1 and RADARSAT-2 missions, and of the ESA satellites ERS-1, ERS-2, and Envisat [12], for Greenland from data of the Advanced Synthetic-Aperture Radar (ASAR) on Envisat, of ALOS-PALSAR, and of RADARSAT-1 [13]. The Sentinel-1 mission ensures the continuity of C-band SAR data provision byESA, following up the SAR sensors operating on ERS-1, ERS-2, and Envisat, with significantly enhanced capabilities in terms of duty cycle and coverage [4].