## 6 Conclusions  
The Sentinel satellite missions, developed and operated by ESA for the European Copernicus Programme under the leadership of the European Commission, provide the science community with powerful and long-term observation capabilities for key variables of the Earth system [2]. Each Sentinel mission is based on a constellation of two identical satellites. The Sentinels, together with the Earth Explorer missions that are addressing specific scientific questions of high urgency, are representing the core of ESA's Living Planet Programme [35].  
In addition to surface motion, the S1 SAR will provide regular repeat observations of other key ice sheet variables, including ice shelf margins, calving fronts of outlet glaciers, extent of surface melt areas, and surface drainage features. The capabilities of the Copernicus space component for ice sheet observations will be complemented by the Sentinel-2 and Sentinel-3 missions [3]. Sentinel-2 is a polar-orbiting, multispectral high-resolution imaging mission at visible and near infrared wavelengths [36]. The first Sentinel-2 satellite was launched on 23 June 2015. Ice sheet applications will include the mapping of outlet glaciers, ice boundaries, surface features and albedo, complementary to the high-resolution radar observations of S1. The Sentinel-3 mission combines optical medium-resolution observations by an imaging spectrometer and a dual-view multispectral radiometer, and altimetry measurements by a dual frequency synthetic aperture radar altimeter (SRAL) [37]. Main products of the optical sensors over the ice sheets will be spectral surface albedo and surface temperature. SRAL, including an along-track SAR capability of CryoSat-2 heritage, will observe surface topography and surface elevation change with increased measurement accuracy and along track resolution when compared to conventional altimetry products.  
In this paper, we highlight the capability of the Sentinel-1 mission for spatially detailed, ice-sheet wide monitoring of surface motion. Sentinel-1 with its IW mode delivers high resolution SAR data at wider coverage and shorter repeat cycle compared to previous SAR missions. Comprehensive mapping of ice motion at regular repeat intervals, as demonstrated with the first S1 ice velocity map of Greenland, will provide an important new database for understanding ice-sheet variability and the processes linking ice dynamics and climate forcing. The long-term S1 acquisition strategy for Greenland plans for two campaigns per year with full coverage of Greenland, comprising about 30 tracks in IW mode. The current plan comprises three consecutive repeat acquisitions, allowing the formation of two 12-day image pairs. The extension to a fourth repeat acquisition is advisable, in order to improve the likelihood of good coherence for ice velocity retrievals in regions of frequent snowfall. In addition to regular ice-sheet-wide acquisitions, it is planned to continuously monitor Greenland's ice margins with six tracks within every repeat cycle, providing 12-day temporal sampling with Sentinel-1A, shortened to six-day repeat once Sentinel-1B will be operational. This data set will be an excellent basis for observations of short-term variability and long-term trends in ice flow and mass export of Greenland's outlet glaciers.  
The work was supported by the European Space Agency, Ice Sheet CCI project (ESA Contract No. 40000104815/11/I-NB), and by the Austrian Research Promotion Agency, FFG, Austrian Space Application Programme (Contract Nr. 844386). The TerraSAR-X data were made available by DLR through the project HYD0096, the ALOS PALSAR data through the ESA ALDEN AOALO 3741 project.  
tools and were in charge of data processing. [PERSON] was responsible for data acquisition. All authors contributed to editing the manuscript.