## 5 Discussion  
Ice sheet monitoring in support of climate change studies is one of the main application domains for the Sentinel-1 mission over land [4]. The high duty cycle of up to 25 min per orbit, the wide coverage of the IW mode as nominal operation mode over land, and the long-term commitment for operation of the mission opens up unique capabilities for building up consistent, long-term data archives on ice sheet parameters. The mission operation is based on stable and predefined observation scenarios and systematic production schemes. The baseline observation scenario over ice sheets applies the IW mode with 250 km swath width and 5 m \(\times\) 20 m spatial resolution. We investigated the feasibility and performance of Sentinel-1 for mapping and monitoring ice sheet motion, based on the first comprehensive ice sheet data set acquired by this satellite. In this section, we discuss properties of the Sentinel-1 ice velocity product of Greenland with reference to ice motion maps from previous years, based on other SAR sensors. [PERSON] and [PERSON] [13] produced a comprehensive digital mosaic of ice motion in Greenland for the IPY assembled from SAR data of ALOS PALSAR, RADARSAT-1, and Envisat ASAR, applying a speckle tracking algorithm. Most of the data were acquired in fall 2009, with some additional tracks from fall 2007 and 2008. The IPY velocity map covers 99% of the ice sheet area. The PALSAR motion retrievals represent the main contribution to the composite ice motion map because of the good temporal coherence of the L-band data (wavelength 24 cm) in spite of the long repeat cycle of 46 days. The C-band signal (wavelength 5.6 cm) is more sensitive to temporal decorrelation caused by meteorological events. For this reason the RADARSAT-1 (repeat cycle of 24 days) and the ASAR (repeat cycle 35 days) ice motion data show large gaps, in particular in the interior part of Greenland [13]. The RADARSAT-1 ice velocity maps of individual winters of [PERSON] _et al._[27] show large gaps, in particular in the central,southeast and south sections of the ice sheet. Whereas L-band SAR offers better performance in terms of coherence, ionospheric perturbations are smaller at C-band [13, 28]. The S1 ice velocity map of Greenland (Figure 1), based on SAR data of winter 2015, shows some gaps in the southeast sections as the RADARSAT-1 ice motion maps of individual winters [27]. However, the S1 velocity shows overall a more extended coverage over the ice sheet. This can probably be attributed to the shorter repeat cycle of S1 (12 days). Further improvements in coverage can be expected after full deployment of the S1 mission when two satellites will fly in coordinated orbits providing six-day repeat coverage, which helps to reduce temporal decorrelation [5]. For velocity retrievals on outlet glaciers the temporal decorrelation is of less concern than in the interior of the ice sheet, because in ablation areas template matching uses mainly conservative amplitude features (crevasses, flowlines, drainage patterns) rather than speckle patterns. A critical issue is the ability to obtain reliable correlation peaks on fast-moving glaciers. The spatial resolution of the sensor and the length of the repeat observation cycle are the main parameters determining the feasibility for resolving velocity fields in zones of strong shear and deformation. The enlarged sections of the ice flow map in Figures 4 and 5 and the velocity profiles in Figure 6 help to illuminate this issue. The S1 offset tracking analysis provides complete motion fields for the glaciers draining into the northern Disko Bugt and for the fast-flowing Store Gletsjer, which has a frontal velocity of 16 m\(\cdot\)d\({}^{-1}\). On Jakobshavn Isbrae the TerraSAR-X analysis of February 2015 yields a frontal velocity of 32 m\(\cdot\)d\({}^{-1}\) (Figure 6), similar to velocities observed in February 2013 and February 2014 [30]. The S1 analysis delivers continuous velocity data upstream of a point 20 km inland where the velocity is 11 m\(\cdot\)d\({}^{-1}\).