## 2 Data Sources  
The ice motion map of the Greenland Ice Sheet (Figure 1) has been assembled from 819 Sentinel-1 (S1) SAR scenes in Interferometric Wide swath (IW) mode. The majority of data was acquired during the period January to March 2015. A few tracks were recorded during October to December 2014 and April to June 2015 in order to fill some gaps on the west and southeast coasts. For retrieval of ice motion S1 SAR images of consecutive passes are used, spanning time intervals of 12 and 24 days. In order to check the impact of spatial resolution, we compare S1 ice motion with ice motion maps generated from 11-day repeat pass SAR images of the TerraSAR-X satellite, acquired in strip-map (SM) mode over several outlet glaciers on Greenland's west coast and with 46-day repeat pass ALOS Phased Array-type L-band SAR (PALSAR) images acquired in Fine Beam Single polarization (FBS) mode. Characteristics of the SAR data used for the study are listed in Table 1.  
The S1 IW mode is operated as TOPS (Terrain Observation with Progressive Scans in azimuth) mode in order to provide a large swath width of 250 km at a nominal ground resolution of \(5~{}\mathrm{m}\times 20~{}\mathrm{m}\) for single look data, offering enhanced image performance compared to the conventional ScanSAR mode [6, 14]. In TOPS SAR imaging mode, the antenna azimuth beam is steered from aft to the fore at a constant rate. As a result, the targets on ground are observed by the entire useful part of the azimuth antenna pattern [14, 15]. This reduces the scalloping effect significantly and leads to constant azimuth ambiguities and signal-to-noise ratio (SNR) along azimuth. On the other hand, the TOPS mode imposes particular challenges regarding accurate co-registration of image pairs and interferometric processing. For producing the Greenland ice motion map, we use Sentinel-1 IW Level 1 single look complex (SLC) products. The data set for generating the ice motion map (Figure 1) comprises 28 different tracks and a total data volume of 3 TByte in compressed form. Each image swath in IW mode is composed of three sub-swaths. Individual swaths are subdivided into slices (frames) of about 350 km length along tracks. Each sub-swath consists of a series of bursts, where each burst is processed as a separate SLC image. For the ice motion map we processed a total of about 28,000 bursts. Most of the swaths over Greenland were acquired at dual polarization (HH and HV). For ice motion retrieval, we use the HH channel, which has higher signal-to-noise ratio and provides better quality of amplitude features than the HV channel. The S1 SLC products are focused SAR data preserving the phase information, processed in zero-Doppler/slant-range geometry and geo-referenced using orbit and attitude data from the satellite. The individually focused complex burst images are put into a single sub-swath image in azimuth-time order [16]. The imaged ground areas of adjacent bursts (with a nominal duration of 2.75 s each) marginally overlap in azimuth, so that contiguous coverage of the ground is achieved. Images for all bursts in all sub-swaths of an IW SLC product are re-sampled to a common pixel spacing grid in range and azimuth. Burst synchronization is ensured, based on a requirement for achieving a synchronization of less than 5 ms between corresponding bursts. This is essential for interferometric processing of burst mode data and is also relevant for accurate retrievals of surface motion applying offset tracking techniques [17]. Figure 1: Ice velocity map (magnitude, in logarithmic scale) of the Greenland Ice Sheet derived from SAR data of the Sentinel-1A satellite, acquired in Interferometric Wide Swath Mode (IW) between January and March 2015. We retrieved velocity maps of the terminus of several outlet glaciers from TerraSAR-X single-look slant-range complex (SSC) images in strip map mode (SM), HH polarization. The SM images extend over a swath width of 30 kilometers.