## 4 Results
### Comparison with Ice Motion from TerraSAR-X and PALSAR Data  
Maps of ice velocity on the outlet glaciers on the central section of Greenland's west coast are shown in Figure 2, derived from SAR data of S1, TerraSAR-X and ALOS PALSAR. The S1 velocity maps are based on repeat pass data pairs of the time spans 22 December 2014-3 January 2015 and 3-15 January 2015, separated up to one month from the TerraSAR-X images. The TerraSAR-X velocity maps are derived from 11-day, respectively, 22-day, repeat pass pairs of single scenes, each of which covers about 30 km \(\times\) 50 km in area. The PALSAR velocity map is based on repeat pass data of a single track, spanning the time period from 20 November 2009 to 5 January 2010. Whereas S1 in IW mode covers the displayed (and actually a much wider) area within a single track, PALSAR in FBS mode covers a strip of about 70 km width which in this case happens to include the terminus sections of the outlet glaciers. The TerraSAR-X scenes cover limited sections of individual glaciers. The wider spatial coverage of S1 IW mode accounts for the lower spatial resolution in azimuth compared to strip map mode data of PALSAR and TerraSAR-X. Spatial resolution is of concern for velocity retrievals on fast flowing outlet glaciers. The enlarged sections of S1 and TerraSAR-X amplitude images of two outlet glaciers (Figure 3) show obvious differences in detail of surface features between the two sensors. In order to examine the impact of spatial resolution on velocity retrieval, we show enlarged S1 and TerraSAR-X velocity maps on two sub-areas: four outlet glaciers draining into the northern Disko Bugt (Figure 4), and the lower terminus of Jakobshavn Isbrae (Figure 5). The spatial resolution of TerraSAR-X is 2.5 times higher in ground range and six times higher in azimuth (Table 1). Nevertheless, on the main sections of the Disko Bugt glaciers the motion fields from the two sensors are of similar quality (Figure 4). Some differences are evident on narrow strips along the glacier margins where S1 tends to flatten the velocity gradients. Whereas the maximum velocity on the outlet glaciers in Figure 4 ranges from 5 m\(\cdot\)d\({}^{-1}\) to 15 m\(\cdot\)d\({}^{-1}\), the velocity on Jakobshavn Isbrae rises up to 30 m\(\cdot\)d\({}^{-1}\) near the calving front. The fast flowing section of the ice stream is only 8 km wide, resulting in strong velocity gradients across. The offset tracking analysis with S1 IW data fails to provide reliable correlation results for the ice stream section where the central flowline velocity is \(>\)\(ca\). 12 m\(\cdot\)d\({}^{-1}\) (flagged out in Figure 5). A similar cut-off velocity is observed for PALSAR retrievals. The PALSAR FBS data have higher spatial resolution than S1 IW, so that smaller matching windows can be used. However, this advantage for velocity retrieval in areas of strong gradients is reduced by the longer time interval for acquisition of repeat-pass data. Figure 6 shows the velocities along the central flowlines on the lower terminus of four outlet glaciers (location in Figure 2), derived from S1, TerraSAR-X and ALOS PALSAR data. Information on the location of the glaciers and on the time span for velocity retrievals is listed in Table 3. Seasonal and interannual variations in ice velocities have been reported for many marine terminating outlet glaciers of Greenland [11, 13, 27, 30, 31, 32]. Here we want to compare the feasibility of the different SAR sensors for tracing ice velocities along the terminal sections of outlet glaciers. Regular observations of velocities on the terminus of outlet glaciers are crucial for quantifying the impact of ice dynamics on mass losses of the Greenland Ice Sheet. The velocity profiles in Figure 6 are from the cold season when short term fluctuations in velocity are modest compared to the melting period. On each of the four glaciers the ice flow speeds up towards the calving front, a typical pattern for Greenland's marine terminating outlet glaciers [13, 27].