# Comprehensive Annual Ice Sheet Velocity Mapping Using Landsat-8, Sentinel-1, and RADARSAT-2 Data
## 4 Discussion
### Implications for (Future) SAR Missions for Ice Velocity Measurements  
Landsat-8 demonstrated great capabilities for ice velocity mapping over ice sheets and the vast amount of data acquired by USGS greatly enhances its utility. There continues to be downsides to optical data, which justifies the continued use of spaceborne SAR data. These downsides include (i) a lack of illumination for part of the year; (ii) cloud cover (which is frequent in some areas); and (iii) lower precision of optical (assuming the same amount of data). We note that all ice motion measurements being discussed here use feature tracking (Landsat-8) or speckle tracking (SAR). This technique is limited by the size of the pixel element (or resolution cell of the data) and data noise. With SAR sensors, however, it is possible to measure ice motion with the interferometric phase instead [12; 17; 42; 43; 44; 45]. The precision of ice motion from the phase is not limited by the pixel size, but by the size of the radar wavelength and data noise. In effect, SAR sensors measure ice motion within millimeters using the phase, or typically one order magnitude better than with tracking [46]. In that case, given the same amount of data and the same pixel size, SAR sensors offer vastly superior quality for ice motion measurements. On the other hand, phase measurements need to be completed along two independent directions (or three if we do not assume surface parallel flow [14]) in order to resolve the vector of ice motion, hence require more data acquisition and tracks [47]. With the interferometric phase, SAR sensors also detect differential movements of the ice surface, e.g., the subtle flapping of floating sections up and down with changes in oceanic tides [41; 48], or the deflation of the ice surface because of drainage of a subglacial lake, at a precision of a few millimeters, which is currently impossible to achieve from optical sensors or altimeters and is a unique capability of SAR sensors. The use of phase data however require twice more data compared to the correlation technique ascending and descending acquisitions are needed. The current SAR sensors (Sentinel-1, RADARSAT-2, TSX) are not ideal for using the phase on a large scale because of their limited coverage and/or their operational mode of acquisition which is not optimum for phase analysis. In the future, the upcoming SAR missions from the CSA's RADARSAT Constellation Mission (RCM) or the NASA/ISRO InSAR mission (NISAR) will provide new opportunities to use phase mapping. NASA and ISRO will launch NISAR ([[https://nisar.jpl.nasa.gov/](https://nisar.jpl.nasa.gov/)]([https://nisar.jpl.nasa.gov/](https://nisar.jpl.nasa.gov/))) in 2020 to completely and continuously survey the ice sheets on a 12-day cycle at the L-band frequency for a nominal mission time of three years [49]. From the archive of ALOS/PALSAR already processed [2; 22; 50], we know that L-band SAR with a cycle of 46 days achieves successful velocity mapping on an area-percentage per scene of 73% in Antarctica. We reasonably expect a higher correlation level for a repeat cycle 4 times shorter. In addition, NISAR will enable phase measurements pole to pole, hence more precise measurements of ice motion, day and night, summer or winter. Along the coastal areas, where ice motion is too large to track with the interferometric phase, we will continue to employ the speckle tracking technique and it is likely that the addition of Landsat-8 data will remain a significant source of additional information on ice motion. Similarly, Sentinel-1a/b has opened new opportunities for mapping ice motion in the polar regions. Since both the Landsat and Sentinel are planned to be long term programs, these assets will remain at the service of the scientific community for decades to come. Figure 9: Composite mosaic of surface ice speed generated from ERS-1&-2/ESA, RADARSAT-1&-2/ CSA, ENVISAT/ESA, ALOS/JAXA, TerraSAR-X/DLR, Sentinel-1/ESA and Landsat-8/USGS in Antarctica acquired between 1992 and 2016.