# Comprehensive Annual Ice Sheet Velocity Mapping Using Landsat-8, Sentinel-1, and RADARSAT-2 Data
## 1 Introduction  
During the last decade, ice velocity mapping at continental scale [1; 2; 3] has allowed major advances in the study of polar regions by providing complete and precise observations of the complex flow pattern of ice sheets in the Arctic and Antarctic, from coastal regions to the deep interior. This has significantly improved our understanding of the physics of ice flow, the determination of the mass balance of ice sheets, and their contribution to sea level. Satellite borne observations in particular have made it possible not only to map ice motion but also to detect significant changes in ice dynamics over the last 40 years, which resulted in an enhanced mass loss of ice sheets into the ocean, and contribution to sea level [4; 5; 6]. Surface motion has been derived from space by using sequential images of spaceborne optical [7; 8; 9; 10] or synthetic aperture radar (SAR) instruments [11; 12; 13; 14]. Although this work initially started from the digital optical images acquired by Landsat [7; 8], since about the 1990s SAR instruments have been the method of choice to map ice motion at the continental scale [1; 2; 3; 4; 12; 15; 16; 17; 18; 19] because of their ability to work irrespectively of cloud cover and solar illumination, their higher temporal and spatial correlation compared to optical sensors, winter and summer alike, and their finer spatial resolution available at the time. The launch of Landsat-8 in 2013 changed this paradigm. First, the use of a 12-bit radiometric quantization compared to the 8-bit used in prior generations made it possible to observe subtle details on the bright ice sheet surface that could not be detected in the past. Second, thehigh rate of data acquisitions reduced the influence of cloudiness. Taking advantage of these new capabilities, [PERSON], [PERSON] et al. [20; 21] showed that mapping of ice flow with Landsat-8 was no longer limited to coastal fields with crevasses and could be extended to large areas in the ice sheet interior traditionally only mapped with SAR sensors. In April 2014 and April 2016, the European Space Agency (ESA) launched Sentinel-1a and Sentinel-1b, a constellation of third generation C-band SAR sensors. These satellites are part of the Sentinel satellite constellation series developed to support the European Union's Copernicus programme. These missions benefit from the success of large-scale data acquisition campaigns during the International Polar Year (IPY) and the mapping of ice motion on both ice sheet that resulted from this effort [2; 3; 22]. Many of the SAR missions (ERS-2, ENVISAT/ASAR, RADARSAT, or ALOS/PALSAR) that supported IPY ended between 2011 and 2013, some of them even before their projected mission end. The only C-band mission which has continued to provide data until Sentinel-1 launch was the RADARSAT-2 mission from the Canadian Space Agency (CSA). CSA has been carrying out several large scale data acquisition campaigns in Antarctica and Greenland in support of the ice sheet science community to fill in the gap between large, non-commercial missions. Between 2013 and 2016, CSA and ESA provided annual coverage of both ice sheets. This is also the time period when Landsat-8 data became widely available. Through their large scale acquisition plans over the ice sheets and free accessibility (open data for Landsat-8 and Sentinel-1, data are available upon request for RADARSAT-2 through an agreement with the data provider), these satellites have offered a novel opportunity to study the ice sheets, to compare ice motion retrieved from multiple sensors, and ultimately, as discussed here, to produce synergistic ice motion products combining all the data in an optimum and seamless fashion. In this study, the main objectives are to (i) investigate the use of Landsat-8 to map ice sheet motion on a continental scale (ii) integrate the results with SAR-derived velocity proucts to obtain comprehensive ice-sheet wide annual time series of ice motion and (iii) discuss the implications of the results for ice sheet studies.