Assessment of Sentinel-3 Altimeter Performance over Antarctica using High Resolution Digital Elevation Models

Abstract. Since 2016, the Sentinel-3 satellites have provided a continuous record of ice sheet elevation and elevation change. Given the unique, operational nature of the mission, and the planned launch of two additional satellites before the end of this decade, it is important to determine the performance of the altimeter across a range of ice sheet topographic surfaces. Whilst previous studies have assessed elevation accuracy, more detailed investigations of the underlying instrument and processor performance are lacking. This study therefore examines the performance of the Sentinel-3 Synthetic Aperture Radar (SAR) altimeter over the Antarctic Ice Sheet (AIS), utilising new detailed topographic information from the Reference Elevation Model of Antarctica (REMA). Applying Singular Value Decomposition to REMA, we firstly develop new self-consistent Antarctic surface slope and roughness datasets. We then use these datasets to assess altimeter performance across different topographic regimes, targeting a number of key steps in the altimeter processing chain. We also evaluate the impact of topography upon waveform decorrelation. We find that, for 90 % of acquisitions, the point of closest approach to the satellite is successfully captured within the Level-1b range window. However, performance degrades with increasing topographic complexity, and this also affects the capacity to record all backscattered energy from within the beam footprint. We find that 24 % of the ice sheet exhibits greater topographic variance within the footprint than can be captured by the range window, and that the window placement captures a median of 90 % of the total possible topography that could be recorded. These findings provide a better understanding of the performance of the Sentinel-3 altimeters over ice sheets, and can guide the design and optimisation of future satellite missions such as the Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL).