Elevation bias due to penetration of spaceborne radar signal on Grosser Aletschgletscher, Switzerland

Kurzfassung

Digital elevation models (DEMs) from the spaceborne interferometric radar mission TanDEM-X (TDX) hold a large potential for glacier elevation change assessments and monitoring. However, a bias is potentially introduced through the penetration of the X-band signal into snow and firn. To improve our understanding of radar penetration on glaciers, we compare DEMs derived from almost synchronous acquisition of TDX and Pléiades optical stereo-images of Grosser Aletschgletscher in March 2021. We found that the elevation bias - averaged per elevation bin - can reach up to 8 m in the accumulation area. Concurrent in-situ measurements (ground penetrating radar, snow cores, snow pits) reveal that the signal is not obstructed by the last summer horizon but reaches into perennial firn. Because of volume scattering, the TDX surface is determined by the scattering phase centre and does not coincide with a specific firn layer. We show that the bias corresponds to more than half of the decadal ice loss rate. To minimise the radar penetration bias, TDX DEMs are recommended to be selected from the same season and over long observation periods. An assessment and correction of the radar penetration bias is recommended, especially when combining optical and TDX DEMs.