TanDEM-X: Deriving InSAR Height Changes and Velocity Dynamics of Great Aletsch Glacier

Great Aletsch Glacier (Grosser Aletschgletscher), the largest glacier in the European Alps, contains 20% of the entire Swiss ice mass. Therefore, it has been selected as a super-testsite for the TanDEM-X satellite mission. Dense time series with a repeat interval down to 11 d were acquired between 2011 and 2019 using two polarizations (HH and VV) and across-track baselines of 0-1 km. To evaluate the use of interferometric single-pass synthetic aperture radar mission for glaciological applications, we implemented a processing pipeline in interactive data language (IDL) and computed 130 digital elevation models (DEMs) from histatic radar interferograms. We present a method to circumvent a common pitfall during orthorectification of radar DEM differences. Regression analysis of DEM time series shows a height loss of up to 8 m a(-1) on the tongue and 1.5 m a(-1) when averaged over the whole glacier area. In spring 2013, we observed X-band penetration depths of 4 +/- 2 m in the accumulation area. For strongly crevassed areas, the coherence drops already to 0.5 for across-track baselines B-perpendicular to > 200 m. With patch-based incoherent offset tracking, we obtained an almost complete 200 m resolution velocity map. Velocities reach up to 0.8 m d(-1), show a seasonal variability of +/- 0.05 m d(-1), and agree within 0.04 m d(-1) (root mean square error) with field measurements. Copolar phase differences suggest an approximation of snow accumulation. We demonstrate orthorectification of the backscatter intensity using simultaneously acquired TanDEM-X interferograms, which allows for the decoupling of horizontal velocity estimates from phase-center height changes due to penetration and ice melt.