Measuring Volume Changes of Alpine Glaciers with SRTM and TanDEM-X Data

Abstract

Interferometric radar data (InSAR) offer the capability of measuring and monitoring the surface topography of glaciers and ice caps at high accuracy. Of particular interest are single-pass spaceborne InSAR systems such as X-SAR and the satellite formation TerraSAR-X (TSX)/TanDEM-X (TDX) because they are not affected by temporal decorrelation and changes in the atmospheric phase delay. The German-Italian dual antenna radar sensor X-SAR operated in February 2000 on the Shuttle Radar Topography Mission (SRTM), acquiring data over swath of 50 km width, mapping about 40 per cent of the land masses between 60°N and 56°S. SRTM/X-SAR delivered DEM data at slightly better relative vertical accuracy (6 m) and horizontal resolution (30 m) than the SIR-C sensor which also operated on SRTM but provided full area coverage. In June 2010 TDX was launched, flying in close formation with TSX, both forming a single-pass InSAR system with the objective of providing a global DEM with 12m × 12m horizontal sampling and <2m vertical relative accuracy. Digital elevation data acquired by InSAR at different epochs enable to map changes of glacier volume and to estimate the net mass balance over the time interval. We analyzed volume changes of glaciers in the Ötztal Alps between February 2000 (SRTM/X-SAR) and February 2011 (TSX-TDX), showing significant down wasting of glaciers as known also from field campaigns and airborne studies. Because radar signals penetrate into snow and ice, the scattering phase centre of the radar signal and consequently the uncorrected topographic data are located up to a few metres below the actual snow and ice surface, the depth depending on the physical state of these targets. Because the topographic data in this study are derived from SAR systems operating at the same radar frequency (X-band), signal penetration is of minor concern for retrieving volume changes. The InSAR data show surface lowering up to 65 metres on the lower terminus of Hintereisferner. The total 2000 – 2011 volume change will be compared to in situ mass balance measurements for the glaciers Hintereisferner and Kesselwandferner.