Speaker
Description
Digital elevation models, derived from single-pass interferometric SAR (SP-InSAR) data, are a main source for mapping surface elevation and its temporal change over ice sheets and glaciers. SP-InSAR data are not affected by temporal decorrelation and variations in atmospheric delay, but effects of signal penetration have to be taken into account. The apparent surface for uncorrected InSAR elevation data refers to the position of the scattering phase centre in the snow and ice volume. Main factors determining the phase centre depth are the position and strength of scattering sources and the losses due to absorption and scattering. The absorption losses can be estimated from dielectric properties which depend on snow density and temperature. The scattering sources and losses can be highly variable, depending on snow microstructure and stratification. We report results of a field study and radar signal propagation modelling in order to assess the factors governing signal penetration and to evaluate the related bias in surface elevation. We performed field measurements on snow structure and stratigraphy on Union Glacier in the Ellsworth Mountains, West Antarctica. The study area comprises ice-free surfaces, bare ice (blue ice), and cold snow and firn with a variety of structural features. Differences in exposure to wind are a main factor for local variations in the accumulation rate and in structural properties of the snow/firn volume. The satellite data base comprises X-band SAR data of the TanDEM-X mission, including amplitude images, SP-InSAR coherence images, across-track interferograms and DEMs, acquired between 2013 and 2017. Several repeat-pass lidar tracks of the ICESat mission crossed the area between 2002 and 2009. These data show very little temporal change of surface elevation so that the ICESat topography can be used as reference. Precise vertical co-registration of the TDM DEMs and ICESat elevation is performed over targets for which the volume scattering contribution is negligible, including bare ground and the blue ice field. In order to assess the impact of snow structure and stratification on signal penetration we performed backscatter modelling for the different field sites using a multi-layer dense media radiative transfer (DMRT) model with the Quasi-Crystalline Approximation (QCA). This shows the dominating backscatter contribution coming from the top 2 m to 3 m and large variations with depth depending on snow metamorphic state. The inversion of this model provides a good estimate of penetration depth. We tested also the feasibility for inverting TanDEM-X backscatter and coherence data with single layer radar propagation models. The single layer approach provides a reasonable estimate of penetration depth and InSAR elevation bias, but shows a trend for overestimating the total span. This can be explained by the typical stratification of the snow/firn medium which accounts for enhanced backscatter contributions of the top snow layers. Even so, backscatter and coherence signals can be used as indicator for detecting changes in penetration. In case of temporally stable signals penetration corrections are not required for deriving glacier surface elevation change (SEC) from SP-InSAR repeat DEMs. This is confirmed by the good agreement of multi-year SEC products from coincident airborne lidar and TanDEM data over Antarctic Peninsula glaciers.
