ALBERT

Description: Ice-penetrating radar is a well-established tool for imaging the interior and the bed of an ice body. The data provide information on ice thickness, but also contain information on englacial stratigraphy, ice flow dynamics and conditions at the interface between ice and the bedrock. During the past two decades, the Greenland Ice Sheet has been extensively measured by radar instruments. However, a large radar dataset has so far been overlooked due to its inaccessibility to modern analysis techniques. This dataset was acquired in the 1970s in a large-scale project that ran over multiple years and covered more than 170,000 km of radar flight lines. While the ice thickness information from the data has subsequently been digitized, the data itself is presently only available as35-mm films, micro-fiche copies of the films and enlarged positives. Here we present the first results from an effort to digitize this unique and rare dataset. We demonstrate how the data may contain valuable information on the state of the Greenland Ice Sheet in the 1970s, a period where observations from the centre of the ice sheet are scarce.

Description: The ESA CryoSat-2 satellite, launched in late 2010, carries a new type of radar altimeter especially designed for monitoring changes of sea and land ice. The radar signal might penetrate into the snow pack and the depth of the radar reflecting surface depends on the ratio between the surface and the volume backscatter, which is a function of several different properties such as snow density, crystal structure and surface roughness. In case of large volume scatter, the radar waveforms become broad and the determination of the range (surface elevation) becomes more difficult. Different algorithms (retrackers) are used for the range determination, and estimated surface penetration is highly dependent on the applied retracker. As part of the ESA-CryoVEx/CryoVal-Land Ice projects, DTU Space has gathered accurate airborne laser scanner elevation measurements. Sites on the Greenland ice sheet, Austfonna and Devon ice caps, has been surveyed repeatedly, aligned with Cryosat-2 ground tracks and surface experiments. Here, we utilize elevation estimates from available Cryosat-2 retrackers (ESA level-2 retracker, DTU retracker, etc.) and validate the elevation measurements against ESA-CryoVEx campaigns. A difference between laser and radar elevations is expected due to radar penetration issues, however an inter-comparison between retrackers will shed light on individual performances and biases. Additionally, the geo-location of the radar return will also be a determining factor for the precision. Ultimately, the use of multiple retrackers can provide information about subsurface conditions and utilize more of the waveform information than presently used in radar altimetry.