Calibration and validation fo the CryoSat radar altimeter: field studies on the Greenland Ice Sheet

Abstract

The ESA CryoSat-2 satellite, due for launch in February 2010, will make measurements of ice sheet elevation and sea-ice freeboard using a new radar altimeter SIRAL. The aim of the ice sheet measurements is to reduce the uncertainty in the ice sheet contribution to sea level to a magnitude similar to that associated with other sources of sea level rise. The international programme of calibration and validation for Cryosat includes field studies by UK scientists on the Greenland Ice Sheet. Loan 867 provided a geodetic GPS for use in North Greenland by the Scott Polar Research Institute as part of the ongoing cal/val activities. The validation concept has been to up-scale surface measurements to those of the satellite via airborne measurements with a SIRAL-like radar, ASIRAS. Pre-launch campaigns have been carried out since 2004 to verify this concept and have largely been successful. They involved measurement of snow density-depth profiles which were then compared with over-snow data from a very high bandwidth (VHB) radar and airborne data from ASIRAS. Peaks in the radar returns could be linked to jumps in density at stratigraphic boundaries. In some areas the largest peak did not come from the snow surface but from an underlying ice layer, so the method of recognising the surface return in SIRAL data, and hence determining elevation, has had to be refined. All field campaigns included an intensive study of a 1 km square test site to investigate spatial variations in snow density and over-snow radar returns. At these sites a radar reflector was erected and its elevation, and that of the underlying snow surface, determined by static survey using geodetic GPS. This allows the ASIRAS measurements to be calibrated.