Modeling the signature of a transponder in altimeter return data and determination of the reflection surface of the ice cap near the GRIP camp, Greenland

Using an active transponder with the ERS-1 and ERS-2 radar altimeters, the distance to the satellite was measured at a location close to the GRIP site , Greenland, at an altitude of 3.2 km. The measurement was executed while t he transponder was in the "ice-tracking mode". It includes a bias due to th e propagation delay. The location of the transponder was determined using t he global positioning system. The transponder signal was modeled and the distance from the altimeter to t he effective reflection point of the transponder was determined. Since the transponder was located within 1 km of the ground tracks, the measurement w as corrected for this offset. A correction was also done for the surface sl ope, resulting in the distance (plus bias) to the closest sub-satellite poi nt on the surface of the (compact) snow The transponder signal was then removed from the radar altimeter waveform, enabling the determination of the distance (plus bias) from the altimeter t o the first reflective surface within the snow. The difference between this distance and that obtained using the transponder was < 2 m. This shows tha t the surface which gives rise to the first return of the reflection agrees with the surface of the (compact, dry) snow at this high-altitude location . This is an important result to be used when studying ice-cap topography u sing satellite radar altimetry.