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

Journal of Glaciology, Vol. 44, No. 148 p. 625-633, 1998 

G. Haardeng-Pedersen
Physics Department, Memorial University of Newfoundland, (SWGC), Corner Brook, Newfoundland A2H 6P9, Canada.
K. Keller, C.C. Tscherning, N. Gundestrup
Departement of Geophysics, The Niels Bohr Institute of Astronomy, Physics and Geophysics, University of Copenhagen.

Using an active transponder with the ERS-1 and ERS-2 radar altimeters, the distance to the satellite was measured as a location close to the GRIP side, Greenland, at an altitude of 3.2 km. The measurement was executed while the transponder was in the "ice-tracking mode". It includes a bias due to the propagation delay. The location of the transponder was determined using the global positioning system.
The transponder signal was modeled and the distance from the altimeter to the effective reflection point of the transponder was determined. Since the transponder was located within 1 km of the ground tracks, the measurement was corrected for this offset. A correction was also done for the surface slope, resulting in the distance (plus bias) to the closest sub-satellite point 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 to the first reflective surface within the snow. The difference between this distance and that obtained using the transponder was <2 m. This shows that 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 using satellite radar altimetry.