Experience with RA Transponder in Greenland

G.Hardeng-Pedersen *, K.Keller (krk@kms.min.dk) **, N.Gundestrup
(ng@gfy.ku.dk), C.C.Tscherning (cct@gfy.ku.dk) ***
* Memorial University of Newfoundland, Canada, ** National Survey
and Cadastre, Denmark, *** Geophysical Department, Juliane Maries
Vej 30, DK-2100 Copenhagen Oe, Denmark.

Abstract:
Using an active transponder with the ERS-1 and 2 radar altimeters
the distance to the satellite was measured at the GRIP site on the 
Greenland Ice Cap at an altitude of 3.2 km. The location of the transponder
was determinde using GPS. The difference between the observed and
computed distance to ERS-1 obtained in 1995 was 0.2 m.

Since the Transponder was located 1 km from the ground track, the
measurements were corrected for this offset. Due to the smoothness of
the surface the error is estimated to be below 0.5 m.

The transponder signal was modelled (without considering the volume
scattering) and then removed from the RA waveform. The difference 
between the corrected distance and that obtained using the corrected
Transponder measurements was 0.1 m for a measurement obtained in 1995.

The two results shows that (a) the transponder can be used as a tracking
device on the ice and (b) that the effective surface of refection
agrees with the surface of the (compact, dry) snow at this high altitude.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry