Within the Gavdos Project (Establishment of a European radar altimeter calibration and
sea level monitoring site for Jason, Envisat and Euro-Gloss), a transponder has been used
for an experimental work package. The instrument has been deployed on the island of
Gavdos beneath the ascending pass of Envisat and was receiving the altimeter pulses,
amplifying and retransmitting them back to the altimeter. The transponder could thus be
easily identified in the altimeter data stream in the characteristic variation of the
waveform power. Acting as a point target for the satellite radar altimeter, the range
measurements from the satellite to the transponder are not subject of "speckle" effects, as
it happens in the case of the sea surface when the multitude of scatters cause interference
effects to the radar echo. The distance measurement has a resolution of < 1 cm [1];
provided that the atmospheric delays are properly modeled, the accuracy of a range
measurement is estimated to 2 cm.
The applications deriving from the use of the transponder in the altimetry context are
based on the accurate determination of the transponder-satellite distance. These include
the altimeter bias calibration, the measurement of the orbit radial errors especially in
places where positioning systems are unavailable, determination of ice thickness, and
height transfer between land and ocean or land and land.
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