ENVISAT - Range Calibration Using a Transponder

Satellite altimetry provides a precise measure of the vertical distance of the satellite borne altimeter to the instantaneous sea surface. The accuracy of this distance depends on the calibration of the altimeter, the quality of the reflecting target and the proper estimate of path delay. Part of the altimeter calibration can be undertaken by comparison against in situ tide gauge data and GPS buoys, by inter-comparison between two altimeter data sets from concurrent satellites, or at crossing points. A different and convenient technique is the use of a dedicated transponder, a device which receives the signal from the satellite, amplifies it and re-sends it back. Opposite to the ocean surface a transponder disposes of a stable and very precise reflection reference (few millimeters), which allows for a very precise determination of the vertical distance between the satellite and the transponder. The accuracy of a such determined range depends on the ability to estimate the path delays caused by the atmosphere, the precision of the orbit and the GPS positioning of the transponder. If the dedicated transponder is operating within the footprint of the altimeter, the corresponding waveforms differ both in power and shape from the other waveforms caused by natural targets. By means of transponder signature analyses, the time of the closest approach of the satellite to the transponder is determined and the range is hence calculated. The transponder is deployed beneath the ascending Envisat track on the Greek island Gavdos. Numerical results are to be presented if relevant data are available.