Altimeter Bias Determination Using Two Years of Transponder Observations

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. The general design characteristics for absolute altimeter calibration imply a large number of measurements to reduce the random errors, diversity of measurement techniques and independent data analysis, to reduce the susceptibility to systematic errors. 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, convenient and independent technique is the use of a dedicated transponder, a device that receives the signal from the altimeter, amplifies it and re-sends it back to the satellite. 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 such a 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. Using precise orbits recalculated for each individual pass, the altimeter bias has been determined using Envisat waveforms produced by the transponder between 2004-2005.