Application of GPS-Buoy Water Level Instrument for Radar Altimeter Calibration

Abstract

This paper discusses contemporary research in GPS-buoy sea level measurements and its application to absolute radar altimeter calibration. Different processing techniques to obtain GPS-buoy water level solutions were studied using differential GPS (DGPS) and using absolute positioning by resolving GPS clock errors, Results of GPS-buoy campaigns in Lake Michigan, USA and near Catalunya, Spain for absolute calibration of the TOPEX/POSEIDON Side B (TSB) radar altimeter are presented. Error analysis indicates that the uncertainty associated with a ``single{''} GPS buoy (water-rider) sea surface height measurement in Lake Michigan is similar to40 mm rms, accounting for a list of estimated fixed and variable errors. The corresponding uncertainty for altimeter bias closure using the ``single{''} measurement is estimated to be similar to62 mm rms. Preliminary studies using altimeter water level measurements and GPS buoy measurements to ``translocate{''} historic TOPEX Side A (TSA) radar altimeter lake level measurements to the Holland West, Lake Michigan tide gauge, to conduct altimeter calibrations indicates that the technique is viable. The resulting TSA bias, based on 112 samples, is estimated to be -122.5+/-41 mm, including an apparent 100 nun offset. The estimated altimeter drift is 4.4+/-2.5 mm/yr. The drift estimate and its uncertainty are in reasonable agreement with results obtained at the dedicated T/P absolute calibration site at Harvest Platform, California, USA.