GPS-buoys for lifetime RA drift monitoring

With the launch of JASON-1 the successful mission TOPEX/Poseidon (T/P) is extended to a second decade. Various analyses have shown that any altimetry mission is subject to performance degradation resulting in an apparent sea level rise. Comparisons of RA with in situ measurements of the instantaneous sea level at Harvest Oil Platform have been used to monitor the absolute sea level measurements of T/P, but not for other missions. However, only a combination of e.g. ENVISAT and JASON-1 gives a sufficient coverage in time and space. For the past and current missions different strategies are used for the calibration and the drift monitoring, e.g. using crossovers or tide gauges as a height reference. The disadvantage of all calibration methods is, that no direct measurement beneath the sub-satellite tracks are available for all missions and, therefore, models have to be used to account for e.g. the sea surface slope or time varying signals. As shown for ERS and Topex/Poseidon, a GPS-equipped buoy, anchored beneath a sub-track, can be used as a height reference. Since GPS-derived coordinates are ITRF-referenced, an absolute calibration is possible. Until today only lightweight buoys were deployed. Therefore, no long-term sensor for the calibration and drift monitoring exists. In May 2002 a ruggedized GPS-buoy was deployed by GFZ in the North Sea in the context of a large German sea level monitoring project. A triple intersect of ERS-2/ENVISAT with a Topex/Poseidon/Jason-1 and a GFO sub-track was chosen. This gives the unique possibility to monitor al active RA missions. The lifetime of the buoy is expected to be several years, therefore, a long-term calibration, drift monitoring and inter-calibration of different missions will be possible. In addition, the buoy is equipped with supplementary sensors, like a dynamic motion sensors and meteorological devices (e.g. wind speed, air pressure sensors), allowing a broader use for calibration, e.g. of wind speed or significant wave heights. All data is stored onboard and, additionally, is transferred ashore by a HF link. Three bottom-mounted tide gauge sensors supplement the calibration site, allowing also to account for the sea surface slope for off-track RA measurements. A wave tide recorder beneath the GPS buoy is used for comparisons of the SWH. First results of the buoy performance will be presented.