GPS-equipped buoys – Sea Level Measurements with cm-accuracy
Tilo Schoene(1) , Saskia Esselborn(1) , and Roman Galas(1)
Various analyses of radar altimetry missions (RA) have shown that performance degradation of the satellite range measurements can result in an apparent sea level change. 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.
As shown for ERS past RA missions and cal/val-campaigns, a GPS-equipped buoy, anchored beneath a sub-track, can be used as a height reference for monitoring the stability and long-term behavior. Since GPS-derived coordinates are ITRF-referenced, an absolute calibration is possible.
In May 2002 a ruggedized toroid GPS-buoy was deployed in the North Sea in the context of a large German sea level monitoring project. After an initial phase, the second deployment was made in 2004. Currently a new system is being developed, integrating the past technical experience.
A close intersection of ERS-2/ENVISAT, TOPEX/Poseidon, Jason-1 and a GFO sub-track near the German coast gives the unique possibility to monitor all active RA missions. The lifetime of the new 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 sensor and meteorological devices, allowing a broader use for calibration, e.g. of wind speed or significant wave heights.
Results of the buoy measurements will be shown.