FTLRS POSITIONING FOR THE EU/NASA ALTIMETER CALIBRATION PROJECT GAVDOS

Abstract

The Eastern Mediterranean area is one of great interest for its intense tectonic activity as well as for its regional oceanography. Recent observations convincingly demonstrated the importance of the area for regional meteorological and climatologic changes. GPS monitors tectonics, while tide gauges record the variations in Mean Sea Level (MSL). Monitoring tide gauge locations with continuous GPS on the other hand, will remove the uncertainties introduced by local tectonics, that contaminate the observed sea level variations. Such a global tide gauge network with long historical records is already used to calibrate satellite altimeters (e.g. on TOPEX/POSEIDON, GFO, JASON-1, ENVISAT, etc.), at present, a common IOC-GLOSS-IGS effort --TIGA. Crete hosts two of the oldest tide gauges in the regional network, at Souda Bay and Heraklion. We recently completed the instrumentation of a third, state-of-the-art MSL monitoring facility in southwestern Crete, on the isle of Gavdos, the southernmost European parcel of land. Our project –GAVDOS, further expands the regional tide gauge network to the south, and contributes to TIGA and MedGLOSS. This presentation focuses on the altimeter calibration aspect of the facility, in particular, its application to the JASON-1 mission. Another component of the project is the repeated occupation of the older tide gauges at Souda Bay and Heraklion, and their tie to the new facility. The Gavdos facility is situated under a ground-track crossing point of the original T/P and present JASON-1 orbits, allowing two calibration observations per cycle. It is an ideal site if the tectonic motions are monitored precisely and continuously. The facility hosts in addition to the two tide gauges, multiple GPS receivers, a DORIS beacon for positioning and orbit control, a transponder for direct calibration and it is visited periodically by Water Vapor Radiometers and solar spectrometers. At frequent intervals we also deploy GPS-laden buoys and conduct airborne surveys with gravimeters and laser profiling lidars for a high resolution and increased accuracy of the geoid and an independent observation of the local Sea Surface Topography (SST). The French Transportable Laser Ranging System (FTLRS) completed recently a co-location campaign at the Chania, Crete base site, which has a long GPS record since 1997. The FTLRS occupation provides us with an absolute SLR-derived position in the ITRF2000 frame, the ability to compare with the GPS-derived position, and improved orbit control over the site during the campaign. This will ensure the best possible and most reliable results from the project. We will present our latest estimates of the FTLRS position and the GPS-derived velocity vectors for the site, and other relevant results.