GAVDOS

• To establish an absolute sea-level monitoring and altimeter calibration permanent facility on the isle of Gavdos, Crete, Greece;
• To conduct tide-gauge measurements as well as direct altimeter transponder, Global Positioning System (GPS), Doppler Orbitography by Radio-positioning Integrated on Satellite (DORIS) and Satellite Laser Ranging (SLR) measurements for altimeter calibration;
• To produce a high-resolution geoid and hydrodynamic levelling;
• To monitor land deformations at the tide gauges and monitor sea level variations;
• To develop high-resolution, regional geoid and sea surface-topography model using dedicated airborne, ground and marine campaign data;
• To ensure the unbiased establishment of the Mean Sea Level, as realised by the globally distributed altimeters;
• To involve this project in other European and international programs (i.e., EU Operational Forecasting, GLOSS, IGS, etc.).

• An absolute sea level and altimeter calibration facility has been established. The facility is operational as of early 2003, is permanent and applicable to many satellite missions;
• The horizontal and vertical land deformation has been monitored using the permanently, collocated with the tide gauges, GPS array on Gavdos (two receivers) and one on Crete;
• The absolute position of the main site is periodically determined with SLR.
• Tectonic motions on Gavdos are also monitored, independently of GPS, by operating a DORIS beacon at the facility;
• Local environmental variations are monitored with a regional network of auxiliary tide gauges and meteorological, oceanographic sensors, on a daily basis, and sea surface topography (SST) with airbone lidars during planned campaigns;
• The Mean Sea Level has been observed and compared to that from altimetry to calibrate the altimeter measurement biases and their potential drifts;
• Maps of the physical oceanographic properties of the area surrounding the Gavdos island have been produced, using data of Conductivity-Temperature-Depth from research cruises to Gavdos, altimetry analysis, and analysis of other satellite data.
• The analysis of the GPS data produced a very high precision position and horizontal velocity vector, comparable to SLR results from nearby sites.
• A state-of-the-art tidal model for the computation of the local tides has been constructed.

• The Jason-1 absolute bias has been determined at the GAVDOS permanent facility. From ground observations and altimeter comparisons and over cycles 70 to 90, the absolute bias for the Jason-1 altimeter was estimated to be 144.7 mm ± 15 mm. The values produced are compatible with other world calibration/validation sites (Corsica, Harvest, Bass Straits, etc).
• The models for the effects of the atmospheric effects have been validated. The instrumental and the geophysical corrections of the altimeter observations from the participating missions have been compared. Comparison of Jason-1 microwave radiometer data from cycles 37 and 62, with locally collected water vapour radiometer and solar spectrometer observations indicate a 1–2 mm agreement.
• An Operations Control Center has been established on Crete at the Technical University of Crete. The OCC receives data, via UHF telemetry, from the field units in Gavdos.
• Data from tide gauges and meteorological sensors are transmitted to the Operations Control Center in near real time (every 6 minutes). Data are archived into the project’s Database. All Partners have access to their data.
• A gravity database and final 1’×1’ geoid models have been created. Comparison of the final 1’ geoid models with T/P indicates a root-mean-square error of about 0.14m.
• During the SLR campaigns the accuracy attained was better than 1cm and the stability of measurement was at 2mm level. Data are available at the International Laser Ranging Service (http://ilrs.gsfc.nasa.gov/stations/index.html).
• A field campaign for airborne gravity, laser altimetry and laser scanning was carried out around Gavdos in January 2003. An Airborne Laser scanner has been used as additional equipment for recovery of the sea state. The Hellenic Navy Hydrographic Ship PYTHEAS took place in the campaign. There has been a close collaboration and help received by the Hellenic Military Geographic Service, Civil and Military Aviation Control Authorities and the Chania Airport. Crossover analysis indicates a noise level of 2.3 mgal for the airborne data.
• GPS-Buoy Experiment. Calibration measurements close to the Souda tide gauge have been carried out simultaneously with the airborne campaign. Continuous measurements were carried out during a leg of the hydrographic survey vessel to the south of Crete. The preliminary airborne laser profiler calibration established with buoy measurements in Souda Bay, Agia Galini and offshore (Pytheas) provided good results, with buoys and laser altitude measurements fitting better than 5 cm.
• A highly accurate and high-resolution map of the marine geoid has been provided (resolution 1’×1’, accuracy ± 10 cm) in the vicinity of the experiment site;
• Estimates of the variations of sea level in the area have been estimated. The two tide gauges in Gavdos are producing measurements every 6 minutes at a resolution of 0.1-mm and thus contributing to the global tide-gauge network, such as Med-GLOSS;
• The vertical land displacement at the facility has been de-correlated from the tide-gauge measured mean sea level of Eastern Mediterranean Sea, using state-of-the-art precise geodetic positioning systems (GPS, DORIS, altimeter transponders);
• The vertical component of the land movement at the experiment site (very significant for seismic work in the area) has been detected;
• Reliable estimates of horizontal land deformation of the facility with respect to stable Europe have been established;
• Any local deformation between the facility and the main island of Crete can now be detected, thanks to the established permanent network of 3 GPS stations, indicating the existence of regional deformation associated with the Hellenic trench subduction zone;
• A high precision, accurate realisation of the ITRF (International Terrestrial Reference Frame) in the area has been provided, facilitating the connection of the local tide gauges in this global reference frame (contribution to GLOSS).

This project is establishing a European calibration and Validation facility for the radar satellite altimeters and contributes Global Change, Climate and Biodiversity and enhances the European component of the global observing systems with a new long-term observing capacity for sea level

Conclusions

The permanent radar satellite altimeter for the calibration and validation of Jason-1 and Envisat satellite at the island of Gavdos, Crete, Greece is operational. Further improvements regarding repote access to the instruments are in progress.

Keywords

sea level, altimeter calibration, radar satellite altimetry, calibration, validation, geodynamics.