Project Summary

Radar altimeter satellites that watch the ocean from space have a limited lifetime. To develop an integrated global observing system, by combining research and observations on the surface and from space, we need to "connect" the results from each satellite altimeter mission to the previous (e.g., TOPEX/Poseidon) and next one (e.g., Jason-1, GeoSat Follow-On (GFO), Envisat, etc.) for as long as possible. Thus, the radar measurements from each satellite mission need to be calibrated and connected continuously and independently of the system that produced them.

Figure 1. A schematic diagram of the permanent facility at Gavdos (copyright AVISO, France)
(click for a bigger view of the image)

The primary objective of this project is the establishment of an absolute sea level monitoring and altimeter calibration facility on the isle of Gavdos, south of the island of Crete, Greece. The GAVDOS project will determine consistently and reliably (1) the altimeter biases and drifts for each of these missions and (2) the bias among different missions. It will also determine the mean sea level and the earth's tectonic deformation field in the region of Crete, Greece.

When the project becomes operational, continuous altimeter measurements of similar quality and reliability and over long time periods will be ensured. It will also supply useful data for global change, geophysics, geodynamics, geodesy, oceanography and seismology.

Scientific objectives and approach: The purpose of this permanent facility is (a) to conduct comparative laser distance measurements between the facility and satellite radar altimeters; (b) to ensure the unbiased establishment of the Mean Sea Level (MSL), as realised by the globally distributed altimeter measurements; and (c) to monitor any altimeter errors. Using this facility, errors from instruments, orbital biases, drifts, delays, etc., are decreased, quantified and monitored. Thus, we improve the accuracy of the oceanographic products, orbit determination, sea-state bias and other issues of algorithm refinements, achieving a global MSL monitoring system with an accuracy of a few mm.

In this experiment, the radar measurements from satellites are to be combined with the satellite ephemeris, the geodetic position of the facility, and the ties between reference points of instruments at the facility. Further, on-site tide-gauge measurements, the determination of a high-resolution regional geoid, hydrodynamic levelling, direct altimeter transponder observations and continuous Global Positioning System (GPS) measurements are planned. Observations for ultra precise orbit determination will be also carried out, using the DORIS (Doppler Orbitography by Radiopositioning Integrated on Satellite) positioning system and Satellite Laser Ranging.

Expected impacts: This integrated facility due to its nature and its open ocean location is appropriate and will be available for use within RTD programmes dedicated in forecasting the Mediterranean and monitoring climate change. Also, the system will be capable for monitoring extreme events (storm surges, floods, etc.) in the proximity. The proposed facility will underpin research relevant to the European component of the global observing systems for climate and oceans and operational forecasting of environmental constraints on offshore activities .

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