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Gavdos-Crete Cal-Val for satellite altimetry missions

“Monitoring, Calibration and Validation of current (i.e., Jason-1, Jason-2/OSTM, HY-2) and future (i.e., Jason-3, Sentinel-3, Altika/SARAL) satellite altimetry measurements using the permanent facility on the islands of Gavdos and Crete, Greece"


Stelios P. Mertikas - (Geodesy and Geomatics Engineering Laboratory, Mineral Resources Engineering Department)

  Antonis Daskalakis
(Space Geomatica Ltd.)


Gavdos-Crete Cal-Val for satellite altimetry missions
The permanent satellite calibration facility for Jason-2 in Gavdos island, south of Crete, Greece
On the small island Gavdos, 40 km south of Crete, the Laboratory of Geodesy and Geomatics Engineering in the Technical University of Crete, Greece, has established in 2001 and has been continuously operating since 2004, a permanent facility for calibrating satellite radar altimeters. The location of the Gavdos Island, in the center of East Mediterranean, constitutes a strategic point for the calibration of satellite altimeters on a world level, and also for monitoring absolute sea level and climate change on a continuous and long-term basis.

The Gavdos facility is, the only Cal/Val site in the world, situated under a crossing point of the Jason orbits and adjacent to the orbits of Envisat, and subsequently of the future SARAL/Altika. The infrastructure of the permanent Cal/Val facility on Gavdos includes tide gauges, 10 permanent Global Navigation Satellite System (GNSS) receivers in Gavdos and Crete, meteorological instruments, a DORIS satellite beacon, an electronic microwave transponder (a new one under testing and calibration at the European Space Agency in the Netherlands and ready to be deployed), communications links for the transmission of data, etc. To further improve the operational capabilities of the Gavdos facility, two more calibration sites have been developed at RDK1 (Jason pass) and CRS1 (cross-over of Sentinel-3 A&B, south west Crete), both in the mainland of Crete. Given the importance of these Cal/Val facilities in Gavdos and Crete Cal/Val, both from the scientific point of view and as a model satellite research centre, the continuation of its operation becomes vital.

The objectives of this proposed research are:

  1. To use Gavdos Cal/Val to continue perform inter-calibration/validation between, mainly, Jason-2/OSTM and Jason-3 missions.
  2. To calibrate current and future satellite altimetric missions, incorporating the newly developed transponder, as well.
  3. To continue providing the services of the permanent facility on Gavdos/Crete for monitoring and determining the absolute systematic errors in satellite altimeters;
  4. To extend the capabilities and calibration measurements of the Gavdos/Crete facilities to other operational, or future satellites, such as, Cryosat-2, Altika/SARAL, Sentinel-3, and the Chinese HY-2 (negotiations with the Chinese are underway), as well as to other measurement techniques, like wide swath, and delay Doppler.
  5. To provide the international scientific community with near real-time meteorological and oceanographic data/products for operational applications.

These objectives are fully aligned with the goals of this OST-ST joint research announcement. This Gavdos/Crete Cal/Val experiment includes the estimation of the satellite altimeter bias, as well as monitoring its drifts and calibrating its radiometer and ionosphere dealys with a permanent network of 10 GNSS sites. At Gavdos, this has been done using two independent techniques. The first one involves direct comparisons between the sea-surface height, measured by the altimeter, and the absolute sea level established by the Cal/Val facility. The second one concerns calibration with the microwave transponder. The benefit of using two independent procedures for calibration, as well as for inter-calibration of different missions can be accomplished with the Gavdos/Crete Cal/Val facility.

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