A Permanent Infrastructure in Crete for the Calibration of Sentinel-3, Cryosat-2, and Jason Missions with a Transponder
Mertikas, Stelios¹; Tripolitsiotis, Achiles²; Mavrocordatos, Constantin³; Picot, Nicolas⁴; Femenias, Pierre⁵; Daskalakis, Antonis^2
1Technical University of Crete, GREECE; ²Space Geomatica Ltd., GREECE; ³ESA/ESTEC, NETHERLANDS; ⁴CNES, FRANCE; ⁵ESA/ESRIN, ITALY

Satellite altimetry provides the only means for monitoring changes in sea level over regional to global scales with [mm/yr] accuracy and with respect to the center of mass of the Earth. The achievement of this level of accuracy requires long-term and consistent ground calibration of the on-board satellite altimeters. The Gavdos Cal/Val facility has been operating and providing absolute biases for the Jason altimetry satellites for more than a decade. It was established in 2001 and has been on continuous operation as of 2004. Lately, the established infrastructure has been modernised and upgraded for satellite altimeter calibration and tectonic deformation monitoring. New equipment have been procured, developed and installed at several field sites in West Crete, as well the central facility has been upgraded. A new prototype microwave transponder has been developed and delivered in 2011 to serve as an alternative and independent technique for calibration of, mainly, European altimetric missions. Calibration of the transponder itself has been conducted at the Compact Payload Test Range facilities in the European Space Agency, in 2012. To extend and strengthen Gavdos operations, three Cal/Val facilities have been developed at several other locations, all on the mainland of West Crete. At first, the RDK1 site has been established in the Central-West Crete and which lies along the Jason No. 109 ascending track, while it serves as extension and validation of the Gavdos Cal/Val procedures. Secondly, in the South-West Crete, the CRS1 site has been created for the calibration of the Chinese HY-2 satellite. Finally, a site, called 'CDN1', located on a triple cross-over of the Jason, Sentinel-3A & 3B and AltiKa satellites, has already been identified, tested, and selected for altimeter calibration. Also, observations from tide gauges and other dedicated scientific sensors (i.e., GPS-Glonass-EGNOS-BeiDou receivers, meteorological, DORIS, SLR, etc.) installed at various Cal/Val facilities over the broader Crete/Gavdos region, will be used to provide a time series of absolute calibration values and drifts for the Sentinel-3 altimeter and radiometer. It will be applied for Sentinel-3A Passes No.7, 71 and 335 and for Sentinel-3B Passes No. 43 and 100. This work will present the preparatory steps taken, and the procedures to be followed for the establishment of a permanent calibration site for Sentinel-3 in the south west of Crete using the developed transponder. Calibration of both Sentinel-3A and Sentinel-3B as well as Jason satellites (and possibly AltiKa) will be performed with this ground infrastructure.