## 2 The Permanent Facility for Altimeter Calibration
### Regional Models and Reference Surfaces  
Calibration of a satellite altimeter is carried out in open sea about 20 km away from the land Cal/Val site to avoid any contamination or interference of altimeter signals. This transfer of precise measurements from the Cal/Val site on land to the open sea requires absolute and accurate knowledge of the reference geoid and mean dynamic topography in the area (MDT). Geoid and MDT models in the region of western Crete and Gavdos have been developed after almost 20 years through dedicated gravity surveys (terrestrial, marine, and airborne), as well as by other independent satellite observations. To determine heights for geoid and MDT at certain locations at sea needed for calibration, various gravimetric and estimation techniques have been applied to original observations. These have been, for example, spatial statistics, colocation, Kriging, remove and restore, and several approximation and interpolation methods. The final produced uncertainty with these geoid models depends upon the instrumentation used for gravity observation, the models applied to convert gravity into heights, and the digital terrain models, including bathymetry, local conditions, etc., for observation reductions. All in all, the final height discrepancy in the absolute sense between the geoid and the ellipsoid is no less than \(\pm\)35 cm as an absolute value, though it does not exceed \(\pm\)1 cm in a relative sense for the region of calibration. Recent observations, for example with GOCE and GRACE gravity satellites may enable us to claim that the final produced accuracy of the absolute geoid height is about \(\pm\)8 cm for the Gavdos/Crete Cal/Val site. The absolute determination of the orthometric height at the Cal/Val site based on these models is not that clear and definitive, even today. It requires extreme and careful fine-tuning for satellite altimeter calibration whose confidence and accuracy improves over time and with experience. Height determination for the  
Figure 21: Cryosat-2 SAR raw waveforms during 10 May 2013 (**left**), and 14 May 2014 (**right**), of transponder calibration at the SLR2 site on the University Campus. The approximate distance between the satellite and the transponder was 236 m and 1100 m, respectively, for those two calibrations. Figure 20: The “SLR2” site was constructed next to the “TUC3” EGNOS site (**left**) on the University Campus as a temporary location for testing and evaluation of the transponder (**right**) before its final deployment at “CDN1” on the mountains of West Crete. Cal/Val observations is an extremely sensitive component in satellite calibration but at the same time it constitutes an influential but less known parameter when establishing a new Cal/Val site. In the past (circa 2010), the relative accuracy of the applied geoid and MDT models and calibrating techniques has been verified by boat, ship, and buoy campaigns, as bathymetry is deep and changes abruptly around Gavdos and western Crete (2010; 2011). These campaigns were made predominately along the satellite groundtracks at sea. In 1990-2008, ocean observations with drifters, but also dedicated cruises, have also established the local ocean circulation and sea conditions. In January 2003, an area of about 200 km \(\times\) 200 km was surveyed by an aircraft with an absolute gravimeter and laser altimetry, along with a dedicated survey ship at sea, around the PFAC (2010) (Figures 22 and 23). The data collected were integrated with previous gravity campaigns using airborne (2011), shipborne, and terrestrial resources to generate, through optimal combinations, several regional geoid models (2011). Details of those reference surfaces currently used to perform sea-surface calibration at the Gavdos/Crete PFAC are presented in Reference (2010). Bias results for all these altimeters (i.e., Jason-1, Jason-2, Jason-3, SARAL, HY-2A, and Sentinel-3A) have been obtained with sea-surface measurements, but also with crossover comparisons at sea.