The Permanent Facility for Satellite Altimetry Calibration in Gavdos/Crete, Greece: Fifteen years of Cal/Val Service.

This work describes the gradual development of the Gavdos/Crete Cal/Val permanent facility as of 2001 in terms of infrastructure, instrumentation and the seasurface as well as transponder calibrations along ascending and descending orbits of satellite altimeters. The difficulties encountered during this period are described, along with measures taken to overcome them. Absolute calibration results are given for Jason-1, Jason- 2 and Jason-3 along their ascending Pass No. 109 together with the descending Pass No.18 for 2004-2017. Sea-surface Cal/Val results for Jason-3 are compared against transponder calibrations carried out along the same orbit at the CDN1 Cal/Val site on the mainland of Crete. Relative calibrations are also given between Jason-3 and Sentinel-3A & 3B simultaneously with seasurface and transponder techniques. Other Cal/Val results for past satellites such as SARAL/AltiKa and the Chinese HY-2 will also be reported. Guidelines for keeping an accurate, reliable, objective, homogeneous and continuous monitoring of oceans, inland waters as well as Polar Regions with altimetry will also be given. Transparent protocols and best practices for establishing new Cal/Val sites according to the standards of Fiducial Reference Measurement for altimetry will be presented.