Craig Donlon (European Space Ageancy/ESTEC, Netherlands); Dimistrios Piretzidis (Space Geomatica, Greece); Constantine Kokolakis (Space Geoamtica, Greece); Robert Cullen (European Space Agency/ESTEC, Netherlands); Pierre Femenias (European Space Agency/ ESRIN, Italy); Marco Fornari (European Space Agency, Netherlands); Francois Boy (CNES, France); Berthyl Duesmann (European Space Agency/ESTEC, Netherlands); Mingsen Lin (National Satellite Ocean Application Service, China); Yongjun Jia (National Satellite Ocean Application Service, China); Lei Yang (First Institute of Oceanography, China); Xenofon Frantzis (Technical University of Crete, Greece); Achilles Tripolitsiotis (Space Geomatica, Greece)

A microwave range transponder has been operating at the CDN1 Cal/Val site on the mountains of Crete for about 6 years to calibrate international satellite radar altimeters at the Ku-band. This CDN1 transponder is part of the European Space Agency Permanent Facility for Altimetry Calibration (PFAC), and has been producing continuously as of 2015 a time series of range biases for Sentinel-3A, Sentinel-3B, Sentinel-6A, Jason-2, Jason-3 and CryoSat-2. A second range and sigma-0 transponder at a crossover location of Sentinel-6 “Michael Freilich” on the island of Gavdos is also operational as of 11-Oct-2021. This work presents a thorough examination of the transponder Cal/Val responses in an effort to understand and determine which factors are responsible for the observed changes in the Cal/Val results. Fourteen uncontrolled and three controlled sources of variations have been investigated.
The latest calibration results for the Jason-3, the Copernicus Sentinel-3A and -3B, Sentinel-6A and the HY-2B radar altimeters will be described based on four sea-surface and two transponder Cal/Val sites of the PFAC in west Crete, Greece. Absolute biases for Jason-3, Sentinel-6A, Sentinel-3A, Sentinel-3B and HY-2B will be given using various techniques, infrastructure and settings.