Craig Donlon (European Space Agency, ESTEC, Netherlands); Dimitrios Piretzidis (Space Geomatica, Greece); Costas Kokolakis (Space Geomatica, Greece); Robert Cullen (European Space Agency, ESTEC, Netherlands); Pierre Femenias (European Space Agency, ESRIN, Italy); Marco Fornari (European Space Agency, ESTEC, Netherlands); Jerome Bouffard (European Space Agency, ESRIN, Italy); Alessandro Di Bella (European Space Agency, ESRIN, Italy); Francois Boy (CNES, France); Xenophon Frantzis (Technical University of Crete, Greece); Achilles Tripolitsiotis (Space Geomatica, Greece); Mingsen Lin (National Satellite Ocean Application Service, China); Lei Yang (First Institute of Oceanography, China)

Two microwave transponders have been operating at the CDN1 Cal/Val site on the mountains of Crete for about 8 years, and at the GVD1 Cal/Val site on the island of Gavdos to calibrate international satellite radar altimeters at the Ku-band. This ground infrastructure is also supported at present by four sea-surface Cal/Val sites operating, some of them for over 20 years, while two additional such Cal/Val sites are under construction.
One corner reflector is already set up at the crossover of Sentinel-6 MF, Sentinel-3A and Sentinel-3b in Crete, while another one is ready to be established nearby. A network of cameras, GNSS- Interferometric Radiometry sites and drones are also to support calibration for wind and wave observations in the strait between Crete and Gavdos. This ground infrastructure 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.

This work presents a thorough examination of the transponder Cal/Val responses in an effort to understand and determine absolute biases in all satellite altimeters overflying this infrastructure.
The latest calibration results for the Jason-3, the Copernicus Sentinel-3A and -3B, Sentinel-6A , CryoSat-2 and the HY-2B, HY-2C 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, CryoSat-2, and HY-2B will be given using various techniques, infrastructure and settings.