Craig James Donlon (ESA-ESTEC, Netherlands); Marco Fornari (RHEA for ESA, Netherlands); Stelios Mertikas (Technical University of Crete, Greece); Laura Fioretti (ARESYS, Italy); Adrián Flores (isardSAT , Spain); Adrià Gómez (isardSAT , Spain); Ferran Gibert (isardSAT , Spain); Sergi Hernández (isardSAT , Spain); Andrea Recchia (ARESYS, Italy); Konstantinos (Costas) Kokolakis (Technical University of Crete, Greece); Demetris Piretzidis (Space Geomatica PC, Greece); Etienne Ferrer (CLS, France); Samira Amraoui (CLS, France); Robert Cullen (ESA-ESTEC, Netherlands); Pierre Féménias (ESA-ESRIN, Italy); Luisella Giulicchi (ESA-ESTEC, Netherlands); Alejandro Egido (ESA-ESTEC, Netherlands); Michele Scagliola (RHEA for ESA, Italy); Francois Boy (CNES, France); Jean Damien Desjonqueres (JPL, USA); Claire Maraldi (CNES, France); Marco Meloni (EUMETSAT, Germany); Cristina Martín-Puig (EUMETSAT, Germany); Remko Scharroo (EUMETSAT, Germany); Walter Smith (NOAA, USA)

The Sentinel-6 Transponder Group is an ESA initiative that was born to share the Sentinel-6 calibration activities, data and results between the different companies and agencies involved on them. We have had a total of 31 meetings, sharing results between the different groups processing schemes

This work presents the calibration results for the Sentinel-6 using the permanent CDN1 transponder Cal/Val facility. Evaluation started on 18-Nov-2020 and continues without interruption till today. The CDN1 Cal/Val is part of the ESA Permanent facility for Altimeter Calibration and serves as a dedicated external calibration source for multi-mission (S3A, S3B, Jason-3, CryoSat-2) satellite altimetry.

Sentinel-6 has passed over the CDN1 Crete transponder during almost 3 years. High-resolution Short Time Critical and Non Time Critical products have been analysed by the transponder and processing experts from the group to assess performance of the Poseidon-4 altimeter. In particular, the instrument performance in the sense of absolute range bias and datation bias were assessed together with the instrument capabilities in terms of achievable resolution in both the along- and the across-track direction. Diverse processing methodologies and algorithms were implemented within this group to cross-validate results.

Together with the CDN1 transponder, GVD1 transponder, installed in the S6 crossover in the island of Gavdos, has been providing performance results for both ascending and descending since October 2021. On top of transponders, corner reflectors has been proven to provide good performances for S6. The Montsec corner has been providing retrievals since September 2021. The diversity of infraestracture provide reliable and redundant capabilities that ensure the long term calibration and validation of Sentinel-6.