Radar Altimetry External Calibration with Passive Elements: Sentinel-6 MF Long Term Monitoring with the Montsec Corner Reflector Facility

External calibration of radar altimeters by means of passive elements such as trihedral corner reflectors only became feasible after the development of Fully-Focussed SAR techniques specific for radar altimeter processors. Since then, corner reflectors have been demonstrated to be very interesting tools for instrument monitoring, with performances comparable to ones from active transponders for range and datation measurements, but also useful for power and radar cross section measurements. The corner reflector deployed by isardSAT in 2021 close to the ground tracks of Sentinel-6 MF, Sentinel-3B and CryoSat-2 has already been operative for more than two years. In this manuscript the main Sentinel-6 MF measurements are reported, showing excellent performance in terms of precision. A first analysis of correlation between the measurables and geometric and geophysical parameters is also provided.