Sentinel-6 Michael Freilich Poseidon-4 Altimeter In-Orbit Performance.

The main payload of the Sentinel-6 Michael Freilich (S6-MF) is the Poseidon-4 Ku- and C-band Radar Altimeter launched Nov 18th 2020 was developed to continue the ocean surface topography reference data record that commenced in 1992 (TOPEX/Poseidon), was followed by Jason-1 (2001), Jason-2 (2008) and Jason-3 (2016). The satellite has been developed to, as a minimum, meet in-orbit performances of the Jason-2 and adopts, for the first time, requirements concerning estimated Global Mean Sea Level (GMSL) drift and stability. The design of S6 has also taken into account the in-orbit performances of the CryoSat-2/Sentinel-3 missions and their altimeter designs. Integrated into the design is the capability to allow for further on-ground processing enhancements that now demonstrate focused processing capabilities that can be performed globally for evolving future applications. This paper provides an overview of the key performances of the mission at the end of its commissioning and tandem phase with Jason-3, that has also allowed for the first time the characterization of both nominal and redundant chains of the altimeter. It is now demonstrated that the mission meets all its commissioning objectives and all mission level requirements are met, though long-term estimates of GMSL stability requires a longer time series of around 3 years. The Poseidon-4 design and its performances has provided the basis for future missions, such as CRISTAL and potentially for the Sentinel-3 and 6 Next Generation systems.