Sentinel-6 MF Poseidon-4 Radar Altimeter In-Flight Calibration and Performances Monitoring

Poseidon-4 is a dual-frequency redundant radar altimeter, embarked onboard the Sentinel-6 Michael Freilich (MF) European Commission Copernicus Programme satellite. In this article, we assess Poseidon-4 main instrumental improvements and performances with the presentation of the more important outcomes from the in-flight internal calibration modes and an external calibration analysis over a transponder. The instrumental performances of the radar altimeter are excellent for both radar chain sides: Poseidon-4 delivers a range/azimuth instrument impulse response (IR) with the highest quality and fidelity in the era of space-borne radar altimetry, and its thermal noise response is almost just random noise. A power decay of the level of the transmitted power in the Ku-band has been detected both for the nominal and redundant sides, which is larger than expected, though it will not violate the requirement of the minimum signal-to-noise ratio over the ocean at the end of the satellite design lifetime. The innovative CAL1 ECHO CAL calibration mode allows for very precisely characterizing the sensitivity of the instrument IR to the in-orbit temperature variations and, thus, correcting for it in the science data as standard practice.