Eric Leuliette (NOAA/NESDID/STAR, United States)

Sentinel-6 was launched on Nov. 2020, follows the same orbits of previous Jason series satellites and provides continuous global sea level height measurements. It is the first altimetry satellite with climate quality of Advanced Microwave Radiometer (AMR-C) aiming for reducing errors in the Wet Tropospheric Correction (WTC) toward accurate global sea level rise determination. The stability requirement for AMR-C contribution to global sea level rise observation is 0.7mm/year, which is more stringent than previous missions. To achieve this goal, Sentinel-6 has an onboard supplementary calibration system (SCS), including a warm calibration target and a cold sky mirror target, and performs frequent satellite maneuver for cold space view, to calibrate AMR-C brightness temperatures in three frequencies: 18.7 GHz, 23.8 GHz and 34 GHz. Due to the large footprint of microwave radiometers and distinct emissivity between land and ocean, altimeter missions often lack sea level estimation over coastal areas. The sentinel-6 has a high resolution microwave radiometer (HRMR), with three channels at 90GHz, 133GHz and 160 GHz and much smaller footprints than AMR-C. Thus, it would be possible to achieve atmosphere path delay estimation with similar accuracy as over open ocean at coastal area. The SCS is only for AMR-C calibration, not for HRMR. The HRMR is an experimental instrument with no stability requirement. However, its stability should be monitored with recent advances in coastal sea level rise estimation from altimetry missions. Due to satellite orbit changes, sensor stabilities, constant calibration algorithm updates, monitoring AMR-C and HRMR stability is necessary for achieving consistent global sea level height measurements and assures climate quality dataset for AMR-C.
In this study, different vicarious calibration means are used to validate the AMR-C/HRMR brightness temperature stability with time series check at on-Earth targets and inter-sensor comparison over more than two years period. The on-Earth targets include the “warm” reference sites, Amazon rainforest/Libya desert, and the “cold” reference, the coldest(driest) ocean surface. The Advanced Technology Microwave Radiometer System (ATMS) onboard Suomi-NPP is also used to validate the AMR-C/HRMR measurements over Simultaneous Nadir Overpass locations for channels with the same frequencies. Comparison with other altimetry missions (e.g. Jason-3) are also carried out to detect the brightness temperature relative bias and stabilities. A website with different validation methods has been set up at NOAA STAR for monitoring the near-real-time and long-term stability of microwave radiometers from different missions with Radar Altimetry Database System (RADS). RADS is NOAA’s operational altimetry database from multiple altimetry missions providing sea level anomaly, wave and surface wind products. The Sentinel-6 HRMR datasets from NASA JPL (different processing versions at different times) is also used for HRMR monitoring.
In addition, ATMS brightness temperatures of two channels at 23.8 GHz and 31.4GHz have been used to simulate the WTC based on the relationship derived from collocated Jason-3 WTC and ATMS brightness temperatures. The Sentinel-6 AMR-C WTC retrievals, the derived WTC using ATMS datasets, and the ECMWF (and Reanalysis V5) WTC simulations are then compared/validated against each other for stability check.