Eric Leuliette (NOAA/STAR, United States)

Sentinel-6B (S6B) was launched on 16 November 2025, marking another milestone in satellite observations of global sea level measurements. At present, it orbits in a tandem phase with its predecessor, Sentinel-6 Michael Freilich (S6MF), during its commissioning phase. While the Poseidon-4 altimeter onboard Sentinel-6 measures the range between the satellite and the ocean surface, the Advanced Microwave Radiometer-Climate (AMR-C) measures the atmospheric conditions to derive a range correction, the wet troposphere correction (WTC), which accounts for the path delay caused by water vapor in the troposphere. Due to the sensitivity of the instrument to environment changes during and after launch, calibration coefficients determined pre-launch often do not fully represent the instrument’s behavior in orbit. Over time, the range correction may exhibit abrupt changes and drifts, which can propagate into the sea level measurements. Therefore, monitoring the stability of the AMR-C and the derived path delay correction, and making readily corrections to the drifts, are import steps to ensure the accuracy of the sea level measurements and reduce the uncertainty in the sea level rise trend determination in regional and global oceans.
At NOAA/STAR the stability of the S6B AMR-C is monitored in real-time using an in-house developed radiometer stability monitoring suites within the framework of the Radar Altimeter Database System (RADS)We will report the stability of the S6B AMR-C brightness temperatures and the WTC time series through comparison with S6MF. The tandem phase between S6MF and S6B facilitates the direct comparison of brightness temperatures and WTC. The real time AMR -C brightness temperature comparisons show significant drift and large jumps at 18.7GHz and 23.8GHz since 29 January 2026 compared to Sentinel-6MF in the near-real-time (NRT) and short-time-critical (STC) products. These brightness temperature jumps and drifts propagate into the WTC difference time series.
Using ground-based vicarious calibration references and modeled WTC from ECMWF (including ERA-5), we assess the relative stability between S6MF and S6B. For brightness temperatures, stability is evaluated using reference targets such as the Amazon rainforest and coldest ocean regions. We also make inter-mission comparisons with Advanced Technology Microwave Sounder (ATMS) onboard three JPSS satellites (SNPP, NOAA-20 and NOAA-21) using Simultaneous Nadir Overpasses (SNOs). For WTC, comparisons with ERA-5 and ECMWF will be presented for both S6MF and S6B.
In addition, both S6MF and S6B carry a High-Resolution Microwave Radiometer (HRMR) to improve sea level measurements near the coast. We will assess HRMR stability for S6MF and S6B, through inter-comparison with JPSS ATMS for common channels. We also developed radiative transfer model for AMR-C/HRMR to quantify the brightness temperature stability in comparison with ERA-5. The brightest temperatures will be validated against the simulated brightness temperature from model.