Assessment of Operational Non-Time Critical Sentinel-6A Michael Freilich Radio Occultation Data: Insights into Tropospheric GNSS Signal Cutoff Strategies and Processor Improvements

Abstract. This study presents an exhaustive assessment of the Sentinel-6A Michael Freilich Radio Occultation (RO) data, focusing on the evaluation of bending angle products derived from the EUMETSAT-provided RO Non-Time Critical (RO-NTC) data collected between September and December 2021.

This analysis confirms the satellite's capability to exceed its target of 770 quality checked bending angle profiles per day, with an availability rate of 99.9 %, demonstrating the robustness of the mission's operational performance. A detailed examination of the Signal-To-Noise Ratio (SNR) and phase noise indicates the high-quality nature of the data. The study also analyses the benefits of employing SNR-based signal cutoff strategies and L2 signal extrapolation in the troposphere, where it is more susceptible to SNR reductions. Furthermore, the paper details some processor enhancements, which led to improved bending angle statistics, particularly below 22 km altitude. Additionally, the analysis revealed terrestrial interference signals on the L2 frequency, confirming that they do not significantly compromise the Sentinel-6A RO data quality.

The validation of the EUMETSAT processed Sentinel-6A RO-NTC data against the European Centre for Medium-Range Weather Forecasts (ECMWF) short-range forecasts and comparisons with Metop-B/C and EUMETSAT-processed SPIRE occultations, highlights the reduction in random error and modifications in the tropospheric bias structure, a result of the enhancements in data processing techniques. This comprehensive analysis confirms the high quality of the EUMETSAT Sentinel-6A bending angle products and underlines the satellite's contribution to the EUMETSAT legacy of precise and reliable radio occultation data for weather forecasting and climate research.