Assessment of operational non-time-critical Sentinel-6A Michael Freilich radio occultation data: insights into tropospheric GNSS signal cut-off 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. The RO instrument has been performing very well since its launch in 2020, consistently surpassing its mission target of providing 770 quality-checked bending angle profiles per day. With a remarkable availability rate of 99.9 % during full operational periods, the mission demonstrates robust performance and reliability. 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 cut-off 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 RO data for weather forecasting and climate research.