Quality Assessment of Spire GNSS-R Ocean Surface Wind Data: Evaluating Accuracy, Uncertainty, Stability, and GNSS Dependence

Global Navigation Satellite System Reflectometry (GNSS-R) is a remote sensing technique that uses surface-reflected GNSS navigation signals to gather information about the Earth’s surface, such as its dielectric properties and roughness state. This technique enables the retrieval of ocean surface wind (OSW) and soil moisture measurements. Spire Global Inc. launched its first GNSS-R satellites in 2019. It delivered Spire-derived ocean surface wind (OSW) data from January to July 2024 to NOAA under the NOAA Commercial Data Program (CDP). A Pilot study is being conducted to assess the quality and impact of Spire GNSS-R observations for measuring OSW and other Earth surface characteristics. This study evaluates the Spire GNSS-R OSW speed data quality by comparing them with collocated ECMWF Reanalysis v5 (ERA5) wind speed data and Jason-3 altimeter wind speed data. ERA5 reanalysis wind data, available at 6-hour intervals and over 0.5-degree grids, are spatially and temporally interpolated for collocated comparisons with Spire OSW. Jason-3 altimeter wind speed is derived from the Ku-band backscatter coefficient and significant wave height. The collocation between Spire and Jason-3 altimeter measurements is constrained to be within 1 hour and 50 km. The accuracy and uncertainty of Spire OSW speed are evaluated by calculating the differences between Spire OSW and ERA5 wind speeds, as well as between Spire and Jason-3 altimeter OSW speeds. These differences are analyzed to understand their dependence on various factors, including specific Spire satellites (FM110, FM146, FM147, and FM172), GNSS transmitters (GPS, Galileo, and BeiDou), antenna beamforming, and incidence angle. Spire/GPS OSW has more data than Spire/Galileo and Spire/Beidou. The stability of Spire OSW data over six months ( v2.07 Spire OSW data from February 15 to July 2024) is assessed through time series analysis of Spire-ERA5 and Spire-Jason-3 OSW biases and standard deviations. Particular interest is given to the impact of data reduction on the overall stability of Spire OSW, especially after mid-June 2024, when three of the four Spire sensors ceased data transmission, leaving only one operational Spire satellite. We examine the functional relationships between OSW and measurement parameters such as Sigma0 (normalized Bistatic Radar Cross-Section) and MSS (Mean Square Slope) to trace Spire OSW retrievals to intermediate measurement parameters. These relationships are assessed by comparing Sigma0 and MSS with collocated ERA5 and Jason-3 altimeter wind data, and we discuss the uncertainties and GNSS-dependence of these functional relationships. This assessment aims to characterize and understand the quality of Spire GNSS-R OSW data, assisting in the preparation for Spire OSW observation error characterization for weather forecasting.