Gulf of Mexico Loop-Current Signature Observed From GNSS-R Phase Altimetry Based on Spire Global CubeSat Data

Global Navigation Satellite System (GNSS) coherent ocean reflections collected at grazing angles by Spire Global's CubeSats are explored to retrieve sea-level anomalies (SLAs) over the Gulf of Mexico (GoM). Dual-frequency Global Positioning System (GPS) carrier phase estimations are derived from the 50-Hz I and Q samples generated onboard the low Earth orbit (LEO) CubeSats. The first-order ionospheric phase advancement, troposphere delay, and precise orbit solutions for GPS satellites and CubeSats mitigate the range measurement errors. Comparisons between the collocated specular reflection tracks from various Spire CubeSats and GPS satellite configurations demonstrate the self-consistency of the resulting SLA solutions. The Spire retrievals are then validated against conventional altimetry SLA products. There is a root-mean-square (RMS) difference of similar to 20 cm between the Spire and the gridded model mesoscale SLAs, unless the reflected elevations are very low. To better evaluate the performance of the Spire retrievals, they are examined over sharp ocean topography. About 40% of coherent reflections with elevations above 12 degrees detected the signature of the energetic loop current during the 2020-2021 period. The Spire retrievals were also compared with the collocated Sentinel-3 tracks. One track with elevations above 12(?) demonstrated high correlation and agreement with the Sentinel-3 retrievals. The other two low-elevation tracks showed large magnitude discrepancies, but similar trends after applying appropriate scaling factors. The study demonstrates the potential for GNSS reflectometry's application to SLA mapping while also highlighting where significant improvements in error mitigations are needed. A particularly large error source is the troposphere error for signals with lower elevations.