Exploring grazing angle GNSS-R for precision altimetry: A comparative study

Sea Level Anomaly (SLA) measurements are essential for understanding oceanic dynamics, climate variability, and climate change impacts. While satellite-based radar altimetry missions are the primary source of such measurements, their spatiotemporal resolution may sometimes be insufficient. This study explores the potential of Global Navigation Satellite Systems-Reflectometry (GNSS-R) as an additional approach for SLA retrieval. It exploits L-Band coherent carrier phase measurements collected by radio occultation receivers in Low Earth Orbit (LEO), known as Grazing angle GNSS-R (GG-R). We compare these GNSS-R measurements with those from traditional radar altimetry, in- cluding Sentinel-3 A/3B, Saral, and Cryosat-2. Our analysis of SLA data spanning from May 2019 to October 2021 reveals an average Root Mean Square Error (RMSE) of ∼47 cm among nearly 10,000 samples. We find that measure- ments derived from both techniques often complement each other when they meet recommended quality standards. Enhancing GG-R estimates could serve as a valuable complement to existing radar altimetry missions, which alone may not provide sufficient data. Furthermore, a comparison exclusively focused on GG-R events has been made to ensure consistency in the Spire GG-R retrievals, resulting in a 25 cm RMSE. Additionally, we conducted an assess- ment to evaluate the coherency and coverage of GG-R measurements. Approximately 24 % of the tracks are coherent, primarily located in the polar regions and calm waters.