On The Relationship Between The GNSS-R Signal SNR and Coherency With Surface Water: A Case Study Over Lake Okeechobee

This paper describes a preliminary investigation into the relationship of Global Navigation Satellite System-Reflectometry (GNSS-R) phase-based coherency and signal-to-noise ratio (SNR) with the extent of surface water contained in the reflected the signal's spatial footprint. Our study focuses on 82 phase-based GNSS-R tracks collected by Spire Global grazing angle satellites in 2021 over Lake Okeechobee, FL. For each track, we calculate its spatial footprint (assumed equal to the First Fresnel Zone (FFZ)) and determine the amount of surface water within the FFZ using a lake mask derived from the 2021 Global Surface Water Explorer (GWSE) Seasonality product. The percentage of surface water in the signal footprint (denoted %F) is compared with the reflected signal's SNR and level of coherency, with coherency measured by the phase-rate circular length. Overall, we see a positive relationship between SNR, circular length, and %F. However, circular length appears to be a more robust indicator of water, with more consistent, high magnitudes once the scattering surface is dominantly water (>70%F). Further, circular length appears to maintain a high magnitude at different distances into Lake Okeechobee when the footprint is full of water, whereas SNR gradually decreases as the signal track approaches the center of the lake. This is most likely a result of wind roughening the lake surface farther away from the shore.