Assessment of Potential GNSS-IR Sites in Existing Tide Gauge Sites in the Philippines

Abstract. GNSS Interferometric Reflectometry (GNSS-IR) converts GNSS signal-to-noise data into decimeter-scale sea level measurements; however, the Philippines currently lacks any operational GNSS-IR stations, leaving a gap in the national sea level observation network. This study evaluates the gap by assessing eight coastal GNSS stations of the National Mapping and Resource Information Authority (NAMRIA) together with 60 primary tide gauge stations to identify where GNSS-IR could be most effectively deployed. Fresnel zone footprints were mapped for each location, and RINEX files from the GNSS stations were processed using the Lomb-Scargle Periodograms (LSP) via the gnssrefl QuickLook module. Results indicate that reflection geometry, rather than receiver specification, governs feasibility: five GNSS stations namely, Davao, Guiuan, Legazpi, Manila & El Nido yields virtually no quality-controlled reflector height estimates, whereas Brooke’s Point, Cagayan de Oro, and San Fernando produce 5-9 valid retrievals per day once the quality control is relaxed. In contrast, Fresnel zone analysis of the standalone tide gauge stations revealed several promising sites for GNSS-IR deployment, such as Baler, Jose Panganiban, and Pasacao. Other sites may become feasible with minor relocations and site-specific planning. The results delineate immediate opportunities and constraints for Philippine GNSS-IR expansion; follow-up work should prioritize on-site reconnaissance, refining equipment, and phased establishment of a dedicated GNSS-IR sea-level network.