ISPRS-Annals ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences ISPRS-Annals ISPRS Ann. Photogramm. Remote Sens. Spatial Inf. Sci. 2194-9050 Copernicus Publications Göttingen, Germany 10.5194/isprs-annals-X-5-W4-2025-181-2026 Assessment of Potential GNSS-IR Sites in Existing Tide Gauge Sites in the Philippines Cruz Leila Micahella D. 1 2 3 Molleno Jasper B. 1 2 Mabaquiao Luis Carlos S. 1 2 Reyes Rosalie B. 1 2 Maclang John Dave C. 1 2 Ngo Dave Liam T. 1 2 Zamora John Michael Rey A. 2 4 Narca Alexi Mae Y. 1 2 Garcia Hanna Mae M. 1 2 Bringas Dennis Arsenio 5 Penaranda Shelmark 5 Algaba Beaverlex 5 Venerable Joshua 5 Department of Geodetic Engineering, University of the Philippines, Diliman, Quezon City, Philippines Training Center for Applied Geodesy and Photogrammetry, University of the Philippines, Diliman, Quezon City, Philippines Marine Science Institute, University of the Philippines, Diliman, Quezon City, Philippines Electrical and Electronics Engineering Institute, University of the Philippines, Diliman, Quezon City, Philippines Physical Oceanography Division, Hydrography Branch, National Mapping and Resource Information Authority, Manila, Philippines 10 02 2026 X-5/W4-2025 181 188 Copyright: © 2026 Leila Micahella D. Cruz et al. 2026 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://isprs-annals.copernicus.org/articles/X-5-W4-2025/181/2026/isprs-annals-X-5-W4-2025-181-2026.html The full text article is available as a PDF file from https://isprs-annals.copernicus.org/articles/X-5-W4-2025/181/2026/isprs-annals-X-5-W4-2025-181-2026.pdf

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 <em>gnssrefl</em> QuickLook module. Results indicate that reflection geometry, rather than receiver specification, governs feasibility: five GNSS stations namely, Davao, Guiuan, Legazpi, Manila &amp; El Nido yields virtually no quality-controlled reflector height estimates, whereas Brooke&rsquo;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.