GNSS4SurfaceWater: an open data hub for rapid GNSS-IR surface water monitoring

GNSS Interferometric Reflectometry (GNSS-IR) has evolved from an opportunistic use of geodetic reference stations towards purpose-built, low-cost sensors optimized for water-surface monitoring. Affordable GNSS-IR instruments are now specifically designed and positioned to observe water surfaces with optimized antenna geometry and controlled viewing conditions. This means we are no longer just picking up reflections when and where they happen to occur but instead purposefully measuring them for hydrological and environmental applications. This also makes GNSS-IR attractive for current and future satellite altimetry validation, particularly in regions where geoid uncertainty, sparse in-situ gauges or complex hydrodynamics limit traditional approaches. With this rapid development and increasing community interest, we present GNSS4SurfaceWater, an open data hub for sharing water-level time series from affordable GNSS-IR sensors following open-science hardware and software principles. The platform provides interactive visualization tools for exploring time series, station metadata, and site characteristics. It works as an independent, ground-based service for monitoring both current and historical surface water levels. GNSS4SurfaceWater highlights ongoing projects using low-cost GNSS instrumentation, promotes reproducible processing workflows and supports community contributions through standardized data upload formats. GNSS-IR sea-level products are also distributed through the Permanent Service for Mean Sea Level (PSMSL) GNSS-IR portal. This portal also aggregates contributions from multiple providers and ensures long-term data continuity. PSMSL focuses on long-term archiving, whereas GNSS4SurfaceWater is designed to provide community-driven near-real-time data availability with low latency to support rapid monitoring and event detection. The two platforms complement each other by supporting open and scalable GNSS-IR surface water monitoring and helping to broaden the adoption of GNSS-IR for hydrological observations.