Continuous coastal sea level measurement in the global geodetic coordinate frame: A realization with GNSS

Sea level is a critical environmental parameter with significant implications for both daily life and global environmental processes. Traditionally, coastal sea levels have been measured using tide gauges. However, tide gauge measurements are affected by the vertical land motion and some sites lack a consistent reference datum on a global scale. GNSS Interferometric Reflectometry (GNSS-IR) offers a promising alternative for continuous coastal sea level monitoring within a geodetic reference frame, supporting sea level studies on a global scale. Despite its potential, GNSS-IR is hindered by various error sources that can degrade and bias sea level measurements. Achieving both precise positioning and accurate sea level monitoring simultaneously remains a significant challenge. To enhance the accuracy of GNSS-IR sea level measurements and align the results within a geodetic reference frame, we developed a dual-antenna GNSS reflectometry and positioning system. The system features a zenith-pointing GNSS antenna optimized for positioning and a horizon-pointing antenna designed for sea level monitoring. These two antennas are tightly connected, with precise measurements of the vertical distance between them. Advanced GNSS-IR error models, including antenna phase center corrections, are incorporated to mitigate errors in the sea level retrievals. The performance of this system was evaluated using nearby geodetic GNSS installations, traditional tide gauge measurements, and leveling surveys. Initial results demonstrate that the zenith-pointing antenna achieves sub-centimeter level positioning precision, comparable to nearby GNSS reference stations. The horizon-pointing antenna provides sea level measurements with an 1σ error of approximately 2 cm on a temporal resolution of 15 min. Notably, the zenith-pointing antenna performs significantly worse for sea level retrieval, while the horizon-pointing antenna is less accurate for precise positioning. These findings highlight the necessity of a dual-antenna system for achieving optimal performance in both positioning and sea level retrieval. The developed system enables continuous coastal sea level monitoring within the GNSS reference frame with centimeter-level accuracy, offering a valuable dataset contributing to global sea level studies.