Accurate Tide Monitoring Using Shipborne GNSS-R Phase Altimetry: A Case Study

Tidal information is a valuable parameter for scientific studies and navigational safety. Despite traditional tide stations and satellite altimeters, shipborne Global Navigation Satellite System (GNSS) altimeters can provide an alternative method for instantaneous measurements. However, due to ship hydrodynamics and draft variations, especially for large vessels, the baseline between the GNSS positioning antenna and the sea surface is always unavailable or less accurate. This case study presents a novel ship-based altimetry method using GNSS-R phase altimetry, which is capable of accurately monitoring tidal information on a moving ship platform. The delay difference between the direct and reflected GNSS paths is obtained from the signal phase difference generated by open-loop tracking through a software-defined receiver. Spectral analysis was used to further solve the integer ambiguity problem of phase measurements, and then, accurate tidal information was obtained based on high-precision GNSS positioning. To evaluate the performance of the system, a case study of a shipborne experiment was conducted. The results show that the ship-based GNSS-R altimetry system can accurately measure the sea surface height variation. The root-mean-squared error (RMSE) is within 3.0 cm compared to the in situ value. This case study demonstrates the potential of ship-borne GNSS-R phase altimetry as an effective and accurate method for tidal monitoring in dynamic maritime environments.