# Development of a GNSS Buoy for Monitoring Water Surface elevations in Estuaries and Coastal Areas
## 1 Introduction  
Real-time tide data for estuaries are usually employed as boundary conditions in simulations of river water levels to assess flood risks. To study sediment transport in coastal areas, data on tides, waves, and currents are required; however, a simple platform for measuring tide in estuaries and coastal areas is not available. Although a pile can be set up and used as an observation platform, piles are expensive, and the piles in such areas are subject to bed scouring, which increases the difficulty of performing long-term monitoring of tides and waves. Floating data buoys have been verified to be reliable platforms for ocean monitoring and they have been deployed worldwide to provide long-term and real-time meteorological and oceanographic data, such as wind speed and direction, barometric pressure, air and water temperatures, wave height, period and direction, and current speed and direction [1]. However, conventional data buoys do not measure tides or water surface elevations. Over the past decade, Global Positioning System (GPS) buoys have been developed to measure tide or ocean surface wave data. [PERSON] et al. [2] developed a GPS buoy with a single-frequency Real-Time Kinematic (RTK) GPS receiver to observe tsunamis in real time. The height of the buoy exceeded 13 m, and the weight was approximately 10 tons. Data from the buoy and a land-based GPS receiver were used to determine the position of the buoy using RTK processing software. In their study, the GPS buoy successfully recorded vertical motion in rough and calm seas. [PERSON] et al. [3] used the GPS buoydeveloped by [PERSON] et al. [2] to observe tsunamis. The GPS buoy utilized by both [PERSON] et al. [2] and [PERSON] et al. [3] was deployed within 20 km of the coast. The submerged structure of their buoy has a depth of 8.96 m, which could make its deployment in estuaries and coastal areas difficult. [PERSON] et al. [4] utilized a low-cost car-navigation-class GPS receiver and developed a high-pass filter for a buoy to observe waves. Because most GPS positioning errors are below 0.01 Hz and the frequency of ocean waves is approximately 0.1 Hz, a high-pass filter was applied to extract the motion of the GPS-equipped buoy under the excitation of ocean waves, thus minimizing the position error. The accuracies reached several centimeters for the wave height and \(5^{\circ}\) for the wave direction. However, the system developed by [PERSON] et al. [4] cannot observe tides. [PERSON] et al. [5] used the GPS system developed by [PERSON] et al. [4] on a moored buoy to measure surface waves. Observations made by the buoy were compared with wave records from a nearby observation tower. Both H\({}_{1/10}\) and T\({}_{1/10}\) were well correlated with the tower observations, suggesting that their GPS buoy determined the wave heights correctly. [PERSON] et al. [6] established a real-time reference station for ground motion detection and reference. In their study, tsunamis were observed in real time, and tides were analyzed, however, the focus of this study was on the analysis of tide data and tsunami warnings rather than wave data from the GPS buoy. [PERSON] et al. [7] utilized velocity signals from a GPS receiver on a buoy to obtain wave data. The one-dimensional wave spectrum was derived from the velocity spectrum. The GPS receiver was installed on a moored buoy attached with an accelerometer-tilt-compass (ATC) sensor. Wave data from both sensors were validated. The results indicated that the wave information derived from velocity signals was a reasonable alternative; however, the developed buoy did not measure tides. [PERSON] et al. [8] evaluated the capabilities of various GPS-tracked buoys for observing ocean surface waves. Various sensors such as ATC sensor, GPS receiver based on the Doppler shift in GPS signals, and GPS receivers based on the satellite-based augmentation system (SBAS) absolute position data, were mounted on test buoys. The researchers found that the SBAS receivers could accurately resolve horizontal wave orbital displacements.