# Development of a GNSS Buoy for Monitoring Water Surface elevations in Estuaries and Coastal Areas
## 2 Methodology  
Real-time centimeter-level accuracy positioning based on GPS measurements was developed in the mid-1990s, and is referred to as RTK positioning [13]. One drawback of this technology is that the distance between the rover station and the reference station must not exceed 20 km. The network-RTK was proposed, expanding the baseline limitation to 50 km. According to [PERSON] [13], four methods are available for transferring network information to the rover, the VBS being the most widely used. This position technology is called virtual reference station (VRS) RTK, or VBS-RTK. Other high-accuracy positioning technologies are currently available, such as precise point positioning (PPP) and PPP-RTK. PPP utilizes only a single receiver for positioning. Unlike those of network-RTK, the correction data utilized in PPP are not obtained from the network. They require precise satellite orbits and clocks that may be obtained from organizations such as the International GNSS Service (IGS). However, establishing a reference network for PPP positioning is not necessary. [PERSON] et al. [14] combined the advantages of network-RTK and PPP and called it \"PPP-RTK\". They assumed that the satellite orbits from the IGS were correct, and utilized data from the ground-based reference network to estimate satellite clock corrections. They also estimated tropospheric delays, receiver clocks and ambiguity parameters. The carrier phase ambiguities were resolved for PPP-RTK clocks corrections, but were not required for PPP clocks corrections. According to [PERSON] et al. [15], the accuracy of PPP and PPP-RTK are at the decimeter and centimeter levels, respectively. The decimeter level accuracy may be viewed as a drawback of PPP. A further drawback of PPP is the long calculation time required for convergence. The integration time for PPP is 30-1800 s, while that for PPP-RTK is 10-50 s. [PERSON] et al. [14] indicated that the main advantages of PPP-RTK over PPP are faster convergence of solutions and improved kinematic positioning. The main advantages of PPP-RTK over network-RTK are the lower bandwidth requirement caused by less data requiring transmission, and that a 1-2 cm horizontal accuracy can be achieved, even for highly kinematic rovers, at a distance of up to 1000 km from the network. PPP and PPP-RTK are not suitable for incorporation into GNSS buoys because their convergence times are too long for the real-time monitoring of tides and ocean waves. Furthermore, Taiwan has no service networks for PPP-RTK; hence, the VBS-RTK positioning technology was adopted in this work. Based on the VBS-RTK positioning technology, this work developed a GNSS buoy for monitoring real-time water surface elevations at the mouths of rivers and in coastal areas. The GNSS buoy contains a buoy hull, data transmission devices, a data logging device, and a RTK GNSS receiver. The methodology for developing a GNSS buoy and the associated theory for obtaining the wave spectrum are discussed in this paper. Section 2.1 describes VBS-RTK positioning technology. To compare the water surface elevations obtained from a RTK GNSS receiver with those obtained from an ATC sensor, the procedures for deriving water surface elevations from the accelerations measured by the ATC sensor are briefly discussed in Section 2.2 and detailed in Appendix A. The procedures for determining the directional wave spectrum and dominant wave direction from the data provided by the GNSS buoy and ATC sensor are provided in Sections 2.3 and 2.4, respectively, and detailed in Appendix B. The wave parameters could also be calculated using the raw data obtained from the ATC sensor. An ATC sensor consists of three accelerometers and three magnetometers, and each of these sensorsis installed along one of three orthogonal axes. The ATC sensor measures accelerations, inclinations, and the azimuth. Inclinations are computed from the accelerometers and the azimuth is calculated from the magnetometers.