# Development of a GNSS Buoy for Monitoring Water Surface elevations in Estuaries and Coastal Areas
## 2 Methodology
### VBS-RTK Positioning Technology  
In this work, the VBS-RTK positioning technology was adopted to determine the position of the buoy. The VBS-RTK system consists of three components, a GNSS base station network, control center, and rover station. Their functions are as follows:  
1. GNSS base station network: Each base station receives GNSS observation data and transmits raw data to the control center continuously. Currently 78 base stations are located in Taiwan. 2. Control center: The VBS-RTK control center for positioning computation used in this work is operated by the National Land Surveying and Mapping Center (NLSC), Ministry of the Interior, Taiwan. After 1 September 2014, the NLSC upgraded the network by replacing a GPS system with a GNSS system. Progressive infrastructure via overlaid technology, the commercial software developed by Trimble Navigation, is run in the center. This software includes three modules: Trimble Instrument Configurator, Trimble Ephemeris Download, and Trimble Streaming Manager. Their main functions are as follows: 1. Connecting the control center and each reference station to enable the automatic receipt, storage, and compression of observations from each reference station. 2. The software not terminating the receipt or compression of satellite signals from reference stations during data download. 3. Monitoring the status of the GNSS receiver of each reference station. The GNSS receiver parameters may be configured including the cutoff angle and sampling interval, etc. 4. According to the carrier phase observations, the software calculates continuously the error caused by the multipath, the ionosphere, troposphere, and ephemeris; as well as the integer ambiguity of the carrier phase of L1 and L2. 5. Generating VBS data in Radio Technical Commission for Maritime Services (RTCM) format and transmitting them to the rover station. When experiments were conducted, the Trimble product GPSNET was running in the control center, and the limit of the baseline length was 30 km. For tropospheric modeling, a modified Hopfield model was utilized to correct for both geometric displacements and tropospheric differences in GPSNET[17]. [PERSON] et al. [18] at Trimble Terrasat presented a model for extracting characteristic \begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **Parameter** & **Specification** \\ \hline \multirow{4}{*}{Acceleration} & range & \(\pm\) 1 g \\  & accuracy & \(\pm\)10 mg \\  & bias & \(<\)\(\pm\)10 mg \\  & frequency response & 20 Hz \\ \hline \multirow{4}{*}{Attitude} & range & \(\pm\)30\({}^{\circ}\) \\  & accuracy & \(\pm\)0.2\({}^{\circ}\) (to 20\({}^{\circ}\)), \(\pm\)0.3\({}^{\circ}\) \\  & frequency response & 0.5 Hz \\ \hline \multirow{3}{*}{Heading} & range & 0–360\({}^{\circ}\) \\  & accuracy & \(\pm\)3.0\({}^{\circ}\) (magnetic inclination \(<\) 75\({}^{\circ}\)) \\ \cline{1-1}  & frequency response & 10 Hz \\ \hline \hline \end{tabular}
\end{table} 
Table 1: ATC wave sensor specifications. parameters to describe the ionosphere across a network of reference stations using GPS measurements. The main concept was an expansion of the ionospheric delay in terms of a series of orthogonal functions across the area spanned by the piercing points of the ionosphere. In the VRS, the rover did not have to deal with the buildup of these models. Furthermore, Trimble VRS enabled the server to compute the network errors at the rover location from a complex physical model by using information from the full reference station network [19]. The data processing in GPSNET used an optimal Kalman filter to process observations from all reference stations and to model all error sources, including ionospheric and tropospheric effects, orbit and clock errors, multipath interference, and reference station receiver noise. This central processing computed the complete state vector by describing all the aforementioned error sources with an update rate of 1 Hz. The approach involved the Trimble patented factorized multi-carrier ambiguity resolution algorithm to resolve the ambiguity.