# Beach Deployment of a Low-Cost GNSS Buoy for Determining Sea-Level and Wave Characteristics
## 3 Results and Interpretation
### GNSS-Derived Tidal Heights  
The RTKLIB software [5] has a number of options for post-processing GNSS data. Table 2 shows the general settings that produced the best overall solutions. GNSS data from GPS and global navigation satellite system (GLONASS) satellites were post-processed in 'kinematic mode' with respect to a land-based reference station. Post-processed kinematic (PPK) mode is an alternative technique to real-time kinematic (RTK) and differs in that the solution algorithms can be applied both forward and backward (to increase the accuracy of the overall solution) and more accurate timing corrections can be applied later. In addition, the elevation mask was set to 15\({}^{\circ}\) above the horizon (i.e., only using satellite data with angles from horizon greater or equal to 15\({}^{\circ}\)). The difference to a previous set-up [8] was to allow the RTKLIB software to use the GLONASS data to resolve ambiguities; this did slightly improve the results with ~5% more fixed solutions. To produce centimetre-accurate solutions, the RTKLIB software uses both the code phase and, in particular, the carrier phase satellite data recorded by the base (static GNSS site at Blackpool) and rover (GNSS buoy) receivers. The software processes the data with an algorithm to determine the exact number of radio wavelengths between the satellites and the base station antenna (a method known as 'ambiguity resolution') and yield either a fixed or float solution. In a fixed solution (referred to as Q = 1, this is generally an indicator of a more accurate positional result), the number of wavelengths is a whole number, or integer, and the algorithm is constrained to yield a whole number. A low number of visible satellites, poor satellite constellation geometry, and distance between the rover and the base station is likely to prevent a fixed solution. If the algorithm does not yield an acceptable fixed solution, then the ambiguity is allowed to be a decimal or floating-point number and the solution is known as a float solution (Q = 2). Generally, results with Q1 solutions should indicate more accurate positional data than Q2 solutions. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Solution & Kinematic \\ Frequency & L1 \\ Sensor dynamics & On \\ Earth tides & Off \\ Ionosphere & Broadcast \\ Troposphere & Saastamoinen \\ Ephemerides & Precise \\ Satellite system & GPS + GLONASS \\ Ambiguities GPS & Fix \& Hold \\ Ambiguities GLONASS & On \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameter settings for the post-processing using RTKLIB. Poorer solutions are often due to cycle slips which occur when the GNSS receiver loses its lock with a satellite; this results in a miss count of carrier phase cycles. These are more likely to occur when the receiver is moving, such as on the GNSS buoy with its horizontal and vertical movements. Cycle slips can be caused by obstructions, high accelerations, reflections, vibration or lack of a suitable antenna ground plane. Once the cycle count is lost the phase-bias estimate (within the RTKLIB software) must be reset and re-converged and this can take up to a minute or longer to recover. The satellite data recorded in Rinex format from both the rover and base stations were processed in two parts, covering tidal cycle A and tidal cycle B, respectively. Table 3 shows the percentage of fixed to float solutions. Q1 percentages shown in Table 3, ranging from 48.4% to 57.0%, indicate that at times during the deployment the GNSS receiver on the buoy was being affected by one or more of the conditions mentioned above. In contrast, the same GNSS buoy was used in a previous deployment in a relatively sheltered part of the harbour at Holyhead [8] and obtained almost 100% of Q1 solutions.