## 3 Structural Monitoring via Global Navigation Satellite System: The Case of High Wind Turbines
The names of the Galileo signals are the same as the center frequencies on which they are carried [47]. The numerical values are reported in the Table 1.  
Figure 2: Schematic of real-time kinetic (RTK)-global navigation satellite system (GNSS) method. This gives more possibilities for the implementation of real-time GNSS solutions to provide continuous monitoring. In order to assist the SA control system in making better decisions by providing precise on-time information, the ability to detect and characterize displacements quickly and to update the position can make a crucial difference in the wind turbine SA control. The availability of low-latency data has a fundamental impact on how we perform the SA control, since the size of latency in practice cam result in wrong real time information. Latency is the time taken for the receiver to calculate and output a position. Lower latencies and higher position update rates mean that the SA control system is more responsive and can be operated without loss of performance. GNSS monitoring receivers are used, typically gathering high-rate data at 10-20 Hz. Acquiring data at the maximum sampling rate, up to 50-100 Hz (ultra-high-rate), the highest performing receivers could identify high structural vibration frequencies of up to 10 Hz [48,49], with latencies of less than 20 ms. Table 2 exhibits up-to-date GNSS receivers, their accuracies for RTK positioning and the maximum sampling rate, based on the brochures provided by manufacturers. The achievable accuracies reported in table are expressed by two terms. The first term is a fixed part while the second varies according to the distance between the rover and the base station, expressed in parts per million (ppm). So, in the case of a baseline of 1 km, 1 ppm means that one millionth of 1 km (1 mm) must be added to the fixed part to get the final accuracy. \begin{table}
\begin{tabular}{c c} \hline
**Signal** & **Carrier Frequency (MHz)** \\ \hline E1 & 1575.420 \\ E6 & 1278.750 \\ E5 & 1191.795 \\ E5a & 1176.450 \\ E5b & 1207.140 \\ \hline \end{tabular}
\end{table} 
Table 1: Galileo carrier frequency per signal [47]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Manufacturer** & **Receiver Model** & \multicolumn{2}{c}{**RTK Position Accuracy (RMS)**} & **Max Sampling Rate** \\ \hline  & & **Horizontal** & **Vertical** & **(Hz)** \\ \hline Leica [50] & GM30 & 8 mm + 1 ppm & 15 mm + 1 ppm & 50 \\ Javad [51] & Delta-3 & 10 mm + 1 ppm & 15 mm + 1 ppm & 100 \\ Navcom [52] & SF-3050 & 10 mm + 0.5 ppm & 20 mm + 1 ppm & 100 \\ Novatel [53] & PwrPak7D & 10 mm + 1 ppm & - & 100 \\ Sepentrio [54] & AsteRx SB & 6 mm + 0.5 ppm & 10 mm + 1 ppm & 100 \\ Sokkia [55] & GNR5 & 5 mm + 0.5 ppm & 10 mm + 0.8 ppm & 100 \\ Topcon [56] & MR-2 & 5 mm + 0.5 ppm & 10 mm + 0.8 ppm & 100 \\ Trimble [57] & R9s & 8 mm + 1 ppm & 15 mm + 1 ppm & 20 \\ \hline \end{tabular}
\end{table} 
Table 2: GNSS Receivers for RTK positioning based on brochures in 2020. Figure 3: Galileo frequency plan [47]. Monitoring and SA Control of a Case-Study Wind Turbine: Sensitivity Analysis for the GNSS System Features  
This section refers to a case study to investigate whether and how the characteristics of the monitoring system of the under-wind loads of a turbine can affect the effectiveness of the SA control strategy described above. In particular, the role that accuracy, latency and frequency of measurement can play in this context is discussed. The case study structure is a prototype real wind turbine (Figure 4a). It is a 3 MW wind turbine with a horizontal power transmission axle, 102.4 m tall, with a variable hollow circular cross-section whose external diameter is variable from 2.30 (top) to 4.15 m (bottom), and whose thickness is variable from 14 (top) to 40 mm (bottom). A lumped mass of 111 t is placed at the top of the tower. The base of the prototype structure is highly stiff and is supported in the middle by a cylindrical steel hinge.