## 3 Structural Monitoring via Global Navigation Satellite System: The Case of High Wind Turbines
Otherwise, a float solution is obtained, which means that only real-valued ambiguity parameters are obtained, and the positioning accuracy degrades to the decimetre level. Therefore, for real-time instantaneous (or quasi-instantaneous) applications, the \"time-to-fix-ambiguities\" (TTFA) is crucial. The AR process becomes more reliable, and the robustness of the RTK increases, the closer the rover is to the base receiver, due to the spatial correlation of the GNSS errors with the exceptions of multipath and receiver noise. Any two receivers that are relatively close together, simultaneously track a particular satellite and they will experience similar errors. The shorter the distance between the two receivers, the more similar the errors, and therefore the common errors will be removed or reduced by the difference between the measurements of the two receivers. Using a single reference base station that is not too far away and provides all rovers on-site continuously with RTK differential corrections, high-end GNSS rover receivers can calculate position using carrier phase measurements in fixed integer carrier phase ambiguity mode with a precision of less than 1 cm (Figure 2). Furthermore, when lock is lost, a cycle slip occurs; this can be a power loss, an obstruction or low SNR. The integer lock is lost and the AR process must be re-initialized. Obstructions hindering satellite visibility (e.g., rotor blades) such as unexpected errors (e.g., multipath) related to observation conditions that are difficult for GNSS could result in incorrect ambiguity fixes, degrading the accuracy of the position even up to tens of centimetres. Therefore, to obtain an accurate and reliable RTK fix and to detect real displacements from the measurements, greater care has to be taken to characterize the RTK performance and to address each individual error present in GNSS data. The multipath is still one of the main error sources in GNSS positioning. It degrades the accuracy of position solutions. The multipath is difficult to eliminate by standard differencing techniques, since it is highly correlated with observation environments [38]. The straightforward multipath mitigation method is to select a favorable environment to reduce the reflections surrounding the antenna, however this possibility is not feasible for wind turbine applications, since the reflection is inevitable due to the presence of blades placed near the tower. Therefore, the demand for high accuracy positioning, along with the improvement in the performance of ambiguity resolution, requires us to address the multipath: wavelet analysis can be used to extract the time-varying frequency and magnitude contents of the multipath [39]. GNSS receivers that are able to track multiple GNSS constellations can lead to improvements in the performance of RTK positioning, since the increased number of satellites makes accurate position measurements more likely and at the same time enables the receiver to exclude satellites with a poor signal, which improves the accuracy and reliability of positioning. The GPS from the USA has led the satellite positioning in recent decades, although lately a number of devices that use the Russian GNSS system named GLONASS have rapidly expanded, since the constellation expanded to 24 operational satellites. Moreover, many devices lately are compatible with the European GNSS system, Galileo, which is currently (March 2020) in its full operational capability (FOC) phase: 22 FOC satellites were launched up to the end of 2019, in addition to the four IOVs launched between 2011 and 2012. However, of these 26 satellites only 19 FOCs and three IOVs are declared usable, as the other four had problems during their life in orbit [40; 41; 42; 43; 44; 45; 46]. The Galileo navigation signals are transmitted in the four frequency bands, depicted in Figure 3 below. These four frequency bands are the E5a, E5b, E6 and E1 bands. They provide a wide bandwidth for the transmission of the five Galileo Signals, whose names are E1, E6, E5, E5a, E5b.