## 3 Structural Monitoring via Global Navigation Satellite System: The Case of High Wind Turbines
_RTK(Real-Time kinematic)-GNSS(Global Navigation Satellite System) in Wind Turbine SA Control_  
GNSS receivers can be categorized by their characteristics. Some receivers (called single frequency) can use only the coarse acquisition code and carrier phase tracking on the L1 frequency. Other receivers track both code and carrier phase signals on two different frequencies (dual-frequency). Finally, the most advanced receivers can track all in-view constellations (GPS, GLONASS, Galileo, BeiDou, QZSS and SBAS) on more than one frequency [31; 32; 34]. The more aspects of the GNSS signal a receiver can employ, the greater its flexibility, but, also, the greater its cost. It is important to understand receiver characteristics and limitations to ensure that the capability of a receiver is matched to the required outcome of the SA control purpose [35]. Global navigation satellite systems provide various types of positioning state solutions, depending on the types of observables used in the positioning and navigation algorithms, the number of observation epochs required to obtain a solution, and the number of receivers needed for the position estimation. In GNSS, there are two types of observables: the code pseudorange and the carrier phase, that is, the basis of the techniques used for high-precision positioning [32]. Real-time kinematic (RTK) positioning makes use of carrier phase measurements in the relative positioning mode. In addition to the rover receiver, used for the local positioning, one reference station receiver, or a network with a priori known coordinates, are employed, along with a data link to continuously transmit measurements from the base station to the rover, sometimes over the Internet using cellular or wireless service, or radio signal. The RTK solutions are derived from the current epoch instantaneously. The basis of RTK is the implementation of the on-the-fly (OTF) ambiguity resolution AR algorithms. It is possible to perform the AR process in less than 10 s under the best circumstances, where the distance between the base and rover receiver is 10-15 km, which can be increased if a network-based RTK service (with a 50-70 km inter-receiver spacing) is available, the receivers are tracking a large constellation of satellites with low GDOP, the receivers are multi-frequency, or the multipath and receiver noise are low [34]. RTK is the most attractive technique for high-accuracy instantaneous positioning, as the coordinate solutions are obtained in the field with a centimeter-level accuracy and can be used immediately for both kinematic and time-critical static applications such as precise navigation, machine guidance and control. As an emerging trend, RTK-based guidance systems are employed, in particular, in all phases of the agricultural operations to perform every activity on the crop, from soil preparation to harvesting, and they help farmers to choose consistent driving strategies, enabling an increase in accuracy and productivity and a decrease in time and operating costs. GNSS-based mechanic guidance systems can be used to provide real-time positioning information to support the precise maneuvering of tractors, avoiding diversions from the desired trajectory [36; 37]. RTK-GNSS measurements have the potential to be considered as a suitable candidate for wind turbine SA control. However, some real-world challenges arise to supply information on the absolute and relative displacements for the SA control implementation. RTK positioning is a much more complicated and vulnerable process, even in favorable conditions. Achieving reliable centimetre-level accuracy with as few as possible data epochs in real-time is not easy in the harsh environment encountered in wind turbines. In RTK mode, as any kind of carrier phase solution, in order to obtain centimeter-level positioning accuracy, an ambiguity-fixed solution is necessary, which indicates that the number of full-phase cycles between the receiver and the satellite at the first instant of the receiver's lock-on are fixed at integer values.