## 3 Structural Monitoring via Global Navigation Satellite System: The Case of High Wind Turbines  
The use of GNSS receivers has long attracted attention in the monitoring of structures subjected to dynamic loads [22], even though they had been developed for use in positioning applications such as geodetic surveying and navigation. In the area of structural health monitoring (SHM), particular research efforts have concerned the real-time measurement of the dynamic response of engineering structures such as skyscrapers, towers, and large span bridges. Typically, an SHM system uses several sensors for the real-time evaluation of the state of health of the structure to which they are applied. Relative displacements can be used to monitor the system performance and to carry out early damage detection. Orthodox monitoring approaches, including accelerometers and robotic total stations, have been used to evaluate structural vibrations and dynamic displacements. GNSS systems have also been employed in the dynamic measurements of flexible structures for SHM [23; 24; 25]. In 1993, the NAVigation Satellite Timing And Ranging Global Positioning System (NAVSTAR GPS) system was for the first time used in order to measure the dynamic displacements of the Calgary tower in Canada. Results achieved, collecting data at 10 Hz, reached a millimeter-level accuracy, showing a vibration frequency, under wind loading, of about 300 mHz in both north-south and east-west directions and the horizontal movement of \(\pm\)15 mm was measured at 160 m above ground level in the north-south direction [26]. Satellite positioning techniques have been largely used for bridge monitoring. In 1996, the Humber Bridge located near Kingston upon Hull, East Riding, of Yorkshire, England was one of the earliest long span bridges to be monitored with real-time trough kinematic GPS methods. The achieved results showed a \(\pm\)1 mm horizontally and \(\pm\)3 mm in height resolution, respectively [27; 28]. More recently, [PERSON] et al. [29] carried out a series of experiments on a medium span suspension bridge, the Wilford bridge located in Nottingham (UK), by using GNSS technology for its dynamic monitoring. Three different GNSS data-processing strategies, real-time kinematic (RTK), network RTK and post-processing kinematic (PPK), were considered. The RTK strategy was developed in the mid-1990s. It involves a reference receiver transmitting its raw measurements or observation corrections to a rover receiver via a data communication link. This strategy can guarantee up to sub-millimetre-level accuracy. The data-processing at the rover site includes an ambiguity resolution of the differenced carrier phase data [30; 31; 32]. Using the multimode GNSS data and a multimode adaptive filtering algorithm, the dynamic displacements of the bridge were identified with sub-millimetre-level accuracy by [PERSON] et al. [29], showing the capability of monitoring the vibration response of medium span bridges. The real-time monitoring of structures like tall buildings and long-span bridges characterized by a long period has become progressively popular due to improvements in the sampling rates of GNSS receivers and developments in integrated systems (see, for example, [25]). Both the quasi-static and dynamic responses are measured either as displacement or acceleration resulting from wind or seismic actions. Compared to orthodox conventional monitoring that used accelerometers, this technology can reliably measure the relative displacements real-time with a sufficient accuracy. Attention to the SHM technology has increased in recent years, including in the wind turbine sector. Although some research has applied GNSS to wind turbines for the SHM application [33], the GNSS use for SA control of wind turbine structures is still rare and there is a lack of knowledge regarding measuring displacements for the SA control purpose using GNSS.