## 1 Introduction
An SA control configuration, presented by the authors in [4,17], is characterized by a cylindrical hinge installed at the base of the wind tower and magnetorheological (MR) devices mounted in parallel with linear elastic springs. The objective is to realize a time-variant base restraint (Figure 1). A properly written control algorithm is needed to reduce the bending moment at the tower's base, by relaxing the base restraint in suitably selected time intervals. This produces a rocking of the tower's base and energy dissipation in the MR dampers. This proposed control configuration involves a specific instrumentation of the tower in order to monitor, in real time, the structural stresses and deformations, and to permit the control logic to decide the optimal calibration of the variable dampers at each instant. Specifically, the measurement of displacements can be carried out in the laboratory on scaled models, by using typical transducers and fixed external reference structures. On the other hand, in real applications of on- or off-shore wind turbines, it is not trivial to make reliable high-frequency measurements of the horizontal displacement of points placed at a height of tens of meters. Very few geodetic technologies can fully satisfy the monitoring requirements for the above goal. Global Navigation Satellite Systems (GNSS) are very advanced for continuous and fully automated monitoring. An overview of GNSS-based dynamic monitoring technologies for SHM list can be found in [18]. Many GNSS-based applications of continuous dynamic monitoring of civil structures were recently developed, mainly for tall buildings and long-span bridges [19]. An advantage of GNSS is its ability to directly measure absolute 3-D position coordinates, thus providing displacement measurements in real-time and at full scale. Usually, GNSS geodetic dual frequency receivers have been used as reliable tool for providing accurate and timely information on the actual status of the structure. The capability of measuring changes in position with a high level of accuracy is extensively achieved by adopting the real-time kinematic (RTK) positioning. In 2019, [PERSON] and [PERSON] [20] applied the RTK GNSS technique to derive the horizontal displacement of a super high-rise structure under construction (the Tianjin 117 tower). In detail, they applied a Chebyshev high-pass digital filter to the results, in order to compensate the measurement accuracy of RTK-GNSS receivers. The same authors proposed in [21] a combined approach based on the complete ensemble empirical mode decomposition with adaptive noise (CEEDMAN) and wavelet threshold (WT) technique to weaken the influence of background noise of sensors, also applying this methodology to the Tianjin 117 tower. With the aim of evaluating the efficiency and feasibility of GNSS-based systems for structural control of wind turbines, this paper tries to obtain insight into the characteristics (receiver type, type of observables, sampling data rate) and data processing techniques that can make the GNSS useful for such applications. The authors have considered a case study structure consisting of a real 3 MW, 102.4-m tall wind turbine. They have performed numerical investigations and evaluated how the specific characteristics of the measurement system may influence the ability of the SA control technique to reduce the structural demand with respect to the fixed base configuration of the same wind turbine. The roles of sampling rate and latency are mainly investigated, with extended research compared to the conference paper [1], highlighting that they may significantly reduce the effectiveness of the SA system. Therefore, after a  
Figure 1: Smart base restraint including semi-active (SA) magnetorheological (MR) dampers: scheme of the idea.