## 3 Structural Monitoring via Global Navigation Satellite System: The Case of High Wind Turbines
### The Role of Latency in Measurement  
Nine cases of SA control have been considered, each time assuming a different latency of the measures from the monitoring system (from 0 to 100 ms). A value of frequency equal to 1000 Hz has been assumed, as well as 1 mm for measurement accuracy (best case). In other words, for each test it has been assumed that measurements are made with different monitoring systems, with different delays in the control chain due to the limited performance of the measurement platform. The 0 ms (no latency) case has been included as an ideal case, which is hard to achieve in real applications.  
It can be observed from Table 5 that latency plays a fundamental role and can greatly influence the response of the controlled system. When the delay in the transmission of the data (time that elapses between the instant in which the measured phenomenon occurs and that in which said information is transmitted to the control system that calibrates the devices) is very low (0-10 ms), the reduction in the demand for bending moment at the base of the tower compared to the case of \"fixed base\" is very relevant (about 80%). This reduction gradually becomes less significant for systems characterized by higher signal latency, reaching values of 50%-60%, then 20% in the worst case (latency 60 ms). The latter case highlights that, in some cases, latency can lead to questioning even the economic convenience in adopting the proposed control strategy, and that it is a figure to be held in great consideration when designing the monitoring system for such applications.  
Figure 7: Response of the wind turbine for different frequencies of measurements: peak top displacement (a), peak base stress (b), No. of state changes (c), peak base rotation (d).  
From the last column of Table 5, one can observe that the base rotation is not significantly different in all the cases, as was already observed in the previous section for variable measurement frequency. The peak values of the top displacement demand result change with latency in the range 25%-50%, which is acceptable considering that the goal of the control technique is mitigating stress demand while preventing the displacement demand from getting worse than in the fixed base case.  
Figure 8 shows the trend of the response parameters over time for some of the above cases (a selection has been made in this case to ensure the readability of the figures). It is worth observing that the demand of base stress over time is much more variable in cases of higher latency of measurement. As discussed in the previous section, this may cause fatigue and may lead to the premature structural collapse of the tower.  
Figure 8: (**a**) Displacement and (**b**) stress response for different latencies of measures for control, (**c**) zoom of panel (**a**), (**d**) zoom of panel (**b**).  
For latency, the above sensitivity analyses were repeated for a wind action equal to 75% and then 50% of the reference EOG case. Figure 9 shows the results obtained for each of the eight latencies. The horizontal lines in parts (a) and (b) of the figure show the peaks in the response for the fixed base configuration and each of the three load cases. It can be deduced that the performance of the control system is more variable with the latency when the intensity of the wind increases, even if the curves are \"flatter\" for lower load cases. This consideration is pointed out in Table 6, where the range of variability of the structural response against the three load cases is shown for the investigated latency range 10-100 ms. This outcome is expected, since a more intense wind gust calls the control system to intervene more frequently, and the latency plays a significant role in influencing the effectiveness of the control system.  
Figure 9: Response of the wind turbine for different latencies of measurements: peak top displacement (**a**), peak base stress (**b**), No. of state changes (**c**), peak base rotation (**d**).