North Hoyle Windfarm Report
2 QinetiQ GPS trials 2.2 Results 2.2.1 Control runs Two control runs were made in order to determine the number of satellites visible when there were no possible obstructions to the line-of-site. The number of satellites locked with time is shown in Figure 2-2 for both the control runs. Here we can see immediately that the visible number of satellites on each control run and for each GPS system is relatively stable in time. Furthermore, the total number of satellites visible is 9 for the GPSIII and 10 for the GPS152. This provides us with an expected number of satellites to work with when considering the different courses in and around the wind turbines. In addition to the expected number of satellites, we are also able to estimate the likely uncertainty in position estimation by the GPS units and compare these to the uncertainties provided when in the wind farm. In the control run, the recorded uncertainty in position was between 4m and 5m. Figure 2-2: Locked satellites on the two control runs In Figure 2-3 and Figure 2-4 examples of the displays for the GPSIII and GPS152 units are shown. It can be seen in the figures that the number of satellites locked onto by the two GPS systems is eleven in each case. Furthermore, a twelfth satellite that is visible to the GPS152 unit. Figure 2-3: The display from the Garmin GPSIII unit during a control run Figure 2-4: The display from the Garmin GPS152 unit during a control run 2.2.2 Trial courses Figure 2-5: The recorded GPS track data for the three routes used in the trial The first course is a path from the northern side of turbine 16 to turbine 20 (as described in [2]). The course runs in a direction parallel to the longest side of the wind farm as is shown by the green and brown lines in Figure 2-5. In 2-6 we present the number of satellites locked onto by the GPS units with respect to time. It can be noted from the plot that for both the GPSIII and GPS152 the number of locked satellites is slightly less consistent than was seen in the control runs. However, for both GPS units between 8 and 10 satellites remains locked at all times providing an uncertainty in the estimated position of between 4m and 6m. It is important to note that for successful operation of a GPS unit, only four satellites are required. A greater number of satellites provide a greater accuracy in position. Figure 2-6: The number of satellites locked onto by the GPS units along course 1 2.2.4 Course two The second course used to test the GPS systems ran parallel to the shortest side of the wind farm from the western side of turbine 3 to turbine 28 (see the blue and purple lines in Figure 2-5). We found that on the course the number of satellites locked onto were 8 for the GPSIII and 10 for the GPS152. The uncertainty in position was recorded as 5m. It is interesting to note that the GPS152 appears to have a consistently higher number of satellites than the hand held GPSIII. However, this is likely to be a result of the elevated position of the GPS152 antenna (on the roof of the launch cabin). The hand held antenna was much lower on the boat and thus more susceptible to shadowing from objects other than the wind turbines. The results for the second course are presented in Figure 2-7. Figure 2-7: The number of satellites locked onto by the GPS units along course 2 2.2.5 Course three The vessel was piloted diagonally through the wind farm from the south of turbine 5 to the south of turbine 26 and the data log of the course is shown in Figure 2-5 (red and light blue lines). Here we find that there is very little variation in the number of locked satellites for either GPS system. The data is shown in Figure 2-8 and it can be noted that the GPSIII has 8 or 9 satellites locked at all times. The uncertainty in the positioning is around 4m. The GPS152 has 8 to 11 satellites locked and because of the variation in satellite number, the uncertainty in position was found to be much more variable, being between 3m and 5m. However, despite this overall operation of the GPS units was not affected adversely at any time. Figure 2-8: The number of satellites locked onto by the GPS units along course 3 2.2.6 Additional tests In addition to the courses described above the GPS units were tested whilst the launch was stationary and adjacent to a turbine. Four turbines (numbers 7, 9, 13 and 17) within the wind farm were used in an attempt to shadow different parts of the sky. We found that regardless of our proximity to a turbine the GPS units operated normally without any undue loss in the number of visible satellites. The results are summarised in Table 2-1. It should also be noted that in each case the estimated error in position with both the GPSIII and GPS152 was between 3 and 5m.
Table 2-1: Summary of visible satellites when adjacent to a turbine The various experiments performed during the GPS trial showed that the wind turbines did not give rise to any loss in the number of locked satellites. The significant outcome of this is that the normal operation of the GPS system was never at risk of failure, due to interference from wind turbines. The additional tests showed that even with a very close proximity of a turbine tower the GPS antenna, there were always enough satellites elsewhere in the sky to cover for any that might be shadowed by the turbine tower.
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