North Hoyle Windfarm Report
5 QinetiQ radar trials 5.3 Radar shadow trial results As outlined above and in more detail in the trial plan, the radar shadow trials involved monitoring the radar display of a shore based radar at Prestatyn. Specifically, the purpose of the trial was to look for signal loss of the target boat, due to the presence of wind turbines. Shadows at the radar frequency of 9.4GHz are deeper than those seen at VHF frequencies (150MHz). If we consider the gain settings of the radar then an estimate of the shadow depth can be gauged. The peak power of the radar is 4kW which corresponds to 36 dB. Assuming a log adjustment to the gain we find that, for example, at 54% gain the power is 19.44 dB. With a gain setting of 54% or 19.44 dB the wind turbines were visible. However, reducing the gain to 44% or 15.84 dB we found that the unshadowed turbines were still visible, but the shadowed turbines had disappeared from the display. The distance behind the shadowing turbine was approximately 1000m. A further reduction of the radar gain to 4% or 1.44 dB, it was found that the unshadowed turbines began to disappear. This can be seen in Figure 5-4 to Figure 5-6 From these observations we find that the difference in power required to detect an shadowed (1000m behind a shadowing turbine) and unshadowed turbine is approximately 14.4 dB. At 1000m the theoretical study[1] suggests that the shadow depth behind a wind turbine is approximately 14.5 dB, which agrees very well with the estimate made using the radar displays and radar gain settings. There were two parts to the radar trials. The first dealt with the clutter effects on ship-borne radar and the second considered shadowing from wind turbines. In the first trial it was found that adjusting the radar gain could reduce the number spurious echoes significantly. However, a consequence of gain reduction is that small targets at long range may no longer be detectable. And at very low gain settings (approximately 34% or less) some shadowed wind turbines start to disappear. The second part to the trial dealt with radar shadows behind wind turbines. It was found that the depth of shadow at a distance of 1000m behind a turbine was approximately 14.4dB. This value was consistent with those determined in theoretical studies undertaken previously [1].
|