Influence of wind turbines on military radar
Radar detection losses and incorrect target detection have been reported above and behind wind farms. This compromises the efficiency of these radar systems. An evaluation tool that is capable of quantifying the expected efficiency loss in radar systems due to the influence of wind turbines was developed within the framework of a measurement-verified study which simulates the dynamic influence of wind turbines on radar.
The German Air Force Command has been monitoring abnormalities in target detection and tracking near wind farms for quite some time already. The detection losses and incorrect target detection identified above and behind wind farms reduce the efficiency of these radar systems.
As a first step, an evaluation tool, which takes account of the topography, the locations and characteristics of military radar systems as well as the locations and dimensions of the planned wind farm, was created to help with the decision as to whether or not parts of a wind farm extend into the detection range of military systems. The next step involves the development of a process that can provide a dynamic description of the influence of wind farms that lie within the detection range of the radar systems and can make a quantitative statement with regard to the expected efficiency loss. The first measurements in and in the vicinity of a wind farm within the detection range of the radar system in Brockzetel have been carried out and analyzed. The recorded data shows clear changes in the signal modulation of the single pulses and the amplitude modulations of the maxima of the single pulses during the observation period both over and behind the wind farm.
At first, measurements were taken with three relatively simply constructed receivers, the antennas of which were mounted on masts at heights of up to five meters. To gain information on the influence of wind turbines and wind farms on the radar signals at greater heights, the FHR airplane »Dolphin« was used for the first time within the framework of a second measurement campaign. A measuring pod, which was specially designed and integrated for wind farm measurements was installed under the airplane.
The first evaluations of the measurements show that the wind turbines not only cause higher propagation losses but also change the modulation of the radar impulses. This can lead to signal power losses. Both of these effects must be taken into account when simulating the influences of wind farms on the radar systems of the military command.
At Fraunhofer FHR, highly efficient simulation programs are developed for the investigation of electromagnetic scattered fields. These, on the one hand, involve so-called numerically exact techniques, which, due to the computing and memory requirements, are only suitable at best for tests or validation computations at individual wind farms. On the other hand, asymptotic techniques capable of modeling very large scenarios with good precision on the basis of certain assumptions and simplifications are also being developed. Discrete rays, which are emitted from one source and are intended to locate all of the propagation paths, form the basis of the simulation program that is used here. This technique is combined with Physical Optics (PO) and the Physical Theory of Diffraction (PTD), thus facilitating the modeling of large scenarios with comparably low memory requirements.
Previous studies on simple generic scenarios as well as complex realistic scenarios show that the effects observed in the measurements can also be qualitatively reproduced in the simulation. The investigations embrace the signal losses and their location relevance in the shadow of the wind farms and also extend to the dynamic effects. The movement of the rotor blades is reproduced through a temporal sequence of static simulation scenarios. The temporal course of these field strengths shows a modulation of the overall field strength, similar to that revealed by the measurements that were carried out.
Two-dimensional analyses of the overall field strength show a very complex propagation and interference pattern in the shadow of the wind farms. The modulated reception field strengths can be explained by the movement of the rotor blades.
All in all, the measurements and the field strength simulations reveal that the influences caused by wind farms can be extremely complex. Further studies aim to provide an efficient, measurement-verified tool to evaluate the influence of wind farms on radar systems.