© PTB

Better safety for 'aviation lighthouses'

The WERAN research project is examining the effects of wind energy installations on radio navigation facilities used by air traffic. The results will allow accurate predictions about potential disruptive effects to be made during the planning phase of new wind farms.

Finding the right answers to the crucial questions of our times is an aspiration shared by every researcher. As renewable energies become increasingly important to our energy supply, new research questions arise, stimulating a search for answers. To expand onshore wind energy, more space is needed – but what minimum distance is required between wind turbines and aviation navigation facilities, for example, and what impact do the former have on the latter? For the first time worldwide, researchers from Germany's national metrology institute PTB are now able to accurately describe the potential extent of disruptions caused by turbines and how these can be precisely measured.

'Aviation lighthouses'

Deutsche Flugsicherung (DFS), Germany's air traffic control company, operates about 60 navigation facilities. These flat ground stations, typically called 'VOR' (Very High Frequency Omni-Directional Radio Range) stations, permanently emit VHF radio signals using a rotational antenna array. In a way that is reminiscent of lighthouses, they enable aircraft to stay on course and thus help to ensure aviation safety. Wind energy installations, however, can disrupt the transmission of the radio waves emitted by VOR stations, distorting the accuracy of the direction signal. When reaching the surface of an installation, radio waves can scatter and be reflected. Due to the angular error that results, the aircraft receives a slightly distorted signal. In fact, it is theoretically possible for the plane to drift off course. This is why wind energy installations to be built within a certain radius of a VOR station are examined very carefully as to the potential disruptions they might cause.

The WERAN research project, which was funded by the Federal Ministry for Economic Affairs and Energy and conducted by PTB and its project partners, looked at the scientific foundations of previous assessment procedures, devised new metrology and developed a 'full-wave simulation' for angular error analysis. Within the framework of WERAN plus, its ongoing follow-up project, researchers are working on a new prediction method that will allow more realistic assessments to be made in advance concerning the potential disruptive effect that wind energy installations may have on VOR stations. The focus is on DVOR (Doppler Very High Frequency Omni-Directional Radio Range) navigation facilities, but conventional VOR stations are also being looked at. The goal is to provide a solid scientific foundation for the identification of necessary examination radii. It is thus hoped that current exclusion zones for wind energy installations can be reduced in the long term without putting aviation safety at risk.

Precision-navigation drones

The scientists developed drones that are capable of precision navigation. Equipped with eight rotors, these 'octocopters' are able to hover and carry out on-the-spot measurements at a height of up to several hundred metres. With the help of specially designed high-frequency metrology and antennas integrated into the drones, the researchers were able to measure precisely how DVOR radio signals spread and are reflected and scattered when they hit an installation. They could also gauge how the reflected signals and the direct DVOR signals overlap to produce an angular error. Furthermore, they showed to what degree wind energy installations are actually responsible for DVOR total angular error as opposed to influences from other sources such as buildings, high-voltage lines or woodland.

Parallel to this work, the project partners from Leibniz University Hannover developed simulations that allow a mainframe computer to calculate the extent of an angular error caused by wind energy installations. The results of these simulations were compared with the detailed measurement data collected by the octocopters. At present, Jade University of Applied Sciences is engaged in an operation to measure (D)VOR signals over longer distances – including at sea – with its manned research aircraft 'Jade One'. Again, the goal is to analyse the interplay between wind energy installations and navigation facilities.

Paving the way for faster and more rigorous decisions on planning applications

The way measurements at wind farms matched the simulations and the results of the improved prediction tool has given rise to a new state of the art that is now being put into practice: Since 1 June 2020, a new formula has been used to calculate disruptive effects on radio navigation facilities (in German only). This formula had been developed by the Federal Supervisory Authority for Air Navigation Services (BAF) and Deutsche Flugsicherung GmbH (DFS) on the basis of PTB's research findings. The Federal Ministry of Transport and Digital Infrastructure (BMVI) and the Federal Ministry for Economic Affairs and Energy (BMWi) have also agreed on further measures. Once the new examination methodology has been satisfactorily confirmed, the radius for examination zones – currently 15 km – is to be adapted.

These decisions are a positive signal for the expansion of onshore wind energy. The new prediction method can make it possible for faster and more rigorous decisions to be made in future on whether planning permission should be granted. In fact, it allows precise forecasts to be made during the planning phase of wind farms on the potential disruptive effects they might have on radio navigation facilities. Application of this method in practice will show what specific improvements this will lead to on the sites of individual VOR stations.

Also, a ring test conducted within the framework of WERAN plus to compare different prediction methods for forecasting the disruptive effects of wind energy installations is expected to provide further insights. It allows institutions that also have a specific interest in the impact of wind energy installations on DVOR signals to reflect on the research results of WERAN plus and relate them to their own findings. Afterwards, the results will be compared with one another.