Unattractive red flashing lights atop wind turbines in the night sky may soon be a thing of the past. German Air Traffic Control (DFS) has approved the passive radar PARASOL for environmental monitoring of wind farms. "The system uses the signals from digital television to switch on the warning lights on wind turbines only when needed, that is, when an aircraft is nearby," explains Prof. Dr. Daniel O'Hagan, Head of Department Passive Radar and Anti-Jamming Techniques at the Fraunhofer FHR.

Wind turbines must be equipped with warning lights above a height of 100 m. "These red flashing lights mark obstacles at specific altitudes for sports pilots and other aircraft," says O'Hagan. However, the red warning lights make acceptance of wind turbines more difficult: Residents feel annoyed by the flashing and the lights are suspected to attract birds and other animals that can be seriously injured in a collision with the large rotor blades.

This is where PARASOL comes in: The system "hears" existing signal sources, so to speak. "Specifically, we use the broadcast signal from DVB-T2. By comparing it to the original signal and what PARASOL receives, the system can determine if a plane is in the area. The signal would be so distorted by the reflection from the fuselage of the aircraft that the location and speed of the "echo" can be determined," explains O'Hagan. However, airplanes have no influence on the TV image quality in people‘s living rooms because the aircraft echoes are negligible compared to the direct transmission signal.

The project partners demonstrated the functionality and reliability of the system in elaborate measurement campaigns. The challenge of the development lay in the clear classification (recognition and categorization) of moving objects in the field of vision, i.e. to distinguish the rotating blades of the wind turbine from an approaching aircraft. And so, Fraunhofer FHR in cooperation with PARASOL GmbH & Co KG, succeeded in using the world's first civil commercial passive radar system.

PARASOL is available through PARASOL GmbH & Co KG. For a wind farm with several dozen turbines, the initial cost is approximately €450,000. This includes installation and test flights, which are required for approval by German Air Traffic Control (DFS) prior to commissioning.

The Fraunhofer Institute for High-Frequency Physics and Radar Technology FHR is one of the leading European institutes and carries out extensive research in the field of high-frequency physics and radar technology. The core topic of research is sensors for the most precise distance or position determination as well as imaging systems. The application spectrum of these devices ranges from systems for reconnaissance, monitoring and protection, medical technology, environmental and geo-monitoring to real-time sensors for traffic and navigation as well as quality assurance and non-destructive testing.