It’s a bird, it’s a plane, no, it’s a wind turbine blade
Aviation and the environment often appear to be in conflict, sometimes in unpredictable ways. One such has led BAE Systems into a collaboration with the UK government and the British Wind Energy Association (BWEA) on a research project that aims to eliminate the interference that the rotating blades on wind turbine installations can cause to air traffic control radars.
The interference is caused by radar returns from the moving blades, which typically are similar in length to a light aircraft and are limited to a tip speed just below the speed of sound, explained Ian Metcalf, business development manager with BAE’s new Integrated Systems Technologies (Insyte) division. Insyte predecessor companies Marconi, Plessey and AMS supplied all but about a dozen of the 80 civil and military ATC radars currently in use in the UK.
Under the terms of the Kyoto protocol, the UK government is committed to generating 10 percent of its energy needs from renewable sources by 2010, and on-shore wind farms are the only technology that could meet that target in the five years remaining, Metcalf said. But the interference problem is so serious that it has led aerodrome operators to object to planning applications for wind farms, causing more than half the applications made to be rejected.
The returns from the blades vary with the angle of the blades as they turn and with their orientation as the turbine platforms respond to changes in wind direction. On radar screens, they show up as multiple targets unpredictably appearing and disappearing, degrading ATC capability to the point at which a wind farm built under an airport flight path would result in the closure of the airport, Metcalf said. And even at the extremes of a typical radar’s 60-mile range, a wind farm under an area where aircraft maneuver would be unacceptable.
BAE’s approach is to demonstrate that the interference can be mitigated to the point at which it is no longer a problem and to convince airport owners, local authorities and the Civil Aviation Authority to withdraw their objections. Its main tool is an advanced digital tracker using fuzzy logic-based algorithms derived from those used in the track extractor fitted to the Type 996 surveillance radars on Royal Navy warships. The track extractor processes the returns to eliminate interference caused by false targets such as flocks of seabirds and high waves.
The next step is a demonstration program funded jointly by the government, BWEA and BAE. This involves a tactical Watchman radar from the Royal Air Force that was stationed on a hill in Wales to record the interference from several wind farms. Next month, BAE engineers will go back to the same location and use the tracker to validate the computer predictions, probably over the course of two or three iterations.
Metcalf expects final tweaking of the algorithms over the next six to 12 months to be followed by an engineering phase involving the recoding of the algorithms in a commercially certifiable form.