Radar signature control of wind turbine generators

Many of the prime locations for wind farms in the UK are on exposed landscapes close to regional airports. However, planning permission for such installations is often being denied because the moving blades of the wind turbines interfere with the operation of airport radar systems. Airport surveillance radars track the position and speed of aircraft in their surrounding airspace using pulsed Doppler radar techniques. The filtering out of signals from spurious stationary and slow moving targets is relatively straight forward as the associated Doppler shift is zero or below a preset threshold. However, when an airport radar receives returns from a wind turbine installation, the motion of the blades can impart to the radar signal a significant Doppler shift that is comparable with that expected from an aircraft. Hence, techniques for reducing the scattering from wind turbines are being actively researched and one approach that we are investigating is the use of active radar absorbing structures based on the concept of a phase-switched screen (PSS) (Tennant, A. and Chambers, B., Proc SPIE, vol.538, 2004; Chambers and Tennant, IEE Proc. Radar Sonar and Navig., vol.149, no.5, p.243-7). We report experimental results obtained from a scaled model windmill with PSS blades operating in conjunction with a 10 GHz Doppler radar system.