Wind Farm Mitigation
What is the problem?
Civil and military radars operate on the principle of detecting the reflected energy from a transmitted radar pulse, and also detecting the frequency Doppler shift of returned radar pulses. Any source of large radar pulse reflected energy and Doppler shift return that emulates the typical signature of a legitimate airborne target generates uncertainty, confusion on an ATC display and creates potential airspace safety and air surveillance security concerns. Wind turbines generate large radar returns and rotating wind turbine blades generate radar pulse Doppler shift and so are a source of interference or clutter on existing radars.
Aircraft – helicopters, aeroplanes or UAV/drones – can be indistinguishable within the clutter caused by the wind turbines. Track seduction can occur, where the aircraft track is displayed incorrectly as the radar confuses the aircraft and the turbine, or it may not be detected at all in the area around the turbines, known as shadowing and obscuration. Holographic Radar™ clearly distinguishes between the wind turbines and the aircraft and feeds only the aircraft positional information without the turbine clutter to the airport radar. The Air Traffic Controllers will then have a consistent and accurate view of the aircraft the entire time it is over the wind farm.
Examples of the problem
This shows the effect a wind farm has on the tracks of two aircraft. The left hand picture shows the "raw" detail and the right hand picture shows the Holographic Radar infill.
How it works
The distinguishing feature of Aveillant Holographic Radar’s solution for wind turbine mitigation is that the surveillance capability is fully maintained at optimum performance levels while any wind turbines detected are not reported to the ATC system.
Aveillant’s approach is insensitive to range and to the size or layout of current and planned wind turbine deployments. It does not depend on high cross-track resolution (which dilutes with range and is inevitable for a beam-scanning radar) or high-range resolution (that consumes radio spectrum).
It is also insensitive to the effects of beam side-lobes and their interaction with time-varying turbine signatures.
The Holographic Radar™ operates by successfully achieving aircraft/turbine resolution in both elevation and the Doppler domain. This does not degrade with range. It also benefits from a narrow spectrum, and is achievable because of the continuous dwell time that is available from a static, staring radar.
Wind turbine clutter suppression
All ground clutter suppression, including wind turbines, is enabled by the very high-precision Doppler measurements and target histories provided by continuous target observation.
Wind turbine radar cross-sections tend to be in the region of 100-1000 square metres, whereas the minimum cross-section required for aircraft targets is 1 square metre. To discriminate aircraft from wind turbines without geographic desensitisation (which leads to reduced probability of detection and vulnerability to extended wind farm deployments), a processing gain of 30-40dB (a factor of 1,000 – 10,000, to yield a 10:1 signal-to-clutter margin) is required.
For a traditional, rotating Primary Surveillance Radar, both aircraft and wind turbine are sampled for between 5 and 50 milliseconds, and typically allow integration of between 10 and 200 pulses. This is insufficient to provide the necessary gain and cannot be improved with a narrow swept beam.
Holographic Radar™ acquires signals over a period of 0.5 to 2 seconds. An aircraft is observed over the whole of this time, compared with the duration of a turbine blade ‘flash’ of 20-50 milliseconds. At the same time, the ‘flash’ yields a Doppler spectrum spread (unevenly) over some hundreds of frequency ‘bins’. This is sufficient to resolve aircraft from turbines even in the worst conditions.
This process is applied over the whole field of view. It is computationally demanding, but HR uses modern graphics processors which provide up to many TeraFlops processing capability (1 TFlop is 1012 operations per second).