60 wind turbines are located at a distance of about 30 km away from a Secondary Surveillance Radar installation; what will be the impact of this?

My less than stellar knowledge of radar systems tells me that there shouldn't be an issue, since its a secondary radar and not a primary radar (the blades are big enough to be mistaken for an aircraft) which relies on reflection.

But the 30 km distances makes me doubt if this will actually be the case, might a 300m high obstacle in a radar's direct line of sight might cause problems?

  • $\begingroup$ It may or may not affect the SSR and it may not matter. As usual: it depends. A few question that pop up in my radar mind: How high is the radar antenna (above reference level, e.g. MSL or WGS84)? How high is the base of the turbine, how high is the hub and what is the diameter of the rotor. How many blades does the turbine have? Is there line-of-sight? What is the operational range of the SSR? Is there regular air traffic in the shadowed area? Is there a far field monitor in the general direction of the turbines? Without an answer to these questions, the answer to your question is: maybe. $\endgroup$ – DeltaLima Jan 28 '19 at 13:13
  • $\begingroup$ Wind turbine length: 225m, wind turbine diameter: 150m, 48 blades, direct line of light. operational range 250NM. yes theres regular traffic over the area. $\endgroup$ – Cyom Jan 30 '19 at 10:24
  • $\begingroup$ 48 blades? Usually there are 3. Sometimes 2. I've seen very small ones with 5. But 48? That's almost a solid disk... Are you sure? $\endgroup$ – DeltaLima Feb 5 '19 at 21:19

No they won't, since secondary surveillance radar (SSR), as you point out, does not rely on reflecting electronic pulses, but rather on digital communication with an aircraft transponder.

Transponder signals do still travel in a straight line (line of sight), so obviously putting a very large obstacle (such as a mountain) close to an SSR will limit the radar coverage in the area behind the obstacle.

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    $\begingroup$ I invite the downvoter to please comment with any factual errors in my answer so that I can correct them, or alternatively post a better answer themselves. $\endgroup$ – J. Hougaard Jan 19 '19 at 15:14
  • $\begingroup$ "Transponder signals do still travel in a straight line" You are minimizing the multipath problem which is essential in radio-communications (refraction), and the Fresnel ellipsoid required clearance (due to diffraction). Doppler shift due to turbine rotor relative speed close to aircraft relative speed may also be something to deal with (SSR receiver must compensate Doppler shift due to aircraft movement). (I'm not the downvoter) $\endgroup$ – mins Jan 20 '19 at 12:04
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    $\begingroup$ @mins Sure, there's a lot more to it (and frankly I'm not qualified to get into all those details). But I think we can agree that - at least from a user (ATC) perspective - some wind turbines 30 km from a radar station is not going to have any noticeable impact on the coverage. $\endgroup$ – J. Hougaard Jan 20 '19 at 16:17
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    $\begingroup$ See also this document from Eurocontrol, where the rational for a recommended 16 km distance between wind turbines and SSR is explained (annex D, page 59). $\endgroup$ – mins Jan 20 '19 at 17:03
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    $\begingroup$ @mins Maybe you would consider posting a more detailed answer, since you know a lot more about the technical aspects than I do? :) $\endgroup$ – J. Hougaard Jan 20 '19 at 17:54

Yes, for planes whose ground vector intercepts any part of the obstacle. By ground vector I mean line-of-sight from secondary antenna to the air plane. Atmospherics may shift the radiation path, and missing the target on one rotation of the antenna likely wouldn’t degrade the confidence factor, except in unusual circumstances, such as a hovering helicopter or a plane blocked by the wind turbine for 3 or more rotations of the antenna.


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