Science Interviews

Radar that can distinguish between planes and turbines

Meera - We all know that the world is running out of fossil fuels and the burning of these fuels is affecting our environment. So we have to run to renewable sources of energy. The problem associated with wind farms is that tall wind turbines that make up the farm are picked up by the radar used by air traffic controllers when they’re looking out for planes. They can’t tell if it’s a plane of a turbine on the radar system. As a result the wind farms can’t be built where aviation radar is nearby which severely limits the areas that they can be built. There could be a solution in the pipeline thanks to a new technology being built by engineering firm, Cambridge Consultants in the UK. I’m at their headquarters in Cambridgeshire with Craig Webster, head of clean technologies. How do the current radar systems work and why can’t they tell the difference between planes and turbines?

Craig - Currently the radars are designed to search across large areas for air traffic so they can control them. I think of it a bit like a spotlight where you’ve got to cover a very wide area with a strong, narrow beam that you sweep across a wide area periodically – once every four seconds. They detect an object and when an object is moving you can tell it’s moving by the presence of Doppler. We all understand as when a train goes past and you hear the perceived change in frequency. You get a reflection of a moving object and when you have movement you then say I have an object of interest which should be an aircraft. With wind turbines they’re also large objects that are all rotating and they have large – structures moving a bit like wings. Unfortunately what they do is they are seen at random. You have unsynchronized rotation and it’s a bit like a very slow strobe light. You see different positions of targets and it’s very had when an aircraft flies over the wind farm to tell which one’s an aircraft and which one’s a wind turbine. This uncertainty is an issue to the air traffic controller.

Meera - Does the fact that it checks it only every four seconds then add to the problem because it could be a quite a long amount of time that a plane ends up travelling over  a wind farm.

Craig - Yes, in some of the examples that we’ve seen the air traffic controller loses control of the aircraft for a minute, maybe more. The uncertainty if they see something that they think might be an aircraft in a wind farm they have to manage the other traffic. They put separation distances – quite often it’s a five mile separation and if that wasn’t really an aircraft and it wasn’t really a wind turbine then that’s causing them some problems: it’s congested air traffic, it uses a lot of fuel. The converse side of that is if you actually didn’t detect an aircraft when there wasn’t an aircraft there that could be a safety situation.

Meera - What have Cambridge consultants come up with to try and solve this problem?

Craig - Well what we have is a radar that doesn’t scan so we’re able to observe the turbines in a way that we can actually measure the speed of the objects that are moving around where a scanning radar, the long-range air traffic control radars, just don’t get the opportunity to dwell on the target for long enough to measure its speed. They can say it’s moving but they can’t tell you how fast it is. Because we can see the speed we can easily tell the way an aircraft moves in a very different way to a wind turbine. If an aircraft is a speed, say it’s 40m/s and it moves 4m in 0.1 seconds (because that’s the measurement interval we have) we know that’s an aircraft. The wind turbine quite simply doesn’t obey the same rules so it has lots of speed – lots of Doppler coming from the wind turbine blades. But they’re moving. They’re moving in circles, the tips move faster than the centre. The blades bend and shift as the blades are loaded up. It gives lots of speed messages but it doesn’t actually go anywhere.

Meera - You say the main reason it can tell the difference is the main reason it can judge the speed at which object s are moving. How does it actually do that?

Craig - It’s a bit like the difference between the search light and the flood light. We have what appears like a low-intensity illumination of the entire wind far. This is illuminated all the time. You might look at this visually you would have time to make the measurements and make the observations. Because we know the speed we can quite easily discriminate be certain it’s not an aircraft and a wind turbine.

Meera - So essentially it is simply because you are constantly watching the area around the turbine. Where have you tested this so far?

Craig - We’ve tested a small-scale prototype in Norfolk. That test has shown very clear differences between a turbine. The next test which we hope to be doing in a few weeks’ time would be to take the same prototype and scale it up so that it’s big enough to include aircraft. We’ll be doing aircraft trials within the next few weeks. We feel this is going to be a really positive step for the wind industry to be able to see differences. This has never been done before.

November 2008