economist阅读写作分析--How to make radar work in wind farms--3.24--vidya

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Nov 6th 2008

From The Economist print edition

How to make radar work in wind farms

WIND farms are controversial. Some people think they are an excellent way to generate green electricity whereas others reckon they are a blot on the landscape and a danger to birds. Such arguments aside, aviation safety is one of the main reasons why plans to build wind farms are held up. The concern is that they interfere with the radar used in air-traffic control. Some people even argue that they could be used as cover by enemy aircraft in time of war. In Britain such worries have caused the shelving or refusal of more than 40 proposed wind farms. Now, however, an Anglo-American company called Cambridge Consultants thinks it has come up with a snazzy way of solving the problem. It calls its invention holographic-infill radar.

The standard radar used by air-traffic control centres can see a long way, but finds it difficult to tell between a moving aircraft and the whirling blades of a wind turbine. Radar works by sweeping a pulsed radio signal around a wide area and then measuring the time it takes for the signal to be reflected back by any objects in its path. This allows the position of those objects to be plotted on a screen. Aircraft can normally be distinguished from stationary objects because the Doppler effect changes the frequency of the returned signal (a familiar example of this effect is the changing pitch of the siren as a police car approaches, passes and recedes).

Although a wind turbine does not change position, its blades are moving and these also cause a Doppler effect. The returning signal from a wind turbine thus creates illuminated blobs on a radar screen that look just like moving aircraft. Moreover, the blobs do not keep still because every four seconds or so, as the radar beam sweeps past, the signal is bounced back by different parts of the turbine’s blades in unpredictable and confusing ways. The clutter this causes on the screen is made worse when the signal is bounced around between different turbines in the same farm. The result is that wind farms can be impossible to filter out because the resolution of a typical long-range radar is not high enough to detect the difference between the Doppler effect caused by an aircraft and that caused by a wind turbine. Holographic-infill radar would deal with this by creating a “patch” covering each farm that could be applied to the wider air-traffic control radar image. The patch would be created by bathing the farm in a continuous stream of radar pulses at short range, rather than sweeping a beam over it from a distance. The reflections of these pulses, when fed into a computer, would provide what was, in effect, a moving radar picture of the farm. If an alien object, such as an aircraft, intruded into this picture, it would be easy to spot. Cambridge Consultants has already tested a small-scale version at the site of a single wind turbine. It was able to show a different Doppler effect for a target moving on the ground from the one produced by the turbine’s blades, according to Craig Webster, who is in charge of clean technologies at the company. The next step is to see if this can be repeated over a wider area and with a flying object; in this case a radiocontrolled model helicopter.

Eventually, Cambridge Consultants hopes to build a full-sized demonstration system at a wind farm. If that works, it would mean that potential blind spots in the radar network could be illuminated, clearing the way for more investment in green electricity.

shelve
vt.搁置
pulse
v.搏动，跳动 n.脉搏，脉冲
illuminate
vt.阐明，解释；照亮
demonstrate
v.证明，论证；示威