A breeze to measure wind

At the Georgia Institute of Technology, researchers have come up with a cheaper, more sensitive and accurate way to measure crosswind speed over long distances, than using mechanical anemometers.

An optical sensor, originally developed for chemical plants, also has applications in aviation, meteorology or aerosol dispersion studies, particularly where erratic winds are prevalent, such as in the cities or widely varying landscapes. It is a low-cost alternative to complex arrays of traditional sensors.

All optics and electronics are mounted onto a large telescope. An inexpensive helium neon laser about two inches in diameter projects a beam of light from this unit onto a target approximately 100 feet away. The target is made of the retroreflective materials common on highway signs.

The method the sensor uses is based on a laser beam degradation phenomenon known as the residual turbulent scintillation effect. The telescope collects laser light reflected by the target and sends it through the series of optics. Amongst those optics are two tiny, horizontally separated detector, each of which monitors a spot on the target inside the laser beam. The detectors pick up shadowy waves or fringes moving across the laser beam. The waves are visible on the target material.

Each of the two detectors in the sensor register the moment at which a dark fringe passes its view. By digitising the points at which each detector picks up a single wave, a computer can measure time and separation. It then computes the velocity of a massive column of air crossing the laser beam.

To duplicate the work the prototype sensor performs, a row of anemometers would have to be placed side by side in a line as long as the laser, a very expensive proposition.

The optical sensor doesn’t work as well in rain or fog, and the sensor only measures the component of wind that crosses the laser beam at right angles.

Figure 1: Beam path in non-Doppler optical sensor

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