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Laser Sensor Monitors Distant Crosswinds

Photonics Spectra
Feb 1997
Charles T. Troy

ATLANTA -- A laser-based sensor can measure crosswind speeds for applications such as aviation safety and meteorology, Georgia Institute of Technology researchers report.
The single-ended, long-path non-Doppler laser wind sensor registers faint wind movements that an anemometer cannot, while matching the anemometer's accuracy at higher wind speeds, said researcher Mikhail Belen'kii.
Originally developed for chemical plants, it is designed for working alongside other sensors that measure airborne chemical concentrations, said principal research scientist Gary Gimmestad. "If you measure the concentration and the crosswind at the same time, you can get a good idea of the rate at which a pollutant is leaving a plant," he said.
Because the sensor measures average wind directions over long distances, designers envision applications in aviation, meteorology and aerosol dispersion studies. It would, the researchers say, be particularly useful in locations where erratic winds are the norm such as tank farms, cities or widely varying landscapes.
The sensor, essentially an optics and electronics package mounted on a large telescope, relies on an effect called residual turbulent scintillation. A helium-neon laser projects a beam onto a retroreflective target 100 ft away. Backscatter passes through a series of optics including two horizontally separated detectors, each monitoring a spot on the target.
The detectors pick up fringes visible on the target. "The fringes look a lot like the shadows of waves created on the bottom of a swimming pool on a sunny day," researcher David Roberts said. "If you look at turbulent wind using a laser beam, you see something very similar to those waves."
Each detector registers the passing of a dark fringe. By digitizing the points where each detector picks up a single wave, a computer calculates time and separation and determines the average velocity of a massive column of air crossing the laser beam.
"Even though air may be flowing erratically -- some going one direction at one end of the beam and some going exactly the opposite direction -- you can get a net flow across the laser beam with this method," Roberts said. The sensor measures wind across, instead of along the beam, as with Doppler. And, unlike conventional lidar systems, it detects turbulence. Drawbacks include poor performance in fog and rain and the need to operate at right angles to the wind.
Researchers plan to test the sensor with technologies that measure airborne pollutant concentrations at a working refinery.

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