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Lasers Listen to Vortices from Airplane Wing Tips

Photonics Spectra
Jan 2006
Hank Hogan

At Flight Safety Technologies Inc. in Mystic, Conn., senior vice president for technology Neal Fine is working to ease airport congestion while increasing safety. His company is using a laser-based system dubbed Socrates to listen for the sound of a vortex generated by wing tips as planes land and take off. Being able to identify when such vortices are present would allow planes to safely take off and land closer together.

Socrates is not an active turbulence sensor, Fine said. “It’s detecting sound on the ground generated remotely from where the airplane passes over.”

That, he noted, is an advantage over lidar-based approaches in which lasers probe upward to track particle movement. Lidar requires that enough particles be present for tracking and that there be no fog, rain, haze or other beam-dimming atmospheric conditions. Because it is passive and remote, Socrates avoids these problems.

The system’s ground-based lasers act like gigantic ears and catch the broadband sound produced by turbulence. This is done by firing a laser at a retroreflector, which can be placed tens of meters away. As the beam traverses the air, its phase changes in response to shifts in the air’s index of refraction, which is affected by sound waves. By the time the beam has returned to the system, it has recorded a snapshot of the sound along the optical path. This integration rejects sound coming from a direction along the optical path, and that leads to better performance than an array of standard microphones can provide.

“It’s that discriminating feature that allows us to use a relatively few number of sensors relative to microphones,” Fine said. He added that another advantage is the need for a smaller amount of dedicated land.

The patented system, which is being built by Lockheed Martin Corp., uses a 1319-nm Nd:YAG laser from Lightwave Electronics Corp. (now part of JDSU) and a receiver from Optiphase Inc. The laser was selected because it is an extremely stable and narrow-linewidth source. The receiver was chosen because it spits out the phase-change information directly.

With an array of 16 such beams, Flight Safety Technologies has been listening to airplane wake vortices at Denver International Airport. As the planes fly overhead on their way to and from the landing strip, they generate a sound spike that decays. The vortex signal shows up as a sound tail that trails away after the plane has passed.

Testing of the system had been scheduled to end last October, but it has been extended for six months. This was done, Fine said, as a way to accelerate the deployment of the next step in the solution, a wake-vortex advisory system.

Such a system would combine Socrates with predictive models to forecast how long vortex conditions would last, based on the weather and the planes involved. With such a system, airport capacity could be increased as much as 25 percent.

Before this solution takes off, the predictive models must be developed and verified. A problem for the sensor is optical turbulence, which is worse in high winds.

One way around this is to discriminate based on speed, because the vortex signal travels at the speed of sound, while the wind moves much slower. Another might be to employ adaptive optics that compensate for atmospheric turbulence, but at present, Fine noted, such optics are too expensive to be used in this application.


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