Lasers are wonderful things. Want to measure the distance to the Moon, or create an artificial guide star for the active optics in a telescope or measure the chemical composition of the air? Then a high-powered laser is what you need.

But you can’t just go blasting the atmosphere willy nilly; the Federal Aviation Authority has rules about this kind of thing. Anybody operating a laser in the atmosphere must have one or more observers looking for aircraft while the laser is in operation. Should an aircraft come within 25 degrees of the laser (as seen by the observer), the light must be shuttered.

That’s a job that is fraught with potential errors so Bill Coles and co from the University of California, San Diego, have built a device that can do the aircraft spotting automatically. The system is based on the ability to detect the air traffic control transponders that are fitted to most aircraft. The idea is to use two antennas, both aligned with the laser beam. One antenna has a broad beam, the other a narrow beam.

As an aircraft approaches the beam, the broader beam picks up the transponder first, followed by the narrower beam. And the ratio of the signal from both antennas is a measure of the distance of the aircraft from the beam.

As soon as the aircraft gets too close, the beam is shuttered automatically.

The team says it has tested the idea at the Apache Point Observatory 3.5 m telescope in New Mexico and says it worked well. In the more congested airspace at UCSD near San Deigo International Airport, the team reported some problems with false triggers due to beam relfections which resulted in the shutter being closed for 80 per cent of the time during the busiest hours. But since most observatories are in more isolated areas, that shouldn’t usually be a problem.

Simple, really.

Ref: arxiv.org/abs/0910.5685: A Radio System for Avoiding Illuminating Aircraft with a Laser Beam