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A Radio System for Avoiding Illuminating Aircraft with a Laser Beam

When scientific experiments require transmission of powerful laser or radio beams through the atmosphere the Federal Aviation Administration (FAA) requires that precautions be taken to avoid inadvertent illumination of aircraft. Here we describe a highly reliable system for detecting aircraft entering the vicinity of a laser beam by making use of the Air Traffic Control (ATC) transponders required on most aircraft. This system uses two antennas, both aligned with the laser beam. One antenna has a broad beam and the other has a narrow beam. The ratio of the transponder power received in the narrow beam to that received in the broad beam gives a measure of the angular distance of the aircraft from the axis that is independent of the range or the transmitter power. This ratio is easily measured and can be used to shutter the laser when the aircraft is too close to the beam. Prototype systems operating on astronomical telescopes have produced good results.