  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.
