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A covert communications system for helicopters and other military aircraft is
revealed in GB 2 320 151. The infrared anti-collision lights currently used on
aircraft are practically invisible to the naked eye, but show up clearly with
night-vision goggles. The intensity or rate of flashing could be varied to carry
messages, but this would not be very secure. So LFD of Gosport, Hampshire,
proposes using two infrared transmitters with different wavelengths, only one of
which is visible with night goggles. The invisible infrared would carry speech
and data superimposed on bursts of 120-kilohertz pulses. The anti-collision
system works as usual and masks the secret information, which is decoded by a
separate infrared detector.