Frequency of a Flashing Light as a Navigational Range Indicator

This study is part of a U. S. Coast Guard evaluation of navigational aids for indicating whether a vessel is proceeding properly along a channel or 'range.' The effectiveness of a single-station flashing or flickering range indicator was examined. This aid would indicate lateral position in the channel by varying the flash frequency, with a steady light seen when on the centerline and an increasing flash rate as the edge of the channel is approached. Positions to the left or right of the centerline would be encoded by the light flashing red or green. The range indicator was simulated by flashing a light at a 50% duty cycle and a 100% square-wave modulation at one of five base frequencies, from 0.5 Hz to 6.7 Hz. Four observers judged when they could detect a just noticeable change in frequency when the rate was slowly increased or decreased.