Navigation Performance Using Parallax Range Lights

At present the U.S. Coast Guard employs a two-station or parallax range method for indicating to a vessel's operator the correct path to follow while proceeding along certain navigation channels. Vertical alignment of two lights, one more distant than the other, indicates that hte vessel is positioned on the range axis, and any deviation from this course is readily apparent. To compare the sensitivity of various methods for range displays, including presumably less expensive single-station ranges, the performance of observers on four parallax range configurations was assessed psychophysically by measuring the observers' ability to determine lateral position in channel and direction of motion across a channel. The currently used two-point fixed range light configuration was found to afford a high degree of sensitivity in determining lateral position and motion. With a two-point flashing range, sensitivity is slightly decreased and errors in judging direction of motion are significantly higher than with the fixed range lights. Two alternatives parallax range light configurations afforded a slight potential improvement in sensitivity over the two-point fixed range lights. Strong evidence was found that training or experience could improve performance regardless of range light configuration. Judgements of motion toward the range axis were significantly more sensitive than judgments of motion away. These findings describe the sensitivity afforded by present parallax ranges and will allow comparison of proposed single-station ranges with present configurations to evaluate their adequacy as navigational aids.