Abstract: In an apparatus for guiding or controlling a vessel over the center of an unmarked channel, a pair of ultrasonic transducers are mounted athwartship on the hull of the vessel and directed downwardly to transmit ultrasonic pulses and to receive corresponding pulses reflected from the floor of the body of water. Time durations between transmitted and corresponding received pulses are measured to determine floor or channel depth below the port and starboard sides of the vessel. The depths are digitally encoded and supplied to a comparator which generates a signal indicating whether deeper water is port or starboard of the vessel. The comparator signal is supplied to a display to instruct the navigator whether to steer port, starboard or to maintain course to guide the vessel over the center of the channel. The comparator signal may be also supplied to a rudder servo to automatically steer over the channel center. A digital filter is provided to eliminate any spurious readings obtained by the transducers.