an acoustic navigation system for marking channels is utilized for provid safe entry and exit from harbors . the system has a plurality of pairs of acoustic projectors aligned on opposite sides of a channel . each pair is spaced from corresponding pairs , a predetermined distance along the sides of the channel . a first projector in each pair provides an acoustic source for a period of time . the other projector in the pair provides acoustic sources spaced at predetermined intervals both before and after the signal from the first projector . a vessel equipped with receiving and display equipment determines its own location within the channel from the received acoustic signals .