Navigational guidance device for ships - uses cables laid along navigated channel supplied with AC to produce alternating magnetic field

摘要

A method of ship navigation involves the use of a cable supplied with an alternating current and laid along the desired path to be taken by the ship. The alternating magnetic field of the cable is detected by a receiver in the ship (31). The cable has two parts (4, 5), supplied alternately from a single generator. Both halves normally lie closely together, but in critical navigation areas (20) the cables diverge to produce warning signals giving on-board directional indications by producing differently oriented electromagnetic fields. The frequency of switching the signal between the two cables must be less than that of the generator, and is less than 10 Hertz. At the ends (9, 10) of the channel the cables (4, 5) diverge and are separately earthed.