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It is an ASW function for navigation between dipping positions for ASW helicopters. A dipping position is the the helo hovers and lowers the sonar. The AFCS or auto pilot flies the aircraft from flight at 200 ft 100 knts to the hover.
Before Helios had radar, Wessex mk 1 the computer was used to work out transit headings and times from dipping posn to dipping post.