From International Dictionary of Marine Aids to Navigation
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A navigation system in which a position fix is obtained by phase comparison of continuous wave transmissions.

Two widely spaced transmitters are used, the beat note between their frequencies being derived on the vessel.

This is phase compared with a similar beat note, produced at a fixed shore location, and transmitted to the vessel on a different radio frequency, to produce a line of position.

A second line is produced by a similar system.

Note: This is similar to the Raydist system (4-4-270).

Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)