Channel Prefixes
In January of 2017, new international regulations changed the numbering scheme for maritime VHF channels. Channels which were previously numbered with a suffix of A or B now have a prefix of 10 or 20.
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Channels formerly noted with an A now have a prefix of 10. These channels transmit and receive using only the ship station frequency and can be used for direct ship-to-ship communication.
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Channels formerly noted with a B now have a prefix of 20. These channels transmit and receive using only the shore station frequency.