New VHF marine radios are now displaying four-digit channel numbers in place of the older two-digit channel number ending with the letter “A”, to better harmonized U.S. use of VHF maritime mobile band with international use of the band, the USCG informed in a recent alert.
Under this new four-digit channel numbering scheme, U.S. two-digit channel numbers previously ending in “A” will now be designated by the same two-digit number, preceded by the digits “10”.
The International Telecommunications Union rewrote their ITU Radio Regulations Table of transmitting frequencies in the VHF maritime mobile band in 2015, making allowances for administrations to make simplex use of duplex channels, just as the US had done fifty years earlier.
Duplex channels use two frequencies, one designated for ship station use, the other for coast station use. Radios operating on a duplex channel cannot communicate with a radio operating simplex on that channel. Since both schemes use the same channel numbering, the Federal Communications Commission added the suffix “A” in 1965 to designate simplex use over the ship station transmit portion of the international duplex channel, to avoid confusion. The prefix “10” serves this same purpose.
The Coast Guard transmits urgent marine information broadcast (UMIB) safety warnings and communicates with mariners on VHF channel 1022 (previously channel 22A).
