of information a carriage return and two line feeds either 5 or more seconds of phasing signal and another message starting with "ZCZC" or an end of emission idle signal α for at least 2 seconds. B1 is an alpha character identifying the station, and B2 is an alpha character used to identify the subject of the message. Receivers use these characters to reject messages from certain stations or if the message contains subjects of no interest to the user. B3 and B4 are two-digit numerics identifying individual messages, used by receivers to keep already received messages from being repeated. For example, a message containing B1B2B3B4 characters of 'FE01' from a U.S. NAVTEX station indicates a weather forecast message from Boston, MA. NAVTEX message example: (phasing signals >= 10 seconds) ZCZC FE01 (message text ...) NNNN (end of message phasing signals for >= 2 seconds before next message) Start of message ZCZC begins the messages. Transmitter identity (B1) This character defines the transmitter identity and its associated coverage area. Subject indicator character (B2) The subject indicator character is used by the receiver to identify different classes of messages below. The indicator is also used to reject messages concerning certain optional subjects which are not required by the ship (e.g. LORAN C messages might be rejected in a ship which is not fitted with a LORAN C receiver). NAVTEX broadcasts use following subject indicator characters: {| class="wikitable" | A | Navigational warnings 1 |- | B | Meteorological warnings 1 |- | C | Ice reports |- | D | Search & rescue information, and pirate warnings 1 |- | E | Meteorological forecasts |- | F | Pilot service messages |- | G | AIS messages (formerly Decca messages) |- | H | LORAN messages |- | I | Not used (formerly OMEGA messages) |- | J | SATNAV messages (i.e. GPS or GLONASS) |- | K | Other electronic navaid messages |- | L | Navigational warnings — additional to letter A (Should not be rejected by the receiver) |- | T | Test transmissions (UK only — not official) |- | V | Notice to fishermen (U.S. only — currently not used) |- | W | Environmental (U.S. only — currently not used) |- | X | Special services — allocation by IMO NAVTEX Panel |- | Y | Special services — allocation by IMO NAVTEX Panel |-