The International Maritime Organization has designated NAVTEX as the primary means for transmitting coastal urgent marine safety information to ships worldwide. In the United States, NAVTEX is broadcast from Coast Guard facilities in Cape Cod, Chesapeake VA, Savannah GA, Miami FL, New Orleans LA, San Juan PR, Cambria CA, Pt. Reyes CA, Astoria OR, Kodiak AK, Honolulu HI, and Guam. The Coast Guard began operating NAVTEX from Boston in 1983, and completed its last installation in Adak Alaska, on the Aleutian Islands, just in time to meet IMO's August 1993 requirement that ships carry NAVTEX receivers.
NAVTEX coverage is reasonably continuous in the east, west and Gulf coasts of the United States, as well the area around Kodiak Alaska, Guam and Puerto Rico. The U.S. has no coverage in the Great Lakes, though coverage of much of the Lakes is provided by the Canadian Coast Guard. Since the U.S. Coast Guard originally only installed NAVTEX at sites where Morse telegraphy transmissions were made previously, propagation analyses show some coverage gaps, particularly in the southeast United States, Alaska, and Guam. NAVTEX broadcasts from Adak were permanently terminated in December 1996 due to closure of the Naval facility there.
Every NAVTEX message is preceded by a four character header B(1)B(2)B(3)B(4). B(1) is an alpha character identifying the station, and B(2) is an alpha character used to identify the subject of the message. Receivers use these characters to reject messages from stations or concerning subjects of no interest to the user. B(3)B(4) are two-digit numerics identifying individual messages, used by receivers to keep already received messages from being repeated. For example, a message preceded by the characters FE01 from a U.S. NAVTEX Station indicate that this is a weather forecast message from Boston MA.
The transmitter identification character B(1) is a single unique letter which is allocated to each transmitter. It is used to identify the broadcasts which are to be accepted by the receiver and those which are to be rejected. In order to avoid erroneous reception of transmissions from two stations having the same B(1) character, it is necessary to ensure that such stations have a large geographical separation. NAVTEX transmissions have a designed range of about 400 nautical miles.
NAVTEX stations in the U.S. use the following B(1) characters:
* - Until a planned new automatic broadcast scheduler is installed, Miami's starting time of 0000will be delayed approximately 5 minutes.
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--Loran C has been phased out). Receivers also use the B(2) character to identify messages which, because of their importance, may not be rejected (designated by an asterisk).
NAVTEX broadcasts use following subject indicator characters:
A = Navigational warnings *
B = Meteorological warnings *
C = Ice reports
D = Search & rescue information, and pirate warnings *
E = Meteorological forecasts
F = Pilot service messages
G = DECCA messages
H = LORAN messages
I = OMEGA messages (note OMEGA has been discontinued)
J = SATNAV messages (i.e. GPS or GLONASS)
L = Navigational warnings - additional to letter A
(Should not be rejected by the receiver)
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
Z = No message on hand
Note: The subject indicator characters B, F and G are normally not used in the U.S. Since the National Weather Service normally includes meteorological warnings in forecast messages, meteorological warnings are broadcast using the subject indicator character E. U.S. Coast Guard District Broadcast Notices to Mariners affecting ships outside the line of demarcation, and inside the line of demarcation in areas where deep draft vessels operate, use the subject indicator character A.
All NAVTEX broadcasts are made on 518 kHz, using narrow-band direct printing 7-unit forward error correcting (FEC or Mode B) transmission. This type of transmission is also used by Amateur Radio service (AMTOR). Broadcasts use 100 baud FSK modulation, with a frequency shift of 170 Hz. The center frequency of the audio spectrum applied to a single sideband transmitter is 1700 Hz. The receiver 6 dB bandwidth should be between 270-340 Hz.
Each character is transmitted twice. The first transmission (DX) of a specific character is followed by the transmission of four other characters, after which the retransmission (RX) of the first character takes place, allowing for time-diversity reception of 280 ms.
Fore more information, see ITU Recommendations M.540-2 and M.476-5, available from the ITU Radiocommunications Sector.
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