NAVTEX
MARITIME SAFETY BROADCASTS
NAVTEX in the United States
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.
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.
NAVTEX Predicted Coverage in the U.S.
These charts will be updated as changes occur.
NAVTEX Message Selection
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)
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.
Subject indicator character B(2)
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). Receivers also use the B(2) character to identify
messages which, because of their importance, may not be rejected (designated by
an asterisk).
NOAA Site-Navtex
Broadcasting Times, Locations, and Recent Updates
Technical Information
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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