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NAVTEX Operation
NAVTEX/AMTEX Theory
NAVTEX transmissions are, in reality, Mode B AMTOR (FEC). What makes NAVTEX unique however,
is the actual message format. NAVTEX stations always transmit on 518 kHz LSB and are typically
located on the coast lines. The same format is now being used by the American Radio Relay
League (ARRL) to transmit their bulletins on a daily basis. These are transmitted during their
scheduled teleprinter bulletins.
At the beginning of a NAVTEX (or AMTEX) transmission, the sending station will send a preamble,
identifying the station that is transmitting, the type of message, and the message sequence num-
ber. The preamble would look like this:
ZCZC AG25
The ZCZC is always present, and the receiving unit sees this as the indication that the identifier is
coming next. The components of the identifier, as defined in the NAVTEX documentation are:
B1 B2 B3 B4
B1 is used to identify the station that is transmitting and consists of the letters A through Z. B2 in-
dicates the message class (A through Z). Note that class A, B and D messages must always be
printed once. B3 and B4 are the sequence number for the message, and will have values from 00
through 99. Messages with sequence number 00 are always printed. When the message numbers
reach 99, they wrap to 01 as the next number.
Currently operating NAVTEX stations are:
Halifax, Nova Scotia Guam
Boston, MA Honolulu, HI
New Orleans, LA Kodiak, AK
Miami, FL Astoria, OR
San Juan, P.R. Long Beach, CA
Chesapeake, VA San Francisco, CA
For NAVTEX messages, the currently assigned message classes are:
A Navigational warnings
B Meteorological warnings (storm warnings)
C Ice reports
D Search and Rescue Information
E Meteorological forecasts
F Pilot messages
G DECCA message
H LORAN-C message
I Omega message
J differential Omega message
Z QRU - no message on hand
K-Y Reserved
NOTE: K has been proposed for "other electronic navigational aid system message".
Once a message has been received from a particular station with less than a specified amount of
errors, it will not be printed again, as the receiving system keeps track of the message numbers it
has received. If it then sees the same message class and number from the same transmitting sta-
tion, it will not output the message to the terminal.
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