NAVTEX differs from regular SITOR-B in the receiving equipment itself. Every NAVTEX message begins with ZCZC and ends with NNNN. This lets the decoder know exactly where in the data stream the messages are. NAVTEX messages have a four-figure message number which is sent right after the ZCZC and which the decoding unit reads and keeps track of. Messages are repeated several times a day, and if the decoding unit reads a previously decoded message number, it doesn't print out that message. This makes it easy for the receiving party (usually a ship's officer) to read new messages without having to wade through all the repeats.

The message number consists of two letters and two numbers. The first letter indicates the transmitting station. Some of the North American stations are as follows:

Station location               Identifier          Sked Times (Z)
----------------               ----------          -----------------------------
Boston, MA                        F                0445 0845 1245 1645 2045 0045
Portsmouth, VA                    N                0130 0530 0930 1330 1730 2130
Miami, FL                         A                0000 0400 0800 1200 1600 2000
Savannah, GA                      E                0040 0440 0840 1240 1640 2040
New Orleans, LA                   G                0300 0700 1100 1500 1900 2300
San Juan, PR                      R                0200 0600 1000 1400 1800 2200
Long Beach, CA                    Q                0445 0845 1245 1645 2045 0045
Astoria, OR                       W                0130 0530 0930 1330 1730 2130
Kodiak, AK                        J                0300 0700 1100 1500 1900 2300
Adak, AK                          X                2340 0340 0740 1140 1540 1940
San Francisco, CA                 C                0400 0800 1200 1600 2000 0000
Honolulu, HI                      O                0100 0500 0900 1300 1700 2100
Prescott, ON                      H                0110 0510 0910 1310 1710 2110

The second letter of the message number gives the message type. These different classes are as follows:

Letter                      Message Class
------                      -------------
A                           Navigational Warnings
B                           Meteorological Warnings
C                           Ice Reports
D                           Search & Rescue Information
E                           Meteorological Forecasts
F                           Pilot Messages
G                           DECCA Messages
H                           LORAN-C Messages
I                           OMEGA Messages
J                           SATNAV Messages
Z                           QRU
K through Y                 Reserved

You can, using the receiving equipment, decide which type of messages to receive or ignore and which stations to receive or ignore. For example, I don't care to receive weather forecasts for all the different oceans, so I set up my decoder to ignore messages of class "E." I can also set up my decoder to ignore certain stations by specifying the letter used by those particular stations, but since I'm interested in receiving all NAVTEX stations I don't use this function.

The two digit number following the station identifier and message type are a sequential number that the decoder uses to keep track of whether or not it received that message before. If so, the message isn't printed. The number starts at 01 and goes to 99. When 99 is reached, the system starts with 01 again and the process is repeated.

Sample NAVTEX message:

ZCZC GA68
CCGDEIGHT BNM 0057-99
LA - GULF OF MEXICO
POOL RIG 54 IS STACKED ADJACENT TO A SAFETY FAIRWAY IN SOUTH PASS
BLOCK 31 IN APPROX POSITION 29-07-00N, 90-11-00W.

NNNN

ZCZC is the message header, and GA68 is the message number. G means the message was sent by the New Orleans station, and A means the message is a navigational warning. 68 is the current message number. When this number reaches 99, the next message of this class will revert to 01.

One helpful feature of receiving NAVTEX is the error-detection mode. NAVTEX is sent in SITOR-B mode which is an error-correction mode. NAVTEX decoders only print out messages they haven't copied before. They also reprint messages that are received with too many errors. Most decoders can be set up so messages with more than the desired percentage of errors will be copied again. Mine is set up by default at 5% errors or less. If a message is copied with too many errors (more than 5%), an error message is printed and the decoder pretends that it didn't receive the message.

Example:

ZCZC GJ63
DGPS BNM 371-99
TEXAS COAST-ARANSAS PASS_N TX
DGPS BROADCAST SITE IS OFF-_-_4
- 9! 212152Z APR 99 UNTIL F_RTHER NOTICE.__
_____T__
NNNN

***TOO MANY ERRORS GJ63

The next time this message is sent, if it is copied with less than 5% errors the decoder will print it out and not copy it again.

I hope this explains how NAVTEX operates. Even if your decoder doesn't have the NAVTEX option, you will still be able to read the messages by using SITOR-B mode. The only difference is that you will see all the message repeats and won't be able to choose what type of messages to receive or ignore. Nor will you have the error detection capability. However, you will still be able to read all the data that is sent. NAVTEX can make for some very interesting monitoring.

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