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Chapter 11 SIAM and NAVTEX Operation
By varying
BITINV
from 0 through 31, you’ll test all the different bit-inversion possibilities that may be used to
encode a Baudot signal.
If only simple bit-inversion is being used, one of the settings of
BITINV
should cause the transmission to print in
plain text. If none of the 32 possibilities reveal plain text, then the transmitting station is likely using a more
sophisticated encryption technique.
About NAVTEX
NAVTEX (NAVigational TELEX)—a form of FEC AMTOR broadcasted in mode-B AMTOR (SITOR) on a
frequency of 518 KHz—is an international, direct printing service designed to broadcast navigational and
meteorological warnings and other urgent information to ships at sea. NAVTEX broadcasts are sporadic, so you’ll
see only information of timely interest and may never see the same message twice.
Recently, the ARRL has also adopted this format for transmitting bulletins. In amateur radio this same format is
starting to be referred to as AMTEX; its transmissions can be found on the ARRL’s bulletin frequencies.
Before proceeding with the rest of this section, review the following commands that are directly applicable to
NAVTEX :
NAVMSG
, and
NAVSTN
. Commands are listed and explained at the end of this chapter.
Setting Up for NAVTEX
(If you’re using an AEA program, follow the instructions in the program’s manual to set up and use NAVTEX.)
To enter the NAVTEX mode, simply enter
NA
at the command prompt. The DSP responds by displaying the
previous mode it was in, then its new mode:
cmd:
NA
Opmode was (old mode)
Opmode now NAvtex
cmd:
Look at your DSPs front panel to verify that you’re in the NAVTEX mode—the left-hand STBY light should be lit.
Next, you can choose which modem you want to use using the
MODEM command, followed by the specific modem
number. For a list of modems available in the DSP, type
DIRECT
at the
cmd:
.
Set your radio to LSB,
WIDESHIFT OFF
.
Receiving NAVTEX Signals
NAVTEX/AMTEX messages are prefaced by the characters “ZCZC”, then a four-character Preamble shown below:
ZCZC AA49
Serial Number 2nd Digit
Serial Number 1st Digit
Message Classification (A to Z)
NAVTEX Station Identification (A to Z)
The first character of the Preamble is a letter that identifies the NAVTEX transmitter, which can be any letter from
A to Z. (This limits the number of NAVTEX stations in an area to 26.)
NAVSTN
can be used to selectively
monitor or reject monitored NAVTEX transmitters.
The second character of the Preamble is the Message Classification. The classifications are:
A) Navigational Warnings
B) Meteorological Warnings (Storm Warnings)
C) Ice Reports
D) Search and Rescue Information
E) Weather Forecasts
September, 05 11-3