NAVTEX Theory
=============

Navtex transmission are, in reality, Mode B AMTOR (FEC).  What makes
this systen unique is the actual message format.  Navtex stations always
transmit on 518 KHZ LSB with transmitters usually 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 regular scheduled teleprinter bulletins.

At the beginning of a Navtex transmission, the sending station will send
a preamble.  This information contains, the station identifier, type of
message and the message sequence number.  For example a preamble would
look like this:

   ZCZC AG25

The ZCZC is always present, and the receiving station sees this as the
indication that the identifier is coming next.  The components of the
identifier are:

   B1 B2 B3 B4

B1 is used to identify the station that is transmitting and consists of
the letters A through Z.  B2 indicates the message class, again the letters
A through Z are used.  B3 and B4 are the sequence number for the message,
and will have a value from 00 through 99.

At the receiving station the teleprinter, or TNC, is programmed to print
a specific class or classes of bulletins or to print all bulletins.  Any
message sequence number 00 is always printed regardless of message class
and message class A,B, and D must always be printed once per station
regardless of message sequence number.


  NAXTEX MESSAGE CLASSES (B2)           CURRENT NAVTEX STATIONS
  =================================     =======================
  A = Navigational warnings             San Franciscio, CA
  B = Storm warnings                    Boston, MA
  C = Ice reports                       New Orleans, LA
  D = Search and rescue information     Miami, FL
  E = Meteorological forcasts           Chesapeake, VA
  F = Pilot messages                    Honolulu, HI
  G = Decca messages                    Kodiak, AK
  H = LORAN-C messages                  Astoria, OR
  I = Omega messages                    Long Beach, CA
  J = Differential Omega messages       Guam
  K = Navigational system messages      Halifax, Nova Scotia
  Z = No message on hand                San Juan, P.R.


  ARRL BULLETIN CODES (B1)   ARRL ASSIGNED MESSAGE CLASSES (B2)
  =======================    ==================================
  A = ARRL bulletins         A = Emergency bulletins
  C = CRRL bulletins         B = Priority bulletins
  I = IARU bulletins         D = Reserved
  J = JARL bulletins         E = DX bulletins
  S = AMSAT bulletins        G = General bulletins
  X = Miscellaneous          K = Keplarian bulletins
                             P = Propogation forcasts
                             S = Satellite bulletins
                             X = Miscellaneous bulletins

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