Commandant (G-TTM)
U.S. Coast Guard                        5 May 1995
Washington DC 20593


NAVTEX Information

(Adapted from the IMO NAVTEX Manual)

The International NAVTEX service means the coordinated broadcast
and automatic reception on the frequency 518 kHz of maritime safety
information by means of narrow-band direct printing telegraphy
using the English language.  The operational and technical
characteristics of the NAVTEX system are contained in
Recommendation ITU-R M.540-2.  Performance standards for shipborne
narrow-band direct printing equipment are laid down in IMO Assembly
resolution A.525(13).

The principle features are:

     1.   The service uses a single frequency with transmissions
from nominated stations within each NAVAREA being arranged on a
time-sharing basis to eliminate mutual interference.  All necessary
information is contained in each transmission.

     2.   The power of each transmitter is regulated so as to avoid
the possibility of interference between transmissions.

     3.   A dedicated NAVTEX receiver which has the ability to
select messages to be printed, according to:

          a.   A technical code {B(1)B(2)B(3)B(4)}, which appears
in the preamble of each message, and
          b.   Whether or not the particular message has already
been printed.

Certain essential classes of safety information such as
navigational and meteorological warnings and search and rescue
information are non-rejectable to ensure that ships using NAVTEX
always receive the most vital information.

     4.   NAVTEX coordinators exercise control of messages
transmitted by each station according to the information contained
in each message and the geographical coverage required.  Thus a
user may choose to accept messages, as appropriate, either from the
single transmitter which serves the sea area around his or her
position or from a number of transmitters.


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 characters 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. OMEGA messages might be rejected in a
ship which is not fitted with an OMEGA receiver).  Receivers also
use the B(2) character to identify messages which, because of their
importance, may not be rejected (designated by an asterisk).

The following subject indicator characters are in use:

     A = Navigational warnings *
     B = Meteorological warnings *
     C = Ice reports
     D = Search & rescue information, and pirate attack warnings *
     E = Meteorological forecasts
     F = Pilot service messages
     G = DECCA messages
     H = LORAN messages
     I = OMEGA messages
     J = SATNAV messages
     L = Navigational warnings - additional to letter A 
          (Should not be rejected by the receiver)
     V }
     W } Special services - allocation by IMO NAVTEX Panel
     X }
     Y }
     Z = No message on hand


     Note: The subject indicator characters B, F and G are normally
not used in the U.S.   Since the National Weather Service normally
includes meteorological warnings in forecast messages,
meteorological warnings are broadcast using the subject indicator
character E.  U.S. Coast Guard District Broadcast Notices to
Mariners affecting ships outside the line of demarcation, and
inside the line of demarcation in areas where deep draft vessels
operate, use the subject indicator character A.

     See also the files NAVTEX.TXT for lists of international
NAVTEX stations, schedules, and transmitter identification
characters; PRODUCT.TXT for meteorological products broadcast by
U.S. NAVTEX stations; NAVTEX-A.GIF and NAVTEX-P.GIF for charts of
U.S. NAVTEX service areas; and IIP.TXT for information concerning
icebergs broadcast over NAVTEX.