Navtex

From Wikipedia, the free encyclopedia

A NAVTEX receiver prints an incoming message.
A NAVTEX receiver prints an incoming message.

NAVTEX is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships. It was developed to provide a low-cost, simple, and automated means of receiving this information aboard ships at sea within approximately 370 km (200 nautical miles) of shore. NAVTEX stations in the U.S. are operated by the United States Coast Guard. There are no user fees associated with receiving NAVTEX broadcasts.

NAVTEX is a component of the International Maritime Organization/International Hydrographic Organization Worldwide Navigation Warning Service (WWNWS). NAVTEX is also a major element of the Global Maritime Distress Safety System (GMDSS). International Convention for the Safety of Life at Sea (SOLAS) mandated certain classes of vessels must carry NAVTEX, beginning August 1, 1993.

NAVTEX receivers which are approved for GMDSS contain an internal printer and cost between $800-$1500. A new generation of NAVTEX receivers intended for non-GMDSS applications such as the recreational community is now entering the marketplace. These receivers include features such as LCD screens and RS-232 output and have a purchase price in the $300-$500 range.

Contents

Every NAVTEX message is preceded by a phasing signal of around ten seconds then a start of message group, "ZCZC", followed by four characters B1, B2, B3 and B4.

B1 is an alpha character identifying the station, and B2 is an alpha character used to identify the subject of the message. Receivers use these characters to reject messages from certain stations or if the message contains subjects of no interest to the user.

B3 and B4 are two-digit numerics identifying individual messages, used by receivers to keep already received messages from being repeated.

A message containing the B1,2,3,4 characters of 'FE01' from a U.S. NAVTEX Station indicate that this is a weather forecast message from Boston MA.

NAVTEX message example:

(start of phasing signals >=10 seconds)

ZCZC

B1,B2,B3,B4

Time of message transmission in UTC (optional)

Message Text......................................

NNNN

(end of message phasing signals for >=5 seconds before next message)

ZCZC begins the message.

This character defines the transmitter identity and its associated coverage area.

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. LORAN C messages might be rejected in a ship which is not fitted with a LORAN C receiver).

NAVTEX broadcasts use following subject indicator characters:

  • A = Navigational warnings 1
  • B = Meteorological warnings 1
  • C = Ice reports
  • D = Search & rescue information, and pirate warnings 1
  • E = Meteorological forecasts
  • F = Pilot service messages
  • G = DECCA messages
  • H = LORAN messages
  • I = OMEGA messages (note OMEGA has been discontinued)
  • J = SATNAV messages (i.e. GPS or GLONASS)
  • L = Navigational warnings - additional to letter A (Should not be rejected by the receiver)
  • V = Notice to fishermen (U.S. only - currently not used)
  • W = Environmental (U.S. only - currently not used)
  • X }
  • Y } Special services - allocation by IMO NAVTEX Panel
  • Z = No message on hand

Note: Receivers use the B2 character to identify messages which, because of their importance, can not be rejected (designated by a 1). 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.

These two characters define the serial number of each B2 message type (class). Generally serial numbers start with the numbers '01', however in special circumstances, the numbers begin with '00'. This forces the receiver to print the message.

The time of the transmission of the message is in UTC.

The full text of the message follows.

The end of the message is asserted when the characters "NNNN" are received.

NAVTEX broadcasts are primarily made on Medium frequencies of 490kHz and 518 kHz, using narrow-band direct printing (NBDP), 7-unit forward error correcting (FEC or, Mode B) transmission and the CCIR-476 character set. This type of transmission is also used in Amateur radio, where it is known as AMTOR. AMTOR Broadcasts use 100 baud FSK modulation, with a frequency shift of 170 Hz.

The international NAVTEX frequency is 518kHz USB. Regional transmission of NAVTEX uses 490 kHz specifically for broadcasts in local languages other than English. It is not used in the U.S.

NAVTEX-type Marine Safety Information (MSI) transmissions also take place on HF at 4209.5 kHz using FEC mode.

Commercial receivers are available for installation into vessels. These are designed to operate unattended and either record messages on to paper or to the receiver's internal memory for later review. There are also a number of software packages available allowing messages to be decoded by a PC with a suitable receiver connected to the computer's soundcard. Any general communications receiver capable of audio reception at 518 kHz or 490 kHz single sideband can be used.

NAVTEX MF Transmitters

NAVTEX decoders

Examples of NAVTEX messages

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