Science Fair Project Encyclopedia
Navtex
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 U.S. Coast Guard. There are no user fees associated with receiving NAVTEX broadcasts.
NAVTEX is a major element of the Global Maritime Distress Safety System (GMDSS). 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 such features as LCD screens and RS-232 output and have a purchase price in the $300-$500 range.
Every NAVTEX message is preceded by a four character header B(1)B(2)B(3)B(4). B(1) is an alpha character identifying the station, and B(2) is an alpha character used to identify the subject of the message. Receivers use these characters to reject messages from stations or concerning subjects of no interest to the user. B(3)B(4) are two-digit numerics identifying individual messages, used by receivers to keep already received messages from being repeated. For example, a message preceded by the characters FE01 from a U.S. NAVTEX Station indicate that this is a weather forecast message from Boston MA.
Subject indicator character 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. LORAN C messages might be rejected in a ship which is not fitted with a LORAN C receiver). Receivers also use the B(2) character to identify messages which, because of their importance, may not be rejected (designated by an asterisk).
NAVTEX broadcasts use following subject indicator characters:
- A = Navigational warnings *
- B = Meteorological warnings *
- C = Ice reports
- D = Search & rescue information, and pirate warnings *
- 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 } Special services - allocation by IMO NAVTEX Panel
- 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.
Technical Information
NAVTEX uses frequencies near the old 500 kHz international distress channel. All NAVTEX broadcasts are made on 518 kHz, using narrow-band direct printing 7-unit forward error correcting (FEC or Mode B) transmission. This type of transmission is also used by Amateur Radio service (AMTOR ). Broadcasts use 100 baud FSK modulation, with a frequency shift of 170 Hz. An alternate frequency of 490 kHz is available for broadcasts in local languages. It is not used in the U.S.
NAVTEX-type Marine Safety Information transmissions also take place on HF at 4209.5 kHz using FEC mode.
Sources
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