Friday, February 15, 2008

Maritime Warning System ( NAVTEX )

NAVTEX is a maritime radio warning system consisting of a series of coast stations transmitting radio teletype (CCIR Recommendation 476 standard narrow band direct printing, sometimes called Sitor or ARQ/FEC) safety messages on the international stan­dard medium frequency 518 kHz. Coast stations trans­mit during preset time slots so as to minimize interference with one another. Routine messages are normally broadcast four to six times daily. Urgent mes­sages are broadcast upon receipt, provided that an another station is not transmitting. Since the broadcast uses the medium frequency band, a typical station ser­vice radius ranges from 100-500 NM day and night. In­terference from or receipt of stations farther away occasionally occurs at night.

Each NAVTEX message broadcast contains a four-character header describing identification of sta­tion (first character), message content (second charac­ter), and message serial number (third and fourth characters). This header allows the microprocessor in the shipborne receiver to screen messages, selecting only those stations relevant to the user, messages of subject categories needed by the user, and messages not previously received by the user. Selected messages are printed on a roll of paper as received, to be read by the mariner at his convenience. Unwanted messages are suppressed. Suppression of unwanted messages is more and more important to the mariner as the num­ber of messages, including rebroadcasts, increases yearly. With NAVTEX, a mariner will no longer have to listen to, or sift through, a large number of irrelevant data to obtain the information necessary for safe navigation.

Vessels regulated by the Safety of Life at Sea (SOLAS) Convention, as amended in 1988 (cargo vessels over 300 tons and passenger vessels, on interna­tional voyages), and operating in areas where NAVTEX service is available, have been required to carry NAVTEX receivers since 1993. The USCG voice broadcasts (Ch. 22A), of more inshore and harbor information, will remain un­affected by NAVTEX. Mariners not able to man a radio on a 24-hour basis in order to hear critical warning messages, commercial fishermen should also find a advantage in owning a NAVTEX receiver. NAVTEX coverage is reasonably continuous to 200nm off the U.S East, Gulf, and West Coast, Puerto Rico, Southwest Alaska, Hawaii, and 300 - 400 nm off Guam.