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Navtex (Navigational Telex) is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent maritime safety information to ships. Navtex 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) off shore. There are no user fees associated with receiving navtex broadcasts, as the transmissions are typically transmitted from the National Weather Authority (Italy) or Navy or Coast Guard (as in the US) or national navigation authority (Canada). Where the messages contain weather forecasts, an abbreviated format very similar to the shipping forecast is used. 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. ==Technical information== Navtex transmissions are also called narrow-band direct printing (NBDP). The transmissions are layered on top of SITOR collective B-mode. SITOR-B is a forward error correcting (FEC) broadcast that uses the CCIR 476 character set. SITOR-B is also used in amateur radio, where it is known as AMTOR-B or AMTOR-FEC. * Navtex / SITOR / AMTOR broadcasts use 100 baud FSK modulation with a frequency shift of 170 Hz. Navtex broadcasts are primarily made on the Medium frequencies of 518 kHz and 490 kHz. The international navtex frequency is 518 kHz,〔The mark frequency is 517.915 kHz, and the space frequency is 518.085 kHz.〕 and these broadcasts should always be in English. National transmission of navtex uses 490 kHz specifically for broadcasts in local languages. It is not used in the U.S. Navtex Marine Safety Information (MSI) national transmissions also take place on HF at 4209.5 kHz using FEC mode. Other transmission modes (like MT63, or Olivia) with better error correction properties have emerged since navtex was made the standard for maritime information transmissions. Overall, with slightly higher transmitter power most Navtex error correction issues tend to be absent. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Navtex」の詳細全文を読む スポンサード リンク
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