NAVTEX is an information system for transmitting MSI (Maritime Safety Information, i.e. navigational and meteorological warnings, meteorological forecasts, and other urgent safety-related messages) and automatic reception of MSI by means of narrow-band direct-printing telegraphy.
NAVTEX transmissions are made and sent by (Radio TELEX) transmitters at CRS (Coast Radio Stations) on:
- 518 kHz MF (Medium frequency) - MSI are transmitted in English - known as International NAVTEX and/or
- 490 kHz MF - MSI and/or other local information are transmitted in the local language primarily for small vessels - known as National NAVTEX and/or
- 4209,5 kHz HF (High Frequency) – MSI and/or other local information are transmitted in the local language. it is used exclusively for NAVTEX-type transmission out of the NAVTEX range where MF reception could be difficult.
NAVTEX is used for transmitting MSI to vessels in coastal waters (approximately within 400 NM of a NAVTEX station). The area covered by NAVTEX may be
- tailored to avoid the possibility of interference between transmitters, for example Oostende CRS (Belgium) coverage range is 50 NM, or
- increased to ensure signal coverage in all coastal waters , for example Azores CRS (Portugal) coverage range is 640 NM,
This is done just by regulating the transmission power, because the range of transmissions in the MF band depends largely upon the power radiated.
Interference between transmitters cannot be absolutely avoided just by regulating their transmission power. They are also time-sharing the frequency. Each CRS has a ten-minute transmitting time slot every four hours. If a CRS is handed a very important message requiring express transmission outside the scheduled transmission time of the station, arrangements can be made to interrupt the scheduled transmission of a nearby CRS enabling the urgent transmission to be made immediately without interference.