NAVTEX is an information system for transmitting MSI (Maritime Safety Information, that are 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.
- 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) - it is used in tropical regions where MF reception could be difficult.
- 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,
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-minutes 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.
NAVTEX receiver sample
- a radio receiver, fixed tuned to the NAVTEX frequencies (it can be single or dual frequency receiver - the last one is capable to receive message on 518 kHz and 490 kHz frequencies without tuning the receiver),
- a signal processor,
- a small keyboard (for setting up and operating the NAVTEX receiver),
- a display and/or a continuous paper feed (for displaying or printing the received MSI),
- a memory for storing received messages and
- antenna system.
The SOLAS convention requirements
SOLAS vessels must carry a dual frequency receiver if reception on 490 kHz is required since availability to receive messages on 518 kHz must be maintained at all times, or they must carry two single frequency receivers tuned on different frequencies. A receiver should comprise (for displaying or printing the received MSI) either:
- an integrated printing device or
- a dedicated display device and printer output port or
- a connection to an integrated navigation system.
On a non SOLAS vessel, it is feasible to have a single frequency receiver. The user can switch to the 490 kHz frequency for reception of a message (at a scheduled transmission time) and then switch back to the 518 kHz frequency when the message has been received.