The NAVTEX system is used for the automatic broadcast of localised Maritime Safety Information (MSI) using Radio Telex (also known as Narrow Band Direct Printing, or NBDP).
The system operates on the Medium Frequency band just below the AM broadcast band. System range is generally 300 or so nautical miles from the transmitter.
The NAVTEX system is designed to be used in GMDSS Sea Area A2, and is utilised mainly by those countries with relatively small areas of coastline and/or sea areas to cover.
Major areas of NAVTEX coverage include the Mediterranean Sea, the North Sea, coastal areas around Japan and areas around the North American continent.
The NAVTEX system has been allocated three broadcast frequencies:
In real terms, 518 kHz is the only NAVTEX channel used - this means that all broadcasts from stations within the same NAVAREA must be coordinated on a time sharing basis to eliminate interference. In addition, power outputs from each station are adjusted to control the range of each broadcast. This is particularly important during night-time hours, as Medium Frequencies tend to travel further after dark.
NAVTEX messages are transmitted using the following format:
Subject indicator characters assigned to the NAVTEX system are as follows:
A - Navigational warnings B - Meteorological warnings C - Ice reports D - Search and rescue information E - Meteorological forecasts F - Pilot service messages G - Decca messages H - Loran messages I - Omega messages J - Satnav messages K - Other electronic navaid messages L - Additional navigational messages V - Special services W - Special services (possible other languages use) X - Special services Y- Special services Z - No message on hand (QRU)
A NAVTEX receiver is normally a compact unit, with an integrated printer, display and keyboard.
Once programmed with appropriate station codes and subject indicator characters, the unit is left unattended - messages are received automatically and printed on the integrated printer.
NAVTEX receivers are supplied with their own antenna system, which is normally a short whip type antenna and a pre-amplifier.