NAVTEX

Introduction

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 mainly operates in the Medium Frequency radio band just above and below the old 500 kHz Morse Distress frequency. 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.

Frequency of operation

The NAVTEX system has been allocated three broadcast frequencies:

518 kHz - the main NAVTEX channel

490 kHz - used for broadcasts in local languages (ie: non-English)

4209.5 kHz - allocated for NAVTEX broadcasts in tropical areas - not widely used at the moment.

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 always travel further after dark.

NAVTEX message format

NAVTEX messages are transmitted using the following format:

Click to enlarge graphic.

in which

ZCZC defines the end of the phasing period,

the B1 character is a letter (A-Z) identifying the transmitter coverage area,

the B2 character is a letter (A-Z) for each type of message.

B3B4 is a two-character serial number for each message, starting with 01 except in special cases where the serial number 00 is used.

The transmitter identification character (B1) is a single letter allocated to each NAVTEX shore station transmitter. NAVTEX receivers allow the operator to accept or reject messages from any station, based on this single letter code.

The subject indicator character (B2) is also single letter code which serves to identify the message subject. NAVTEX receivers also allow the operator to reject certain messages based on the subject indicator character. However, messages concerning Navigational Warnings, Meteorological Warnings, and Search And Rescue information can not be rejected.

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)

Shipboard NAVTEX EQUIPMENT

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 receiver front panel - note inbuilt printer, keyboard and display.

NAVTEX receivers are supplied with their own antenna system, which is normally a short whip type antenna and a pre-amplifier.

Click to view amendments to the Navtex manual - which came into effect in January 2002 (PDF format).

GMDSS is not perfect - let's look at some flaws - and possible solutions...