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I. Principal features of NAVTEX

The service uses a single frequency (518 kHz), with transmissions in English from nominated stations within each NAVAREA being arranged on a time-sharing basis to eliminate mutual interference. All necessary information is contained in each transmission. The power of each transmitter is so regulated as to avoid the possibility of interference between transmitters.

A dedicated NAVTEX receiver which has the ability to select messages to be printed, according to (a) a technical code (B₁, B₂, B₃, B₄), which appears in the preamble of each message and (b) whether or not the particular message has already been printed. Certain essential classes of safety information such as navigational and meteorological warnings and search and rescue information are non-rejectable to ensure that ships using NAVTEX always receive the most vital information.

NAVTEX coordinators exercise control of messages transmitted by each station according to the information contained in each message and the geographical coverage required. Thus a user may choose to accept messages either from the single transmitter, which serves the se area around his position, or from a number of transmitters as appropriate.

II. Message priorities

Three message priorities are used to dictate the timing of the first broadcast of a new warning in the NAVTEX service. In descending order of urgency they are:

VITAL – for immediate broadcast, subject to avoiding interference to ongoing transmissions,

IMPORTANT – for broadcast at the next available period when the frequency is unused

ROUTINE – for broadcast at the next scheduled transmission period

Note: Both VITAL and IMPORTANT warnings will normally need to be repeated, if still valid, at the next scheduled transmission period.

III. The standard format of NAVTEX messages

Phasing signals >10 sec

ZCZC (Start of message group) B₁, B₂, B₃, B₄

(B₁ – transmission identity,

B₂ – Subject indicator,

B₃, B₄ – consecutive number)

(Time of Origin) – Optional

Series Identity + Consecutive Number

Message Text

NNNN – end of message

Idle signals aa…a >2 sec

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