Principle Of Navtex System; How Navtex Works; Navtex System Operation - Furuno NX-300-D Operator's Manual

1. PRINCIPLE OF NAVTEX SYSTEM

1.1

How NAVTEX Works

There are many types of navigational and meteorological information available on radio, such
as NAVAREA, HYDROPAC, etc. However, these systems rely heavily upon the operator's
experience and skill in tuning the radio and interpreting messages. In addition, constant
monitoring to pick up wanted information among a vast volume of messages is not practical with
a limited radio staff.
To provide all mariners with up-to-the-minute information automatically, the NAVTEX system
was developed.
NAVTEX is an acronym meaning Navigational Telex, and as its name shows, it is a kind of
narrow band radio teletype system for sending (by frequency shift keying) text messages
expressed in a 7-unit code. The difference is that a NAVTEX transmitter transmits nine control
characters (header code) ahead of the main message, so that the receiver can identify the
station, message type and serial number automatically.
1.2

NAVTEX System Operation

For navigation purposes, the world is divided into 16 areas (called Navareas) as shown in the
figure below. Each Navarea has multiple NAVTEX stations and each NAVTEX station has an
identification code, from "A" to "Z." The frequency assigned to NAVTEX is only one (518 kHz),
and many stations exist in the same Navarea. (Some stations use 490 kHz also.)
If the stations were to transmit without any rule, the system would collapse due to mutual
interference. To avoid this problem, the following rules apply.
• The transmission schedule is determined so that two or more stations having a common
service area may not overlap in time.
• Each station transmits with minimum required power to cover its service area (200 nautical
miles nominal).
Figure 1-1 NAVTEX area
1-1