Furuno NX-700B - Manual (Page 13)

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1. PRINCIPLE OF NAVTEX SYSTEM
1.1 How NAVTEX Works
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 as shown in the
figure below. Each Navtex station has an identification code, from “A” to “Z”. The
frequency assigned to Navtex are 518 kHz and another (490 or 4209.5 kHz),
and many stations exist in the same service coverage.
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 to 400 nautical miles nominal).