NT-2000 REF Manual
Theory of Operation
8-1
8 . S u m m a r i z e d T h eo r y o f O p e r a ti o n
8 . 1 . W h a t
i s
NAV T E X ?
NAVTEX, a system of broadcast and automatic reception of global maritime safety
information by means of direct display/printing telegraphy, is part of the
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GMDSS
infrastructure, defined by
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IMO Assembly resolution A.706 (17). Broadcasting is
accomplished by a worldwide network of dedicated coastal stations regularly sending
weather information, navigational warnings, search–and–rescue (SAR) information and
other related messages, all in an easy–to–read text format, to mariners free of charges.
8 . 2 . O p e r a ti n g
F r eq u en ci e s
At present three frequencies are officially allocated by the
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ITU for NAVTEX
broadcasting service: 518 kHz, 490 kHz and 4209.5 kHz. All
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SOLAS convention
vessels are required to carry a receiver dedicated to receiving NAVTEX transmissions
automatically on all the above frequencies.
Transmissions on 518 kHz are in English and of general interest to vessels navigating
international waters, while the 490 kHz service is primarily targeted at those working
most of the time in areas close to transmitting stations, providing more localized
information in national languages. Some stations also broadcast information to local
users on 4209.5 kHz. Stations on each frequency transmit NAVTEX messages usually
for a period of 10 minutes on a time–sharing basis to avoid cross–station interference.
8 . 3 . S e r vi c e
Ar e a
The coverage area of a 518 kHz station is normally 200 to 400 nautical miles in daytime
hours, depending on the transmitting power level, and a signal travels along the surface.
At night some of the signal often travels more than double the daytime coverage by
being reflected a number of times between the surface and the ionosphere, a band of
ionized atmospheric layers 50 to 400 km above the surface. The 490 kHz service area is
less than the 518 kHz coverage due to lower power levels, but the nighttime coverage
often extends greatly because of ionospheric reflections. The 4209.5 kHz broadcast,
though catering mainly for inshore users and covering much less areas in the daytime,
can be received from more than 1000 miles away during hours of darkness.
8 . 4 . E r r o r
Co r r e c t i o n
Signal propagation conditions constantly vary with time of the day and also with the
season. The signal arriving at the receiver can be corrupted or distorted at any time by
static discharges, natural noise, abnormal solar activity, unstable ionospheric conditions,
interference between the signal coming directly from the transmitter and the one
reflected from the ionosphere, etc. It is therefore likely that a vital NAVTEX message
can be missed due to signal corruption or distortion at the receiving end.
In order to ensure that each message is properly received within its area of coverage, a
station sends out the same message twice, some hours apart, each with the station’s
identification code, message type indication and a serial number. In addition, the
system employs a mode of transmission called forward error correction (FEC),
transmitting each alphanumeric character in the message twice at very short intervals,
thus enabling the receiver to have a chance to receive either the first character or the
next one even if it fails to receive either one correctly. In this way, a NAVTEX message
transmission is designed to be received without fail and with minimum number of
erroneous characters.
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