METEOHYDROGFigure 9.1
The NAVTEX Networknavigation warning etc. information from different sources and sends
that information to the N AVTEX station for broadcast. The Meteo Office
sends weather information to the NAVTEX station for broadcast. Each
NAVTEX station is allocated six time slots every day and all stations in
a particular area are allocated different times to avoid interfering with
each other (times for individual stations are included in ALRS Vol. 5 -
immediately following the NAVTEX maps).
Message type A, Band D (navigation warnings; weather warnings;
and SAR messages - see below) are broadcast on receipt and, where
appropriate, at subsequent allocated times. The 'on receipt' broadcast
will only be made if there are no scheduled transmissions from other
stations in the area. Messages are transmitted using a form of radio telex
error correction known as Forward Error Correction (FEC). This does
not actually correct errors -but it does detect where errors have occurred.
With FEe, each character is transmitted twice (e.g. GGAALLEE
WWAARRNNIINNG) and your receiver, if it receives a duplicated
character, will print the character out. If it does not received a duplicated
character it will print an asterisk e.g. GALE WA NING. This shows that
there was interference when the letter R was received and the receiver
detected the interference, printing an instead of guessing what the letter
should be.
NAVTEX transmissions use ground wave propagation to cover
the service area from the transmitter. At night, on these frequencies, you
will experience sky wave interference from distant stations. If the
interference is at the same time as your programmed station, your
messages will be corrupted (many asterisks). If the sky wave interference
happens outside of your local station broadcast times, you should be
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