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Section 3 14 navtex
Choose NAVTEX from the DIGITAL row. It is the maritime safety broadcast every coast station in the world sends: navigational and meteorological warnings, search-and-rescue bulletins, ice reports, pilot notices.
Receive only, and deliberately. The service belongs to coast stations and sits next to distress traffic; an amateur transmitting on it would be putting out safety information nobody may act on. There is no transmit half of this panel and no callsign to set.
The channels. The band buttons are the three services: 518 kHz (international, always English), 490 kHz (national language, so whatever your nearest coast station broadcasts in) and 4209.5 kHz (tropical). What they tune is 1.7 kHz below the channel, because the quoted frequency is the assigned one — the centre of the two tones — and a receiver in upper sideband has to sit below it to put them at 1615 and 1785 Hz. That arithmetic is done for you, and the readout still says where the signal is.
A dial that is a little off still decodes. The receiver tracks the tones itself, so a readout a few hundred hertz out — a rig that will not set exactly, or one of the tuning figures that rounds differently in different publications — is pulled in over the first second or so instead of returning nothing. It is not a reason to be careless: a mistuned signal still carries more errors than a clean one, and past about ±350 Hz the pull-in gives up.
What you see. Two panes:
- MESSAGES — one entry per message, newest first, headed by the four characters the service identifies it with: the transmitter (a letter the NAVAREA co-ordinator allocated), the subject, and a serial number. Navigational warnings, meteorological warnings and search-and-rescue bulletins are shown in red — a ship's receiver is not allowed to filter those three out, and sdroxide does not offer to either. A message that was cut short, or that lost characters, says so under its heading.
- READING — the message selected, or the one arriving. Monospaced and unwrapped, because a NAVTEX message is laid out in columns by the station that sent it and reflowing it destroys the only formatting it has. With nothing selected and nothing arriving it shows everything decoded, message or not, which is the honest view when a header has been missed.
The time a message states is shown beside its heading, where the body names
one — 1200 UTC, 0900Z, 06:30 UTC. It is read out of the text, not a field
of the format: a gale warning states when it was issued, a station's time
broadcast is an ordinary message that happens to carry a clock reading. Only a
four-digit time marked as one is taken, so a position (5103N 00109E) or a
serial is not mistaken for a time of day. It is shown for the reader and
nothing else — sdroxide never sets the system clock from it.
A station repeats itself, and that is the point. Each transmitter has a ten
minute slot every four hours, and it sends the same messages again on the next
one. sdroxide keeps one entry per message — a repeat replaces the copy already
held when it is better, meaning it reached its NNNN where the first did not, or
lost fewer characters. So a warning that faded halfway through fills itself in
over the next few hours instead of appearing four times half-copied.
The two numbers beside SYNC are what the mode's error correction actually
did. NAVTEX has no checksum: every character is sent twice, five character times
apart, in an alphabet where exactly four of seven bits are marks — so a single
bit error makes a code that cannot exist, and the repeat is what replaces it.
Repaired counts the characters taken from the repeat and lost the ones where
both copies were bad; a lost character stands in the text as *, marked rather
than hidden, because half a gale warning has to look like half a gale warning.
REV swaps the mark and space tones, for a signal received on the other sideband. Off is upper sideband on the channel, which is what every published tuning instruction for the service says.
From the User Manual. Home.