NAVTEX on 518 kHz: navigational warnings for ships in SITOR-B

What you hear

There is no voice on 518 kHz. What comes out of the headphones is a warbling two-tone chatter, slightly uneven, that runs for a few minutes and stops; on the waterfall it is a pair of thin lines 170 Hz apart. That is SITOR-B, a teletype mode at 100 baud that sends every character twice with a delay between the copies, so that a ship's receiver can fill in what a burst of noise took out. Decoded, the stream is plain English in capital letters: a header of four characters, then a warning about a buoy that is unlit or a firing exercise in progress, a gale warning, a forecast for the sea areas near the station, a search and rescue notice, and NNNN to close.

The four-character header tells you who is speaking and what about. The first letter is the station, the second the kind of message: A for navigational warnings, B for weather warnings, D for search and rescue, E for forecasts. The last two characters are a serial number, so that a ship's NAVTEX printer can skip a message it has already printed.

Who transmits and why

NAVTEX is the maritime safety information service that every ship under the international safety conventions must be able to receive: a small dedicated receiver on the bridge listens to 518 kHz all the time and prints what matters for the ship's area. The broadcasts come from coast stations run by national maritime authorities, coastguards or navies, each assigned a letter and a ten-minute slot that repeats every four hours, so that stations within range of each other never overlap. Transmitter power is modest and the range by design is a few hundred nautical miles, enough to cover the coastal shipping lanes.

518 kHz is the international frequency, always in English. The same stations often run a second broadcast on 490 kHz in the national language, and 4209.5 kHz carries NAVTEX in the tropics.

When and where to listen

Which station you hear depends on where the receiver is and on the time. By day, within a few hundred kilometres, you get the local station in its slot and little else; at night the signal travels much further by skywave, and a receiver on the North Sea coast can log stations from Norway to Spain in one four-hour cycle. In Europe the usual catch is the British stations, the Netherlands Coastguard, the German station at Pinneberg and the Scandinavian ones, each in its own ten minutes. The slots are fixed to the UTC clock, so once you have identified a station you know exactly when it will be back.

The catalogue uses USB with a narrowed passband, which is how most listeners receive teletype: the two tones come out as audio and a decoder reads them.

In ShackDeck

Station catalogue → Aviation → NAVTEX (518 kHz). The program picks a public receiver that covers 518 kHz, tunes it in USB and narrows the passband; no receiver, antenna or account of your own is needed. ShackDeck does not decode SITOR-B, so what it gives you is the sound and the waterfall, which is enough to watch the stations come and go in their slots; to read the text you need a NAVTEX decoder fed from the audio. Because the dial sits exactly on 518 kHz, which leaves the tones below the audio passband, step the frequency down to about 516.5 kHz to hear them rather than only see them.