Commercial and maritime signals

The three levels of distress

Almost everybody knows SOS. Very few people know it is the top of a ladder with three rungs, and that the two below it were used far more often. The hierarchy still exists today; only the technology under it changed.

Each was sent three times, then DE and the station's callsign. Three times because once could be missed and twice could be a coincidence.

SOS does not stand for anything

Not "Save Our Souls", not "Save Our Ship". Those were invented afterwards, by people who assumed a three-letter signal must be an abbreviation. It was adopted at the Berlin radiotelegraphic convention of 1906 and came into force in 1908 for one reason: the pattern is unmistakable, symmetrical, and almost impossible to send wrong under pressure.

It is also not three letters. It is sent as one continuous run with no gaps between the S, the O and the S — which is why it is properly written <SOS> with a bar over it, and why it sounds so different from the letters S O S sent normally. Listen to the two next to each other:

Same nine elements, entirely different signal. The first is unmistakable in noise; the second is three ordinary letters that could be part of anything.

CQD, and why Titanic sent both

Before SOS there was CQD, introduced by the Marconi company in 1904 — the general call CQ with a D for distress appended. It was a company convention rather than an international one, and it was never "Come Quick, Danger"; that is another backronym.

When Titanic struck ice in April 1912 the changeover to SOS was only a few years old and the Marconi operators aboard were company men. Her callsign was MGY, and she sent both:

The auto-alarm

A small ship might carry one radio officer. One person cannot keep watch for twenty-four hours, so there was a signal designed not to be read by a human at all: twelve dashes, each four seconds long, one second apart. It tripped a relay in every receiver in range and rang a bell in the radio room and in the operator's cabin.

It is the only signal in morse that carries no characters. Its entire job was to be unlike everything else on the band, so a mechanical device could recognise it and wake somebody up. A distress call followed.

The silent periods

Twice an hour, for three minutes, every station on 500 kHz stopped transmitting and listened. From a quarter past to eighteen minutes past, and from a quarter to until three minutes to. A ship in trouble with a failing generator and a weak signal had two guaranteed windows every hour in which nothing else was on the frequency.

Radio room clocks were made with the segments marked on the face, so an operator could see at a glance whether it was time to shut up. It is a beautifully simple piece of design: no technology, no protocol negotiation, just everybody agreeing to be quiet at the same moment.

The working grammar

Most traffic was not dramatic. It was a coast station and a ship agreeing who talks next, how many messages there are, and whether the last one arrived. That needed a grammar, and this is it — the signals that carried the conversation rather than the content.

What a routine exchange sounded like

A ship with one message for shore, and the coast station taking it. Nothing is wasted; every transmission ends by handing control to the other station.

1. CQ CQ CQ DE GKA GKA GKA QSX 500 KPortishead, calling anyone, listening on 500
2. GKA GKA DE GBTT GBTT QTC 1 KA ship answering — I have one message
3. GBTT DE GKA QSW 512 KMove off the calling frequency to work
4. MSG NR 42 12 0930 GMT <BT>Message 42, twelve words, filed at 0930
5. R R QSL NR 42 TU <AR>Received, acknowledged, thank you
6. GKA DE GBTT QRU TU <SK>Nothing further. End of work

The stations

Coast stations had three-letter callsigns and were places with people in them, working shifts, for decades. An operator at sea knew these the way you know phone numbers you no longer need.

How it ended

The Global Maritime Distress and Safety System came fully into force on 1 February 1999. Satellites, digital selective calling and automatic position reporting did the job better than a person listening on 500 kHz, and they did it without needing anybody to be awake. The maritime morse watch stopped.

Coast stations closed one by one through the 1990s, and several signed off with a last transmission written for the occasion rather than a routine CL. It is worth knowing that the people involved understood exactly what was ending.

What survives is amateur radio, where CW is used more now than it was twenty years ago; the aviation beacons that still identify themselves in morse round the clock; and a handful of preserved stations kept on the air by volunteers who were there the first time.

Corrections welcome. This is history, and a good deal of it is remembered rather than documented. If you worked these circuits and something here is wrong, please say so and it will be fixed.