Later Developments
Additional warning and distress signals followed the introduction of SOS. On January 20, 1914, the London International Convention on Safety of Life at Sea adopted the Morse code signal TTT ( — — —), three letter T's (—) spaced correctly as three letters so as not to be confused with the letter O (— — —), as the "Safety Signal," used for messages to ships "involving safety of navigation and being of an urgent character."
With the development of audio radio transmitters, there was a need for a spoken distress phrase, and "Mayday" was adopted by the 1927 International Radio Convention as the equivalent of SOS. For TTT the equivalent audio signal is "Securité" for navigational safety. It is interesting to note that "Mayday" is actually the French "M'aidez (help me)" and that there is a third, lesser after "Securité" distress call: "Panne (French for 'breakdown')" spelt "Pan" in English. French was the international language at the time that these were formalised.
During the Second World War, additional codes were employed to include immediate details about attacks by enemy vessels, especially in the Battle of the Atlantic. The signal SSS signalled attacked by submarines, whilst RRR warned of an attack by a surface raider, QQQ warned of an unknown raider (usually an auxiliary cruiser), and AAA indicated an attack by aircraft. They were usually sent in conjunction with the SOS distress code. All of these codes later switched from three repeats of the letter to four repeats ("RRRR", etc.).
None of these signals were used on their own. Sending SOS as well as other warning signals (TTT, XXX etc.) used similar procedures for effectiveness. These were always followed correctly. Here is an example of an SOS signal; the portions in brackets are an explanation only.
SOS SOS SOS de (this is) GBTT GBTT GBTT (call sign of the QE2 repeated 3 times) Queen Elizabeth 2 (name of ship) psn (position) 49.06.30 North, 04.30.20 West. Ship on fire, crew abandoning ship (nature of distress) AR (end of transmission) K (invitation to reply).
Ships and coastal stations would normally have required quiet times twice an hour to listen for priority signals. However, many merchant vessels carried only one or two radio operators in which case the SOS may not be heard by operators off duty. Eventually equipment was invented to summon off-duty operators by ringing an alarm in the operators berth. This was triggered by the operator of the ship in distress transmitting twelve long dashes of four seconds duration each. These were sent prior to the SOS hopefully ringing the automatic alarm in ships so equipped. If possible a short delay was given before transmission of the SOS proper. This was to give those off watch operators time to get to their radio office.
Read more about this topic: SOS
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