I thought SOS means "save our souls", but apparently it doesn't.
But ships send SOS when in danger and they used Morse code. Why do airplanes use MAYDAY?
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The difference here isn't between ships and aircraft: it's between Morse code and voice.
The SOS signal is only for Morse code. It's short, easy to send, and easy to recognise. But it's not as convenient to say. It doesn't actually mean "save our souls". The letters were chosen just to form the simple Morse pattern, and "save our souls" is a backformation: it was made up by sailors later, partly as a joke, partly as a mnemonic.
"Mayday" is an English-looking spelling of French m'aidez, "help me". Spoken out loud, it's short, easy to send, and easy to recognise. It doesn't have any sounds that some nationalities can't say (such as r, th, or v). It's a good signal to use as voice, but would be much worse as Morse code, because it's too long.
Back in the days when aircraft were equipped with Morse code transmitters, they would have sent SOS as a distress call, just like a ship. And a ship with voice radio would send "Mayday" instead of SOS. Now that Morse code has fallen out of use, SOS is also disused. You only hear it in movie plots where the plucky hero doesn't have a working radio but can somehow improvise a way of signalling Morse code (usually by holding two wires together on a broken radio).
Ships use Mayday.
This is the transcript of M/S Estonia disaster from 1994, a major passenger ferry sunk.
01:23.11 Estonia> Europa, Estonia. 01:23.15 Estonia> Silja Europa, Estonia. 01:23.19 Europa> Estonia, this is Silja Europa replying on channel 16. 01:23.27 Estonia> Silja Europa * 01:23.34 Europa> Estonia, this is Silja Europa on channel 16. 01:23.55 Estonia> Silja Europa, Viking, Estonia. 01:23.59 Mariella> Estonia Estonia. 01:24.02 Estonia> MAYDAY MAYDAY. 01:24.07 Estonia> Silja Europa, Estonia. 01:24.10 Europa> Estonia, Silja Europa. Are you replying— calling Mayday?
You can also listen the same in Youtube:
SOS is for morse code, Mayday for voice.
Merely being a ham operator for 58 years, this is my understanding. "SOS" means nothing. It's easily sent and easily copied (understood), and it follows "CQD" which was also used on the Titanic. CQD is most aptly "COME QUICKLY DISTRESS" (the D has various meanings / uses).
The senior radioman on the Titanic told the jr. to go ahead and try the new call. As an aside; a young Welsh lad, about 15 being a young ham, heard the Titanic's distress call, sic (ITS A CQD DE MGY CQD DE MGY SOS AM SINKING HAVE STRUCK A BERG). ("DE" means from,- MGY is the TITANIC'S Marconi owned wireless station call letters). He copied the text and went to the local police. When he told them, they laughed at him saying sic. ("the Titanic is unsinkable, go home").
They were surprised the next morning to hear / read the news. MAYDAY is from a French word "French "m'aidez", "mayDAY," [one word], and means in actuality - "help me". It is a short form of venez m'aider - come and help me'.
This first URL is a simulated spark gap transmission.
The second is an exchange between the Titanic and the Carpathia.