American steamer Arapahoe in 1909
It is believed the first ship to have sent out an SOS signal was the American steamer Arapahoe in 1909. When the Titanic was sinking in 1912, its operator first sent out CQD and then SOS, alternating. CQD persisted, particularly among British operators, for many years. But SOS was a landmark in global communication.
What is SOS Signal
SOS is the International Morse code distress signal which means "Save Our Souls" or "Save Our Ship". This distress signal was first adopted by the German government radio regulations effective April 1, 1905, and became the worldwide standard under the second International Radiotelegraphic Convention, which was signed on November 3, 1906, and became effective on July 1, 1908. SOS remained the maritime radio distress signal until 1999, when it was replaced by the Global Maritime Distress and Safety System.
A SOS signal consist of three dots which makes S followed by three dash which represents O followed by three dots again which again represents S i.e. "…---…".