CFR /  Title 46  /  Part 161  /  Sec. 161.013-7 Signal requirements.

(a) An electric light must have a flash characteristic of the International Morse Code for S-O-S and, under design conditions,

(1) Each short flash must have a duration of \1/3\ second;

(2) Each long flash must have a duration of 1 second;

(3) The dark period between each short flash must have a duration of \1/3\ second;

(4) The dark period between each long flash must have a duration of \1/3\ second;

(5) The dark period between each letter must have a duration of 2 seconds;

(6) The dark period between each S-O-S signal must have a duration of 3 seconds.

(b) The flash characteristics described in paragraph (a) must be produced automatically when the signal is activated.