Sierra, in the NATO Phonetic Alphabet, stands for the letter "S." Spartan-117 is the Master Chief's callsign. Spartan obviously begins with "S." Thus, "Sierra-117" equals Spartan-117.
Actually, "Sierra" doesn't stand for Spartan. Otherwise, B312 would be S312. Chief is S117.
It does stand for spartan, B312 becomes SB312. Master Cheif Petty Officer John -117 is S117.