was there not already a ship in halo three that started with sierra? so it might be a ship
but also if you remember the level, sierra 117, it has to do with the MC and his being 117 so it could be a spartan.
but when he says WE have spartans on the ground, i doubt he is talking AS a spartan
EDIT: oh snap! i just almost contradicted myself. so as i said in another post on some topic, the military uses words instead of letters. so sierra, starting with S, could mean spartan. beacause it also starts with.... S haha so there is just one more reason it could be a spartan.
[Edited on 07.03.2009 2:41 PM PDT]