Navy Ranks

(Enlisted Ranks) 

Crewman (seaman, airman, construstionman, and medic)

Petty Officer 3rd class

Petty Officer 2nd class

Petty Officer 1st class

Cheif Petty Offficer

Senior Chief Petty Officer

Master Chief Petty Officer

(Officer Ranks)

Ensign

Lt. Juniour Grade

Leutenant

Lt. Commander

Commander

Vice Admiral (lower)

Vice Admiral (Upper)

Admiral

Fleet Admiral (only in a time of war)  ---- in this case there are Many Fleet Admirals in the Halo-verse

Most of the UNSC mainly consists of Navy/Marine Corps Personnel. That goes for Enlisted and Officers as well.  Everyone how plays Halo (including my master chief)  seem to nit pick at the Ranks and Chevrons (Ranking Simbols),  Traditional  Dress uniforms, ect. a lot.  If you look closely, Srg. Johnson is wearing a P.O. 3rd class chevron on his hat.  I constantly have to remind my fellow Marine friends and shipmates that it's just a game, and that it's also future posibility.  Most of the Navy/Marine Corps today work hand in hand, very much like halo.  Hence why it's so popular in the military.