This subject has been discussed time and again by Halo fans who are determined to discover the structure of the UNSC's rank system, its insigne, and how it corresponds to real-life systems. By consensus, based on information from author Eric Nylund, UNSC ranks are based on the system used by the modern-day United States Armed Forces.
However, there are several problems that must be resolved. First of all, the UNSC Marine Corps enlisted rank insigne.
The insignia of Marine enlisted ranks has never been revealed. It is often assumed that it is exactly the same as U.S Marine insigne. We have but one passage in Halo: Contact Harvest to give us some guidance.
From Halo: Contact Harvest, page 47:
"...the Lieutenant still hadn't quite accepted the drunk outside his office was the Marine Corps Staff Sergeant indicated by the four gold chevrons on his filth dress-coat's sleeve."
If the UNSC was to use the American system, it should properly be three gold chevrons and one rocker to identify the rank of Staff Sergeant, so we have little to go on.
However, consider this screenshot of the Rookie: http://screenshots.teamxbox.com/screen/80438/Halo-3-Recon/ Halo 3: Recon Screenshot] - Upper right chest plate has clear sign of one chevron and two crossed rifles
Thanks to CommanderTony for providing the link.
So, the UNSC Marine Corps does apparently use U.S Marine enlisted insigne. We should the appropriate images to the articles with all haste (pending approval from the administrators, of course). The only problem comes with the Sergeant Major of the Marine Corps, as we have no idea what the emblem of the UNSCMC is.
Next, Miranda Keyes' insignia. We are told that she is a Commander in Halo 2, which fits the U.S system in that she wears three gold stripes and a silver oak leaf on her collar. However, Halo 3 shows Keyes with the three gold stripes of a Commander, but the gold oak leaf of a Lieutenant Commander. This lead to a lot of confusion on the talk pages.
The Art of Halo 3 tells us that Keyes is actually a Lieutenant Commander, presumably having been demoted some time after the events of Halo 2. Or perhaps she was always a Lieutenant Commander, with UNSC Lieutenant Commander stripes being three gold braids. But then that throws Halo 2's silver oak leaf into question. As long as we stick to the belief that the UNSC uses the American system, it's difficult to reconcile. Most likely the animators simply screwed up.
We may be able to find a solution to this by looking at the insignia of UNSC flag officers. From Halo: The Fall of Reach, page 95, on Admiral Stanforth:
"The insignia on the forearm of his jacket, however, was unmistakable: the single gold star of a Rear Admiral."
Read that again. Gold stars represent UNSC Admirals. Not silver as in the U.S Navy. In the American military, silver is considered higher than gold, but if it's good enough for UNSC Admirals, why not Commanders? Perhaps the insignia of Commander is in fact three stripes and a gold star. Interestingly, our articles on Admirals seem to have ignored this insight into the UNSC rank structure, instead using the silver stars of the U.S Military.
Your opinions on all the subjects, please.