chris
An INS is telling you where it "thinks" you are because it knows where you started off ( that's if you aligned it properly) and has integrated every single acceleration you've made since brake release.
Any error in the above processes, (e.g. in measuring the accelerations) will result in a mismatch between where the INS "thinks" you are and where you really are in the world.
It was not unknown, in the days of flying around with a single 1960s build INS to see mismatches of tens of miles between the calculated position and your real position...the modern kit is much better, a good ADIRU or IRS position might only be adrift by a couple of miles after a long flight. Even so if you let down on raw INS alone (highly unusual these days of GPS but....) you are letting down on a "theoretical" position. FWIW some companies (most?all?) forbid letting down on the basis of a raw INS position, so I guess you can draw your own conclusion..
ALL IMHO as an end user, not as a INS techie.
Last edited by wiggy; 28th Nov 2011 at 17:16.