BOAC:
Sorry to say but if you can't make up your opinion on how it works and answer most of your questions on ISIS, then I am at loss to help out.
Following the latest ISIS post by h3dxb (#1951), and taking into account other well informed posts (even excluding mine if you wish), you can have a pretty good idea on how the ISIS functions by now.
Key fact is it contains a gyro, indicating attitude. I never said it's an inertial plateform !! Aguadalte implied it got its attitude info from ADIRUs, and that's plainly false. He standed corrected earlier. No problem.
Fact is, tech info on ISIS is not disseminated as profusionnally as one would wish.
Personnally, I learned more now on ISIS than on any other ground course I followed recurrent or not !