Sorry to sound pedantic, Cornish Jack, but I don't think that it's the IRS which holds both the raw and the refined positions. That's what used to happen on the older system of INS, where you actually updated the INS present position.
With the Honeywell Boeing system, it is the FMC which holds the refined position. All the IRS holds is the raw position. Thus each IRS has its own raw position, but the FMS holds 4 positions at the same time, left IRS, right IRS, DME/DME, and FMC position (the final computation used). If you select the "Present Position" page on the CDU you can actually see and compare all 4 positions.
It may be all different on the L1011, of course.
Last edited by oxford blue; 23rd May 2002 at 21:31.