TO: Flight Path and BOAC.
It takes a team to forumate operations procedures such as what we (in this case "we" being a team of industry experts with different backgrounds, and the FAA) who cobbled together the criteria and general obstacle clearance and performance parameters for RNP AR. Two of our experts were FMS engineers. We left the details about how IRUs perform the position calculation and where in the "black box" stream the GPS sensors updated the final position calculation; the FMS position.
Others of us had flight operations backgrounds and a few of us had TERPs experience, either formal (FAA) or in my case informal.
Throughout all of this all I needed to know is that the IRU postions were massaged by the FMS (sort of like triple mix in the L1011s I flew) and GPS, when available, indeed performed a postion update at some point. Whether it actually tweaked the IRUs or the FMS positon was immaterial to us operations types unless our FMS engineers told us it matter. They spent a career being trained, then working on that stuff.
Personally, I often see operations folks such as pilots trying to be avionics engineers when they aren't. Then again, they fly airplanes a whole lot better than FMS engineers and they, the pilots, have invaluable input as to how the airplane will behave in the performance based nav world.
I don't know which of you is right, and it is unimportant to me for my purposes.
