le Canard
Quote from Spitoon:
As I understand it, an IRU maintains a position value for the aircraft.
[Unquote]
I think you are putting the cart before the horse. The inertial platform is initialised while the vehicle (aircraft) is stationary. [Admittedly, it is aware immediately of the local vector (i.e., direction and speed) of the earth's rotation, and uses it to calculate true heading in relation to the poles. But I don't think that has any bearing on my following argument.] Like you, my limited knowledge does not stop me from arguing a case, and any INS references are not to hand - nor easily accessible on this dial-up connection, so I stand to be corrected.
From the moment the aircraft starts to move in relation to the earth, each and every acceleration VECTOR is sensed, and used to update TRK and GS. The IRU computer runs a dead-reckoning "plot" of these multiple tracks and distances. This enables it to calculate the curent position at any moment ("PPOS").
TRK and GS inevitably degrades with time. PPOS degrades as a function of all the miniscule - but rising - errors in TRK and GS.