Greetings,
FMS selects the update source from the most precise to the least, i.e., and in order please, ILS, GPS, DME DME, DME VOR (VOR VOR is a NO NO situation) then IRS.
On the Smiths FMS world they use Kalman Filtering (So does Litton on the Flag Ship AIME) this enables an error modeling from which the position will be corrected
IRS Alignement on the ground it is highly recommended (even for a fast align) NOT TO MOVE THE AIRPLANE
An error in position input during initial alignment, a part from obvious bias, will give the wrong "G" initial value and induce wrong vertical acceleration (delta between the sensed real one and the erroneous one)
throughout the flight dixit Korean Air B747
