IRS update in flight.

[lonerider]

I understand that the IRS/INS cannot be updated in flight- and can only be aligned on the ground when the aircraft is stationary.

My question is how then does the IRS/INS keep the radial error down- if it cant update its position in flight?? I know that the FMC provides cross cuts and the most reliable source of updating comes from DME-DME and then DME-VOR. What happens to this information? Does it by-pass the IRS completely, and is only mixed in the FMC along with GPS information etc?

Thanks

lonerider