B737 FMC Question 2
This is from the 737 NG manual. Hope it helps
In flight, the FMC position is continually updated from the GPS, navigation
radios, and IRS. Updating priority is based on the availability of valid data from
the supporting systems.
FMC position updates from navigation sensor positions are used in the following priority order:
• GPS
• two or more DME stations
• one VOR with a collocated DME
• one localizer and collocated DME
• one
Radio position is determined by the intersection of two DME arcs. If the DME radios fail, or if suitable DME stations are not available, FMC navigation is based on IRS position information only.