The UAS procedure in the A380 states: FPV Avail
Normally the FPV uses ADIRS info, with all 3 ADR’s off it’s a GPS / IR hybrid, however when there is a static problem it displays incorrect information in the SIM, and I assume it is the same on the actual aircraft.
Maybe the catch is the word “Avail” as opposed to “Reliable” for instance. Equally the FCOM mentions its display with the ADR’s is available, not accurate.