Presumably using the backup network requires pilots to be trained and preferably current on the aids?
Has a simulation been done to see whether ATC and the backup network has the capacity to cope with a widespread outage affecting all GPS navigation in e.g. the Sydney area?
I guess jets would be OK using e.g. INS navigation and vectors to an ILS, but what about GA aircraft? Do you need to mix GA and RPT jets on the same aids in the backup network?