STBYRUD - you said
On most of our planes the DME-DME updating is off by default
Why is that?? I don't have a lot of experience of state-of-the-art navigation, but on the only type I have flown with FMS updated by GPS, DME/DME, and laser inertial, the manual definitely said that DME/DME is the most accurate - makes sense that it would be more accurate than GPS as it is basically the same method of determining position, but from fixed position beacons rather than ones flying round in space..........