Aye you are meant to use the raw nav data to back up all the other whizz bang nav kit.
Its in our part A that we have to. Usually its only displaying the vor's/DME that we have selected on the MAP mode and they should overlay where the GPS thinks the point is or roughly where the VOR is if its off the track. If they don't overlay we have to trust the VOR not the GPS. The scary bit is when the GPS is saying your tracking what the NDB plate says to the beacon and your RMI is pointing that you are off by 15 degrees. Legally you have to follow the RMI.
And i think the other poster was on about the 2 signals which are broadcast from the GPS sats. The main signal which the US goverment can fanny around with and alter the timing of which increases the error. And the second encrypted signal which then tells the military GPS what the fanny around factor is with the first signal to cancel it out. To my knowledge they stopped the fanny around factor a few years ago because the mil sets where to expensive and everybody had bought civi models for personal use in the field. So if they fanny it around the troops don't know where they are either.
Personally I don't have as much faith in GPS as IO540. Its a good tool in the tool box. But it has its issues.
MJ