RNP and full-scale deflection
We have a little discussion here at my school regarding RNP. As I understand it, RNP is based on the accuracy of the equipment in the airplane, the navaids on the ground (or GPS) and flight crew errors. So, if you are flying a GPS approach with RNP 0.3 and you have a full-scale deflection or less, you should not be further than 0,3 NM from the centerline. Is this correct?
Another theory I've heard is that the system can "lie" to you with 0.3 NM. So if you are on the same approach with a full-scale deflection, you could be as far 0.6 NM from the centerline. This doesn't seem to make sense, but some of the stage checkers here thinks that is correct.