You make a good point on the GPS systems. By having the two GPS radios I would assume it makes it very diffiuclt then the get the ANP to rise significatly to where it challenges the RNP. In the PMDG, and of course this is more of a question to those on the beta, is it possible to fly a NGX that is not equiped with GPS and thus rely only on the IRS and the pilot entered initial position?

GPS comes standard with the 737NG, however, there is a switch in the FMC to exclude it from the calculated position in one of the POS REF pages, labelled "GPS INHIBIT". Now, of course, I couldn't know for sure if PMDG have done it, but I would bet a copy of the NGX that when you inhibit GPS updating, the ANP will gradually rise. Cheers DZ

I do believe the 747x from PMDG had a GPS inhibit. I will certainly give this a look and see how it goes. Would be interesting if this is indeed the case, not neccessarily to inhibit the GPS, but should the GPS radios fail to give a position, to see how the rest of the system reacts.