ATC Watcher,
Accuracy 95% of the time has nothing to do with GPS. That lost 5% can not be affected by a USAF general !
ANP is a measure of the FMC POSITION accuracy not GPS. The FMC generates its position using a combination of inertial/DME-DME/LOC-DME/VOR-DME/GPS etc. The GPS is just one sensor that it can use but the ANP is a measure of FMC ability 95% of the time not GPS. Latest Boeing FMC's are authorised to 0.3 without GPS even being fitted !
and without GPS a bad radio update can certainly affect position even if they are cemented into the ground. That's why radio inhibit/purge functions are fitted to FMC's.
Anything less than 0.3 RNP (RNP-AR) and yes we currently have to use GPS but that's because ILS's don't bend around corners. Yes most of us have GPS fitted these days but lets not forget its the FMC that ultimately produces the position not the GPS.
I understand the argument between ground based and shall I say "aircraft based approaches" but ground based systems are far more expensive and limited in there geographical options and dependent on variable servicing standards throughout the world. There are almost daily reports of scalloping/interference etc. The FMC/GPS approaches are statistically more reliable and we never have and never will operate to risk free standards. All we do is manage the risk. Every approach in Pans-OPS or TERPS has a calculated probability of crashing. It may be small but its there.
Last edited by 8che; 9th Mar 2015 at 10:56.