8che
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.
Sorry, but I can only assume you have never seen the system in operation.
The fact of the matter is that the FMC position will closely shadow the GPS fixes, to the detriments of all other aids. The IRS and VOR-DME fixes can be miles away, and the FMC will still follow the GPS. I think the system bias must be 95% to the GPS fixes.
And therein lies another problem, because if the GNS detects a RAIM error it will stop using the GPS and 'revert to the IRS/VOR fixes', as the manual says. Yet the IRS/VOR positions could be miles away. The manual rather unhelpfully does not say how fast this transition will take, but if it is within a minute or so, you could end up with a dramatic map-shift just when you do not want it.
Regards the 95% probability, this is a sum of ALL the errors that can effect the system, especially when on the approach. It is not a measure of GPS accuracy
per-se. It includes errors from the VOR-DME fix, GPS signal ghosting or reception errors, satellites not in optimal positions, insufficient satellites, atmospheric refraction, hardware noise, IRS heading errors and much else besides. Grind all that together, and you have only a 95% probability of your position being within 0.3nm. And don't complain to me about this - that is what the system says in the manual. If you don't like it, complain to the OEM manufacturer.
And finally, regards Capt Blogs. Sorry, but if you are doing a 0.3nm rnp GNS approach, that is all you are going to get. You cannot assume that the runway will always be in front of you. Why? Because that is what the system says on the tin. The 0.3 nm rnp is a pseudo non-precision approach, and not a pseudo ILS. Especially if you have no WAAS system (which Europe does not have) and don't know if someone has a $50 jammer on the airfield boundary. And if you don't like the 15 degree turn to achieve the runway, speak to the CAA who designed that approach, or speak to your flight manager and get an extra 'company 200ft' added to the minima. But whatever you do, don't always expect the GNS approach to be perfect, if you do not have WAAS augmentation and checking.