Originally Posted by IO
BRNAV is RNP5 I think. PRNAV is RNP1, I think. GPS approaches are RNP0.3 or something like that, which is why PRNAV (which requires weird equipment and crew certification, despite being less accurate than GPS approaches for which even EASA has a straight path today) is now completely irrelevant. But does that stop the PRNAV machinery in its tracks? No way. It will be 10-20 years before they realise that the "RNP" boat has left the port (sunk, actually) a few years ago.
The RNP is about how close the aircraft is to where it is supposed to be, not about how accurately the nav kit measures the actual location.
So RNP 0.3 means you are within 0.3 nm 95% of the time and, I think, 'never' (ie. 99.999% sure) be more than 0.6 nm from your programmed track. This needs to be true for all of the supported profiles (turns, DME arcs, etc.) this is why there was the debate on PRNAV if autoslew HSI was requried
(How many people can make a 45 degree course change in IMC never moving more than 0.5 mile from the specified flight path using an NDB or VOR?)
The US version of PRNAV is called RNP xxx. It is on trial in Alaska with IMC approaches consisting of multiple fly past waypoints weaving up mountain gorges all below the mountain ridges.
The use of GPS ranges from the use RNP SAAAR approaches, through WAAS approaches, into normal GPS approaches, RNAV, GPS monitoring, VFR situational awareness with a quality aviation moving map (with up to date data) down through a hacked car GPS with a 5 year old map on down to a camping GPS with a lat lon display. So making any comment about Good/Bad on 'GPS' is not very illuminating with out contexting the type of equipment.