Silvertate, I don't want to be harsh but it seems that you and I have been on different GPS-Approach planets for over a decade. I have done dozens and dozens of these over a decade or more using three different systems and the navigation accuracy is "on the centreline", and they were not RNP-AR systems.
I am just saying that for all the GNS approaches I have done, 0.3 nm was quite sufficient because of the offset and high missed approach point.
Most of ours (RNP APCH LNAV) have MDAs of around 450-500ft. Very few are offset. 0.3nm is
not "quite sufficient" to be able to get in if you pop out at the MDA.
I note on page 2 of this thread, that the GNS approach at VNKT is actually straight in for the last 3 nm, with a decision of just over 300 ft. That is a max 15º lateral turn to achieve the threshold, if you are at the margins of the 0.3 nm error for some reason. Not too bad.
Are you serious?? 0.3nm offset from the centreline "at just over 300ft". I defy anybody to safely get a medium/big jet onto the runway from that far off the centreline. Have you actually tried it? I am
sometimes 0.1nm off and it is very hard work to get onto the runway. BTW, there is no offset on the approach posted on page 2. The runway QFU is 022° and the approach track is also 022°.
But do bear in mind that the rnp is only a 95% probablity, so five times in every hundred approaches you could be off by more than that.
That's nice to know; we and I assume hundreds of other operators around the world are using dual-GPS sensors for sole-means approaches. No other navaids, and no alternates. If a VOR, NDB or ILS approach had such bad "reliability" it would never be certified.