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.
What Silvertate said is quite correct . It is 95% of the time for RNAV and no, VOR DME or ILS Approaches performances are not the same because they are fixed based and their exact position is known 100% of the time..GPS stand alone APP is no ILS substitute . The 5% uncertainty are still there and will remain there as long as GPS ( as opposed to EGNOS , etc..) is used.
I am very surprised this fact is not known to Pilots operating the sytem.
Just a reminder for the younger generation :
When RNAV/RNP was designed RNP1 was the lowest. RNP 5 the norm aimed at. . "Nav capabilities based on sensors able to calculate your position within 5 ( or 1) NM radius 95% of the time" .
This was designed initially to reduce lateral separation on non-radar enviroment, and allow the design of closely parallel routes in radar airspace.
RNP 0.3 was intially never designed for precision APP but for helicopters in Terminal aereas. How it progressed to what it is now is due to the pressure ( lobby) of operators ( Alaska Air was the first if I remember correctly to test it in remote places in Alaska where no ground aids were available)
Nobody at the time expected the thing to be used in KTM by a non-based airline in bad weather /visibility. ( no pun meant against TK, just to illustrate a foreign crew using a system in a difficult airport a few times a year and again , not suggesting this is the cause of this accident.)
RNP 0.3 may be used in KTM in 2015 with 900m RVR successfully by many, but it is still a GPS and 95%, plus a having a USAF general somewere having access to a button that can further degrade accuracy (SA). But this is probably included in the 5%.
