Old Akro, sorry mate, but you're rapidly losing the plot.
Maybe, but not completely, I'm just trying to learn & getting some bits wrong.
I screwed the MDA's - you are correct.
I pretty sure I'm correct about DME feeding the FMS - how else does it know the track miles to run? The INS needs updating. It can come from GNSS or DME. I'm guessing the FMS uses DME as default.
By definition, a precision approach requires an additional altitude reference. ILS does this with the radio based glideslope. GPSS does not have sufficient altitude accuracy, but it does when augmented with ground based reference (WAAS). The other alternative is a sensitive barometric input which I presume is something sophisticated that exists in FMS world only.
Without an additional altitude reference, I think we strictly speaking have RNP - LNAV approaches. This allows curved approaches and the fuel saving of shorter " Green" track approaches that AsA boasts about as well as reduced lateral obstacle clearances, but does not have the vertical accuracy of a precision ILS approach unless it has altitude augmentation.
My reading (mainly of FAA material) is that not all RNP approaches are alike. The CASA CAAP identifies 4 flavours of RNP approach. Only 2 of the four has ILS equivalent MDA's and accuracy - one using WAAS and one which requires localiser augmentation. See AC 91U-II-C-5. Its categorisation of sensitive barometric input differs from the design notes that I read on the Queenstown RNP design - it might just that the NZ stuff is old.
If you look at the YMML RWY 27 ILS & RWY 27 RNAV -M (RNP), the RNP approach has a DA(H) - being careful to get it right - for Cat D is 610ft & 931ft respectively. This reflects the greater altitude uncertainty of the (non WAAS) RNP NPA approach.
I am presuming (although I may be wrong) liability issues will mean that aircraft systems will not display vertical guidance ( a la ILS) for approaches with only C129a GNSS input. I contend it is the absence of this vertical guidance that contributed (or at least exacerbated) to the Thai incident.
But, this is off-topic, although its been interesting.
My fundamental thesis is that (whether through ability or not) the Thai crew joined a list of others who messed up an NPA approach to RWY 34. Rather than saying they are just not good enough and we are better, I'm saying there have been enough Australian, US & Thai crews to make similar mistakes that we should get on board with the rest of the world and put in more ILS approaches. If we can't do it for less than the cost of the carpark modifications at Melbourne, then we should be asking why we can't install them at International benchmark costs. If anyone mentions us being a small market, I'll scream. This level of equipment is an international market. A bunch of airfares & some airfreight from the US might add $100k, not $millions. The reference I found from the FAA was no less than Randy Babbitt about a year ago.