Doesn't GPS actually measure relative, rather than absolute, altitude - that's to say relative to a reference ellipsoid that approximates the earth's surface ?

I can't speak for specific applications, but I have a GPS tracker system that I use in my high power rocketry. When I power it up, it reads that altitude of the launch site relative to MSL. There is a button that I then push that resets that altitude to zero so that subsequent readings are relative to the altitude of the launch site. The GPS can then be used to determine things like max altitude of the flight (and speeds - although during the ascent the speeds change to rapidly that the update rate of the GPS makes it less than completely useful).
As MechEngr notes, if that's how the system normally works, it would require a pretty detailed terrain map to reliably determine altitude to the local ground level.

In which case all that needs to be done is align the GPS '0' to the same reference zero that has been in use since forever to calculate elevations and altitudes for the standard old-school aviation data-set? Within the original scope of the thread - not it's the altitude which was wrong already, no need to evaluate height.

My personal favourite is to mandate the ANSPs to publish radio-height at the 1000' AAL checkpoint for every IAP. Darn easy, GPWS, EGPWS and ACAS also started mandatory for big aeroplanes only. I mean, if the issue is flying into terrain and RAs exist, why insist on devising another tool or gadget instead of learning how to utilize the full potential of what we already have?

For the aeroplane discussed, an electronic comparison cross-check between FCU BARO REF and MCDU APPR PG seems a no-brainer.

I think there are several datums that are available for reporting, including the GPS default ellipsoid WGS-84 (apparently), but a GPS nav unit is initially calculating a 3D position based on the orbits of the GPS satellite constellation. The report should be consistent for a consistent location, such as a if one is on a radio tower or mountain peak. The pressure altitude will vary and the difference with some other fixed location will vary for those fixed locations.

The more I look at it the more I don't want to look at it. https://geodesy.noaa.gov/GEOID/index.shtml It starts to glaze over the brain when considering that we live on a lumpy blob that is changing shape in ways that are measurable, but not reliably so from any location on the planet. Worse, where the satellites are is affected by the lumpy gravity of that lumpy blob. Gah!

There appears to be a project to convert to using the constellation directly as the reference; there is also a project to generate and equi-potential (they call it orthometric) model that includes differences in local gravity that is very useful in predicting water flow and flood plains. https://geodesy.noaa.gov/GRAV-D/pubs...2007_12_19.pdf

Easy when you know what to look for :)

While I found the video from Mentour Pilot to be useful, it has left me wondering if his videos sometimes don't include important information such as this quote from post #4 on this thread: "9 seconds between minima and TOGA". Isn't that of significant importance.

To repeat - slightly edited - my earlier comment (post #133).
"the preliminary report seems to be clear that ...the crew flew straight through (what they believed to be) the MDA at a constant descent rate of 12 ft/sec, without having any visual references. It appears that the need for a go-around was not manifested for a further 6 seconds (72 ft further descent), when the Captain disconnected the autopilot and made a nose-up input, but a full go-around was not initiated for a further 3 seconds when TOGA was applied. So even if the altimeters had been set correctly obstacle clearance would have been significantly infringed – at the lowest point the aircraft would have been 123 feet below the correct MDA, apparently in full IMC. From the Captain’s actions (autopilot disconnect and stick back and then TOGA power 3 seconds later) it seems likely that he was not mentally prepared for a go-around.

So regardless of the altimetry issue, we have a prima facie serious breach of AOM in that the aircraft descended far below the indicated MDA without the Captain having seen visual cues which should already have “been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position in relation to the desired flight path.” (ICAO definition of reqjired visual reference)."
To repeat: it's "DECISION to land or go around height/altitude", not "start looking for cues altitude".