Converting "true" to "indicated" instead of the other way round.

i have one question though :

GPS altitude is WGS-84 altitude, nothing to do with actual… So what altitude would you be referring to ?

I think that would be the case if it were invented today.

However to change over… everyone would need to change over at the exact same time, which means the complete installed base of aircraft (millions of them) would need an approved, accurate GPS (WAAS). In some countries, even for the highest class of commercial operations you still don’t require a GNSS. It also includes all private/LSA/microlight etc. etc. Who is going to pay for that eh?

Plus the requirements for accuracy over time (e.g. 50 feet vertical accuracy for 99.99% of the time) may be higher than what GPS could ever offer.

Now the good old altimeter, if it’s accuracy starts to fade… can just be replaced by another one which has been tested accurate……………..

In the end it only matters what reference one uses. It’s impossible for two aircraft to hit one another if the one happily follows a 300 mb level and the other happily follows the 320 mb level.

I think we would still use barometric altimeters today, at least above transition altitude. There’s still discussion about how to avoid becoming too dependent on GPS for navigation, and when the sunstorm comes and the whole network goes down, what will plan B be for altitude? Obviously (as Jan mentioned) it’ll be to fall back on the barometric altimeter, which will have to be provided anyway. It’s a simpler question than for navigtion, where setting up parallel systems is expensive.

It’s at low-level that barometric pressure changes make life complicated and risky, why not have the GPS set a flag when it thinks the altimeter is reading below QNH?

I think we would still use barometric altimeters today, at least above transition altitude

Do you perhaps mean below the TA?

Above it, it is easy to make a very accurate, redundant, whatever you want, pressure transducer based on 1013mb. You don’t need GPS, and if you used GPS you would then need to correct its reading for temperature to get the FL. It would be just messy.

Below the TA, a GPS gives you a very accurate altitude AMSL (even my old KLN94 is more accurate than a fresh KEA130A altimeter) but with all the obvious caveats which as noted above will ensure it never happens.

No, I can see an advantage to GPS below the transition altitude (you’re not going to fly into terrain because of forgetting to set the altimeter correctly) but don’t really see the advantages when you’re above. Or am I missing something?

I can’t help feeling that with today’s technology, it would be cheaper and more accurate to get true altitude from GPS and communicate that to ATC and/or to other pilots.

Electronic barometric pressure sensors are still cheaper then GPS units. However, you’d have to claculate all errors into the system to make it compatible to existing aircraft.

Then again, why would you want to have that precise altitudes? You can’t fly that precise anyway.

I have never been to any airport where the GPS altitude was “wrong” for the published airport elevation.

I think the WGS84 to AMSL error is very small. But haven’t we done this before here many times? Isn’t there a correction table? The old SIRF-2 chip had a ~150ft error it in. All current products have the correction in them. It is normally done as a lookup table (the earth’s surface is divided into lots of squares, with interpolation) and there is even a quadratic formula which works over most of the earth.

Electronic barometric pressure sensors are still cheaper then GPS units.

Do you have part numbers?

While it’s true that there are lots of very cheap sensors for engine applications,those that are calibrated to the same accuracy as a TSO’d altimeter are still quite expensive

Many reasons for keeping the altimetric system as the basis, but why could not GPS altitude be more integrated in the FMS? Just like a multitude of sources is utilised in determining the horisontal position, why not use GPS altitude as a control parameter to help determining if there is a blocked static or a seriously mis-set QNH? That could have prevented a few accidents I can think of. Or it could be of help in such cases where there is no reliable QNH and the flight is close to terrain. In case of blocked pitot it could provide a backup altitude that would be a life saver. (Although the manifold pressure meter can be used as a crude altimeter also in constant-speed pistons.)

Usually when I have checked GPS altitude during run-up, when setting QNH for take-off, the on-board (non-WAAS) GPS has been more accurate than the barometric altimeter. It has always been within 60 ft, usually within 30 ft. It also is capable of computing its own integrity.