Airbubba : we are talking 2 different things, I was reffering to the global 0.1NM EMBEDDED in the FMS to counter GPS nav accurracy. ( disccussed in ICAO on initiative IFALPA ATS committe after the Sioux Lookout collision in 1995.) long debate about it, also here on PPrune . But off topic .
Sunamer:
ADS-B format retains both ways of reporting barometric as well as gps altitude
Yes but the ICAO standard chosen was Mode S/Baro, not the GPS Alt. No single a/c ADS equipped flying today uses and transmit GPS Alt.
Ian W :
the precision switch is not so critical and discussed these days.
Agreed , and this will disapear completely with Gallileo coming in. However a future hypothetical switch from barometric to GPS Alt would have to be globally coordinated on a specific date ( similar to RVSM) and old airframes retrofitted, which has always been THE main problem , especially on your side of the Atlantic (since you still use Jurrassic jets overthere
)