Bill,

GPS altitude gives you absolutely NO more headroom below Class A
airspace, since FL180 is a PRESSURE altitude (referenced to 29.92),
not an absolute altitude above sea level (which is approximately what
GPS altitude indicates). At 17,990 ft you have 9 feet of headroom,
you still can't go above 17,999' without an IFR clearance or wave
window) regardless of what the GPS is telling you.

GPS is probably more accurate for final glides.

There is practically no chance that ATC will switch to GPS altitude
for airspace control, since it would require ALL aircraft to have WAAS
GPS with RAIM and all that kind of fancy "stuff". Whereas a simple
pressure altimeter, good for +-75'when set to the local altimeter
setting, works fine for traffic separation - and doesn't require an
electrical system.

Kirk
66

"Bill Daniels" wrote in message ...
I have seen GPS altitude at 19,200 when the panel altimeter set to local
pressure said 17,990. I figure that gives me about 1200 more feet of
"headroom" to play in below Class A airspace in the USA. In the high
mountain country of the western US, GPS altitude gives much better final
glide calculations than pressure altitude.

Hopefully, the feds won't take away this extra useable attitude by switching
to GPS altitude for ATC.

Bill Daniels