Mike Rapoport wrote:

Many, if not most, turbine airplanes have this but it requires an airdata
computer system. You can set it to arrive at a particular point at a
specific altitude. You need an airdata system as long as you are climbing
or descending to a pressure altitiude. WAAS could only guide you to a GPS
altitude.

Perhaps I'm missing something basic, but don't we specifically adjust the
kollsman window to convert values provided by a barometric altimeter to a
true altitude (at least below 18,000')? Of course, you're quite right that
the GPS would need "correction" to yield the pressure altitude used above
18,000 (which I didn't consider because I've never flown there myself {8^).

- Andrew