Originally Posted by
BackPacker
And where would this gadget get its input data from?
From the aircraft's static ports.
The point about GPS derived altimetry is that it removes the need for altimeter setting, with its attendant drawbacks. Every time a subscale setting change is required there is the risk of mis-setting or omission of resetting. In European airspace, with low transition altitudes, the authorities are obliged to publish notams every time a deep low pressure weather system passes through, warning pilots about the dangers arising from failure to reset when there are large QNH/1013 differences.
The transition process will be similar to the gradual adoption of RVSM and RNAV approaches, with the procedures introduced initially into low traffic density airspace to prove efficacy.