Stilton:
I'm probably missing something but if this could be done you would eliminate pressure data derived errors and there would be no need for transition altitudes / levels.
You could retain the standby pneumatic altimeter as a back up.
Since GPS derived altitude and pressure derived altitude give different values it is not possible to replace the pressure altimeter with GPS. The GPS altitude and pressure altitude are just not compatible. If your GPS would fail, you can't fall back on the barometric altimeter, because you would end up at a different altitude. That is simply not acceptable in a crowded airspace.
Consider the following:
- The barometric altimeter is very sensitive to vertical deviations (think RVSM). Unaugmented GPS is far less accurate in the vertical plane. For vertical separation the barometric altimeter is the preferred solution.
- The barometric altimeter does not require a power source. It helps to have passive instruments you can rely on in case you loose electrical power. (again think RVSM).
- GPS is dependent on satellites, the barometric altimeter on the presence of the atmosphere. Atmosphere has proven to be more reliable than satellites.
- GPS measures absolute altitude, the barometric altimeter assumes Standard Atmosphere. For your height above the hills, obstacles, runway etc, a reliable GPS can give a solid indication. Barometric altimeters depend on the correct setting (QNH), and suffer from temperature induced errors, so can be less accurate when it comes to absolute height.
So when it comes to vertical separation between aircraft, the barometric altimeter is and will always be the preferred solution. For operations where the vertical distance between the aircraft and ground is of the essence, (augmented) GPS may become an alternative. But still you will always need the altimeter as a backup.
ATCast