Altimeters. They do not depend on bits outside the aircraft, same as INS.
Meaning what, exactly? If the assertion is that GPS is a better source of altitude information, then I ask again, why is it not used for RVSM?
Your answer is that it isn't used because it employs no external reference (you're not familiar with baro-aiding, I take it). If GPS is indeed more accurate and therefore the better altitude reference as some have asserted, then why is it not used for the primary altitude reference? What has external measurement got to do with the price of tea in china?
In fact many GPS units do consider barometric pressure as an input, but that's neither here nor there. Whether the GPS display considers barometric pressure or not, or uses any external input, shouldn't have any bearing on the reason that it is or isn't used as the primary altitude standard.
The fact is that despite most all aircraft in RVSM airspace, where separation standards are reduced and aircraft operate closer together in the IFR environment, are equipped with advanced GPS and FMS equipment, GPS is still not used for altitude reference.
We do use that, and other altitude data as an unofficial reference sometimes when giving metric altitudes in Chinese and Russian airspace; it's handy for giving a "passing through" altitude on our way to an altitude assigned in meters. Otherwise, GPS altitude remains little more than an interesting tidbit.
Suppose that should read QNH? Mixing up QNH with QFE, perhaps?
No, QNE...which is what's set once one flies above the transition level.
While 1013.25 might be a technical standard, it's not used for flight above the transition level. I set 29.92 or 1013 on the altimeter going up, and QNH on passing transition altitude on the way down.
QFE is an entirely different animal, used by Russians and parachutists.