The technical limitation is the basic geometry of GPS. Satellites are at ~20,000km, aircraft at ~10km max. And you want 0.0003km resolution in altitude. We bare

The technical limitation is the basic geometry of GPS. Satellites are at ~20,000km, aircraft at ~10km max. And you want 0.0003km resolution in altitude. We barely even have 0.003km in the lat/long plane, and it's only that good because of fancy DSP algorithms which lean on special modeling of atmosphere and the satellites themselves - I.e. assumptions about a 3rd-party system, wholly inappropriate for safety critical stuff. In fact, cruising altitude (~"full span" indication) is roughly the magnitude of the specs for error in lat/lang back when GPS was first commissioned (IIRC).