It’s a combined signal system, using pressure based sensors + gps.

And inertial guidance too?

I don’t know what airbus uses I only looked into the schematics of commercial avionics like Garmin. I doubted though IMU drift and calibration introduce more error than they can provide in useful signal, old school pressure sensors + gps adjusted manually or automatically for regional pressure settings (pilots get these numbers through radio when they enter a new pressure area) is accurate enough (~1m). I’ll let a real avionics engineer correct me here, I’d be curious if that signal is worth the hassle + I can imagine such tiny SMD sensors ARE the biggest victims of radiation hallucination.