Not my area at all, but I'm extremely surprised that a fly-by-wire system would use GPS as an altitude reference. Is that really the case?

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.

i would expect a huge shift like that to violate the gaussian assumption of the kalman filter? (which i guess is what you're checking, sort of?). regardless i would expect the kalman filter to smooth the shift over some substantial time at least?

I think it more likely these receivers fell for a spoof GPS signal or some software bug internal to the receiver than a solar bitflip.

In our case I don’t believe it was solar bitflips but rather wildly changing ionospheric conditions. I was primarily pointing out how you can have a software-only fix for problems like this.

Without going too far into the weeds, the fact that the receivers in question were reporting high accuracy under uncertainty is definitely a software bug in the receivers from my perspective. There was a different receiver with a completely different chipset in it on-site too that was experiencing similar issues but was reporting low accuracy. Without going into too much detail, I’ve got pretty good reasons to believe it wasn’t spoofing.