Dead reckoning works surprisingly well at least in Teslas, of which the precision can be seen when they move around in underground facilities and finally resurface: the car pops up pretty close to the entry point.

In any case, aren't all self-driving vehicles equipped with lidars and work in pre-mapped regions? I would 100% expect they are able to just use those maps with lidar to precisely geolocate themselves, probably more precisely than with GPS. And, indeed, GPS itself can at times be inaccurate or simply not available due to obstructions.

So what I'm saying, maybe it's wasn't the GPS.