Can't spoof gps all the time everywhere. Once you have a general idea where you are (usually because that's where you were when you stopped yesterday), you can compare what you can see with hd maps, and fix your position exactly. You know you're not in a building, but on a drivable surface for a start.
My 2006 car was fine driving underground for 10 mins with no gps signal, but got confused when I drove it onto a train and it moved 30 miles without the wheels turning. Then after a few minutes got a gps signal again and fixed itself. Had an option to manually set the position and heading too.
My 2006 car was fine driving underground for 10 mins with no gps signal, but got confused when I drove it onto a train and it moved 30 miles without the wheels turning. Then after a few minutes got a gps signal again and fixed itself. Had an option to manually set the position and heading too.