Hey, Thought you might enjoy this video of SS in action in an underground parking lot. Despite the (probable) lack of GPS the car has no issues navigating the lot ☺️ Due to our silly regulations what you see in the video is the max allowed distance of 20m (66 feet).
The GPS isn't needed for distance travelled... The odometer and other sensors in the vehicle are easily able to relay distance travelled to whatever process needs it. They don't rely on the GPS. In the Model X, if you pass under an obstacle or entry way (like a garage or low overhang), the falcon wing doors won't open fully unless you've travelled a certain distance beyond them when you've cleared them. All without GPS.