Good question about GPS corrections, I have no idea what Google does (n addition to terrain shadowing there is also
intentional jamming*). There are two different map problems: 1) making a good map with whatever data you can get beforehand, and then 2) using the map with live sensor data to locate and drive a car in real time.
If your mapping car has at least occasional good GPS fixes, and you also have a lot of really high-resolution visual + LIDAR data (and a lot of CPU power), you can piece it all together like a jigsaw puzzle using self-consistency into a map with very good local (relative) accuracy, and good-enough absolute accuracy based on GPS. Likewise the self-driving car could work even with iffy GPS, if visual and other sensor data could be used to key into the map well enough.
* It's an interesting article:
"...most of the jamming Gostomelsky saw was done for the sake of profit or paranoia. Gostomelsky lives near Interstate 476 in Pennsylvania, and would regularly detect GPS jammers in use as tractor-trailers went through the toll booths. Because the toll-taking for commercial trucks relies on GPS tracking, they can avoid paying through jamming. If a $45 device made your daily commute free, you too might be tempted to commit a federal crime. [...]
Other jammer use-cases that Gostomelsky saw were more mysterious. A couple of jammers who were flagged by his system in Philadelphia had unusual electronic signatures emanating from their cars. They appeared to have both smartphones and burner phones, based on their communication with cell towers, which he was also monitoring. He was intrigued, so he started moving his listening stations around to figure out the cars’ driving patterns, to zero in on where these jammer users lived. He wound up tracking one of the jammers to a house in New Jersey. He went digging through public records to see who lived there and figured out the property owner was related to a police officer.
He is now convinced that some of the people using illegal jammers were police officers. They may not have seen a
2014 warning from the FCC that the prohibition on jammer use applies to “state and local government agencies, including state and local law enforcement agencies.”
“I think they’re using GPS jammers on their vehicles so that they can’t be tracked home,” Gostomelsky said. An undercover officer, for instance, might worry about a suspicious target sticking a GPS tracker under his bumper to find out if he’s a cop. "
https://gizmodo.com/jamming-gps-signals-is-illegal-dangerous-cheap-and-e-1796778955