Russian officials are seriously talking about shutting down US GPS ground stations within their borders and Deputy Prime Minister of Russia tweeted on the subject.
What will happen to the GPS? My assumption would be that its accuracy will get worse, at least in some parts of the globe. If it is correct, then where, and by how much exactly?
I have previously posted this question on physics.stackexchange.com and here are two answers I have received there. They seem to be a little contradictory, though.
Usually ground stations are alternative points for a 3rd/4th signal since it is a fixed (known) location. Russia, at least in the 90s, didn't have a way to connect to the 24 satellites & depended on the stations; I doubt that they are of any use anymore given GLONASS
There are two types of GPS ground stations for the USA system. Main stations about 5 around the world provide corrections to timing data due to relativistic effects, clock drift, ephemeris, or ionospheric delays that are timing constants rebroadcast by the satellites to receivers for corrections. The types of stations in Russia are for augmented gps called Global-DGPS . DGPS improves local GPS accuracy from about 3m to 10 cm. However this lack of support in Russia's areas only effects civilian use of the US system in those areas. The Chinese, EU, and Russian systems won't be effected.
"Employing a large ground network of real-time reference receivers, innovative network architecture, and award-winning real-time data processing software, the GDGPS System provides decimeter (10 cm) positioning accuracy and sub-nanosecond time transfer accuracy anywhere in the world, on the ground, in the air, and in space, independent of local infrastructure." (gdgps.net)