Found a map of the four locations that can upload corrections to the satellites (surprisingly to me via direct s-band). From 1995
Colorado Springs (Colorado, US),
Ascension Island (South Atlantic),
Diego Garcia (Indian Ocean),
Kwajalein (North Pacific).
(Hawaii cant upload as it is only is part of the monitoring network)
Edit Replaced map due to AFB naming error (Thanks ArmySGT). Cape Canaveral went online in 2001 so please ignore
http://www.navipedia.net/index.php/GPS_Ground_Segment
Might be an interesting Last Submarine Scenario to go to either Ascention Island or Kwajalein (I've always assumed Diego Garcia was nuked) to get either data, personnel, or equipment to bring back to Colorado Springs to help keep the satellite update system there running.
Again in my game the satellites were generally shredded by a low tech ASAT solution (otherwise how can you explain the 8th Mech getting totally lost) right after the initial US Strikes (when ICBM launches are not unexpected), but the game as written has the players being responsible for transporting the inventors of low tech cold fusion technology, and bringing a microchip replacement to life, so helping to bring out of sync satellites back into service seems like an equally plausible adventure seed.