Not long after GPS became operational with the 24 sat cluster, the DOD added SA to downgrade civil use to about 100mts. Some clever chappie realised that if you placed a receiver at an accuratley surveyed location, you could work out the psuedo range errors and transmit them to a sat nav receiver which would then correct its readings and the accuracy then improved to abt 6mtrs. This was known (and still is) as differential sat nav. Intitially developed for marine use
the corrections were overlade on the marine beacon frequencies (200 to 400 Kcs). Nowadays surveying equipment can get down to centimeter accuracy using this system. That basically why SA was removed as it no-longer degraded anything. (The accuracy of an ordinary, non mil, unit is about 20 mtrs)
Hope this helps