Until 2000, the accuracy of the GPS civil signal was reduced using SA, so that a normal satnav receiver would be accurate to only 100m or so. There was, and remains, a separate military signal, encrypted so only those with appropriate keys - such as the US forces and their allies - can use it.

Despite SA, many civil users were able to achieve excellent accuracy using differential GPS, where a ground station at a known location calculates the SA error and transmits corrections to a mobile civil receiver in real time.

SA was switched off in 2000 on the orders of president Clinton, but it remained an option. In the years since, the US government has sought to assure civil GPS users that SA would never be used again, but its presence has remained a concern.