A function in the GPS navigation system that deliberately introduced random errors for civilian GPS receivers. It was implemented to prevent enemy troops on foreign soil from using the GPS system to their advantage, while allowing friendly troops to obtain the true signals in GPS receivers that supported military encryption. Selective Availability (SA) was temporarily disabled in the Gulf War, because so many U.S. troops were using consumer-grade GPS receivers, and SA was disabled permanently in 2000. Because of the lack of accuracy for the consumer market, ground-based augmentation systems were created to correct the discrepancies. See GPS augmentation system.