During the introduction to GPS class yesterday we discussed the issue of Selective Availability (SA), which was discontinued in May, 2000. SA is the purposeful introduction of an error in the timing signal so that civilian users cannot get an accurate signal without using differential GPS. Specially equipped military GPS can remove the error on the fly.
The decision to turn off SA was made to promote GPS for civilian use. As more users came on board the degraded signal actually became a big obstacle. Also, the military developed other ways to limit the degradation of signal to battlefield areas. There is an interesting article in Wikipedia that describes SA and why it was eliminated.
The Wikipedia article notes that President Reagan directed that GPS be opened to civilian use as a public resource after the downing of the Korean Airlines 747 by the Soviets in 1983. I didn’t realize that was the reason this high security system was opened up.
Currently, although highly unlikely, SA could be turned back on by the Department of Defense. However, DOD announced in fall 2007 that the new GPS III generation of satellites would not have SA capability at all.
GPS is a great example of how military technologies eventually make their way to civilian use for the benefit of all. It would be great if that tremendous research and development capabilty could be musted without the need for wars.