writer: Edward Carolipio, Ph.D.
Several months ago, there were some murmurs about "something" happening to GPS that was supposed to be "good" for GPS-based navigation. The event was on local and national news broadcasts, but the explanations weren't very detailed and the buzz was nonexistent. In fact, that "something" was the removal of Selective Availability, or SA, from the GPS signal and the reason why it was a "good" thing is because GPS-based navigation gets almost a five-fold increase in accuracy with existing equipment. Believe it or not, this event was probably as significant to GPS users as the first time the constellation was declared usable. For current and future owners of car navigation units, understanding this technology and how it affects these products are important. This article gives a brief history of GPS and SA, a description of how SA works, and the effects of removing SA on automotive users.
In its infancy, GPS was envisioned as a navigation system primarily for the military. The downing of Korean Airlines flight 007 in 1983 changed this approach. The Boeing 747 had drifted over Soviet airspace as it flew south towards the Korean peninsula and was shot down by Soviet fighters. More accurate radio navigation aids with global coverage such as GPS would have prevented such a mishap. Because of this tragedy, President Reagan issued a decree that the United States would be committed to providing the GPS signal, free of charge, to civilian users worldwide and recommended to Congress that the civilian use of GPS continually be expanded.
The military's concern about providing the maximum accuracy for everyone is that "the bad guys" would use our own navigation system against us. Consequently, the initial deployed design called for a Precise Positioning Service (PPS) for military and authorized civilian users and a Standard Positioning Service (SPS) for all other users. The GPS signal would be crippled using a technique called Selective Availability, so SPS users would be guaranteed accuracies to only about 100 meters. In comparison, PPS is guaranteed accurate to 15-30 meters, though in practice accuracies of 10 meters have been noted. PPS also has protections against "spoofing," or the enemy purposely tweaking the GPS signal to fool or eliminate the use of the signal by our troops; of course, this protection method is called "Anti-Spoofing."
Providing SPS is part of this continued expansion of civilian access to GPS. The increasing reliance of so many industries on GPS technology puts pressure on the government to provide the maximum available accuracy of the system. Congress eventually mandated that SA be turned off as soon as issues regarding national security were resolved. This order was executed a few minutes past midnight EDT on May 1, 2000.