Definition
Selective Availability was a feature of the Global Positioning System (GPS) operated by the U.S. Department of Defense that intentionally degraded the accuracy of civilian GPS signals by introducing controlled errors into the satellite timing and position data. It was used to limit the precision available to non-military users for national security reasons. Selective Availability was discontinued on May 1, 2000, and civilian GPS receivers have since received the full unrestricted accuracy of the system.
Plain English
Selective Availability was a deliberate fuzzing of GPS accuracy that the U.S. military used to keep civilian GPS less precise than military GPS. It was switched off in 2000, so today civilian receivers get the full accuracy.
Context Anchor
Seen in discussions of GPS accuracy, older navigation material, and the history of satellite navigation.
Derivation
‘Selective’ means choosing who gets what. ‘Availability’ refers to what level of accuracy was made available. Together, the term describes a system where full accuracy was selectively granted to military users and withheld from everyone else.
Why Pilots Care
Explains the accuracy limits of pre-2000 GPS units and why current GPS performance supports more precise navigation.
Intuition Check
Do not read “availability” as meaning whether GPS is turned on or off. Here, Selective Availability means the GPS signal was available, but its full accuracy was intentionally limited for some users.
Example Sentence 1
Before Selective Availability was turned off in 2000, civilian GPS positions could be off by up to 100 meters.
Example Sentence 2
The end of Selective Availability allowed general aviation pilots to rely on GPS for more accurate position reporting.