Definition
Selective Availability (SA) was a U.S. Department of Defense feature that intentionally degraded the accuracy of civilian GPS signals by introducing small, controlled errors into the satellite timing and position data. It was used to prevent adversaries from exploiting the full precision of GPS for military purposes. SA was set to zero (turned off) on May 1, 2000, and the capability to reintroduce it was permanently removed from later GPS satellites.
Plain English
It was a deliberate fuzzing of GPS accuracy for non-military users. The government built it into GPS to keep enemies from getting pinpoint accuracy, then switched it off in 2000 so civilian GPS — including aviation receivers — could be much more accurate.
Context Anchor
Seen in discussions of GPS accuracy, especially when comparing older GPS limitations with current GPS performance.
Derivation
Selective' means chosen or applied to certain users, and 'availability' refers to how much of GPS's true accuracy was made available. Together it described a system where full accuracy was selectively given — to the military — and withheld from everyone else.
Why Pilots Care
It once limited the precision of GPS navigation until the feature was discontinued in 2000, after which civilian GPS accuracy improved significantly.
Analogy
It is like giving one person a sharp, clear map and giving another person the same map with the details slightly blurred on purpose. Both maps point to the same general place, but one is intentionally less precise.
Intuition Check
Do not read this as ordinary “availability,” meaning whether GPS is working or not. Here it means that full GPS accuracy used to be made available only to selected users, while others received a deliberately less accurate signal.
Example Sentence 1
Before Selective Availability was discontinued in 2000, civilian GPS positions could be off by up to 100 meters.
Example Sentence 2
The end of selective availability allowed instrument pilots to rely on much more precise GPS data for approaches.