Definition
A method formerly used by the U.S. Department of Defense to intentionally degrade the accuracy of civilian GPS signals by introducing controlled errors into the satellite timing and position data. Selective Availability was discontinued on May 1, 2000, after which civilian GPS receivers gained access to the full accuracy of the system.
Plain English
A deliberate fuzzing of GPS accuracy that the U.S. military used to apply to civilian GPS signals so they would not be as precise as the military version. It was switched off in 2000, so civilian GPS is now far more accurate than it used to be.
Context Anchor
Seen in GPS and navigation accuracy discussions, especially in older explanations of satellite navigation limitations.
Derivation
‘Selective’ means chosen or applied to some but not others — in this case, applied to civilian users but not military ones. ‘Availability’ refers to how much of the system’s full accuracy was made available. Together: the level of accuracy selectively made available to different users.
Why Pilots Care
Explains why pre-2000 GPS position data carried larger uncertainty and why current unaugmented GPS performance is significantly better for navigation and approaches.
Intuition Check
Selective availability does not mean GPS works only in selected places. It means full GPS accuracy was once selectively limited for public users.
Example Sentence 1
Before selective availability was turned off in May 2000, civilian GPS positions could be off by up to 100 meters.
Example Sentence 2
After selective availability was turned off, the same GPS receiver provided consistent accuracy suitable for non-precision approaches without additional corrections.