selective availability

Selective Availability (S/A) was a feature that intentionally degraded the accuracy of the civilian Global Positioning System (GPS) signal. This intentional degradation was terminated in 2000, partly due to safety concerns arising from the increasing use of GPS by civilian aircraft and vessels. Before S/A was turned off, unaugmented civilian GPS had a horizontal error variance of 30 meters (100 feet) with 95% probability, meaning the reported location was within 30 meters of the true location 95% of the time. After S/A was terminated, typical horizontal accuracy improved to about 10 meters (30 feet).1

GPS is a space-based radio-navigation system that uses satellites to provide accurate positioning, navigation, and timing information to users on or near Earth. A GPS receiver calculates its location by measuring the time it takes for signals from at least four satellites to reach it, determining the distance to each satellite and finding the intersection point of spheres representing these distances.1