A deliberate blur in civilian GPS
Early civilian GPS receivers were not simply bad at reading the sky. The U.S. military deliberately made the public signal less precise through a policy called Selective Availability. Until it was turned off in 2000, the system introduced small, shifting errors into satellite timing and position information available to civilian users.
GPS works by measuring how long signals take to travel from satellites to a receiver. Because those signals move at the speed of light, even a tiny timing error can make the receiver calculate a noticeably wrong location. The receiver combines signals from several satellites to estimate its position; if their timing or position data has been deliberately blurred, the resulting estimate wanders too.
Why a small error put a car in the wrong place
A location error that seems modest on an open field can be glaring on a road. A car may be physically on a narrow street while its displayed position lands on a sidewalk, in a nearby yard, or on a parallel road. Early navigation systems then compared that estimate with their stored map. If the map’s road line and the calculated position did not line up, the screen could make the car appear to leave the road.
The military signal was protected, while ordinary receivers used the less accurate civilian signal and could not simply remove the intentional distortion. Other errors—such as poor satellite visibility or an imperfect road map—could add to the mismatch, but Selective Availability was a major reason civilian GPS locations were so unreliable.
What changed in 2000
The U.S. government ended Selective Availability on May 1, 2000, allowing civilian receivers to use a much more accurate signal. That did not make every GPS device instantly perfect: receiver quality, buildings, trees, and map data still mattered. But it removed a deliberate source of error, making it far less likely that a car’s position would drift across the map and appear to drive off the road.
