GPS was conceived in the 1970s, and is controlled by the United States Department of Defense. Although GPS was initially envisioned for military use, the Government realized early on that there would be numerous civilian applications as well. Subsequently, the Department of Defense created two transmission codes; the P code (Precision code) for military use, and the C/A code (Civilian Access code) for civilian use.
The highest accuracy levels were to be reserved for the military so as to prevent hostile enemy attacks against the U.S. using our own navigational system. However, once in operation, the civilian GPS receivers using the C/A code proved to be more accurate than the D.O.D. had intended. Consequently, the military developed a system for randomly degrading the accuracy of the signals being transmitted to civilian GPS receivers. This intentional degradation in accuracy is called Selective Availability or S/A. This reduced the civilian GPS accuracy levels to being within 100 meters or less, 95% of the time. However, typical accuracy for most users averaged between 20 and 50 meters the majority of the time. You could easily see the effects of S/A on a GPS receiver when you were not moving. Typically there would be random movements in speed, altitude and position readings, along with slow position "wandering" on the plotter trail, easily seen when you are on a .1 or .2 mile Plotter scale, and not moving. For example, while parked at the dock in your boat, you would see unexplainable changes in your digital speed-readings up to a few miles per hour, even though you were not moving.
Effective May 2, 2000 selective availability (S/A) has been eliminated. The United States Department of Defense (DOD) now has the technology to localize the control system to deny GPS signals to select areas. It is not often that your electronics products increase in value after you've purchased them. Now boaters, hikers, hunters, and outdoor enthusiasts of all types can locate their position up to ten times more precisely (within 10 to 20 meters) and navigate their way through unfamiliar terrain. Anglers can now return to their favorite spot on a lake or river instead of just their favorite area.
The decision to allow civilians so much accuracy in location information was finally completed because GPS is continually playing a more important role in the lives of people around the world - it's becoming a national utility. GPS is the global standard in navigation because it is completely free of charge to the public.