The Global Positioning System (GPS) is a satellite based navigation system developed by the United States Department of Defense. It is widely used for civilian navigation and positioning, surveying and scientific applications.
Twenty-seven satellites orbiting the earth are responsible for the location-tracking information any GPS receiver – be it in a car or a ship – receives. Of these, at least 24 are operational at any time, and the remaining ones are used as backups.
These satellites continuously send out string of bits that synchronies with every active GPS receiver. They also send out information about where they are.
Thus, a GPS receiver always has information about the location of satellites within its range.

Point positioning and navigation

Prior to 1 May 2000 the GPS specifications stated that under conditions of 'Selective Availability' a single horizontal GPS position will have an uncertainty of 100 meters at 95% confidence. However, on 1 May 2000, 'Selective Availability' was turned off by the United States Government, so that best practice is observed, a single GPS position will now have an uncertainty of about 10-20 meters.