A satellite navigation or sat nav system consists of a receiver and a system of satellites that, when combined with a computerized map, can pinpoint an individual or a vehicle's location to within 30 feet.  Most sat navs use the United States NAVSTAR Global Positioning System (GPS), which is made up of around 30 US military satellites orbiting the earth.  They constantly beam high frequency radio signals to earth. Each satellite contains an electronic clock, accurate to 1 second in 138 million years.