The system is so precise that adjustments must be made because of the speed of the orbiting satellites. You see their speed changes the rate that time passes for the equipment on board compared to equipment on the ground. If the time rate change was not taken into account then the location data would be way off.
http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html
The article leaves out a lot of important details (it’s the LA Times, so that’s to be expected). The key predecessor system was NRL’s TIMATION, which used atomic clocks on satellites at mid altitudes (8 hour orbits preferred). GPS uses 12 hour orbits. See my article http://www.thespacereview.com/article/626/1