GPS is based on satellite ranging. This technique of measurement is based on timing how long it takes a radio signal to travel from the satellite to the GPS receiver and then using this time to calculate the distance. There are basically two factors involved. The speed of the radio signal (approximately 300,000km/s) and the time it takes for the signal to reach Earth. The timing is the critical part and requires the use of a very accurate clock, and to facilitate this the satellites do have atomic clocks on board. "Because if the timing is off just a thousandth of a second, at the speed of light, that translates to almost 200 miles of error" (Trimble).