3 - TIMING

If measuring the travel time of a radio signal is the key to GPS, then our stop watches had better be really good, because if their timing is off by just a thousandth of a second, at the speed of light, that translates into almost 300 Km of error! On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board. But our receivers here on the ground cannot have this sort of clocks...