If measuring the travel time of a radio signal is the key to GPS, then our stop watches had better be darn good, because if their timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error!
On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board.
Atomic clocks don't run on atomic energy. They get the name because they use the oscillations of a particular atom as their "metronome." This form of timing is the most stable and accurate reference man has ever developed.
But what about our receivers here on the ground?
Remember that both the satellite and the receiver need to be able to precisely synchronize their pseudo-random codes to make the system work. (to review this point click here)
If our receivers needed atomic clocks (which cost upwards of $50K to $100K) GPS would be a lame duck technology. Nobody could afford it.
Luckily the designers of GPS came up with a brilliant little trick that lets us get by with much less accurate clocks in our receivers. This trick is one of the key elements of GPS and as an added side benefit it means that every GPS receiver is essentially an atomic-accuracy clock.
Using GPS for Timing
We generally think of GPS as a navigation or positioning resource but the fact that every GPS receiver is synchronized to universal time makes it the most widely available source of precise time.
This opens up a wide range of applications beyond positioning. GPS is being used to synchronize computer networks, calibrate other navigation systems, synchronize motion picture equipment and much more.
The secret to perfect timing is to make an extra satellite measurement.
That's right, if three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing.
This idea is so fundamental to the working of GPS that we have a separate illustrated section that shows how it works. If you have time, cruise through that.