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.
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.
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.