Measuring the travel time of a radio signal is the key to GPS. If one clock varies from the other by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error!
Satellites have incredibly precise atomic clocks on board. But what about our receivers here on the ground?
Atomic clocks cost upwards of $50,000 to $100,000, making GPS with an atomic clock cost-prohibitive.
GPS scientists determined a way for the ground receivers to have less accurate clocks – use a fourth satellite measurement. If three perfect measurements (triangulating) locate a point in three-dimensional space, then four imperfect measurements can do the same thing. The extra measurement corrects for any timing offset between the receiver and the satellite.
If our receiver's clock was perfect, then all our satellite ranges would intersect at a single point (which is our position). With imperfect clocks, a fourth measurement, done as a crosscheck, will NOT intersect with the first three. The receiver's computer sees the discrepancy and locates the single correction factor it can make to its timing measurements to bring all four measurements to a single point.
That correction brings the receiver's clock back into sync with universal time. This gives the receiver atomic accuracy. Once it has that correction it applies it to all the rest of its measurements for precise positioning.