The (~0.1sec) travel time has to measured to exquisite accuracy.
? The GPS signals travel at the speed of light – one foot per
billionth of a second (nanosecond).
? The satellites have four atomic clocks on board – which in turn are synchronized with terrestrial time standards. They keep time to a few ns.
? However, your GPS receiver has an inexpensive quartz clock – little better than your wristwatch. The distance inferred from a satellite’s transmission time and the receiver’s receive time is called the pseudo-range. Four pseudo-ranges enable the receiver to determine position, completely eliminating the clock error!