http://www.wired.com/2011/06/st_equation_gps/
"Your GPS unit registers the exact time at which it receives that information from each satellite and then calculates how long it took for the individual signals to arrive. By multiplying the elapsed time by the speed of light, it can figure out how far it is from each satellite, compare those distances, and calculate its own position. (...) According to Einstein's special theory of relativity, a clock that's traveling fast will appear to run slowly from the perspective of someone standing still. Satellites move at about 9,000 mph - enough to make their onboard clocks slow down by 8 microseconds per day from the perspective of a GPS gadget and totally screw up the location data. To counter this effect, the GPS system adjusts the time it gets from the satellites by using the equation here. (Don't even get us started on the impact of general relativity.)"
The speed of light varies with both the speed of the emitter/observer and the gravitational potential. Einsteinians multiply "the elapsed time by the speed of light" but wrongly assume that the speed of light is constant (they have no other choice - Divine Albert said so). In order to fix the wrongness, "the GPS system adjusts the time it gets from the satellites", even though all clocks tick at exactly the same rate (there is no time dilation).
Pentcho Valev