A satellite navigation receiver does not measure distance directly. It listens for a timing signal from each satellite it can see and works out how long the signal took to arrive. Because the signal travels at the speed of light, an error of a single millionth of a second puts the answer out by three hundred metres. The satellites therefore carry atomic clocks accurate to a few billionths of a second a day, and the ground stations correct them constantly. What looks to the user like a simple question of position is answered, underneath, as a question about time.