Let's suppose you're trying to time a 100 yard dash based on sound. Someone fires a gun, the race starts, and when the first runner crosses the finish line anot

Let's suppose you're trying to time a 100 yard dash based on sound. Someone fires a gun, the race starts, and when the first runner crosses the finish line another gun is fired, you determine the elapsed time. There are obvious things to correct for -- e.g. if you're standing at the finish line the sound from the finish line will take 1/3 of a second (roughly) to get to you, so you need to adjust your calculation. Now suppose that you are standing off on a barge during the race. You know the distance to the start and finish lines but ignored the drift of the barge because you figured it was insignificant. We're talking 60 nanoseconds. The satellites are moving at tens of thousands of miles an hour.