Correct me if I'm wrong, anyone who is more informed on this than I am, but wouldn't it be because of the magnetic fields and whatnot? The longer I think on it, the more I feel like I'm wrong, so don't take my word for it 100%
The Global Positioning System, receives signal from satellites over head, the difference in time, for the signal to reach the receiver on earth, will be different from each satellite and can be measure!
