" The general principle is called passive ranging. Imagine you and a friend who lives far away each possess highly accurate synchronized clocks. In addition to your clock, you have a live video feed showing the face of your friend’s clock. You notice, from looking at the feed, that your friend’s clock is just slightly off from yours. What does this tell you? Perhaps one of your clocks is malfunctioning. But if you can rule out that error, and know with absolute confidence that both clocks are working perfectly, this discrepancy becomes information. The lag is caused by the time required for the image of your friend’s clock, traveling at the speed of light, to reach you. The speed of light is constant and stable. Your clocks are constant and stable. The lag is directly related to the distance between you and your friend. You now have tools in place for a satellite-based passive positioning system. "

― Greg Milner , Pinpoint: How GPS is Changing Technology, Culture, and Our Minds