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What happens when a policeman with a love for the outdoors gets a distress call from a hiker who has lost her way? If you’re Mount Vernon, Washington, officer Tom Wenzl, you activate your cellphone’s “geocaching” application and hit the trail. Hiker Brenda Johnston had called 911 after becoming disoriented in the early December evening darkness. In so doing, she had helpfully provided her precise coordinates to police dispatchers—not that she had any idea of her longitude and latitude, but her phone did, thanks to the Global Positioning System (GPS). 
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From Isaac Newton to Albert Einstein, physicists have dreamed of a day when they could pinpoint where an object is and when. GPS has turned that dream into a reality. Collect a sequence of such coordinates, and you can calculate an object’s position, velocity, and acceleration at any moment. Going further, you can deduce the forces at work on that object.
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How does GPS work? At its heart lies a fleet of 31 satellites orbiting 20,000 km above ground—around four times as far from the center of the Earth as you are. When at least four of these satellites are above the horizon, a commercial GPS receiver can pinpoint your location on Earth with an accuracy of a few meters. Each GPS satellite constantly transmits its location and tags that information with a precise time. When your GPS unit receives that signal, it instantly “knows” how long the signal took to reach you. Divide that time interval by the speed of light, and you know the satellite’s distance. Repeat the procedure with several other satellites, and your GPS-enabled device calculates your latitude, longitude, and elevation.
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GPS receivers once were the sole province of secretive military personnel. Initially built by the U.S. Department of Defense, the system was invented to track and guide nuclear submarines and intercontinental ballistic missiles. Now GPS sensors are standard equipment in many cellphones. You can buy a satellite-navigation system for your car for the price of a small television.
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