Answer Geek: How GPS Works

Q U E S T I O N: I have a handheld GPS device that can tell me, within a few dozen yards, where on earth I am. Can you tell me how on earth it works? — Justin D.

A N S W E R: Actually, Justin, much of the work of GPS takes place not here on earth, but out in space.

To pinpoint your location somewhere — anywhere — on the face of the earth, the Global Positioning System uses a network of 24 satellites, each of which circles the earth twice a day, from an orbit about 10,900 miles high in space. Placed in orbit by the United States Department of Defense, they are the backbone of a system that can pinpoint your location with remarkable accuracy.

GPS is also the culmination of a centuries-long quest by humankind to develop tools for navigation that allow us to get from one place to another without fear of getting lost. Among the most important navigational inventions in human history were the compass, which made it possible to know in which general direction one was heading, and the sextant, which helped determine one's latitudinal location. With the invention of the chronometer in 1761, which can calculate both latitude and longitude, sailors could finally cross oceans with some certainty that they would end up at their preferred destination.

Radio Days Early in the 20th century, radio-based navigation systems superceded those navigational wonders. Originally built for military purposes, the Global Positioning System was put into place beginning in the mid-1970s. A decade later the system saw civilian uses: now GPS not only makes it even easier for modern sailors to navigate their way across the seven seas, but it also helps hikers find their way in the woods and road-weary travelers negotiate their way to the motel of their choice at the end of a long day on the highways.

Here’s how it works: each one of those 24 satellites — which weighs in at about a ton and measures 17 feet across with its solar panels fully unfurled — constantly transmits radio signals earthward. Those signals, which travel at the speed of light, include timing and positioning data. The timing information is crucial for the GPS receiver to figure out how far it is from the satellite — this is the first step to getting an accurate fix on your location. Time-keeping accuracy is so critical that each satellite is equipped with not one, not two, not even three, but four, count ‘em, four atomic clocks. Those clocks allow the satellite to measure time with a margin of error that is three nanoseconds or less. How accurate is that, you ask? How about 0.000000003 of a second?

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