QuickStudy: Global positioning systems

Like many people, I've come to take for granted the availability of navigation systems in cars and handheld devices based on the Global Positioning System. But it was all abstract until I recently acquired a modern GPS myself. My reaction reminded me of Arthur C. Clarke's Third Law: "Any sufficiently advanced technology is indistinguishable from magic."

How the Magic Works

The basis for GPS is 29 satellites orbiting the Earth 12,000 miles up; five of them are spares. The US military began launching them in 1978, and it took until 1994 to get 24 in orbit -- enough to calculate a position anywhere in the world. These 2,000- to 4,000-pound satellites are 17 feet wide with their solar panels extended. They traverse six separate orbits, and each orbit has four satellites chasing one another.

The satellites are positioned so that any ground-based GPS receiver can always "see" -- receive data from -- at least four of them. A master control station in Colorado Springs and five unstaffed monitor stations around the world track each satellite's orbit precisely. If they find a satellite out of position, they command its booster rockets to nudge it back on track.

Using just a 50-watt radio transmitter, each orbiting GPS satellite continuously broadcasts signals containing a pseudorandom code that provides its identity and position and the time (maintained by an atomic clock). When data from at least three satellites is available (four is much better, and six or seven provide even more accurate results), a GPS receiver uses relatively simple geometric calculations to determine its own latitude, longitude and altitude. Comparing successive readings against time, it can also calculate ground speed and direction. The GPS receiver uses the satellite data to reset its own clock and saves the data for use in calculating position. Newer GPS receivers use a multichannel design in which five to 12 receiver circuits operate simultaneously, each able to lock onto a different satellite.

Built-in Errors

GPS data is never totally accurate. Radio waves travel at the speed of light (186,000 miles/sec.) in a vacuum, but the Earth's atmosphere slows them down. Further delays occur when signals bounce off buildings, hills and trees. Even a millisecond discrepancy can create a 300-meter positioning error. Moreover, until 2000, the public GPS was purposely made less accurate than it could be: Because GPS was originally designed for military use and the U.S. government didn't want enemy forces to have position information as good as that of the U.S. military, it introduced deliberate errors into the system. This process resulted in GPS calculations that could be off by 100 meters.

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