GPS works by using the 29 satellites orbiting the Earth 12,000 miles up. Five of them are redundant. The U.S. military began launching them in 1978, and it took until 1994 to get the required 24 in orbit which was enough to calculate a position anywhere in the world. These 2,000- to 4,000-pound satellites are 17 feet wide. They traverse six separate orbits with each orbit having four satellites chasing one another.
The satellites are positioned so that any ground-based GPS receiver can always 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. The controls are correcting in that if a satellite is out of position, they command its booster rockets to nudge it back on track.
Using 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. An accurate data location requires at least three satellites and incrementally 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. By comparing successive readings as compared with time, it can also calculate ground speed and direction.
Errors persist since many people have noticed that GPS data is never totally accurate. Radio waves travel at the speed of light or 186,000 miles/second. And in a vacuum, the Earth's atmosphere slows waves down. Further delays occur when signals bounce off intervening obstacles such as buildings, elevated geography, and trees. Until 2000, the public GPS was purposely made less accurate because GPS was originally designed for military use and the U.S. government didn't want enemy forces to possess better position information. Thus, the U.S. military introduced deliberate errors into the system. This process resulted in GPS calculations that could be off by 100 meters.
Differential GPS (DGPS) now corrects for measurement errors by comparing the GPS positions recorded at designated reference stations with the accurately known positions (determined through careful surveys) of those stations. The improved calculations result in accuracy within a yard or two.
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