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What Is GPS... |
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The Global Positioning System (GPS) is a constellation of 27 Earth-orbiting satellites (24 in
operation and three extras in case one fails). The U.S. Military originally developed and
implemented this satellite network as a military navigation system, but soon opened it up to
public use. Each of these 3,000 to 4,000 pound solar-powered satellites orbits the globe at about 12,000 miles, making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky. A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration. In order to calculate its location, the GPS receiver has to know two things: The location of at least three satellites above the receiver The distance between the receiver and each of those satellites The GPS receiver figures both of these things out by analyzing high-frequency, low-power radio signals from the GPS satellites. Radio waves are electromagnetic energy, which means they travel at the speed of light (about 186,000 miles per second). The GPS receiver figures out how far the signal has traveled by timing how long it took to arrive. The satellites emit a long, digital pattern called a pseudo-random code and the receiver compares this pattern against its own. The length of the delay between the patterns is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond and require atomic clock accuracy. Atomic clocks cost somewhere between $50,000 and $100,000, which makes them much too expensive for everyday consumer use. The Global Positioning System has an effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. The receiver looks at incoming signals from four or more satellites and uses this information to gauge its own accuracy. The receiver is programmed to do this whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites. In order for the distance information to be of any use, the receiver also has to know where the satellites actually are. This isn't very difficult because the satellites travel in high and predictable orbits. The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. The Department of Defense constantly monitors the exact position of the satellites and transmits any adjustments to all GPS receivers as part of the satellites' signals. Once these calculations are made, a GPS receiver can tell you the latitude, longitude and altitude (or some similar measurement) of its current position. Taken over a period of time, information such as speed and direction of travel can be obtained. This data can then be superimposed on a map to provide a user friendly interface with extremely accurate and detailed information on the receivers’ present location, direction of travel, and history. |
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