| To explain how GPS system works - a minimum of 24 GPS satellites are in orbit at 20,200 kilometers (12,600 miles) above the Earth. The satellites are spaced so that from any point on Earth, at least four satellites will be above the horizon.
Each satellite contains a computer, an atomic clock, and a radio. With an understanding of its own orbit and the clock, the satellite continually broadcasts its changing position and time. (Once a day, each satellite checks its own sense of time and position with a ground station and makes any minor correction.)
On the ground, any GPS receiver contains a computer that calculates its own position by getting time signals from three of the four satellites, using a process called trilateration (similar to triangulation). The result is provided in the form of a geographic position - longitude and latitude - to, for most receivers, within 100 meters.
If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map.
If a fourth satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position.
If you are moving, your receiver may also be able to calculate your speed and direction of travel and give you estimated times of arrival to specified destinations. |
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