Global Positioning System

The GPS is owned and operated by the U.S. Department of Defense but is available for general use around the world. Briefly, here's how it works:

• 21 GPS satellites and three spare satellites are in orbit at 10,600 miles above the Earth. The satellites are spaced so that from any point on Earth, 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, it 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 "triangulates" its own position by getting bearings from three of the four satellites. 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.

The GPS is being used in science to provide data that has never been available before in the quantity and degree of accuracy that the GPS makes possible. Scientists are using the GPS to measure the movement of the arctic ice sheets, the Earth's tectonic plates, and volcanic activity.

GPS receivers are becoming consumer products. In addition to their outdoor use (hiking, cross-country skiing, ballooning, flying, and sailing), receivers can be used in cars to relate the driver's location with traffic and weather information. Here are some Web locations that describe GPS receiver products:

Read more about Global Positioning System: - DeLorme (http://www.delorme.com) sells the Tripmate, a receiver that attaches to a PC and shows its results inside a map display. - Trimble (http://www.trimble.com) sells the ScoutMaster Flash, a portable GPS receiver that can be carried by hikers.

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