Definition

Global Positioning System (GPS)

Contributor(s): Moshe Peleg

The GPS (Global Positioning System) is a "constellation" of approximately 30 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location. The location accuracy is anywhere from 100 to 10 meters for most equipment. Accuracy can be pinpointed to within one (1) meter with special military-approved equipment. GPS equipment is widely used in science and has now become sufficiently low-cost so that almost anyone can own a GPS receiver.

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.

Mobile GPS technology has enabled today's smartphones with convenient and highly efficient means for end users to receive navigating instructions via a global positioning system process called "trilateration." A phone's built-in GPS receiver also communicates with an array of satellites which provides navigation instructions for those either in an automobile or on foot. More technologically advanced phones can identify individual streets and attractions on maps, as well as provide narrated tracking capability. 

This was last updated in July 2016

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