Definition: Also known as Navstar GPS, an electronic device that may rapidly and accurately determine its own geographical location by receiving signals from a network of orbiting satellites.

Overview

There are a few dozen special-purpose satellites, launched and maintained by the US military, in constant Earth orbit. They contain extremely accurate clocks, programmed knowledge of their own orbits (which is constantly corrected by signals from ground stations), and high-frequency radio transmitters. These satellites transmit a constant stream of data, which describes the exact time and their position, and can easily be picked up by terrestrial receivers.

A GPS receiver listens to the stream of data from a number of these satellites, and from it, using relatively simple geometry, can calculate its own exact position (longitude, latitude, and elevation) to an accuracy of just a few feet. These receivers can be made very small (around one square inch) and moderately cheaply (tens of dollars). Their primary use is as an aid to navigation (in aeroplanes, in smart guided missiles, and in hand units for use by people), but they may also be embedded into larger systems. By comparing a sequence of positions, a GPS unit may also determine its own speed and direction of movement, and the relevant portion of a digitized map may be automatically displayed.