GPS

GPS, in full Global Positioning System, space-based radio-navigation system that broadcasts highly accurate navigation pulses to users on or near Earth. In the United States’ Navstar GPS, 24 main satellites in 6 orbits circle Earth every 12 hours. In addition, Russia maintains a constellation called GLONASS (Global Navigation Satellite System), and in 2007 the European Union approved financing for the launch of 30 satellites to form its own version of GPS, known as Galileo, which began operations in 2016. China launched two satellites in 2000 and another in 2003 as part of a local navigation system first known as BeiDou (“Big Dipper”). In 2006 China, which had a limited participation in Galileo, announced plans to expand BeiDou to a full GPS service known as the BeiDou Navigation System. In 2007 China began launching a series of 14 second-generation satellites, known as BeiDou-2, or Compass, to provide services in China. A third-generation constellation of 30 satellites, BeiDou-3, was completed in 2020 and provides global service.

A GPS receiver operated by a user on Earth measures the time it takes radio signals to travel from four or more satellites to its location, calculates the distance to each satellite, and from this calculation determines the user’s longitude, latitude, and altitude. The U.S. Department of Defense originally developed the Navstar constellation for military use, but a less precise form of the service is available free of charge to civilian users around the globe. The basic civilian service will locate a receiver within 10 metres (33 feet) of its true location, though various augmentation techniques can be used to pinpoint the location within less than 1 cm (0.4 inch). With such accuracy and the ubiquity of the service, GPS has evolved far beyond its original military purpose and has created a revolution in personal and commercial navigation. Battlefield missiles and artillery projectiles use GPS signals to determine their positions and velocities, but so do the U.S. space shuttle and the International Space Station as well as commercial jetliners and private airplanes. Ambulance fleets, family automobiles, and railroad locomotives benefit from GPS positioning, which also serves farm tractors, ocean liners, hikers, and even golfers. Many GPS receivers are no larger than a pocket calculator and are powered by disposable batteries, while GPS computer chips the size of a baby’s fingernail have been installed in wristwatches, cellular telephones, and personal digital assistants.