The Global Positioning System (GPS), whose satellites replaced the Transit series, uses a web of 24 satellites in 12-hour orbits; additional satellites orbit in reserve. First satellite in the development series was launched in 1978. The first satellite in the second generation, for a worldwide GPS system, was launched in 1989, and the system began full operation in the 1990s. The third generation of satellites had its first launch in 2018. The satellites broadcast time and position messages continuously, and GPS receivers employ the more accurate triangulation method to determine position; the signals picked up by a GPS receiver can calculate position to within a few yards.
GPS receivers and navigation software may be incorporated into aviation and marine navigation equipment, built-in vehicle dashboard devices, smartphones, computer tablets, and other equipment. Standalone GPS devices are manufactured for use in vehicles and by hikers, but the increasing prevalence of GPS receivers and navigation software on smartphones offers a reasonable alternative to such units. The GPS system can also be used for nonnavigation purposes, such as surveying, tracking migrating animals, plotting the crop yields of small sections of farmland, and determining an individual's or vehicle's location or movements.
The Soviet Union (now Russia) established a Navstar-equivalent system known as the Global Orbiting Navigation Satellite System (GLONASS). Russia's GLONASS uses a similar number of satellites and orbits similar to those of the Navstar satellites. China's Beidou navigation satellite system began operations in 2011 with 10 satellites and now has 35 satellites, and the European Union and European Space Agency's Galileo system began operations in 2016 with 18 satellites and will eventually have 30 satellites.
The widespread use of GPS for navigation and location has led to the development of defense systems that interfere with GPS signals, disrupting the proper functioning of GPS equipment. The jamming tactic, which is known as GPS spoofing, uses strong, locally generated false GPS radio signals to overwhelm the ability of GPS equipment to find the satellite signals, resulting in unreliable GPS navigation.
See T. Logsdon, Understanding the Navstar: GPS, GIS, and IVHS (1995); B. Hofmann-Wellenhoff, Global Positioning System: Theory and Practice (1997).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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