navigation satellite, artificial satellite designed expressly to aid the navigation of sea and air traffic. Early navigation satellites, from the Transit series launched in 1960 to the U.S. navy's Navigation Satellite System, relied on the Doppler shift. Based on the shift in the satellite's frequency, a ship at sea could accurately determine its longitude and latitude.

The Global Positioning System (GPS), whose satellites replaced the Transit series, uses a web of 24 Navstar satellites in 12-hour orbits. The first Navstar satellite was launched in 1989, and the system began full operation in the 1990s. 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 former Soviet Union 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, or Compass, navigation satellite system began operations in 2011 with 10 satellites, and the European Union also is developing a navigation satellite system.

See T. Logsdon, Understanding the Navstar: GPS, GIS, and IVHS (1995); B. Hofmann-Wellenhoff, Global Positioning System: Theory and Practice (1997).