This 1967 U.S. Navy training film describes the Transit satellite navigation system, which went into service in 1964, providing global coverage with position fixes every several hours at the equator and more frequently at higher latitudes. Its original mission was providing position information to Polaris ballistic missile submarines, and was later adopted by a wide variety of surface ships. The architecture was very different from GPS, and relied upon Doppler measurement of transmissions from a constellation of five satellites in low polar orbit. Each satellite transmitted an ephemeris of its position, updated daily from a computer centre on the ground and a network of tracking stations, from which shipboard computers could calculate position based upon the observed Doppler shift. Only one satellite was needed to provide 2D (latitude and longitude) accurate to originally 400 metres but eventually improved to around 200 metres as the system was refined. The Transit system remained in service until 1996, when it was retired in favour of the Global Positioning System (GPS), which had become fully operational (continuous global coverage) the preceding year, but since the 1980s had provided better and more frequent 3D position fixes than Transit.