Radionavigation systems using artificial satellites as sources of radio signals and position references. Development of satellite-based systems for global positioning and navigation began almost immediately after the launch of Sputnik I by the Soviet Union in 1957. The U.S. Navy Navigation Satellite System, better known as Transit, became operational in 1964. The Soviet Union responded with its own satellite navigation systems, Parus and Tsikada. All three systems were based on measurement of Doppler shift in signals received from a satellite, and were aimed mainly at offering navigational guidance to ships and submarines.