In October 1957, the Soviet Union launched the world's first artificial satellite, "Sputnik." Scientists at Johns Hopkins University's Applied Physics Laboratory took a series of measurements of "Sputnik"'s doppler shift yielding the satellite's position and velocity. This team continued to monitor "Sputnik" and the next satellites into space, "Sputnik II" and "Explorer I". In March 1958 the idea of working backwards, using known satellite orbits to determine an unknown position on the Earth's surface began to be explored. This led to the "TRANSIT" satellite navigation system. The first "TRANSIT" satellite was placed in polar orbit in 1960. The system, consisting of 7 satellites, was made operational in 1962. A navigator using readings from three satellites could expect accuracy of about 80 feet.