How it began
In November 1972, Air Force Col. Bradford Parkinson was tasked with overseeing the satellite navigation program. He led a team that combined the best aspects of three earlier efforts, Transit, Timation and Project 621B. The proposal received Department of Defense approval in December 1973. In 1974 the Air Force began developing the first Navstar satellites, the ground control system and military user equipment. The first developmental satellite was launched in February 1978, with three more by the end of that year.
Before it existed
After the Soviet Union launched Sputnik in 1957, the United States developed Transit, the first global satellite navigation system. The Navy's Timation satellites later proved that accurate clocks could be placed in space. Ground-based radio navigation systems such as LORAN and the Decca Navigator dated from the early 1940s. Limitations of these systems drove the need for a more accurate and universal solution.
How it works
The system approved in 1973 was a passive, one-way ranging system of 24 satellites. Each carries atomic clocks and travels in a medium Earth orbit with a 12-hour period. Receivers only listen to the satellites' signals and do not transmit. The system was designed to offer two services, a Standard Positioning Service for the public and a Precise Positioning Service reserved for the military.
How it spread
In 1983 President Ronald Reagan authorized the use of Navstar by civilian commercial airlines to improve navigation and safety. The move followed the loss of Korean Air Lines Flight 007, which was shot down after straying into prohibited airspace. The full constellation of 24 satellites was available by December 1993, and full operational capability was declared in 1995. President Bill Clinton ended the deliberate degrading of the civilian signal on May 1, 2000, which increased its accuracy fivefold.
Where it stands today
GPS is owned and operated by the United States government. By 2000 it was recognized that the system needed to be modernized to meet expanding military and civilian uses, and newer generations of satellites have followed. The Aerospace Corporation describes uses ranging from telling the time on a phone or computer to guiding self-driving cars.
