The Global Positioning System is a global navigation satellite system (GNSS) developed by the United States Department of Defense and managed by the United States Air Force 50th Space Wing.
It is the only fully functional GNSS in the world. It can be used freely, and is often used by civilians for navigation purposes. After it became fully operational in 1993, GPS has been used as an aid to navigation worldwide. It is also a useful tool for map-making, land surveying, commerce, scientific uses, and hobbies such as geo-caching. Also, the precise time reference is used in many applications. For instance, the scientific study of earthquakes. GPS is also an essential synchronization resource of cellular networks, like the Qualcomm CDMA air interface used by many wireless carriers in several countries.
The first satellite navigation system, Transit, was used by the United States Navy. It was first successfully tested in 1960. Seven years later, the U.S. Navy developed the Timation satellite. With this satellite, the ability to place accurate clocks in space was acquired. It is this technology that GPS relies on. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first worldwide radio navigation system. The design of GPS is based partly on similar ground-based radio navigation systems, like LORAN and the Decca Navigator which were developed in the early 1940s, and used during World War II. Further inspiration for the GPS was provided by the launch of the first Sputnik by the then Soviet Union in 1957.
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In 1983, Korean Air Lines Flight 007 was shot down after it strayed into the USSR’s prohibited airspace. After this incident, President Ronald Reagan issued a directive making GPS freely available for civilian use. The satellites were launched between 1989 and 1993. Initially the highest quality signal was reserved for military use. The signal available for civilian use was intentionally degraded (“Selective Availability”, SA). Selective Availability was phased out in 2000. This improved the precision of civilian GPS. One important factor for the function of GPS is the placement of atomic clocks in the satellites. This was first proposed by Friedwardt Winterberg in 1955.
In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive declaring GPS to be a dual-use system. He also established an Interagency GPS Executive Board to manage it as a national asset. The most recent launch was on March 15, 2008. The oldest GPS satellite still in operation was launched on November 26, 1990, and became operational on December 10, 1990.