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.