Brief History about GPS
The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first worldwide radio navigation system. GPS requires the equations of general relativistic corrections to these atomic clocks in satellites for sufficient accuracy. The general relativistic equations correcting signals of atomic clocks in satellites were first published in 1956 by Friedwardt Winterberg.
The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. Additional inspiration for the GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik’s radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.
Initially the highest quality signal was reserved for military use, while the signal available for civilian use was intentionally degraded (”Selective Availability”, SA). Selective Availability was ended in 2000, improving the precision of civilian GPS from about 100m to about 20m. In a contemporary sense, GPS is used often in vehicles and portable devices like the Bushnell Handheld GPS.