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It is a tremendous year for GPS. On May 2, 2000 the White House freed up a civilian GPS signal, with accuracy to 3 meters. There was jubilation, as it officially had until 2006 to do so. A multi-billion dollar industry was launched. Satellite constellations are becoming international. Most people are aware of GPS as consumer items now, maybe own a device, or have heard of earthquake monitoring and environmental applications. The commercial applications are integrated into mobile phone platforms, transport industries, communications, with the the iddy-biddiest of receivers. It's quite the decade. [GPS: Global Positioning System]

It starts with atomic clocks...

The clocks were created by physicists for both scientific and military purpose, with no conception at the time that their technology would one day lead to a global system of navigation. The current GPS constellation consists of 32 Block II/IIA/IIR/IIR-M satellites.

Where Am I ?

The GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude).

International Definitions

GPS: Global Positioning System (US), NAVSTAR* program
GLONASS (Russia)
COMPASS/Beidou (China)
INSAT (India)
QZSS (Japan)
GNSS: generic, international term 'Global Navigation Satellite System'
ICG: International Committee on GNSS

GPS: A Brief Atomic Clock Timeline

GPS World's article in the May 2010 issue, "The Origins of GPS: And the Pioneers Who Launched the System," sheds new light on this traditional history of GPS.

1938 Between 1938-1940, I.I. Rabi invents molecular-beam magnetic resonance at Columbia University in 1938. He and his colleagues apply magnetic resonance to fundamental studies of atoms and molecules. Possibility of atomic clock to measure gravitational red shift is discussed. Rabi is awarded the Nobel Prize for this work in 1944.
1949 Norman Ramsey invents separated-oscillatory-field resonance method at Harvard University, for which he was awarded the Nobel Prize in 1989. Jerrold Zacharias proposes using Ramsey's method to create cesium-beam "fountain" clock that would be accurate enough to measure gravitational red shift.
1949 National Bureau of Standards operates atomic clock based on microwave absorption in ammonia gas. Work starts on cesium-beam atomic clock.
1954 Charles Townes at Columbia University demonstrates operation of the first maser based on emission of radiation from ammonia molecules. Townes shared the 1964 Nobel Prize in physics.
1954 Between 1954-1956, Zacharias and National Company develop the first self-contained portable atomic clock, the Atomichron.
1957 Sputnik is launched in October by the Soviet Union. Satellite Doppler tracking is inaugurated at MIT Lincoln Laboratory and Johns Hopkins Applied Physics Laboratory (APL). Navy Transit program is started at APL in December.
1959 Albert Kastler and Jean Brossel, working in Paris and at MIT, develop methods of optical pumping. Kastler is awarded the Nobel Prize for this work.
1960 Ramsey and students Kleppner and Goldenberg operate hydrogen maser at Harvard University.
1960 Between 1960-1965, rubidium optically pumped clock is introduced. Cesium frequency standards are installed in most international time-standard laboratories.
1964 Between 1964-1965, first position fix from a Transit satellite is computed aboard Polaris submarine.
1967 Transit system is made available to civilian community.
1968 Standards of a Defense Navigation Satellite System are defined.
1973 Development of Navstar GPS is approved by the Department of Defense.
1974 First GPS test satellite, from Timation program, is launched to test rubidium clocks and time-dissemination techniques.
1977 Test satellite incorporating principal features of later GPS satellites, including first cesium clocks in space, is launched.
1978 Between 1978-1985, ten prototype GPS satellites are launched, built by Rockwell International.
1989 Between 1989-1993, series of 24 satellites are launched at about 6 per year.
1993 Final GPS satellite is launched on June 26, 1993.
1996 White House announces that a higher level GPS accuracy will be available to everyone.

*NAVSTAR, the official U.S. Department of Defense name for GPS.


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