Changes

== History ==

== History ==

When the [[Soviet Union]] launched the [[satellite]] [[Sputnik]] in 1957, American scientists of the [[Johns Hopkins University]] [[Applied Physics Laboratory]] used ''Sputnik's'' [[radio]]-signal's  [[Doppler Effect]] to track its position. They realized that a similar procedure for satellites with well known orbits would allow to calculate an unknown position on earth, and so the idea for the '''Transit''' was born: the first operational satellite-based navigation system. It consisted from seven low-altitude polar-orbiting satellites, and one of its main purposes was to provide navigation data for the [Submarine|Polaris missile submarine]]s. Though it had a couple of disadvantages, the ''Transit'' system was in use until 1996.

When the [[Soviet Union]] launched the [[satellite]] [[Sputnik]] in 1957, American scientists of the [[Johns Hopkins University]] [[Applied Physics Laboratory]] used ''Sputnik's'' [[radio]]-signal's  [[Doppler Effect]] to track its position. They realized that a similar procedure for satellites with well known orbits would allow to calculate an unknown position on earth, and so the idea for the '''Transit''' was born: the first operational satellite-based navigation system. It consisted from seven low-altitude polar-orbiting satellites, and one of its main purposes was to provide navigation data for the [[Submarine|Polaris missile submarine]]s. Though it had a couple of disadvantages, the ''Transit'' system was in use until 1996.

Another project was [[United States Air Force]]'s '''System 621B''', which - other than ''Transit'' - allowed for three-dimensional navigation. The [[United States Army|army]] had its own project: '''Army-SECO''', while [[United States Navy|navy]]'s '''Timation''' program used the most accurate clocks, deploying its first satellite in 1967 with [[quartz]]-[[crystal]] [[oscillator]]s, but developing [[atomic clock]]s for its last two satellites.

Another project was [[United States Air Force]]'s '''System 621B''', which - other than ''Transit'' - allowed for three-dimensional navigation. The [[United States Army|army]] had its own project: '''Army-SECO''', while [[United States Navy|navy]]'s '''Timation''' program used the most accurate clocks, deploying its first satellite in 1967 with [[quartz]]-[[crystal]] [[oscillator]]s, but developing [[atomic clock]]s for its last two satellites.

#first  cesium  frequency  standard  in  space

#first  cesium  frequency  standard  in  space

#verification  of  relativity theory

#verification  of  relativity theory

Until 1985, the eleven ''Block I'' satellites were built. This number was increased until 1989 by the ''Block II'' satellites. GPS today uses 24 satellites and numerous ground stations.  

Until 1985, the eleven ''Block I'' satellites were built. This number was increased until 1989 by the ''Block II'' satellites. GPS today uses 24 satellites and numerous ground stations.

 

==External Links==

==External Links==