BACKGROUND ON GPS
Global Positioning System History
http://www.nasa.gov/directorates/heo/scan/communications/policy/GPS_History.html
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GPS has its origins in the Sputnik era when scientists were able to track the satellite with shifts in its radio signal known as the "Doppler Effect." The United States Navy conducted satellite navigation experiments in the mid 1960's to track US submarines carrying nuclear missiles. With six satellites orbiting the poles, submarines were able to observe the satellite changes in Doppler and pinpoint the submarine's location within a matter of minutes.
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In the early 1970's, the Department of Defense (DoD) wanted to ensure a robust, stable satellite navigation system would be available. Embracing previous ideas from Navy scientists, the DoD decided to use satellites to support their proposed navigation system. DoD then followed through and launched its first Navigation System with Timing and Ranging (NAVSTAR) satellite in 1978. The 24 satellite system became fully operational in 1993.
Initial Satellites: Block II series
The Block II satellites were the first full scale operational GPS satellites, designed to provide 14 days of operation without any contact from the control segment. The prime contractor was Rockwell International, which built a SVN 12 qualification vehicle after an amendment to the Block I contract. In 1983 the company was awarded an additional contract to build 28 Block II/IIA satellites.
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Block II spacecraft were 3-axis stabilized, with ground pointing using reaction wheels. Two solar arrays supplied 710 watts of power, while S band communications were used for control and telemetry. A UHF channel was use for cross-links between spacecraft.
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The first of the nine satellites in the initial Block II series was launched February 14, 1989; the last was launched October 1, 1990. The final satellite of the series to be taken out of service was decommissioned on March 15, 2007, well past its 7.5 year design life.