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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.
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. A hydrazine propulsion system was used for orbital correction. The payload included two L band navigation signals at 1575.42 MHz (L1) and 1227.60 MHz (L2). Each spacecraft carried 2 rubidium and 2 cesium clocks, as well as nuclear detonation detection sensors, leading to a mass of 1,660 kilograms (3,660 lb).[5]
The first of the nine satellites in the initial Block II series was launched February 14, 1989; the last was launched October 1, 1990