| GPS Block 2 and 2A |
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The Navstar GPS constellation worked in concert with ground receivers to give precise location information to military and civilian users anywhere in the world. Use of GPS receivers revolutionized civilian and military operations of every kind by providing precise location information. GPS Block 2 was the Operational system, following the Demonstration system comprised of Block 1 spacecraft. The Block 2A were "Advanced" versions of this spacecraft (mass 1816 kg). The complete basic constellation had 24 spacecraft in 6 high-altitude orbit planes. (4 slots per plane). In addition, on-orbit spares were placed in some planes in a fifth slot. The spacecraft was 3-axis stabilized, nadir pointing using reaction wheels. Dual solar arrays supplied 710 watts (EOL). S-Band (SGLS) communications were used for control and telemetry. A UHF channel provided 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. Also carried were nuclear detonation detection sensors. Launches were originally scheduled for Proton, Atlas, and Delta. Satellite costs $ 50 million for Block 2 and $ 40 million for Block 2A. Rockwell was the prime contractor, building a single Block 2 SVN 12 qualification vehicle under a 1981amendment to the Block I contract. This was followed in 1983 with a contract for 28 Block 2 and 2A satellites. Design Life: 7.5 years. Typical orbit: 20,200 km / 12 hr period; 55 deg inclin; 6 planes. Length: 3.40 m (11.10 ft). Span: 5.30 m (17.30 ft). Mass: 1,665 kg (3,670 lb). Main Engine: Star 27. Electrical System: 2 solar panels. Electric System: 0.71 average kW. Associated Launch Vehicle: Delta 6000, Delta 7000. GPS Block 2 and 2A Chronology
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