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GPS.gov: Space Segment
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Scientists create their own GPS by spying on internet satellites | Science | AAAS
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How is GPS used in spaceflight? | HowStuffWorks
What would happen if all satellites stopped working? - BBC Future
Look up: It's a satellite! | Human World | EarthSky
Satellite Navigation - GPS - How It Works | Federal Aviation Administration
Global Positioning System
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Space debris: what can we do with unwanted satellites?
