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MERCURY Geostationary Collection Satellite

MERCURY is the public name most often used for the last major generation of the Chalet/Vortex geostationary SIGINT satellites. This entry explains what MERCURY appears to have been, why Menwith Hill mattered to it, how it differed from the Pine Gap Orion line, and why it remains one of the least fully acknowledged but most important overhead collection systems in public intelligence history.