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Senior Airman Trey Barnes, 2nd Space Operations Squadron space systems operator, sits in front of a model of a Block IIF GPS-satellite in the Master Control Station on Schriever Air Force Base, Colo., Sept. 4, 2014. Barnes maintains contact with more than 30 satellites to ensure the overall accuracy of the GPS signal for the more than 3 billion users around the world. (U.S. Air Force photo/Airman 1st Class Krystal Ardrey)
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