Spy Satellite Calibration Crosses

via wired.com 
Venture into the Sonoran Desert about an hour south of Phoenix 

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and you’ll eventually stumble upon a concrete cross. More than 100 dot the terrain, each of them 60 feet across and spaced precisely one mile apart. The government used them to calibrate the world’s first spy satellites as they peered down on Russia and China while photographing more than 750 million square miles of the planet.

The 95 satellites of the once top-secret Corona project sent their last images in 1972, but the calibration markers still stand in a grid that once measured around 16 square miles. “It’s such a massive thing in terms of its scale, but it pales in comparison to the scale of history that it’s a part of,” says Julie Anand. She and Damon Sauer have spent the better part of three years photographing the markers and mapping the satellites that pass over them for Ground Truth: Corona Landmarks.