The grids of zigzagging white lines seen in two of the images — the strangest of the various desert structures — are spy satellite calibration targets. Satellite cameras focus on the grids, which measure approximately 0.65 miles wide by 1.15 miles long, and use them to orient themselves in space. [Gallery: Mysterious Structures In China’s Gobi Desert]

The existence of these calibration targets may seem suspicious or revelatory, but Hill said it really isn’t; China was already known to operate spy satellites, and many other countries (including the United States) do so as well. In fact, the U.S. also uses calibration targets. “An example I found just now is a calibration target for the Corona spy satellites, built back in the 1960s, down in Casa Grande, Ariz., [at coordinates] 32° 48′ 24.74″ N, 111° 43′ 21.30″ W,” Hill told Life’s Little Mysteries, a sister site to LiveScience…

Another strange image taken not far away shows a Stonehenge-like arrangement of objects radiating outward, with fighter jets parked at its center. “This is almost certainly a calibration/test target for orbital radar instruments,” Hill said. “Since a significant amount of radar return is due to differences in surface roughness, they’re probably testing ways of making the areas around planes ‘bumpy’ enough that the planes are partially masked.”