What is this strange structure in the Arizona desert?

Normally when you see something bizarre like this on Google Earth, you want to believe it's for secret spy stuff. But the skeptic in you says, "Naw, this is just for irrigation." In this case, however, your spy dreams would be right.

This is an "aerial photo calibration target," and there are dozens of them all over the United States. People built them during the 1950s and 60s and they were used for testing aerial photography and surveillance aircraft. That's right — this is basically a test pattern for spies. Below, you can see another one in Maryland.

What is this strange structure in the Arizona desert?

According to the Center for Land Use Interpretation:

Most of them follow the same general form established by the Air Force and NASA (and prior to 1958, its precursor agency, NACA): a concrete or asphalt pad constructed flat on the ground, 78 feet by 53 feet, coated in a heavy black and white paint. The pattern painted on the targets is sets of parallel and perpendicular bars duplicated at 15 or so different sizes, and, sometimes, a large white square. The configuration is sometimes referred to as a 5:1 aspect Tri-bar Array, and follows a similar relative scale as a common resolution test chart known as the 1951 USAF Resolving Power Test Target, conforming to milspec MIL-STD-150A. This test pattern is still widely used to determine the resolving power of microscopes, telescopes, cameras, and scanners.

The targets function like an eye chart at the optometrist, where the smallest group of bars that can be resolved marks the limit of the resolution for the optical instrument that is being used. For aerial photography, it provides a platform to test, calibrate, and focus aerial cameras traveling at different speeds and altitudes. The targets can also be used in the same way by satellites.

Many of these resolution test targets are found in the Mojave desert of California, one of the principal development and test areas for surveillance aircraft. of the most sophisticated aircraft made by the nation, like the A12 and SR-71 Blackbird and the U-2, were unarmed, and designed to be used only as flying cameras. The photo targets were most certainly used by these planes.

What is this strange structure in the Arizona desert?

This is an up-close view of an aerial photo calibration target in California. Many of these landmarks are still in use today. (Spotted on Google Earth Blog!)