Landscapes Made for Satellite Eyes: Calibration Targets, Resolution Tests, and Giant Desert Compasses
In 2011, when Google Earth hit the scene, armchair explorers began discovering all kinds of strange stuff scattered across the globe. Seen from space the planet could be very weird indeed. One category that has launched a thousand conspiracy theories are the enormous patterns, pictures, and measuring tools carved or painted directly onto the landscape. Many of these are enclosed within military compounds or are far out in inhospitable terrain, lending them an even greater air of mystery.
A few years ago Gizmodo ran a slightly feverish roundup of user-submitted photos of large patterns and structures in Chinese deserts: an 18-mile-long grid, a bright blue decoy airport, a 1:20 scale model of border terrain, a massive series of squiggles that had a clearly discernible pattern.
Satellite calibration targets in the Gobi Desert (Google Maps image via the Blaze)
Many guesses were presented as to what these were for — including that they were a QR code to be read by aliens — but according to a researcher at the Mars Space Flight Facility (which operates space cameras for NASA), this one, at least, is merely a satellite calibration target. These were created to give aircraft-mounted cameras something on which to focus their lenses, allowing them to test their resolution and ability to take clear pictures at high speeds. The resulting properly calibrated satellites could then go on to do the work for which they were meant: spying.
Not that China was alone in that pursuit. In the 1950s and 1960s, the United States’ Corona Satellite System included 144 satellites, many of which were used to track weapons development and nuclear threats from other countries, primarily China and Russia. Due to the technology of the time, these satellites required a system of fixed points to focus their lenses on, so a series of huge concrete targets were poured throughout the Arizona desert by the US Army Map Service, which leased the land ostensibly for office space.
Corona Satellite Calibration Targets, Casa Grande, Arizona (image via Google Maps)
There are many other kinds of calibration targets, some acting like a giant eye chart for airplanes and other spacecraft. This type has tri-bar markings in decreasing sizes; the smallest one that the satellite can focus on indicates the greatest distance it can go and still take discernible pictures. These too are scattered throughout the United States, with the highest concentration at Edwards Air Force Base in Southern California, and many more on airport runways and in deserts from Florida to Arizona.
Tri-bar satellite calibration targets in Fort Huachuca, Arizona (image via ilikethisart.net)
Of course, satellites by now have plenty of uses other than covert operations. Environmental scientists rely on them for many things, such as measuring the changing level of the ice shelf in polar regions. In order to be able to determine the change, satellite-based altimeters need a stable reference point that hasn’t risen or fallen since the last time the satellite looked at it. Such as, for example, the Salar de Uyuni.
Salar de Uyuni (image via Alfredobi / Wikimedia)
Once part of a giant prehistoric lake, the massive salt field — nearly 4,000 square miles, about the size of New Jersey — is the one of the world’s flattest places. In 2002 the entire expanse was mapped with both fixed and kinematic GPS, yielding a topographical precision accurate to 2.2 centimeters which is a perfect reference point for satellite calibration.
Similar to the Bolivian salar, the Railroad Valley in Nevada is “flat, bright, spatially uniform, and spectrally stable over time,” and it has also proven to be an excellent reference calibration site for satellites and other “large footprint sensors.” It was used to calibrate ground temperature measurements in 2000, to measure solar radiation in 2004, and to measure Japan’s greenhouse gas emissions in 2009.
Railroad Valley, Nevada (image via J Brew / Flickr)
The majority of the decades-old calibration targets are now technologically obsolete, and many have been destroyed. Those that remain have become relics and curiosities, conspiracy-theory fodder and the stuff of legends. Some do receive the respect they deserve, though; one of the oldest, which was built in the 1930s for use by airplane pilots long before satellites existed, is the compass rose painted onto the enormous Rogers Dry Lake at Edwards Air Force Base in California.
World’s Largest Compass Rose (image via NASA / Wikimedia)
It’s the largest compass rose in the world, boasting a diameter of more than 4,000 feet, and is accurately inclined toward magnetic north. To honor the fact that when it was made, it was at the very cutting edge of aerospace and aviation technology, the rose was designated a National Historical Landmark in 1985.
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