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

President Eisenhower authorized the Corona project in 1958 to photograph Earth from space to keep tabs on Russia and China. The idea was simple, even if the execution was not: Satellites with one (and later two) cameras loaded with 70mm film would orbit the planet, snapping pictures and periodically ejecting a pod of exposed film. As the film fell toward earth beneath a parachute, a passing plane would snatch it out of the sky.

As crazy as it sounds, the system worked. Eventually. After 13 failed launches, Corona flew its first successful mission on August 18, 1960, when it photographed 1.6 million square miles of Soviet territory. The images proved the Russians were greatly exaggerating their intercontinental ballistic missile capabilities.

Over the next decade, the Air Force, the CIA, and private contractors continued refining the technology. Among other things, Corona revealed a Soviet bomber base 400 miles west of Nome, Alaska, mapped Russia's 25 long-range missile bases, and revealed when and where China would conduct its first nuclear explosion. It also photographed much of the United States, images that provided the basis for many government maps.

As to those enormous crosses, the government built more than 250 of them in 1967 to measure the scale of objects in the photographs. You can think of the concrete and stone markers as a ruler visible from space. Although the feds removed many crosses when Corona gave way to Landsat, more than 100 remain. “They’re unsung monuments to the Cold War," Anand says.

Anand and Sauer live in Phoenix and heard about the crosses in 2011. Two years later, they decided to make a trip into the desert, where they found one cross along a lonely highway and another on Casa Grande Mountain. Intrigued, they did a little research and started mapping the remaining crosses on Google Earth. They've photographed more than 40 so far.

They trek into the desert each spring and fall when the weather is mild, hauling a medium format camera, tripods, and a jib. They shoot as many as 12 images of a single cross, stitching them together in Photoshop to create a high-resolution image. Then they plug the GPS coordinates and altitude, along with the date, time, direction, and angle of the photograph, into the satellite mapping system SkySafari. That provides a list of satellites orbiting overhead. Anand and Sauer use that data to draw arcs representing each satellite’s trajectory. "It's making a technology that was invisible, visible," Anand says.

Their sweeping images juxtapose the past and present in a reminder of how ubiquitous satellite technology, once the stuff of science fiction, is today.