In early 1977, the new president, Jimmy Carter, and his CIA director, Adm. Stansfield Turner, were both fascinated by spy satellites, especially the revolutionary KH-11 that transmitted electronic images directly to the ground, rather than using the cumbersome previous method, in which film canisters were ejected from a satellite and captured by airplanes on descent.

The KH-11 images could be seen in real time instead of days or weeks later. “It was a marvelous system,” Turner later recalled, “much like a TV in space that sent back pictures almost instantly.”

But satellites had their limitations in collecting intelligence. They could count missile silos and track military equipment. But they could not see inside a file cabinet, or inside the mind of a Politburo member. George J. Tenet, who was CIA director in the Clinton years, recalled: “From the mid-1960s on to the Soviet collapse, we knew roughly how many combat aircraft or warheads the Soviets had, and where. But why did they need that many or that kind? What did they plan to do with them? To this day, intelligence is always much better at counting heads than divining what is going on inside them.”

This is why Adolf Tolkachev was so valuable as an agent for the United States, a radar engineer working deep inside the Soviet military-industrial complex. His story is told in my new book, “The Billion Dollar Spy: A True Story of Cold War Espionage and Betrayal.”

In the secret documents which he copied with a simple Pentax 35mm camera clamped to the back of a chair, Tolkachev opened a window on Soviet research and development plans a decade into the future. He provided the kind of intelligence that no satellite could ever hope to capture.

Espionage can be split into “operational intelligence” – carrying out the spying – and “positive intelligence,” the secrets that are stolen.

Tolkachev’s “positive intelligence” was a goldmine, particularly for the U.S. Air Force.

On June 17, 1980, Tolkachev met his CIA case officer, John Guilsher, in Moscow. He gave Guilsher 179 rolls of 35mm film containing documents about Soviet airborne radars and weapons control systems. According to a CIA memo, Tolkachev’s delivery included the first documentation about a new Soviet Airborne Warning and Control system, or AWACS, and extensive information about a new modification of the MiG‑25 high-altitude interceptor. And, the memo said, Tolkachev had documented “several new models of airborne missile systems and technical characteristics of other Soviet fighter and fighter/bomber aircraft to be deployed between now and 1990.”

Tolkachev was providing a road map to the United States for compromising and defeating two critical Soviet systems: the radars on the ground that defended the country from attack, and the radars on warplanes that gave it capacity to attack others. His espionage put the United States in position to dominate the skies in aerial combat and confirmed the vulnerability of Soviet air defenses—that in the event of any war, American cruise missiles and bombers could fly under the radar.

The evaluations of Tolkachev’s material from the U.S. military were glowing. They said he had provided the “first information” and “only information” about certain Soviet weapons systems; that “time saved on research and development of U.S. countermeasures to these systems has been reduced by [a] minimum of 18 months, for one system as much as five years,” and his material had led to a 180-degree turnabout in a $70 million radar. In addition to blueprints, documents and schematics, Tolkachev twice turned over to the CIA actual circuit boards from Soviet radar design projects.

At one point, the CIA asked the Air Force to estimate what Tolkachev’s intelligence was worth, in a broad way. Could they put a dollar amount on how much they had saved in research and development costs? The answer came back: “somewhere in the neighborhood of $2 billion.”

That was before they even looked at the 179 rolls of film delivered to Guilsher in the briefcase.