Tales of espionage are filled with lanky men in trenchcoats walking through cold Berlin streets at the height of the Cold War. But the most important intelligence—in terms of volume and reliability—was gathered by reconnaissance satellites far overhead. These satellites were precise, they collected vast amounts of information, and unlike spies, they did not forget, embellish, lie, or go rogue. Photographic reconnaissance satellites like CORONA, GAMBIT, HEXAGON, and KENNEN were in many ways the most prolific spooks. But they were also accompanied by other satellites, signals intelligence, or SIGINT, satellites that listened for the electronic whispers of radars and radios, engaged in a high-tech war of electrons against an enemy that could vanish and emerge at will.
During the Cold War the United States intelligence community gathered signals intelligence from the Soviet Union via a variety of means. These included ground stations, cable-tapping and bugging operations, airborne platforms such as the RC-135 Rivet Joint and RB-47 Stratojet, and signals intelligence satellites. Any history of SIGINT satellite operations during the Cold War is going to be limited in scope because much of the story remains classified, and unlike the reconnaissance photographs, signals intelligence is an arcane and esoteric subject.