KH-7 Gambit
Codenamed Gambit, the KH-7 (Air Force Program 206) was a reconnaissance satellite used by the United States from July 1963 to June 1967. Like the older CORONA system, it acquired imagery intelligence by taking photographs and returning the undeveloped film to earth. It achieved a typical ground-resolution of 2 ft (0.61 m) to 3 ft (0.91 m). Though most of the imagery from the KH-7 satellites was declassified in 2002, details of the satellite program (and the satellite's construction) remained classified until 2011.
Read more about KH-7 Gambit: System Configuration, Mission, Functionality, ELINT Subsatellite, List of Launches, History, Cost, Other U.S. Imaging Spy Satellites