KH-9 (BYEMAN codename HEXAGON), commonly known as Big Bird or KeyHole-9,[1] was a series of photographic reconnaissance satellites launched by the United States between 1971 and 1986.

The highest ground resolution achieved by the main cameras of the satellite was 2 ft (0.61 m),[3] though another source says "images in the "better-than-one-foot" category" for the last "Gambit" missions.

[1] The satellites were an important factor in determining Soviet military capabilities and in the acquisition of accurate intelligence for the formulation of U.S. national policy decisions as well as deployment of U.S. forces and weapon systems.

The satellites were instrumental in U.S. National Technical Means of Verification of Strategic Arms Limitation Talks (SALT) and the Anti-Ballistic Missile Treaty (ABMT).

Lockheed Missiles and Space Company (LMSC) was awarded the contract for the Satellite Basic Assembly (SBA), Perkin Elmer for the primary Sensor Subsystem (SS), McDonnell for the Reentry Vehicle (RV), RCA Astro-Electronics Division for the Film Take-Up system, and Itek for the Stellar Index camera (SI).

The USS Trieste II (DSV-1) retrieved its payload in April 1972 after a lengthy search, but the film had disintegrated during the nine months underwater, leaving no usable photographs.

[10] The Satellite Control Section (SCS), which forms the aft part of the SBA, started as Air Force Project 467.

SCS was intended as a more capable replacement for the on-orbit propulsion, which had been provided by the Agena upper stage for previous generations of reconnaissance satellites.

The SCS incorporated a freon gas system for backup attitude control inherited from the Agena, commonly referred to as "lifeboat".

[15] SCS was equipped with deployable solar panels and an unfurlable parabolic antenna for high data rate communication.

The system aperture is defined by a 20 in (510 mm) diameter aspheric corrector plate, which corrects the spherical aberration of the Wright design.

[15] The forward section of KH-9 housed four McDonnell Douglas Mark 8 satellite reentry vehicles (RV), which were fed film exposed by the main cameras.

[15] Missions 1205 to 1207 carried Doppler beacons[24] to help map the atmospheric density at high altitudes in an effort to understand the effect on ephemeris predictions.

[27] Missions 1203, 1207, 1208, 1209, and 1212 to 1219 included Ferret ELINT sub-satellites, which were launched into a high Earth orbit to catalogue Soviet air defence radars, eavesdrop on voice communications, and tape missile and satellite telemetry.