The KH-9, also known as Hexagon or "Big Bird," was a large reconnaissance satellite designed to replace the earlier KH-4B Corona satellites. Unlike its predecessors, it utilized a new maneuverable bus called SCS (Satellite Control Section) and featured four return capsules for extended operational lifespan. Manufactured by McDonnell Douglas, the Mk.8 return capsules facilitated mid-air recovery of film capsules, similar to Corona satellites. The camera system, built by Perkin-Elmer, comprised two panoramic cameras for area surveillance, capturing images at altitudes ranging from 90 to 200 miles. These cameras utilized an f/3.0 folded Wright Camera optical layout with a 1.5 m focal length and achieved ground resolutions better than 0.61 m, scanning areas up to 120° wide.

S3 (Small Secondary Satellites) were launched as piggy back payloads on some KH-9 spysat launches. Each one had his own solid fuel motor to perform injection into an higher orbit.