Corona (satellite) - Technology

Technology

The Corona satellites used special 70 millimeter film with a 24-inch (61 cm) focal length camera. Manufactured by Eastman Kodak, it was initially 0.0003 inches (0.0076 mm) thick, with a resolution of 170 lines per 0.04 inches (1.0 mm) of film. The contrast was 2-to-1. (By comparison, the best aerial photography film produced in World War II could produce just 50 lines per 0.04 inches (1.0 mm) of film.) The acetate-based film was later replaced with a polyester-based film stock that was more durable in outer space. The amount of film carried by the satellites varied over time. Initially, each satellite carried 8,000 feet (2,400 m) of film for each camera, for a total of 16,000 feet (4,900 m) of film. But a reduction in the thickness of the film stock allowed more film to be carried aloft. The amount of film carried was doubled (both by reductions in film thickness and by the addition of additional film capsules) by the time the fifth generation of the satellite (the KH-5s) was built, to 16,000 feet (4,900 m) of film for each camera for a total of 32,000 feet (9,800 m) of film. Most of the film shot was black and white. Infrared film was used on mission 1104, and color film on missions 1105 and 1008. Color film proved to have lower resolution, however, and was never used again.

The cameras were manufactured by the Itek Corporation. A 12-inch (30 cm), f/5 triplet lens was designed for the cameras. Each lens was 7 inches (18 cm) in diameter. They were quite similar to the Tessar lenses developed in Germany by Zeiss. The cameras themselves were initially 5 feet (1.5 m) long, but later extended to 9 feet (2.7 m) in length. Beginning with the KH-4 satellites, these lenses were replaced with Petzval f/3.5 lens. The lenses were panoramic, and moved through a 70° arc perpendicular to the direction of the orbit. A panoramic lens was chosen because it could obtain a wider image. Although the best resolution was only obtained in the center of the image, this could be overcome by having the camera sweep automatically ("reciprocate") back and forth across 70° of arc. The lens on the camera was constantly rotating, to counteract the blurring effect of the satellite moving over the planet.

The first Corona satellites had a single camera, but a two-camera system was quickly implemented. The front camera was tilted 15° forward, and the rear camera tilted 15° aft, so that a stereoscopic image could be obtained. Later in the program, the satellite employed three cameras. The third camera was employed to take "index" photographs of the objects being stereographically filmed. The J-3 camera system, first deployed in 1967, placed the camera in a drum. This "rotator camera" (or drum) moved back and forth, eliminating the need to move the camera itself on a reciprocating mechanism. The drum permitted the use of up to two filters and as many as four different exposure slits, greatly improving the variability of images that Corona could take. The first cameras could resolve images on the ground down to 40 feet (12 m) in diameter. Improvements in the imaging system were rapid, and the KH-3 missions could see objects 10 feet (3.0 m) in diameter. Later missions would be able to resolve objects just 5 feet (1.5 m) in diameter. A single mission was completed with a 1 foot (0.30 m) resolution but the limited field of view was determined to be detrimental to the mission. 3 feet (0.91 m) resolution was found to be the optimum resolution for quality of image and field of view.

The initial Corona missions suffered from mysterious border fogging and bright streaks which appeared irregularly on the returned film. Eventually, a team of scientists and engineers from the project and from academia (among them Luis Alvarez, Sidney Beldner, Malvin Ruderman, and Sidney Drell) determined that electrostatic discharges (called corona discharges) caused by some of the components of the cameras were exposing the film. Corrective measures included better grounding of the components, improved film rollers that did not generate static electricity, improved temperature controls, and a cleaner internal environment. Although improvements were made to reduce the corona, the final solution was to load the film canisters with a full load of film, feed the unexposed film through the camera onto the take-up reel with no exposure. This unexposed film was then processed and inspected for corona. If none was found or the corona observed was within acceptable levels, the canisters were certified for use and loaded with fresh film for a launch mission.

The first satellites in the program orbited at altitudes 100 miles (160 km) above the surface of the Earth, although later missions orbited even lower at 75 miles (121 km)). Originally, Corona satellites were designed to spin along their main axis so that the satellite would remain stable. Cameras would take photographs only when pointed at the Earth. The Itek camera company, however, proposed to stabilize the satellite along all three axes—keeping the cameras permanently pointed at the earth. Beginning with the KH-3 version of the satellite, a horizon camera took images of several key stars. A sensor used the satellite's side thruster rockets to align the rocket with these "index stars," so that it was correctly aligned with the Earth and the cameras pointed in the right direction. Beginning in 1967, two horizon cameras were used. This system was known as the Dual Improved Stellar Index Camera (DISIC).

Film was retrieved from orbit via a reentry capsule (nicknamed "bucket"), designed by General Electric, which separated from the satellite and fell to earth. After the fierce heat of reentry was over, the heat shield surrounding the vehicle was jettisoned at 60,000 feet (18 km) and parachutes deployed. The capsule was intended to be caught in mid-air by a passing airplane towing an airborne claw which would then winch it aboard, or it could land at sea. A salt plug in the base would dissolve after two days, allowing the capsule to sink if it was not picked up by the United States Navy. After Reuters reported on a reentry vehicle's accidental landing and discovery by Venezuelan farmers in mid-1964, capsules were no longer labeled "Secret" but offered a reward in eight languages for their return to the United States. Beginning with flight number 69, a two-capsule system was employed. This also allowed the satellite to go into passive (or "zombie") mode, shutting down for as many as 21 days before taking images again. Beginning in 1963, another improvement was "Lifeboat", a battery-powered system that allowed for ejection and recovery of the capsule in case power failed. The film was processed at Eastman Kodak's Hawkeye facility in Rochester, New York.

Coronas were launched by a Thor-Agena rocket, which used a Thor first stage and an Agena booster (which served as the second stage of the rocket lifting the Corona into orbit). With the implementation of the J-1 camera system in 1963, a Thorad rocket was used for the first stage, leading to large improvements in launch reliability. Later launches were made using the Thrust Augmented Thor (TAT). Maneuvering rockets were also added to the satellite beginning in 1963. These were different from the attitude stabilizing thrusters which had been incorporated from the beginning of the program. Corona orbited in very low orbits to enhance resolution of its camera system. But at perigee (the lowest point in the orbit), Corona endured drag from the Earth's atmosphere. In time, this could cause its orbit to decay and force the satellite to re-enter the atmosphere prematurely. The new maneuvering rockets were designed to boost Corona into a higher orbit, and lengthen the mission time even if low perigees were used. For use during unexpected crises, the National Reconnaissance Office (NRO) kept a Corona in "R-7" status, meaning ready for launch in seven days. By summer 1965 NRO was able to maintain Corona for launch within one day.

The procurement and maintenance of the Corona satellites was managed by the Central Intelligence Agency, which used cover arrangements lasting from April 1958 to 1969 to get access to the Palo Alto plant of the Hiller Helicopter Corporation for the production. At this facility, the rocket's second stage Agena, the cameras, film cassettes, and reentry capsule were assembled and tested before shipment to Vandenberg Air Force Base. In 1969, assembly duties were relocated to the Lockheed facilities in Sunnyvale, California. (The NRO was worried that, as Corona was phased out, skilled technicians worried about their jobs would quit the program—leaving Corona without staff. The move to Sunnyvale ensured that enough skilled staff would be available.)

The decisions regarding what to photograph were made by the Corona Target Program. Corona satellites were placed into near-polar orbits. This software, run by an on-board computer, was programmed to operate the cameras based on the intelligence targets to be imaged, the weather, the satellite's operational status, and what images the cameras had already captured. Ground control for Corona satellites was initially conduct from Stanford Industrial Park, an industrial park on Page Mill Road in Palo Alto, California. It was later moved to Sunnyvale Air Force Base near Sunnyvale, California.