Subject:      New article on CORONA history
From:         thomsona@netcom.com (Allen Thomson)
Date:         1997/02/05
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The issue of Physics Today that showed up in the mailbox yesterday has an interesting article (*) on the CORONA spysat program by Albert D. (Bud) Wheelon. Wheelon was the head of the CIA's Directorate of Science and Technology from 1962 to 1966. The article is said to derive from his talk at the amazing symposium given at GWU on 23 May 1995. However, there are bits and pieces that are new, or that I didn't catch at the time. Herewith are some snippets from the article. Checks against other information would seem to be a good idea. "The Thor [booster] would burn out at an altitude of 70 miles -- well short of the reconnaissance orbit. An additional stage was thus needed to lift Corona. The Agena upper-stage vehicle had been in development at Lockheed for two years but had not yet flown.... Agena was the logical choice to provide the additional velocity, and Lockheed became the second [Corona] team member [after Douglas Aircraft]..." I'd always assumed that Lockheed got into the spysat business as a logical extension of the Skunkwork's development of the U-2 for the CIA. Probably this did play some part in it, but Wheelon's description makes it appear as if the availability of the Agena was the deciding factor. Considering the two gigabucks/year Lockheed now brings into Santa Clara county, I think the local officials should put up a shrine to the Agena. :) "Color and infrared film were tried, but they seemed not to increase the intelligence value of the photography." This is kind of interesting. I wonder if today's military users of SPOT imagery order mostly black and white pictures. "As we became comfortable with Corona operations, the CIA began to look ahead. It was apparent that the photointerpreters were having difficulty finding strategic targets in the vast amount of 70 mm film that was arriving. I asked the [Sidney] Drell [technical advisory] group to examine this problem, and I put two questions to it: What resolution do the photointerpreters need to find and identify strategic installations in broad area coverage; can Corona be improved to provide that level, or must we begin a new system? "We approached these questions by preparing simulated satellite photographs from high-resolution U-2 coverage and giving them to the photointerpreters to see how recognition varied with resolution. The experiments confirmed my impression that a substantial improvement in resolution was needed. The Drell group judged it unlikely that we could push the Corona system to that new level by further improvements. Corona's basic design had inherent limits, and we had reached them." The limits he's talking about clearly have to do with resolution, and the article gives some notion of what numbers should be assigned to them. At one point he notes, "The satellites were built around a panoramic camera that operated near diffraction limits." The camera itself had a focal length of 24 inches (61 cm) in an f/5 system, implying that the objective lens had a diameter of 4.8 inches (12.2 cm). The diffraction limit for this system would be 4.98 microradians or about one arc second. At the operational altitude of 100 miles (160 km), diffraction-limited resolution at nadir would be around 80 cm. A separate check can be made from the focal length of the camera, the orbital altitude, and the film characteristics given in the article: at high contrast the film gave 280 lines per millimeter. Projecting 280 lines per millimeter through the 61 cm focal length system on the ground 160 km away gives 1.07 lines per meter, or 94 cm resolution -- basically the same result as obtained from the diffraction limit calculation. So the system probably was designed to operate at the diffraction limit, and the film was matched to the optics, just as it should have been. Initial resolutions were only some 20 ft, but Wheelon says that stabilization and other improvements got it down to 6 feet, about 2 - 2.5 times the diffraction limit. Whether this discrepancy is meaningful isn't clear. In any event, it looks as if the basic Corona design should have been improvable down to about a meter ground resolved distance, or NIIRS 5 in today's parlance. This is a peculiar result in light of the first question put to the Drell panel: "what resolution do the photointerpreters need to find and identify strategic installations in broad area coverage" and the answer, "The experiments confirmed my impression that a substantial improvement in resolution was needed." If you look at almost any task-vs-resolution table, it's clear that one or even two meter resolution is more than adequate for "finding and identifying strategic installations." What he should have been asking was either "how can we increase area coverage while keeping enough resolution to discover strategic installations?" or "how can we increase resolution to do technical intelligence and other fine-scale photointerpretation tasks?" These are different and somewhat antithetic questions. The answer he says they got pertains much more to the second of these should-have-askeds, and leads down the path to the KH-8. The broad area search question takes one in the direction of the KH-9 and away from very high resolution. I suspect there are many more fascinating details to be revealed about the programatics and politics of this era. Finally, Wheelon gets a vote of thanks from me by including a section called "Vulnerability," a topic I've been known to obsess on from time to time. "From my first days in government service, we all worried a great deal about the vulnerability of Corona. We recognized that it was an easy matter for the Soviets to locate these satellites and predict their orbits... "It is an easy matter to destroy satellites in low Earth orbit if there is an incentive to do so... We recognized that it would be a simple matter for [Soviet nuclear ABM interceptors] to eliminate Corona. Had they done so, we would have had to conduct our affairs in almost total ignorance of the Soviets' activities -- as we did prior to 1960. "The destruction of a low-altitude satellite does not require nuclear weapons. Because such vehicles travel in Earth orbit at speeds of approximately 17,000 miles per hour, it is only necessary to stand in their path to destroy them... "A wide range of defensive measures were examined... "These proposals were not implemented, primarily because they required a great deal of payload weight. We opted each time for increased film loads and prayed that Corona would not be attacked. As the Soviets developed their own reconnaissance satellites, both parties saw benefit in avoiding space warfare. A climate of mutual forbearance set in, which now serves both parties extremely well." Which illustrates, IMNSHO, the extremely contingent character of the Cold War's tacit agreement not to harm satellites, and how little we can count on other nations, in other circumstances, to honor it. (*) "Corona: The First Reconnaissance Satellites by Albert D. Wheelon Physics Today, February 1997, pp. 24-30 ISSN 0031-9228