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This is Volume One of The Army Air Forces in World War II, titled Plans and Early Operations, January 1939 to August 1942, edited by Wesley Frank Craven and James Lea Cate. The University of Chicago Press first published it in 1948, and the Office of Air Force History reprinted it in 1983. The front matter explains how the AAF historical program was set up during the war. The index covers Pacific, Atlantic and European operations, and nothing in the text concerns unidentified aerial phenomena.
“Cairo”26 pages
D E F E N S E O F T H E W E S T E R N H E M I S P H E R E soon defined the elements which would be required in a defense sys- tem. It would be necessary to know in advance of the approach of enemy craft. Next, there would have to be some method for con- tinuous tracking of the planes as they came toward their objective, and this would require some device to supply information regarding the height, direction, speed, and approximate size of a raiding force. Finally, there would need to be some system for identifying friendly fighters and of communicating to their pilots the information gathered on the movement of enemy planes. Given all these elements, a com- mander on the ground might then control the interception and bring friendly fighters into contact with any large attacking force. Such a system might not prevent an occasional raider from breaking through, but it would make it possible to limit the advantage of complete sur- prise which some students of the problem thought the airplane would always enjoy. With the technical equipment which existed in 1918, no adequate defense against air raids was possible, but attempts to perfect new devices occupied much of the time of military planners through the two decades between World Wars I and 11.'' T h e first detectors used such equipment as parabolic reflectors to pick up the sound of an on- coming plane, but results were so poor that the United States Army in 1933 abandoned research in the field of auditory detection.lo0Several other lines of investigation, notably in the possibility of using infrared heat waves, attracted attention, but ultimately interest came to be centered in discovery of a more exact method of detection by use of radio waves. A t first, ordinary radio waves were employed; but severe technical limitations were encountered, and the true origins of modern military radar-"radio detection and ranging"-dated from I 93 5, when practical microwave radio sets were developed. Four basic steps were involved in the improvements which gave radar its great military sig- nificance: the discovery that radio energy of very high frequency was reflected by objects in its path rather than being absorbed as was the case with waves of lower frequency; the development of a trans- mitter to emit pulses of radio energy in a sharply defined directional beam, which, with allowance of a time lapse between emissions, per- mitted a receiver to pick up the energy reflected from the target; the perfection of a cathode ray tube, or oscilloscope, on which the small time lapse between the outgoing pulse and the reflected image could be measured; and, finally, the invention of the cavity magnetron tube, 2 8 7
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