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,
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