Before electronic sensors guarded the approaches to North America, before satellite warning systems peered down from space, before air defense aircraft carried identification equipment, the US had the Ground Observer Corps. In World War Ⅱ, and then again during the early years of the Cold War, the nation's air warning system lay largely in the hands of the corps, a US military adjunct composed almost entirely of volunteers, intently studying wall charts and model airplanes to memorize the characteristics of "ours" and "theirs."
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