In spite of radar, U.S. air defense still needs volunteer "spotters" like those who watched the skies from rooftops and lonely hills during World War II. Last week the Air Force estimated that an all-out war would call for 500,000 spotters to plug the unavoidable gaps in the U.S. radar network.
The trouble with radar is that it is subject to blind spots. Its waves go out in straight lines, like television waves; they cannot duck down behind buildings, hills or other obstacles, and they cannot follow the curvature of the earth (see diagram). So a radar station works best against high-flying...
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