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World War One was the first major conflict that saw the use of airplanes. During the early years of the fighting, aircraft were mostly used for surveillance and observation, but as the war progressed, airplanes took on an offensive role. By the end of the war, airplanes had become so deadly that it became necessary for the Allies to develop new tactics for air defense, if the next war was to be won.

The most important ingredient of an effective air defense system is an early warning system that could detect and track enemy aircraft before they arrive. But in the days before radar, the only way to detect enemy aircraft was to listen to the sound of approaching airplanes.

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Photo credit: Paul/Flickr

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