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In page Civil defense siren:

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These tones would be initiated by the Royal Observer Corps after spotting Luftwaffe aircraft coming toward Britain, with the help of coastal radar stations. The red warning would be sounded when the Royal Observer Corps spotted enemy aircraft in the immediate area. The sirens were tested periodically by emitting the tones in reverse order, with the all clear tone followed by the red warning tone. This ensured the public would not confuse a test with a real warning.[citation needed]