A place used to watch for, or intercept, signals of an enemy attack, sometimes called a 'Y-station'.

This could be a Roman or medieval tower from which a view could be obtained and a signal sent, by a beacon or flags. Signal stations were used in World War I to intercept radio signals, so as to plot the movements of ships, aircraft and airships by taking a bearing, (using another station's bearing), and using triangulation. In all cases defences could be readied and the alarm raised quickly.

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