Headache was an underappreciated British system for monitoring German naval voice communications. German operators often transmitted at high power and talked freely enough for ships and shore stations to gather useful movement information. Some Royal Navy vessels carried German speakers who could interpret the traffic directly. Combined with Enigma intelligence, that radio picture allowed Admiral Latham to withdraw vulnerable anti-submarine patrols and arrange a destroyer interception instead of stumbling into the German force by chance.
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Headache exploited German radio habits
Enigma and the less famous Headache listening system exposed German movements before Allied destroyers made radar contact at Ushant.