In World War II, a branch of the Royal Navy known as 'Y' intelligence and commonly referred to as 'HEADACHE', was charged with the task of intercepting and reading German low-grade radiotelephone traffic which had been steadily increasing since 1941. Headache operators usually came from a Special Branch of the RN and were fluent in German. Every important warship in the D-Day armada and the bombing force was provided with a Headache Unit for the interception and interpretation of enemy air and naval R/T on VHF. Fifty warships in all were fitted with Headache units'.
The Hallicrafters S-27 receiver shown in this photo was used to monitor tactical voice traffic from the Luftwaffe. In the event of air attack, the operator would inform the bridge thus allowing the Officer of the Watch to take evasive or defensive tactics. On one occasion, enemy aircraft were giving a sighting report on HAIDA's position while she was berthed in Plymouth, England in 1944. As best as we can figure out, the Headache office was located in the Navigator's office, directly below the bridge.