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Shore High-Frequency Direction-Finding in the Battle of the Atlantic:
An Undervalued Intelligence Asset
by RALPH ERSKINE -- abstract

This article reviews the operations of British and allied shore HF-DF nets during the Battle of the Atlantic in WWII. It includes Kriegsmarine assessments of shore HF-DF, and describes the measures, including off-frequency and “spurt” transmissions, adopted by the Kriegsmarine to counter HF-DF. It shows that shore HF-DF indirectly advanced the breaking of naval Enigma by about 12 months during 1941 and 1942, and that it was crucial to the breaking of the four-rotor naval Enigma cipher, Triton (codenamed Shark by Bletchley Park), at the height of the Battle of the Atlantic in the first half of 1943.