Battle of the Beams

From September 1940, Bletchley Park’s decryption of the German Luftwaffe (Air Force) ‘Brown’ Enigma key played a crucial part in protecting the UK during the Battle of Britain and the Blitz. Brown was used by the German Air Force’s radio and research regiment. In 1940, they were responsible for two beam navigation systems used by the Luftwaffe to direct their night bombing raids over Britain. Brown messages gave valuable information about the systems, allowing the British to jam navigational beams and divert German bombers away from their targets. They also helped identify the target for each night’s raid – if Hut 6 was able to decrypt the messages in time. But the Brown network contained more information than could be read in the messages themselves. Traffic analysis at Y (wireless intercept) stations listening to Brown traffic built up a detailed picture of German navigation beam operations, from transmitter locations to operational information.

This album was first published on 14 September 2020.

The ‘Note on the Brown Group’, dated 24 December 1940, was compiled by staff at Chatham, then home of the principal Army Y (wireless intercept) Station. This station was the first to intercept the Luftwaffe’s Red Enigma key and by late 1940 was also intercepting Brown. The Note shows how much information Chatham was able to deduce from traffic analysis on Brown messages during the Battle of Britain and the Blitz.

As a Y Station, Chatham would have access to all the unencrypted parts of intercepted messages as well as frequencies, call-signs and operator chat – all useful information for traffic analysis. However the document, marked ‘Secret’, also shows the compartmentalisation of knowledge between Bletchley Park and Chatham. The compiler of the Note had no access to the encrypted texts of the messages, which were read only at Bletchley Park, or to the intelligence derived from those texts.

Chatham observed that the Vannes station was using both the Brown and Red Enigma keys, which was unusual. Other transmission stations using Brown did not also use Red. Vannes was the home of KG100, the Luftwaffe pathfinder bomber squadron which was responsible for leading beam-directed raids on the UK. As an active bomber base, Vannes would have needed to communicate on Red for normal administration as well as on Brown for beam navigation.

The station at ‘LEIPZIG Area’ refers to Köthen, near Liepzig in Germany. This was the location of the HQ of the Luftwaffe Nachtrichten Versuchs Regiment (the Air Force radio and research regiment).

A peak of activity in the early afternoon (1200 – 1400 GMT) matches the times when raids were being prepared for that evening. Operational information, such as navigational beam angles for that night’s target, would be communicated to German bomber units.

The locations of Brown key users identified by Chatham include ‘VANNES, MORLAIX, CHERBOURG, BOULOGNE, TEXEL’. These correspond with the base of the KG100 pathfinder squadron, and other bases that transmitted the navigational beams for bomber units to follow during raids.

The security of the Brown Enigma network was notoriously bad, according to the History of Hut 6 – giving the codebreakers and traffic analysts a helping hand. As an experimental, secret unit, Brown operators were not subject to the usual scrutiny applied to Luftwaffe signals regiments and were also mostly personal friends. Plain language chat was common, as were basic security breaches such as repeatedly using their own initials as Enigma rotor settings.

Chatham notes that the three-letter group ‘FDL’ interpreted as Feindlich (Enemy), is associated with certain call signs and frequencies. This may be a reference to British efforts to jam the various navigational beam systems.

The German beam navigation systems used during night raids in late 1940 were codenamed Knickebein and X-Gerät – likely the XBT identified in this section of the Note. Antennae on the French, Dutch and Norwegian coasts transmitted navigational beams across the UK. These guided bombers to their targets, pinpointed by the intersection of two beams. The longer-range X-Gerät system also used beam intersections to tell aircrews when to release their bombs for maximum accuracy.

Chatham had also identified aircraft movements and callsigns in the Brown messages – particularly when operators bypassed security measures by sending unencrypted messages over the network. The callsigns probably refer to aircraft in KG100 squadron, which led flying operations based on the beams.