Traffic Analysis

SIXTA was the largest of several sections at Bletchley Park dedicated to Traffic Analysis. Traffic Analysis examined the volume, direction, broadcast patterns, call signs and other characteristics of enemy message traffic, rather than the message content. This information gave vital insight into how enemy forces were organised and administered, and where they were located. It added useful context to decrypted message content and allowed intelligence to be gleaned even from unbroken messages. Each Traffic Analysis section worked on specific message types. SIXTA was part of the Hut 6 German Army and Air Force Enigma codebreaking operation, specialising in analysis of this traffic.

This Collections Uncovered album was first published on 25 February 2019.

SIXTA staff in Block G at Bletchley Park. Most SIXTA personnel were British Army men and women, serving in the Army’s No. 6 Intelligence School. Not all were Army, however. The woman on the far left of the image is Miss Elisabeth Roscoe, a Foreign Office civilian.

The SIXTA unit moved from Beaumanor to Bletchley Park in April 1942. It was initially housed in Huts 15A, 15B, 15C and 15D, and moved to Block G in November 1943. Previously known as Military Wing Central Party, then Special Liaison Party, it was christened SIXTA (Hut SIX Traffic Analysis) that same month. This photograph shows one of the section’s Block G offices.

Auxiliary Territorial Service (ATS) Sergeant Jeanne Lindley (née Cammaerts) worked as a Log Reader in SIXTA. Log Readers analysed logs of intercepted enemy wireless communications, compiled by Intercept Operators at Outstations such as Beaumanor, Chicksands and Cheadle. From this, SIXTA could reconstruct the communications networks over which the messages were sent.

Warrant Officer II James “Jimmy” Thirsk was another SIXTA Log Reader. He transferred to Bletchley Park from Beaumanor, with the rest of the Military Wing Central Party, in May 1942. In his book “Bletchley Park: An Inmate’s Story” Jimmy recalled: “Each of us was allocated a particular network, or perhaps several networks, used by units of the German Army or Air Force… We log-readers would make a diagram on a pro forma sheet, drawing lines between the various [enemy] outstations and their headquarters… By placing arrows on the lines, using coloured pencils, we could indicate each message and the direction it was travelling… Under our diagrams we wrote notes, in pencil, about the activities of our stations during the past twenty-four hours. We also noted any unusual flows of traffic and any changes from normal procedure.”

This is a Morrison Wall, named after Major Eric Keir Morrison of SIXTA. It is a systems diagram, used to represent the enemy’s communication network. The wall shows communication links between enemy units, and the location of each unit.

ATS Subaltern Adrienne Gurr spent a few short weeks training at Beaumanor before being posted to Bletchley Park as a Log Reader in 1943. She recalls: “I was first put into the ‘Search Room’, a job that no one much liked. This involved receiving quantities of random Logs about which there had been some doubts as to their accuracy, or that they were known to be corrupted, or that were clearly wrong on the part of the wireless operators. The message Logs were in code and our job was to try to solve the discrepancy and to make sense of the string of code. To do this we looked for cribs and repetitions etc. I must say that in my experience we were not very successful on the whole but we did try.”

ATS Warrant Officer II Pamela Hobbs (née Kanis) was responsible for a card index that recorded the broadcast frequencies used by enemy units. She recalls: “We had little shoeboxes for our filing, with lots of cards, a little bit bigger than a postcard. We had to find out what enemy unit was using which frequencies and record these on the cards, using the names of flowers to designate the units, for example Narcissus and Daffodil.”

The SIXTA Direction Finding Plotting Room in Block G. SIXTA was able to pinpoint the locations of individual enemy radio transmitters by plotting and comparing the directions from which a transmitter’s signals were intercepted. Combined with other information, this helped to build up a picture of the enemy’s organisation and dispositions.

ATS Subaltern Joan Thirsk (née Watkins) worked in a SIXTA sub-section known as the Fusion Room. Here, information from Log Reader reports was combined with intelligence from Enigma decrypts to give as clear a picture as possible of the enemy’s formations. Joan worked particularly on the analysis of German Air Force units and their activities. Joan would later marry Jimmy Thirsk, a SIXTA Log Reader, and go on to have a distinguished academic career. She was awarded a CBE in 1994.

FO Civilian Jean Dearden (née Robson) was part of the Quiet Room, a sub-section of Hut 6 that became the link between SIXTA and the rest of Hut 6. The Quiet Room (later known as Traffic Identification Section 2) also carried out Traffic Analysis research, typically looking at longer-term problems and unusual trends.