Even without understanding or decoding the message content, the silent listeners obtained staggering amounts of actionable information from the flow of signals themselves. It’s called radio traffic analysis or T/A and has two components.
First, what party is sending the signal and to whom? What is the frequency and the call sign? Where is the message coming from and where is the destination? Are there code names for participants or locations? Are there networks of participating stations that can be identified? What are the schedules, volumes and statistics of the message flow? Is radio silence suddenly started or ended? If the messages are in low grade codes or plain text, what do they say?
Now opposing forces frequently changed call signs, frequencies, code names and locations. This confuses the enemy. Or does it?
That’s were the second type of T/A comes in – radio fingerprinting. This applies to equipment and operators. On the equipment side, does the transmitter have any unique characteristics? For example, the Morse Code wave form, the frequency stability or chirping, or any particular buzz or hum or power supply ripple. On the operator side, each human sender has a “fist” or a characteristic melody to how he sends the code. Some are slow or jerky. Some provide different timing or spacing between dots and dashes. Individual chatter and personal comments are included.
So radio fingerprinting can help you understand how call signs, frequencies and networks are changed to create uncertainty. Morse code and teletype signals were frequently recorded on paper tape or film for later study. Listeners used an undulator to make these recordings.
All of this information was recorded, centralized and shared across listening posts around the world. Perhaps T/A was the first instance of “big data”.
Radio Traffic Analysis as Important as Ultra
From T/A, wireless intercept provided information about the enemy order of battle. What were the units, where were they located and how were they organized? Who was in command? When was movement or attack building up? Where were convoys moving?
Also, traffic analysis provided context information the Bletchley part to assist with the decoding. In return, GCCS provided tactical listeners with low level enemy codes so they could decrypt messages in the field.
Author David Kahn tells a typical story of T/A in his article The German Comint Organization in World War 2: “The traffic evaluators then listed on cards all stations using a certain frequency, all stations called by a particular call sign, and all known locations of the different stations. They diagrammed these relationships and saw what patterns emerged. One station addresses messages to many others, which seldom intercommunicate; direction finding shows it further to the rear. The evaluators deduce that it represents a higher echelon commanding the others. A higher volume of traffic on one circuit may portend an attack, a withdrawal, a relief, perhaps only a disciplinary problem, but almost certainly something. The evaluators followed the movements of the stations. Often they were extremely acute.
Once the Russians sent an armored army toward Stalingrad while leaving some of the army’s radiomen in its old location to give the impression that it was still there. But on the march, one of the radiomen who had gone along inadvertently transmitted. The Germans picked up his transmission, recognized him, and concluded that the armored force was moving south, probably toward Stalingrad.“