Traffic Analysis

Photograph of Japanese
        "Adcock" direction finder

Japanese "Adcock" direction finder station

U.S. Navy. Via

Traffic analysis is the art of deducing an enemy’s dispositions and intentions through analysis of radio traffic.  It is distinct from cryptanalysis in that the messages are not actually decoded.  Instead, information is gleaned from knowledge of the sender and addressee (which is relatively easy to deduce) and from the volume of traffic.  Traffic analysis is most effective when the physical location of a transmitter can be determined using direction finding gear. 

Both the United States and Japan conducted traffic analysis with success.  For the United States, traffic analysis was a vital adjunct to cryptanalysis. American signals analysts were able to determine that an operation was under way when sea units ceased responding to messages from their corresponding shore units.

For Japan, traffic analysis was usually the best intelligence available.  Japanese traffic analysts warned Yamamoto that American units in the Hawaii area appeared to be going on alert just before the battle of Midway, but the warning was ignored.   Later, Japanese traffic analysis was successful in predicting American carrier strikes at the Philippines.  When the Americans realized this, they increased their radio security, and the next strike caught the Japanese by surprise and destroyed many Japanese aircraft on the ground.

Japanese Navy traffic analysis was centered at the Owada Receiving Station, near Tokyo, which was considered an unusually favored site for radio reception. From 1940 on, the station personnel deduced the approximate distance to American transmitters by determing the difference in arrival times of echoes off of different layers of the stratosphere. This allowed the station to work out the movement patterns of the U.S. Pacific Fleet from Lahaina, and may have provided useful intelligence to the Pearl Harbor attack planners. However, triangulation from widely separated stations was more reliable, and the five-man L Kikan team in Mexico was particularly valuable in extending coverage to the Atlantic. The Centrifugal Offensive gave Japan control of numerous suitable locations for intercept stations.

Radio direction finders were also mounted on ships and aircraft to obtain tactical intelligence. Because there was regular heavy tanker traffic between Japan and the U.S. West Coast during the interwar years, the Japanese Navy installed extensive electronics gear on the Shiretokos for signals intelligence. These oilers regularly conducted clandestine monitoring of U.S. fleet activities between 1924 and 1939. The Japanese made use of the Type 1 Ku Mark 3 Radio Direction Finder on numerous aircraft models throughout the war. This device could zero in on nearby transmissions in the frequency band from 0.17 to 1.2 MHz.

The Japanese Army also made use of traffic analysis, but primarily against the Russians and Chinese. Such intelligence was designated as C-jo. This produced a significant false alarm on 2 August 1941, when a sudden drop in the volume of Russian signals traffic in Siberia put Kwantung Army on full alert. The drop in volume of traffic turned out to be a consequence of poor atmospherics. On the other hand, traffic analysis pointed to the Manchuria offensive of August 1945, but this intelligence was essentially ignored by the Army General Staff.

Like the Japanese, the U.S. Navy had conducted clandestine prewar signals intelligence operations. One of the most successful was Operation GOLD STAR, conducted from auxiliary Gold Star, which was based on Guam and made regular supply runs for the island garrison throughout east Asia. With this cover, the ship closely monitored Japanese naval maneuvers during the early 1930s.

U.S. radio intelligence in the Pacific was centered at Oahu and Corregidor, with additional listening stations in Alaska and the West Coast. With the fall of the Philippines, the Corregidor personnel were relocated in Australia. The Hawaii station eventually grew into the enormous Joint Intelligence Center, Pacific Ocean Areas (JICPOA), which eventually had a strength of 1,767 specialists drawn from all branches of the armed forces.

Identifying Sender and Receiver. A radio message must necessarily identify both sender and receiver, but the identifying codes will be chosen to conceal this information from the enemy and will likely be changed regularly. However, radio direction finding can determine the approximate location of the sender and receiver, and the pattern of communications will usually allow major headquarters to be identified, since headquarters tend to initiate more messages and send them to a larger number of regular recipients.

In addition, a particular radio operator can often be identified by his "hand", that is, the style with which he works a telegraph key. For example, American traffic analysts claimed that the radio operator on Akagi was unmistakable for his heavy "hand." Since radio operators tended to remain with a particular unit for some time, this provided important clues on the identities of mobile units. Of course, radio operator unit assignments could be deliberately changed to confuse the enemy. Signals intelligence derived from identifying operator "hands" was known as TINA.

It is also the case that a radio transmitter has a subtle electronic "fingerprint" that can be identified by sufficiently sophisticated electronic analysis. This technique was employed as early as the First World War, when radio transmitters were still not well tuned. The crystal sets of the Second World War were very stable in frequency and it was correspondingly harder to distinguish one from another, but of course the methods of analysis were also more sophisticated.

The Japanese indulged in some practices that compromised their security. Although major Japanese units might have as many as seven call signs assigned, the messages from a particular unit were always numbered sequentially. Thus, an operator who changed call signs in the middle of a sequence of messages was likely to compromise the new call sign.

Another clue to Japanese traffic was the Japanese system of regional recruitment. Each regiment was tied to a geographical district, and even trivial personnel matters were routinely reported back to the home district. This allowed the traffic analysts to identify units from the home district they were associated with.

The Americans, by contrast, took strong measures to conceal their call signs. Following the attack on Pearl Harbor, a system was devised for call signs that made them very difficult for the Japanese to break. The Japanese were often left with nothing better than volume of traffic from each geographical area, though even this could be useful for deducing American intentions.


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