Throughout the period prior to World War Two, all OTRG operators were busy following the patterns of communications operated by the Japanese Navy.
In addition to the recovery and of identity of the radio station callsigns, the operators were able to gather useful information on the format of traffic including the addressee and originator systems.  The format used in maneuvers and exercise were catalogued by most operators.  Movement reports, ships’ schedule and position reports all used a format which was carried into the encrypted traffic before and during hostilities started (WWII).

One of the first applications of encryption of the noon position reports by the Japanese took place in 1938 when the Japanese Navy Training Squadron was dispatched to Honolulu for a courtesy visit.  However, OTRG operators were able to track the squadron in both directions primarily because the position reports were encrypted in a very simple substitution system.

At Station “B” on Guam, in 1937, the OTRG operators had intercepted and translated a plain language report from a Japanese destroyer operating in the Shanghai area during the Sino-Japanese War in 1937.  The message was a battle report after an exchange of fire with Chinese shore establishments.  This occurred just prior to a scheduled inspection of Station “B” by the Commandant of Guam, a Navy Captain.  Taking advantage to impress others, the OTRG operators arranged the traffic file so that the battle report was the top message.  Sure enough, the Captain picked up the file, read the message, and asked, “Do you really intercept many message like this?”  “Absolutely Captain,” one of the operators exclaimed.

As Duane Whitlock explained traffic analysis in the 1991 Naval Cryptologic Veterans Association Special edition on Alaska, plain language messages more or less disappeared from Japanese naval communications and was replaced by message enciphered in increasingly sophisticated cryptographic systems.  Some of these systems remained readable, notably one system used for enciphering ship movement reports.  Being unable to read any part of the message they were intercepting, a few of the U.S. Navy intercept operators began to develop an advanced art of traffic analysis. And gradually they became quite skilled in deducing much of the content of encrypted messages without the benefit of being able to decrypt them.  They accomplished this by breaking or “recording” the Japanese secret callsign system, then studying the message headings (preamble) to determine originators, radio traffic routing and addressees.  An example:


This message is addressed to the cruise Idzumo (KAMO8), station ship in Shanghai and for information (TUHO) to Sasebo Naval Base (EEI) and to the Asama Maru (KERE0), originated by Naval Headquarters in Tokyo (HAFU6), and had already been delivered to the Asama Maru as indicated by the  double slant sign that precedes that ship’s callsign.  With the experience obtained while “reading”  plain language traffic, from this message heading a traffic analyst would probably deduce that the Asama Maru is departing Tokyo Bay  (probably from Yokosuka) en route Shanghai with a stopover scheduled at the Sasebo Naval Base on the island of Kyushu.

Source: NCVA/ Carl Jensen
Edited By Mario Vulcano