Communication Intelligence Summary, 1
General.-All service radio calls of forces afloat changed promptly at 0000, 1 December. Previously, service calls
changed after a period of six months or
more. Calls were last changed on 1 November, 1941. The fact that service calls lasted only one month
indicate an additional progressive step
in preparing for active operations on a large scale. For a period of two to three days prior to the change of calls,
the bulk of the radio traffic consisted
of dispatches from one to four or five days
old. It appears that the Japanese Navy is adopting more and more security provisions. A study of traffic prior
to 0000, 1 December indicates that an
effort was made to deliver all dispatches using old calls so that promptly with the change of calls, there would be a minimum of undelivered dispatches and
consequent confusion and compromises.
Either that or the large number of old messages may have been used to pad the total volume and make it
appear as if nothing unusual was
First Fleet.-Nothing to indicate that this fleet as a fleet is
operating outside of Empire waters. It
is believed that such a large percentage of the First Fleet is operating with the Second Fleet Task Force that
this fleet has ceased to operate in a
Second Fleet.-This fleet is believed proceeding from the Kure-Sasebo area in the direction of South China and
Indo-China. Takao does not appear to
play an important role in today's traffic; consequently, the assumption is made that his fleet is passing
up Takao. Certain units of the Second
Fleet Task Force are definitely in the Indo-China area (Cruiser Division Seven and Destroyer
Squadron Three most prominent).
Third Fleet.-Nothing to report except that the same association of Second, Third Fleets, and Combined Air Force
with South China and Indo- China Forces continues.
Fourth Fleet.-No change in the Fourth Fleet or Mandates area.
Fifth Fleet.-Nothing to report.
Submarines.-Large number of the Submarine Force believed to be in the area to the eastward of Yokosuka-Chichijima
and Saipan Flagship somewhere in this
Combined Air Force.-No change.
U.S. CONGRESS JOINT COMMITTEE ON PEARL HARBOR ATTACK HEARINGS; PT. 35, THE CLAUSEN INVESTIGATION, PP. 62-81.
EXHIBIT "A" DAILY COMMUNICATION INTELLIGENCE SUMMARIES)