The Japanese Offensive: Gearing up U.S. Naval Radio Intelligence Efforts
 
   On 10 December 1941, OP-20-G directed Hypo, later known as the Fleet Radio Unit, Pacific or FRUPAC, to join the attack on available JN-25B messages from its local intercept site and work on the unproductive Admiral’s code was abandoned.  Washington also referred to as Station Negat, and Cast accelerated their work on JN-25B, while SIS assumed primary responsibility for diplomatic traffic.  By the end of January 1942, Negat, Cast and possibly the British may have made fragmentary JN-25B decrypts with the new key list.  However, the first reported JN-25B decrypt was made by Cast on 16 March and it identified AF as being Midway Island. Soon, Hypo and Cast were vying to first report Japanese fleet movements with Negat generally being third in timely reporting. Nevertheless, Negat’s larger personnel base and greater machine capability permitted them to have the best database of information to support the decryption efforts of all stations, which benefited everyone.  By this time, the Japanese strike forces and land based naval aircraft had sunk the British ships Repulse and Prince of Wales and others as well as several Allied combatants like the Houston, Perth, and several smaller ships on their way to conquering Southeast Asia and the Philippines.

   Both Hypo and Negat began an urgent campaign to recruit personnel with all the various skills necessary to support their radio intelligence collection, direction finding, analysis, decryption and reporting.  With Admiral Nimitz’s support, Hypo obtained former band personnel from the California and Oklaholma for IBM machine operations as well as radiomen and other technical personnel rushed to Pearl Harbor after the attack or without a ship assignment.  Out of Hypo’s personnel, a new organization, Combat Intelligence Center (CIC) began to emerge to work directly with the Fleet Intelligence Officer. Negat grew tremendously with reserve personnel and large numbers of Waves.  Substantial percentages of school graduates, particularly radiomen, yeomen and maintenance personnel were allocated to OP-20-G requirements for U.S. and overseas billets.   In addition to recruiting Japanese linguists, a Japanese language school was established at the University of California at Berkley.  It later moved to Boulder, Colorado and provided OP-20-G with many additional language officers.

   From 5 February to 8 April 1942, Cast personnel were evacuated from Corregidor to Australia in three submarine trips.  Their new station, which eventually was called Fleet Radio Unit, Melbourne (FRUMEL), was set up in Melbourne so as to take over Cast’s cryptologic duties when the last group departed Corregidor.

   With Admiral Nimitz’s support and approval, Hypo commenced a program of providing major fleet commanders with a small on board Radio Intelligence Unit (RIU) for cryptologic support.  This started out with a junior language officer, a Chief Radioman and two or three radiomen.  The program began with the early carrier raids on the outlying islands of Marcus and Wake, targets in the Marshals and Gilberts, and, of course, the Halsey/Doolittle raid on Japan.  The success of these RIU’s varied with the confidence or lack thereof the fleet commander had in such units.  Most fleet commanders soon saw the tremendous advantage of having such units aboard particularly for their early warning capability.  As time went on, more and more fleet commanders insisted on having such units and more personnel for each unit.
By the end of the war, expanded RIU’s were provided to all carrier task forces, battleship force commands and some smaller commands.

    As personnel became available, Hypo commenced copying all the Japanese weather broadcasts and soon their weather code was broken.  U.S. weather observations were augmented by these Japanese weather reports and the resulting forecasts were sent out to all U.S. forces by the CinCPac weather unit.