Missing Pieces: - Royal Australian Navy

76 Missing Pieces
A significant Allied Sigint breakthrough was the solving of the IJN digraph/trigraph
system of geographical designators in mid-March. Although some eluded solution,
the association of ‘RZP’ with Port Moresby was accepted by early April. On 9 April an
intercepted message gave the Allies an order of battle for the MO Operation, revealing
that the Occupation Force was to be supported by an Attack Force and a Striking Force
containing aircraft carriers. Principal targets of the Striking Force were US carriers:
since they had been missed in the Pearl Harbor attack, the IJN Combined Fleet had
made the sinking of these its top priority. 215 A further Support Force and two further
Occupying Forces were also included, although their targets were not immediately
recognised, but these were Tulagi and the Gilbert Islands. By 15 April it was learned
that two more IJN carriers had been assigned for duties south of Truk, and Nimitz was
able to warn that an offensive in the South West Pacific appeared imminent. This was
supported by traffic analysis showing a building up of air assets in Rabaul, confirmed
by land-based air reconnaissance.
The USN interpretation of the Japanese objectives was not wholly shared by GHQ
SWPA. MacArthur’s staff believed that northeastern Australia was the target, as carrierborne
air power was considered unnecessary for any assault on Port Moresby, and
they said so in a report for the chief-of-staff on 21 April. There was, indeed, a facet of
Operation MO which called for Rear Admiral Takagi’s carriers to attack Allied bases
fronting the Coral Sea, and this was indicated as a Japanese aim in the COIC report of
25 April. However, Takagi objected to this as a distraction from his principal aim of
destroying USN carriers and the secondary aim of supporting the assault on Port
Moresby. He was successful in having this task deleted from his orders. 216 MacArthur
himself believed that his air reconnaissance assets, primarily B-17s of the USAAF and
RAAF Hudsons, would detect any approaching Japanese force.
By 23 April the Allies knew the composition of each of the MO forces in accurate
detail. A change of the locator system used by the IJN on 30 April, compromised by the
interception of messages containing both the old and new designators, was correctly
interpreted to mean that Operation MO had been launched. On 2 May FRUMEL
intercepted and recovered a message detailing the role of the Striking Force in the
operation, and another indicating that the progress of MO was being delayed by bad
weather. 217 On 3 May, X-day (the day of the assault on Moresby) was deduced to be
the 10th of the month, with air attacks commencing on X-3. On 4 May, GHQ SWPA
began to report the movements, composition and identities of Japanese forces from
aerial reconnaissance. On 5 May, the Occupation Force schedule and its broad route
were recovered by Sigint, including its position for 0600 (Tokyo time) for that day. 218
MacArthur’s B-17s began bombing attacks on the Japanese shipping on 6 May (without
significant result) and GHQ advised that an intercept revealed that the IJN Fourth
Fleet knew there were USN carriers in its vicinity. These reports emanated from the
Japanese force at Tulagi. Fletcher initially gave scant credit to any intelligence support
from SWPA, but later recanted. With the IJN forces observing radio silence and nothing