From Operation Order No. 1 and related messages that followed in quick succession, CDR Rochefort’s analysist pieced together the Fourth Fleet’s order of battle, the disposition of its forces and objectives.
What emerged from these and other decrypts was a picture of three invasion convoys assembled at Truk and Rabaul, all under VADM Inoue, getting ready to move, some of them already steaming from Truk toward Rabaul and points farther south.
The largest convoy was destined for Port Moresby, the campaign’s ultimate objective. The MO invasion force, covered by the light carrier Shoho and her escorts, would steam southward down the eastern coast of New Guinea; capture the islands of Deboyne, Samarai, and other islands in the Louisiades, at the easternmost tip of New Guinea; then enter the Coral Sea, en route to Moresby.
As the MO force steamed toward the Coral Sea, a second invasion force would sail eastward and, on X minus seven days (May 3), seize Tulagui (RXB) in the Solomons, desired by the Japanese for its excellent harbor. Once that island was occupied, part of the Tulagi convoy would rejoin the MO invasion force near New Guinea. Meantime, other elements of the Tulagi force would steam east from the Solomons into the south central Pacific, where it would capture Ocean and Nauru (RY).
Providing extra security for each attacking force would be a striking force consisting of Fifth Carrier Division (Zuikaku and Shokaku), two heavy cruisers, and six destroyers. If all preceded the way Station HYPO expected, the force would enter the Coral Sea from the northwest, escorting the invasion transports, all the while watching for the approach of any Allied navy forces in the area. Operating in tandem, the three forces – invasion, occupation, and striking – would slip around New Guinea’s eastern tip and steam north toward Moresby.
CDR Rochefrot and his Station HYPO analysis didn’t get everything right. As already noted, they put more carriers in VADM Inoue’s Fourth Fleet armada than he possessed. A more serious error was Station HYPO’s failure to note that the Zuikaku and Shokaku would not advance with the invasion force from the northwest, but separately from the northeast, from RADM Fletcher right flank. RADM Fletcher would find out that for himself in a few days.
Mistakes aside, Station HYPO now provided Commander in Chief, United States Pacific Fleet and Commander-in-Chief, U.S. Navy with a steady stream of estimates pinpointing the direction and timing of Fourth Fleet ship movement. On April 27, Station HYPO reported that the Japanese appeared ready to change their codes and ship call signs – move that usually preceded a major attack. The Japanese did change their call system but, astonishingly, they retained their old JN-25(b) code for another month, a fatal mistake that enabled U.S. Navy codebreakers to continue reading major portions of Imperial Japanese Navy traffic.
As ship’s coming and going disclosed Fourth Fleet schedules, CDR Rochefort reported the new developments to ADM King and ADM Nimitz. On April 30, he altered ADM King, ADM Nimitz and Station CAST to “RZP” Occupation Force Operation Order #1,” which reported that two Imperial Japanese Naval troop transports would “depart Rabaul Xray minutes seven day and rendezvous off Deboyne Islands with the Saipan Base Force scheduled to arrive Deboyne Xray minutes five days.”
From this radio intercept, the departure of the transports from Rabaul was fixed at May 3, the rendezvous of the transports with the Saipan force – the light carrier Shoho and her escorts – at May 5, and the invasion of Port Moresby at X day, or May 10. The force that earlier had invaded Tulagi would then move against Nauru and Ocean Islands and, if all went the way the Japanese planned, seize both on May 15.
Source: Elliot Carlson, “Joe Rochefort’s War”