Station “Hypo”
27 MAY 1942
STATION “HYPO”
Next week will mark the 83rd anniversary of the Battle of Midway, one of the most significant events in our Navy’s history. Many are aware that ADM Chester W. Nimitz was aided in this victory by fleet intelligence, who had broken the Japanese naval code, JN-25. The man responsible was LCDR Joseph Rochefort of the Naval Intelligence facility at Pearl Harbor, codenamed “Station Hypo.”
In the months that followed the December 7th attack, Rochefort and his cadre of industrious cryptographers poured over volumes of intercepted Japanese code. To assist, they employed a new, top-secret high-speed data processing technology (primitive IBM punchcard computers). By the first week in May Rochefort, who was himself fluent in Japanese, began to notice frequently recurring phrases such as, “expedite,” “fueling at sea,” and “current scheduled operation.” He correctly reasoned these were pieces of battle orders initiating Yamamoto’s long expected “crushing blow” against the Americans. But where would that blow fall?
It was known that JN-25 used two-letter designations for locations, and Rochefort noticed recurring references to location “AF.” To the dismay of Naval Intelligence back in Washington, DC, he deduced that “AF” referred to Midway. In a flurry of heated messages however, Washington failed to see why “AF” might not be Australia, the Panama Canal, or possibly Hawaii. Nimitz found himself caught in the middle. Rochefort then hatched an ingenious scheme to trick the enemy into revealing “AF.” He asked the garrison on Midway to broadcast an uncoded radio message indicating that their freshwater evaporator was not operational. As Rochefort suspected subsequent Japanese message traffic included a reference to “AF” being short on fresh water. With additional work, by this date in late May Nimitz knew the complete Japanese Midway plan.
Even during the battle itself, Nimitz was kept informed by Station Hypo. Rochefort intercepted and decoded messages from VADM Nagumo about the fatal bombing and fires on the carriers AKAGI, KAGA, and SORYU. Nimitz knew when HIRYU was fatally hit the following day and later heard Yamamoto call off the attack. Clearly the work of Rochefort and Station Hypo had been invaluable, and Nimitz recommended him for the Distinguished Service Medal.
But the outspoken LCDR Rochefort had by then made enemies in Washington. Not only was his award disapproved, but Rochefort was transferred to sea duty in command of the floating drydock ABSD-2, where he remained for the duration of the war. Fortunately, Rochefort’s monumental work has not gone permanently unrewarded. In 1985, following his death, Rochefort was finally awarded his Distinguished Service Medal by President Reagan.
Watch for more “Today in Naval History” 2 JUN 25
CAPT James Bloom, Ret.
Blair, Clay, Jr. Silent Victory: The U.S. Submarine War Against Japan, Vol 1. New York, NY: J.B. Lippincott Company, 1975, pp. 236-39.
Potter, Anthony R., (prod.). “Station Hypo”. Spies Video Series, Columbia House Company, 1992.
Prados, John. Combined Fleet Decoded: The Secret History of American Intelligence and the Japanese Navy in World War II. New York, NY: Random House, 1995, pp. 314-35.
