Owada Unit Identifies Atomic Bomb Carrier Aircraft

The Owada Communications Unit was the Imperial Japanese Navy's specialist unit for intercepting foreign radio traffic. Its main station stood at Nishibori, Owada-machi, Saitama Prefecture (now Niiza City), extending into Kiyose and Kurume villages in Tokyo. It grew out of the Owada Receiving Station of the Tokyo Naval Radio Station, set up around 1936 and in full operation from 1937; at the outbreak of the Pacific War in 1941 it became independent of the Tokyo Communications Unit as the Owada Communications Unit, the Navy's only communications unit whose main body was charged with interception. Intercepts were passed by dedicated line to the Naval General Staff in Kasumigaseki for analysis, and by the end of the war it was one of the Navy's largest receiving stations.

After US forces took the Marianas in summer 1944 and built B-29 bases on Saipan, Guam and Tinian, tracking B-29 communications became a major task. A cultural-properties bulletin of the Higashikurume City Board of Education introduces a Navy document dated 15 July 1945 held by the National Archives of Japan (JACAR), 'On the B-29s in the Marianas', which lists units, aircraft numbers and call signs island by island, showing that the Navy was systematically following B-29 unit call signs.

As to the atomic-bomb aircraft, the Japanese Wikipedia article on the Owada site states that the unit, through interception of B-29 communications from Tinian, recognised the group to which Enola Gay and Bockscar belonged as 'suspicious aircraft' near Tinian, and that after the Hiroshima attack it determined from a coded transmission sent over Hiroshima that Enola Gay's call sign was '5V625' and that it had taken off from Tinian; the article cites no source for this. The earlier text of this record stated that Owada analysts identified 2-3 aircraft with distinctive call signs that never participated in regular bombing raids, and that when these aircraft appeared over Hiroshima on August 6, 1945, and over Kyushu on August 9, the unit recognised them, though warnings could not be communicated in time to prevent the Nagasaki bombing. No primary source for these details could be found in the material consulted, and they are best treated as postwar testimony and secondary accounts.

What is certain is that, whatever indications Navy or Army signals intelligence had, no corresponding warning reached the people of Hiroshima or Nagasaki before the attacks of 6 and 9 August. The Owada facilities were handed over to the US Army's 1st Cavalry Division on 13 September 1945 and became the US forces' Owada Communication Site, part of which remains in use in Niiza; part of the former sub-station in Kiyose is now the Japan Meteorological Agency's Meteorological Satellite Center.