Imperial Japanese Navy Owada Communications Unit

The Owada Communications Unit originated in 1936 as a naval receiving station built at Owada, Saitama Prefecture (now Nishibori, Niiza) on the border with Kiyose, Tokyo. Established as a subordinate facility of the Tokyo Naval Wireless Telegraph Station to receive radio traffic from across the Asia-Pacific, it became the Owada detachment of the Tokyo Communications Unit in 1937 and achieved independent status in 1941 at the outbreak of the Pacific War. Technically designated the Central Radio Intelligence Group, it worked under the Naval General Staff's special section for communications intelligence and was the Navy's central intercept station.

Sited on the Musashino plateau away from urban radio noise, the station covered about 1.2 million square metres and had some ninety wooden antenna masts. The unit operated 23 large and about 200 small receivers, intercepting communications from six countries, and employed approximately 500 personnel including specially recruited Nisei for English-language intercept work. Couriers carried radio intercepts from Owada to the Navy's central offices in Tokyo for intelligence analysis.

Because the Navy did not break high-grade American ciphers, Owada relied mainly on traffic analysis, call-sign identification and direction finding to infer the movements of U.S. fleets and air units. Late in the war it tracked the call signs of B-29 units in the Marianas; Japanese accounts state that its operators noticed a small unit on Tinian using unusual call signs and picked up related signals around the time of the Hiroshima bombing, but such intelligence was not turned into effective air-defence measures.

The unit was dissolved upon Japan's surrender in August 1945. The U.S. Army took over the site that September and it later passed to the U.S. Air Force; as the Owada Communication Site it remains in joint use by Japan's Ministry of Defense and U.S. Forces Japan and is closed to the public. A few boundary markers survive in Niiza and Kiyose, making it one of the few former naval intelligence facilities in the Tokyo area still in operation.