Japan was behind the UK, US and Germany in developing radio location; pre-war proposals met with little interest, only after a military mission to Germany in early 1941 did the Japanese take a keener interest in radar. The mission saw some British radar equipment left behind at Dunkirk, had some limited opportunities to see German equipment, and over the first half of 1941 news and rumours of British use of radar in engagements such as the sinking of the Bismarck and Battle of Cape Matapan prompted a ¥11 million investment in radar from the Japanese Navy in August. The first prototype air warning radar was set up in September, the first production unit was deployed in November 1941 at the Katsuura Lighthouse, shipboard radars followed in 1942 but were not in service at the time of Pearl Harbor.

(From A Radar History of World War II, Louis Brown.)