Japanese aware of positions of Allied submarines according to the U.S. Joint Intelligence Committee. Daily summary dated 31 January 1942

An item referred to Chinese sources reporting that a partly descripted Japanese message pointed out that the Japanese were aware of the positions of 10 Allied warships mostly submarines. This confirms earlier reports dealing with the high efficiency of enemy radio direction stations of which large numbers were available in the Mandates, on board of surface units and probably in occupied territories. Between 1-24 January 1942 Japanese submarines destroyed 13 Allied ships in Asiatic waters, 10 with torpedoes, 3 with gunfire. It was believed that 10 Japanese submarines were involved in hunting merchant shipping and the scale of operations was increasing.

Source

Map Room Papers (Roosevelt Administration), 1942 - 1945 mr0422. U.S. Joint Intelligence Committee. Daily summary No. 52 dated 31 January 1942