Japanese navy guarding Great Circle route approaches according to the U.S. Joint Intelligence Committee. Daily summary dated 26 January 1942

An item reported that locations of Japanese radio admitters in the Gulf of Alaska pointed out the existence of an enemy force along the longitude 155 degrees observing the Great Circle route approaches to Japan from the American north west coast. The J.I.C. commented that such observing complicated surprise raids against Japanese objects.

Source

Map Room Papers (Roosevelt Administration), 1942 - 1945. U.S. Joint Intelligence Committee. Daily summary No. 47 dated 26 January 1942.