Yoshikawa began gathering intelligence in earnest by taking auto trips around the main islands, touring Oahu in a small plane, and posing as a tourist. He visited Pearl Harbor frequently and sketched the harbor and location of ships from the crest of a hill. Once, he gained access to Hickam Field in a taxi and memorized the number of visible planes, pilots, hangars, barracks and soldiers. He also discovered that Sunday was the day of the week on which the largest number of ships were likely to be in harbor, that PBY patrol went out every morning and evening, and there was an antisubmarine net in the mouth of the harbor. Information was returned to Japan in coded form in Consular communications and by direct delivery to intelligence officers aboard Japanese ships calling at Hawaii by consulate staff.
