aerial raids in history and was in effect the first aircraft carrier of the Imperial Japanese Navy. By the end of the siege the aircraft had conducted 50 sorties and dropped 200 bombs, although damage to German defenses was light. Further developments (1916–1918) In 1916, the Commission on Naval Aeronautical Research was disbanded and the funds supporting it were reallocated for the establishment of three naval air units (hikotai) which would fall under the authority of the Naval Affairs Bureau of the Navy Ministry. The first unit was established at Yokosuka in April 1916, however, the lack of a specific naval air policy in these early years was made apparent by the fact that the Yokosuka Air Group operated with the fleet only once a year when it was transported briefly to whatever training area the IJN was then using for maneuvers. Japanese naval aviation, though, continued to make progress. In 1917, officers at the Yokosuka Naval Arsenal designed and built the first Japanese seaplane, the Ro-Go Ko-gata reconnaissance seaplane, which was much more useful at sea and much safer than the Maurice Farman aircraft that the navy had been using up to that point. The aircraft was eventually mass-produced and became the mainstay of the navy's air arm until the mid-1920s. Japanese factories by the end of the war, in increasing numbers, were beginning to turn out engines and fuselages based on foreign designs. A major expansion in Japanese naval air strength was part of the 1918 naval expansion program which made possible a new air group and a naval air station at Sasebo. In 1918, the IJN secured land around Lake Kasumigaura in Ibaraki Prefecture, northeast of Tokyo. The following year, a naval air station for both land and sea aircraft was established, and subsequently, naval air training was transferred to Kasumigaura, from Yokosuka. After the establishment of a naval air training unit at Kasumigaura, the air station became the principal flight training center for the navy. Interwar development Sempill mission The Japanese navy had closely monitored the progress of aviation of the three Allied naval powers during World War I and concluded that Britain had made the greatest advances in naval aviation. They had also learned a good deal about naval aviation through their contacts within the Royal Navy. In 1920, a representative had also been sent to Britain to observe air operations off the decks of