Originally, BDA was required due to the disconnected nature of aerial bombardment during World War I. It became necessary to send ground forces to an area to determine whether the damage was effective, or to overfly the target again. Either situation was difficult to accomplish safely. Starting with this conflict, specialized equipment has been used specifically for BDA type missions. Originally these were aircraft which were converted fighters or bombers (which in the USAAF were given a new "RQ" designation). Photographic reconnaissance was used extensively in World War II. From the 1950s onwards satellites became available, as well as high altitude surveillance planes such as the Lockheed U-2 and the Lockheed SR-71. During the Viet Nam War, U.S. special operations and regular ground components often performed on-the-ground BDAs in operational areas where U.S. forces had a presence.