The 1956 United States Army aircraft designation system In 1956, the U.S. Army adopted a new, and relatively simple, designation system for its aviation assets. Aircraft were divided into three different types - 'A' for fixed-wing aircraft, 'H' for helicopters, or 'V' for V/STOL aircraft, and then were given a mission modifier, which, unlike the USAF system, came after the type code: 'C' for transports, 'O' for observation and reconnaissance aircraft, 'U' for utility types, and 'Z' for experimental aircraft. Aircraft types designated in this system were numbered sequentially.[1]
In the late 1950s, the Army Combat Surveillance Agency was assigned responsibility for developing a series of reconnaissance drones for U.S. Army use. While the projects developed by the Agency did not receive designations in the Army manned aircraft system, the drones developed did receive sequential designations in the AN/USD- series, shortened to SD- in practice, and are therefore listed here.[2]
Newcome, Lawrence R. (2004). Unmanned Aviation: A Brief History of Unmanned Aerial Vehicles. Reston, Virginia: American Institute of Aeronautics and Astronautics. ISBN978-1-56347-644-0.