Whilst unmanned aircraft (UMA) can be said to encompass missiles, targets for weapon training, preprogrammed reconnaissance drones and civilian systems such as radio-controlled models used to carry cameras, for the purpose of this paper we will restrict ourselves to aircraft which are capable of sustaining autonomous flight and accepting navigation commands whilst airborne. The terms applied to unmanned aircraft are variously UMA, remotely piloted vehicle (RPV), Drone and Unmanned Aerial Vehicle (UAV).