The system architecture (Figure 1) is built around a radio controlled (RC) helicopter. Navigation is primarily ground-based, requiring only a laser reflector and a ``dumb'' acoustic transducer to be mounted on the vehicle. Mission planning and guidance functions reside on the ground computer, a Silicon Graphics Indy workstation. Based on the state estimate from the navigation subsystem and a waypoint from the mission planner, the guidance subsystem up-links three attitude angle commands and an altitude command, but receives no feedback about the vehicle's actual attitude or altitude. Inner-loop flight control and altitude control are accomplished entirely on-board; the vehicle does not know its own position over the field. This architecture requires only ground to air communication. The disk retrieval mechanism involves few moving parts and is contained entirely on-board.
