While unmanned aerial vehicles (UAVs) are a promising technology, they suffer from fundamental fuel and payload restrictions. A system of UAVs can be used to address these limitations. In this paper, we consider an automated system of UAVs seeking to provide an uninterrupted, or persistent, security presence to customers in an outdoor environment. We define the goals of such a system and provide architecture to achieve them. Various components such as the central director, mission planner and UAV service stations are introduced and analyzed. Following the architecture, we constructed prototypes of each component and implemented a small scale outdoor experiment. The experiment included receiving customers' orders, algorithmic implementation of UAV tasks and UAV control.