Ship patrol: Multiagent patrol under complex environmental conditions

In the problem of multiagent patrol, a team of agents is required to repeatedly visit a target area in order to monitor possible changes in state. The growing popularity of this problem comes mainly from its immediate applicability to a wide variety of domains. In this paper we concentrate on frequency-based patrol, in which the agents' goal is to optimize a frequency criterion, namely, minimizing the time between visits to a set of interest points. In situations with varying environmental conditions, the influence of changes in the conditions on the cost of travel may be immense. For example, in marine environments, the travel time of ships depends on parameters such as wind, water currents, and waves. Such environments raise the need to consider a new multiagent patrol strategy which divides the given area into regions in which more than one agent is active, for improving frequency. We prove that in general graphs this problem is intractable, therefore we focus on simplified (yet realistic) cyclic graphs with possible inner edges. Although the problem remains generally intractable in such graphs, we provide a heuristic algorithm that is shown to significantly improve point-visit frequency compared to other patrol strategies.