Adaptive mission planning for cooperative autonomous maritime vehicles

This paper addresses the problem of developing an autonomous cooperative multi-vehicle system composed of one simple low-cost unmanned underwater vehicle (UUV) guided by a more capable, sensor-equipped, surface or underwater vehicle. Specifically, in this work we consider a system composed of an autonomous surface vehicle (ASV) able to localize a small-sized UUV via a multibeam sonar and, then, to guide it via an acoustic link to reach a target. The UUV is equipped with low-cost navigation sensors (a compass and a depth sensor), and to estimate its own position relies on the data received by the ASV (range and bearing from the ASV and ASV position). This paradigm was applied in the domain of mine countermeasures (MCM) to realize the NATO CMRE Autonomous Mine Neutralization System. In this system the low-cost feature of the UUV is of the most importance since the UUV is supposed to be expendable. In this approach, however, there may be cases in which the underwater vehicle cannot be detected by the sonar for extended time periods causing drifts in the UUV position estimate, potentially compromising the mission. This paper presents a set of behaviors to address this situation. The behaviors coordinate ASV circular search patterns together with sending to the UUV purposely generated navigation updates. The aim is to limit the distance between the two vehicles, increasing the probability of sonar reacquisition, and at the same time ensuring that the UUV progresses toward the target. Results from sea trials held in Elba island (Italy) during ANT12 experiments (in June 2012) are reported, and demonstrate the approach is effective and can push our system toward full autonomy.