Multiple UAVs cooperatively circumnavigating a group of non-cooperative targets in a GPS-free environment via a range-only distributed controller

This article proposes a distributed controller for multiple unmanned aerial vehicles (UAVs) cooperatively circumnavigating a group of non-cooperative targets only using range measurements. Unlike most existing methods that either only consider a single target or know the global locations of the UAV and target beforehand, this article investigates a more general situation to circumnavigate multiple unknown targets in a GPS-free environment. First, based on the range measurements, a fixed-time sliding-mode observer and a relative position observer are designed to estimate the relative position, thus eliminating the requirement for global position measurements. On this basis, each UAV can reconstruct the relative geometric center of multiple targets. Second, an observer-based range-only control method is developed to drive a UAV to encircle a group of unknown targets. The results show the UAV velocity does not affect the global convergence of the range-only controller. Subsequently, a range-only distributed controller is proposed to coordinate the velocities of multiple UAVs so that they can encircle multiple targets with a desired angular spacing between them relative to the geometric center. Furthermore, using the cascade system theory and Lyapunov stability analysis method, the proposed distributed controller is demonstrated to exhibit global asymptotic convergence. Finally, the effectiveness and robustness of the proposed method are verified via numerical simulations.