TICOP: Time-Critical Coordinated Planning for Fixed-Wing UAVs in Unknown Unstructured Environments

Safe coordination of fixed-wing UAVs in unstructured environments poses challenges due to the intricate coupling of UAV cooperation, obstacle avoidance, and motion constraints. One task is time-critical coordination, which means that all UAVs can safely reach their destinations simultaneously. Existing methods, which rely on a spatio-temporal decoupling framework and design the coordination law at the control layer, often compromise maneuverability and safety in coordination scenarios. To address the challenges, we propose a planner-based framework that enables fixed-wing UAVs to navigate in unknown, unstructured environments with time-critical coordination. This framework, which is proposed with theoretical analysis, coordinates the UAVs at the planning layer rather than the control layer. Moreover, a differential-flatness-based trajectory planning method is presented within this framework, after which a two-step method is designed to undermine the problem of local minima. The proposed methods are validated through software-in-the-loop (SITL) simulations and semi-physical experiments utilizing general flight control units (FCUs), verifying the effectiveness of time-critical coordinated planning of fixed-wing UAVs in challenging environments.