A Pre-Tactical Conflict-Free 4D Trajectory Planning Method for Multi-UAVs

With the continuous growth of the number of unmanned aerial vehicles (UAVs), the low-altitude airspace is poised to face a complex operational scenario characterized by high traffic flow, high density, and close intervals. Pre-flight trajectory planning for UAVs has thus become crucial to alleviate conflict pressures in subsequent actual operations. This paper proposes a pre-tactical conflict-free four-dimensional (4D) trajectory planning method for multi-UAV route operations. The method first generates initial 4D trajectories integrating flight plans, performance parameters and environmental constraints. To reduce multi-UAV conflict detection complexity, a hierarchical strategy is adopted: a 4D grid-based model performs preliminary conflict screening, followed by geometric-based precise determination of potential conflicts. A dynamic grouping strategy is introduced to group conflicting trajectories, within which an improved genetic algorithm (GA) optimizes take-off times for conflict resolution. Through the coevolution of grouped trajectories, high-quality 4D planning is achieved to ensure conflict-free multi-UAV operations. Simulation results demonstrate that the proposed method outperforms comparative algorithms in terms of average fitness value, average delay time, and convergence speed, thereby alleviating potential flight conflicts and ensuring low-altitude flight safety.