In the complex urban environments, owing to the presence of dense, irregular obstacles and the strong nonlinear dy-namics, the trajectory planning for fixed-wing UAVs poses significant challenges of safety risks and low solving effi-ciency. To address these issues, this paper investigates a spatial-temporal hierarchical trajectory planning method for fixed-wing UAVs based on differential flatness. A hierarchical planning framework of “path planning, safe corridors generation and trajectory optimization” is constructed. Firstly, the heuristic graph search is utilized to obtain the refer-ence path as the initial motion. Guided by the path planning results, the safe flight corridors are generated through the iterative regional inflation method, providing feasible regions for trajectory optimization. Subsequently, to reduce the complexity of trajectory optimization, a differential flatness-based spatial-temporal trajectory parameterization model for fixed-wing UAVs is established, enabling the elimination of nonlinear dynamics, terminal and safety constraints. Additionally, through designing penalty costs to handle flight performance constraints, the original problem is trans-formed into an unconstrained nonlinear optimization problem with analytical gradients. Finally, a differential flatness-based spatial-temporal hierarchical trajectory planning algorithm (STH-DFTO) is proposed to achieve efficient trajecto-ry optimization. The simulation results illustrate that the proposed STH-DFTO has the superior efficiency, which only takes 10-2-10-1 s to generate the flight trajectories for fixed-wing UAVs in complex urban environments, meeting the requirements for online trajectory planning in practice.
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