Fractional-order sliding-mode guidance control for rendezvous of unmanned aerial vehicles during air refueling

A guidance law based on fractional-order sliding-mode control is developed to deal with the terminal angle and velocity constraints of unmanned aerial vehicle (UAV) in automatic rendezvous during air refueling. This law makes the UAV possess the ability to rendezvous with the tanker aircraft without active cooperation from the tanker aircraft. By considering the relative motion in three-dimensional space between the UAV and the tanker aircraft, we adopt the proportional navigation guidance (PNG) law in three-dimensional space to make the UAV fly to the desired formation point for refueling. Then, by making use of the robustness of the sliding-mode control and the meticulous characteristics of the fractional calculus, we develop a sliding-mode control law with fractional order to restrict the chasing angles of the UAV based on the variable structure control theory and the Lyapunov stability theory. The obtained sliding-mode control law with fractional order is employed to modify the PNG law, the resultant law will make the UAV possess the ability to fly to the desired formation point with the desired terminal chasing angle. Finally, a velocity command generator is developed to constrain the terminal velocity of the UAV. Simulations are carried out to validate the proposed guidance law; the results show that the designed guidance law achieves good control performance and satisfies the terminal angle constraint and the terminal velocity constraint.