Abstract: To meet the requirement of missiles attacking high maneuvering targets with desired impact angles during terminal guidance, a second-order sliding mode guidance law satisfying impact angle constraint is proposed by combining the super-twisting algorithm with adaptive sliding mode disturbance observer. The disturbance caused by target maneuver leads to the unknown upper bound of system disturbance. Taking target acceleration as system disturbance, an adaptive sliding mode disturbance observer is designed for on-line system disturbance estimation. By adaptively adjusting the observer gain, the defect that the traditional observer depends on the upper bound of disturbance when selecting the gain can be overcome.The improved super-twisting algorithm is designed as the reaching law of the guidance law, enabling the guidance law to make full use of the overload capacity of the missile while reducing the chattering, thus improving the convergence speed of the system, and solving the problem of slow convergence speed when the system state is far from the equilibrium point in the traditional super-twisting algorithm. Based on the Lyapunov stability theory，it is proven that the guidance system can converge in finite time. Finally, the effectiveness of the adaptive sliding mode disturbance observer and the designed guidance law is verified by mathematical simulation. The adaptive sliding mode disturbance observer can accurately track system disturbance, and the designed guidance law can meet the expected terminal angle constraint and has high impact accuracy, with the miss-distance smaller than 0.2 m.

Key words: terminalguidance, adaptiveslidingmodedisturbanceobserver, super-twistingalgorithm, chatteringproblem, lyapunovstability