Abstract: When the radar seeker overlooks and detects an ultra-low altitude target, the tracking precision is greatly reduced because of the existence of multipath interference. In order to minimize the interference, the line-of-sight (LOS) angle must be constrained to Brewster’s angle. A nonsingular fast terminal sliding mode guidance law is designed based on the terminal sliding mode control method. Aiming at the problem that the target acceleration is difficult to be accurately obtained using the guidance law, a sliding mode disturbance observer (SMDO) is designed to estimate the target acceleration. A composite nonsingular fast terminal sliding mode guidance law is designed by introducing the estimated value of the observer and the sign function with switches coefficient into the nonsingular fast terminal sliding mode guidance law. It is proved by Lyapunov theory that the proposed composite guidance law enables the LOS angle to converge quickly to the Brewster angle and the LOS angular rate to converge to a small neighborhood near zero in finite time. Simulated results show that the miss distance is reduced within the kill radius when the proposed guidance law is used in the ultra-low altitude target interception. Compared with the traditional sliding mode guidance laws, the convergence time of the system state can be reduced about 4 s by the proposed guidance law.Key

Key words: guidancelaw, multipathinterference, finite-timeconvergence, slidingmodedisturbanceobserver