Terminal Guidance Law with Impact Angle Constraint for the Fixed-Trim Reentry Warhead

As the fixed-trim reentry warhead only has a single axis of rolling control, a novel terminal guidance law with impact angle constraint is proposed in this paper. The three-dimensional terminal guidance law combines the traditional circular navigation guidance approach with impact angle constraint and the improved cross-product guidance method without impact angle constraint for fixed-trim warheads. According to the theoretical analysis, it has been proved that the new guidance law will guarantee zero miss distance and impact angle constraint by choosing proper guidance parameters. For improving the robustness of the guidance law, a linear disturbance observer is combined with the variable structure control to estimate the system uncertainties and disturbances during flight. Simulation results show that the proposed terminal guidance law can satisfy both miss distance requirement and impact angle constraint perfectly.