Robust Near-Optimal Nonlinear Target Enclosing Guidance

This paper proposes a nonlinear optimal guidance law that enables a pursuer to enclose a target within arbitrary geometric patterns, which extends beyond conventional circular encirclement. The design operates using only relative state measurements and formulates a target enclosing guidance law in which the vehicle's lateral acceleration serves as the steering control, making it well-suited for aerial vehicles with turning constraints. Our approach generalizes and extends existing guidance strategies that are limited to target encirclement and provides a degree of optimality. At the same time, the exact information of the target's maneuver is unnecessary during the design. The guidance law is developed within the framework of a state-dependent Riccati equation (SDRE), thereby providing a systematic way to handle nonlinear dynamics through a pseudo-linear representation to design locally optimal feedback guidance commands through state-dependent weighting matrices. While SDRE ensures near-optimal performance in the absence of strong disturbances, we further augment the design to incorporate an integral sliding mode manifold to compensate when disturbances push the system away from the nominal trajectory, and demonstrate that the design provides flexibility in the sense that the (possibly time-varying) stand-off curvature could also be treated as unknown. Simulations demonstrate the efficacy of the proposed approach.