This paper proposes a computationally efficient algorithm for nonlinear optimal guidance with a predefined final flight path angle. Although numerous impact angle guidance methods based on optimal control theory exist, a lack of efficient calculation procedures remains for the exact nonlinear engagement model, leaving practical hardware implementation challenges for the end-user. A fixed-structure algorithm with deterministic computational burden is developed for real-time onboard integration. The performance and optimality of the algorithm are verified through a comparative study with established guidance laws. Unlike methods relying on line-of-sight rate or time-to-go estimations, the proposed approach uses a closed-feedback form based on standard navigation data. A closed-form solution is derived for the climb phase to the cruise altitude. Practical feasibility is demonstrated on a microcontroller-based onboard computer, with execution times analyzed for flight software compatibility. The robustness of the proposed framework is validated via high-fidelity hardware-in-the-loop tests for two distinct scenarios: a multi-phase cruise mission and a short-range ballistic trajectory subject to propulsion uncertainties. Results confirm high precision and accurate impact angles across vastly different flight regimes, ranging from low-altitude cruise to high-dynamic reentry.
Miličić et al. (2026) studied this question.