Unified Lateral-Axial Nonlinear Guidance Law for Fixed-Wing Path Following

Path following for fixed-wing aircraft near performance limits requires guidance that is robust to disturbances and demanding geometry. Existing [Formula: see text] nonlinear guidance laws exhibit undefined geometry beyond the lookahead distance, overshoot scaling with initial cross-track error, and decoupled lateral and speed subsystems. This paper introduces a unified framework coupling a lateral law ([Formula: see text]) using arc-length parameterization and a speed law ([Formula: see text]) enforcing curvature-feasible airspeed limits. A Lyapunov direct-method proof establishes asymptotic stability of [Formula: see text] on a straight path for any positive lookahead distance, and an analytical inflation condition prescribes the lookahead for curvature-reversal handling under saturated first-order roll dynamics. The adaptive time constant holds intercept overshoot below 30 m for cross-track errors from 50 m to 10 km. The speed law anticipates upcoming curvature, wind, and turbulence, adjusting airspeed to maintain turn feasibility while detecting path infeasibility before arrival. Unlike model predictive control, computations are closed-form with deterministic execution suitable for certified systems. A 500-trial Monte Carlo on a flight-test-validated Learjet six-degree-of-freedom (6DOF) simulation spanning 50 varying-curvature paths, winds of 0–50 kt, four turbulence severities, three altitudes, and three weights produced zero tracking failures with [Formula: see text] average cross-track error.