Disturbance-observer-augmented Backstepping control for sea-skimming UAVs under severe wave uncertainties

A sea-skimming unmanned aerial vehicle (UAV) is a specialized type of aircraft capable of performing tasks at low altitudes above sea level. However, under extreme sea conditions, wave disturbances introduce significant errors in measuring the relative distance between the UAV and the sea surface, which complicates the control system and degrades flight performance. While existing research primarily relies on filtering or adaptive control methods, their effectiveness in handling non-Gaussian disturbances remains limited. In this paper, a mathematical model for the altitude and pitch motion of a sea-skimming UAV is established, taking into account external disturbances caused by sea wave height. A control method integrating Backstepping theory with a nonlinear disturbance observer (NDO) is designed to ensure system stability. The key innovation of this work lies in using the NDO to estimate and compensate for aggregated disturbances, thereby enabling the Backstepping controller to achieve exponential stability without a parameter adaptation process. Furthermore, a quasi-adaptive coefficient function is incorporated into the NDO to adjust its gains in real time, improving estimation performance. The stability of the proposed method is rigorously proven using Lyapunov theory. Simulation results demonstrate the effectiveness of the NDO-based Backstepping control scheme, showing that it maintains a tracking accuracy of 98.52% even under severe level 7 sea condition.