This paper presents a positioning control method for underactuated unmanned surface vehicles (USVs) subject to environmental disturbances and wave-contaminated measurements. In underactuated dynamic positioning, surge motion and yaw motion can be directly regulated by the propulsion system, whereas sway motion cannot be directly controlled by an independent lateral thrust. Therefore, the lateral environmental-force component is utilized to induce the vehicle’s dynamic response to sway. To achieve this objective, a position-keeping guidance system taking into account the desired heading and the lateral positioning error is introduced in this paper. With this guidance mechanism, the lateral environmental-force component can drive the USV to reduce the cross-track error, thereby enabling underactuated positioning. A rotated coordinate system is established around the desired position, and the positioning error is decomposed into along-track and cross-track components. To improve the transient response, an error-rate feedback term is introduced into the rotated-angle update law for yaw-heading guidance design, which enhances the damping of the cross-track dynamics. Meanwhile, a wave-filtering observer is designed to make low-frequency position and velocity estimates for feedback control. Simulation results under multiple operating conditions show that the proposed observer reduces the amplitude and high-frequency variation of the control signals compared with the existing wave-filtering observer, and the proposed positioning control method achieves smaller positioning errors than the existing nonlinear positioning control (NPC). The comparative results also indicate that the proposed method is suitable for position keeping under constant or slowly varying environmental loads, moderate model uncertainty, and wave-contaminated measurements, whereas rapidly varying load directions may degrade the positioning accuracy.

Autonomous underwater vehicles (AUVs) have been extensively employed in diverse marine applications, where depth-tracking control remains a critical technical challenge. Although previous studies have investigated various guidance law designs for underactuated AUVs, compensation for the attack-angle effect induced by heave velocity remains underexplored, particularly in the presence of extreme time-varying external disturbances and sharp path curvature variations. Existing line-of-sight (LOS) based methods, including integral LOS (ILOS) and adaptive LOS (ALOS), rely solely on the integral action of the pitch-tracking error to estimate the attack angle, which inevitably introduces inherent time-delay characteristics and induces overshoot and chattering in the depth-tracking response. Given that a system’s position and its velocity are uniquely determined by its acceleration, a response-tunable adaptive LOS (RTALOS) guidance design is proposed for AUV depth-tracking control task. This guidance law incorporates both the depth-tracking error and its velocity term as boundary constraints, generating an acceleration reference through a time-based planning polynomial to achieve accurate attack-angle estimation. The proposed method redefines attack-angle estimation as a new paradigm based on a second-order depth-tracking kinematic system, where the introduced velocity term acts as an adjustable damping effect to effectively suppress the depth-tracking overshoot and oscillation. Simulations were conducted to verify that the proposed guidance design could make the depth-tracking motion converge within a given time duration if the state-constrained acceleration was promptly executed. Compared to the selected ILOS and ALOS methods, the proposed RTALOS method can rapidly and accurately compensate for time-varying attack-angle effects via the estimation of attack angle, resulting in precise depth-tracking control performance, even under severe time-varying disturbances and when the pitch-tracking control performance is unsatisfactory.