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This paper presents a new approach for wave filtering and state estimation in regulation and dynamic positioning of marine vessels. In this approach, low-frequency (LF) and wave-frequency (WF) motion components, LF disturbance forces, and vessels velocities devoid of oscillation are estimated from the measured position signals. The proposed system is composed of a modified notch filter and a nonlinear observer. In this method, filtering is independent from the vessel and LF disturbances models, and therefore it is not affected from modeling uncertainty. Furthermore, the proposed structure is furnished with an estimation law of fundamental frequency of WF vessel motion that improves its performance considerably. It is shown that the basic structure is globally exponentially stable and its adaptive version is locally asymptotically stable. Simulations illustrate the desirable performance of the proposed methods, the …