## 1 Introduction  
In recent years, unmanned surface vehicles (USV) have received significant attention, due to their broad applications in the marine environment, such as ocean surveillance, gas exploration, and marine transportation [1; 2; 3]. In these applications, the high tracking performance and complex external environment make the USV path following control challenging. Path following is a motion control scenario where the USV has to follow a predefined path without a temporal constraint [4; 5; 6]. At present, various control methods have been proposed in the literature. In [7], an adaptive control approach based on the backstepping technique was developed for an unmanned marine surface vessel, which was also suitable for other unmanned vehicles. Using the dynamic surface control (DSC) method, a practical path following control scheme was developed for underactuated ships in [8], whose advantage was that it could deal with the practical condition \"waypoints based navigation\". The paper [9] presented an effective control approach for path following of marine vehicles, where the input-output feedback linearization method being applied by defining \"hand position\" as the output. In addition, a popular and effective guidance method adopted in path following is line-of-sight (LOS) guidance [10; 11; 12; 13]. The main idea is to implement a lookahead based LOS guidance law mimicking a helmsman and generate a desired heading angle, which is fed into the inner dynamics loop. In [14], an integral LOS (ILOS) guidance law based on monotone cubic Hermite interpolation was developed, and the main innovation was that it proposed a time-varying equation for the lookahead distance. In [15], a novel guidance control method was presented for a marine surface vehicle (MSV), in which the unknown sideslip angle was estimated using a reduced-order ESO. The paper [16] proposed a sideslip-tangent LOS (SLOS) guidance strategy, where the sideslip angle was eliminated by constructing a finite-time sideslip observer (FSO). Compared with [14; 15], the advantage was that it could handle a large sideslip angle. The paper [17] proposed an adaptive LOS (ALOS) algorithm for unmanned aerial vehicles, in which an adaptive law was constructed to handle the unknown sideslip angle. On the other hand, in order to deal with complex interference, scholars have put forward many excellent solutions [18; 19]. A concise path following controller was designed for a podded propulsion USV in [20], in which unmodeled dynamics and disturbances were handled by introducing the neural network minimum parameter learning method. The paper [21] proposed a robust compound control method subject to bounded uncertainties, where LESO was used to estimate the system uncertainties. A novel sigmoid function based disturbance observer was designed to eliminate various uncertainties effectively in [22], which was proven to be an efficient observer. The paper [23] developed an online constructive fuzzy approximator by the decoupled structure learning mechanism, which was incorporated into the dominant adaptive controller to identify unknown nonlinearities and unknown external disturbances exactly. Besides, input saturation [24; 25] is common and inevitable, which may lead to system instability. In [26], a path following controller was presented for a surface vessel, in which the auxiliary system developed could cope very well with input saturation without any downtime. Considering disturbances and input saturation, the paper [27] addressed the course tracking problem of ships, where the rudder saturation was compensated using an auxiliary dynamic system. The paper [28] developed a novel trajectory tracking control method, in which a Gaussian error function was introduced to approximate input saturation. Motivated by the above observations, considering unknown dynamics, disturbances, and rudder saturation, this article presents a novel path following control strategy for the USV. This paper covers the following three contributions:  
1.