## 1 Introduction
### Wave Glider (WG)  
The WG is an autonomous unmanned marine vehicle comprising two modules: a surface module (_float_), performing motions under the action of waves, and an underwater (_winged_) module connected with the latter through umbilical and comprising a wing system which generates thrust due to the motions of the upper module in the waves. The WG is a perfect interface platform designed to provide a gateway between the agents of the global robotic information system operating in the upper and the lower spaces. The platform is driven entirely by the renewable energy of the ocean, converting wave power into the translatory motion of the system and using solar panels fixed on the upper deck to feed instrumentation. Wave-energy-harvesting principle has been implemented in marine robots exemplified by the wave glider (WG) of Liquid Robotics, introduced in 2005-2007, and an USV named _AutoNaut_, of the University of Southampton (2016). The Liquid Robotics-type WG has two modules. The upper module is subject to the action of waves and transmits its motions through a umbilical to the deeply submerged lower thrust-generating wing module which propels the vehicle, controlled by the rudder fitted on the surface platform. AutoNaut has a floating elongated displacement hull which is equipped with springed high-aspect ratio wings fitted at the extremities. Hereinafter we provide a short review of some relevant publications on the WGs. [PERSON] et al. (2016) and [PERSON] (2017) state that the key innovation of the Liquid Robotics Wave Glider (LRWG) is its ability to harvest the abundant energy in ocean waves to provide an essentially limitless propulsion. Ocean waves possess substantial power, and the WG harnesses this power to maintain an average reported forward speed of 0.78 m/s (1.5 kts) in seas with a 0.5-1 m wave height. Note that the LRWG has been designed to withstand large open-ocean waves and strong winds with its low-profile surface float, high-strength tether and robust submerged glider. In 2007, during Hurricane Flossie, it demonstrated an ability to operate in seas with wave heights over 3 m and winds greater than 21 m/s (40 kts). While surveying along the Alaska coast, the LRWG successfully operated in waves over 6 m high and winds greater than 26 m/s (50 kts). The robustness of the LRWG design has led to many successful missions over long periods in strong and weak seas with many different deployment configurations. [PERSON] and [PERSON] (2007) defined their work as estimating the WG dynamics for precise positioning and developed a 2D model in longitudinal motion. They calculated the wave and driving force with the use of empirical data, and produced a simulation for some concrete values of wave height and period. [PERSON] et al. 's (2018) study focuses on the Wave Glider Platform from Liquid Robotics and is targeted at determining a kinematic model for offline planning that provides an accurate estimation of the vehicle speed for a desired heading and set of environmental parameters. Given the significant wave height, ocean surface and subsurface currents, wind speed and direction, the authors present the formulation of a system identification to estimate the vehicle's speed over a range of possible directions. Based on the data collected over long-duration missions, it has been observed that the LRWG is able to maintain an average speed of approximately 0.8 m/s. [PERSON] et al. (2018) applied Gaussian process models to existing wave data to predict the performance and carried out an effective method for forecasting WG velocity. In [PERSON] et al. (2018), the nonlinear dynamic model of a wave-driven unmanned surface vehicle (WUSV) in two dimensions was established based on the analysis of its driving principle in the longitudinal direction. The authors calculated the wave and driving force, determined the hydrodynamic coefficients with the use of empirical data obtained with an experimental platform of WUSV, and, finally, presented the simulation results of the model.