## 1 Introduction
### Underwater Glider (UG)  
The UG is an autonomous underwater vehicle propelled by gravitational forces produced through an excess of weight and/or volume, resulting in a positive or negative buoyancy. In terms of technology evolution, the UG is a further step from the so-called _diving buoys_, whereby, due to the installation of lifting elements, the vehicle acquires a horizontal component of motion. Moving under water in a _dead reckoning mode_, the UG needs to surface regularly in order to transmit collected data to the satellite and receive further instructions and corrections of the trajectory. Eventually, the underwater gliders perform saw-like motions limited by the design depth from below and the free water surface from above or other designated depth corridor. Driven by a small excess of buoyancy, the UGs are designed to move slowly but for large distances, which makes them an economically viable ocean monitoring and data collection tool. These unique capabilities of the UG can be multiplied through the use of their _swarms_, thus opening remarkable perspectives for efficient spatial-temporal measurements. The concept was demonstrated by [PERSON] in 1989, followed by the emergence of the first vehicles shaped as bodies of revolution with high-aspect ratio wings (see reviews [1; 2; 3; 4; 5; 6; 7]). Typical UGs of this type were developed by the University of Washington (\"Seaglider\", 2001), Webb Research Corporation (\"Slocum\", 2001) and Scripps Institution of Oceanography (\"Spray\", 2001). The new types are represented by the Deepglider ([PERSON] and [PERSON], 2007) with 6000-m operating depth, the Z-Ray ([PERSON] et al., 2005) with heavy payload capability and the Sea-explorer ([PERSON] et al., 2014) with high-gliding velocity. In April 2018 China's _Haiyan_ series UG has become the world's deepest underwater glider by diving to 8213 m in the Mariana Trench. In December of the same year _Haiyan_ series UG set a new endurance record after working for 141 days and sailing 3619 km in the South China Sea. As the trials showed a direct dependence of the range of UG upon their lift-to-drag ratio, further R&D in this field resulted in the emergence of _flying wing_ configurations of UGs, exemplified by several vehicles of the _Libedale_ class [8], such as \"StingRay\" (2004), \"XRay\" and its modifications (2005-2008). Note that _Libedale_-class vehicles distinguish themselves by their improved hydrodynamic characteristics (e.g., lift-to-drag ratios exceeding 17-20) and better payload ratios. It was shown in the course of the UGs monitoring operations that for the observation of some larger-scale and longer-lasting ocean phenomena a much larger riding range is required, which can be achieved by increasing the battery capacity in the limited internal space of the UG and reducing energy expenses wherever possible. Despite the fact that underwater gliders emerged relatively recently, quite a number of papers have been published on different aspects of their development. Some of selected papers, reflecting the initial and recent developments of the UG, can be found in the reference list and are briefly analyzed below. A whole range of issues related to the composition of the UG and the concretization as to the choice of the hull form, the geometry of the fixed wings and rudder, the variable buoyancy arrangements, the stability at the desired equilibrium and the motion control have been considered in [PERSON] et al. [9]. A comprehensive review of UG technology development was published by [PERSON] [10]. An important contribution to the UG dynamics fundamentals was made by [PERSON] [11], in his Princeton dissertation, 2005. As follows from relevant papers, the hydrodynamic characteristics of the UG have been obtained experimentally, computationally and by means of identification from full-size trials data. [PERSON] et al.