METHODS FOR ENSURING THE NAVIGATION SAFETY OF UNMANNED SURFACE VESSEL

When creating unmanned surface vehicles (USV), special attention is paid to the safety of navigation. One of the main threats at sea is the threat of collision. Two main directions of ensuring the safety of navigation can be distinguished. The first is legal regulation and a number of International documents that are binding on all ship-owners. The second is technical control systems and software, the purpose of which is ensuring the safety of navigation. This work is devoted to the issue of determining the level of collision danger and reaction to this danger from the system of automatic control of the course and speed of the USV, which acts as the object of study. The subject of research is management processes and algorithms. Given the significant danger that automatic mobile systems at sea can pose, maritime safety issues are a priority.The analysis of effective control systems for autonomous mobile vehicles shows that their creation is based on relatively simple, but fairly accurate abstract models of interacting media (physical and informational). Such models are the starting point for the creation of automated and automatic systems, which include USV as well. Paying attention to the technical side of the problem, it should be noted that determining the level of danger and the reaction to it from the side of the USV control system also requires some formalization.In this paper, a method is proposed for determining the danger of USV collision with other moving and stationary marine objects. The generalized algorithm of the control system for the course and speed of the USV is determined. The reaction of the propulsion (propulsive) system and the necessary composition of on-board equipment to ensure the safety of navigation are determined. It should be noted that in the work under the USV let's mean small-tonnage (up to 1 t) surface self-propelled floating craft of the boat or boat type.The research results will be useful in constructing control systems based on fuzzy or neuro-fuzzy controllers.


Introduction
The goal of creating unmanned surface vessels (USV) is increasing the effectiveness of state regulation in such an important area as environmental protection and monitoring the environmental safety of water areas and coastal zones. In addition, USV can be used to protect protected water areas, hydraulic structures and communications, as well as sunken historical monuments. With the help of USV, hydrographic work can be performed (mapping the bottom surface, controlling the depth of fairways, straits), search and rescue operations, exploration of natural resources, and the like. But when using USV in any of the above cases, it is necessary to ensure their safe and trouble-free operation. The main danger at sea is the threat of collision with other objects. It can be other watercraft, hydraulic structures (bridge supports, breakwaters, lighthouses), navigation aids (buoys), natural formations (rocks, shallows), coastline. To prevent the risk of collision, determining the degree of danger and how to reduce it is important.
As evidenced by numerous information sources, this problem is actively discussed by all stakeholders around the world. There are many approaches and solutions. In [1], a modern overview of various approaches to preventing collisions with several unmanned aerial vehicles is presented, as well as a classification according to the algorithms and structures used, their main features are discussed. In [2], the option of installing a specialized radio navigation system for collision warning on all ships of the world is considered. In [3], it is noted that the development of a high-level autonomous collision avoidance system for ships operating in an unstructured and unpredictable environment is a complex task. The authors propose solutions to this problem by establishing intelligent control systems based on neural networks that are pre-trained in difficult situations and tested in real-life conditions. In work [4] of the SACAS system (shipborne autonomous collision avoidance system), an architecture of parallel trajectory planning is proposed. Such a system includes a scheduler based on a modified RRT (a rapidly exploring random tree) algorithm designed to find the optimal global ISSN 2664-9969 trajectory at a low re-scheduling frequency. The system also contains a scheduler based on a modified DW (Dynamic Window) algorithm. The latter is used for parallel trajectory planning in order to generate an optimal local trajectory at a high redevelopment frequency and to counteract the unexpected behavior of dynamic obstacles in the immediate vicinity of the vessel. Numerous studies of maritime safety related to solving specific problems [5,6], where the problems of creating automatic motion control systems for unmanned boats and other self-propelled surface vehicles (USV -Unmanned surface vehicle) are solved. Separate studies [7] prove that humans can be a key factor in successfully preventing collisions in future operations using MASS (maritime surface autonomous ships). Thus, at the moment there is no single approach to solving the problem of navigation safety. Search and development of effective systems for preventing collisions at sea is also part of the subject matter of such scientific developments [8]. The need to create such systems is due to the constant increase in the number of autonomous vessels in the vast oceans [9].
Therefore, it is urgent to develop a method for ensuring the safety of navigation for unmanned surface vessels.
Thus, the object of study is an unmanned surface vessel. And the purpose of the work is to establish a method for determining the level of danger of collision of the unmanned surface vessel with other moving and stationary marine objects, engineering structures and natural formations.

Methods of research
Obviously, for the safety of USV navigation, it is necessary to give the control system (CS) of the ability to preidentify obstacles that are or may arise on the trajectory of its movement and actively evade them. It is important that the process of avoiding obstacles ends with the restoration of the main task. To do this, it is necessary to equip the USV CS: -system of sensors for the effective detection of surface/underwater obstacles; -reliable and productive computing equipment and peripherals; -software with algorithms that are based on information from sensors about the environment and mission goals, capable of generating adequate commands to executive devices. Currently, the most effective sensor systems are considered to be radar stations (radar) for illumination of surface conditions and sonar stations (SS) for illumination of underwater environment. It is also possible to use lidar (laser scanners) and vision systems in various optical ranges. These sensor systems must satisfy a number of conditions, but the main ones are the detection range, the resolution in range and the angle resolution of the obstacle.
The main actuator is the propulsion-steering complex (PSC), which should ensure the movement of the USV from the fluid spatial coordinate of the given spatial coordinate along a given path and a given period of time taking into account external disturbances.
The choice of engine type for the USV is a complex scientific, technical and engineering task, the solution of which should provide the necessary speed of the USV movement and its maneuverability. These characteristics directly affect the safety of navigation, but also determine the cost-effectiveness (autonomy), reliability, controllability and compactness of the entire system as a whole (Fig. 1).
An important issue when creating a USV with rowing electric installations (REI) is the choice of power source (PS). Currently, the main sources of electricity for autonomous vehicles are chemical batteries of various types (acidic, alkaline, silver-zinc, nickel-intentional, lithium-intentional). It is necessary to use of fuel (hydrogen-oxygen) elements. As additional sources, smallsized electromechanical converters (for example, a piston engine generator), wind, wave or photoelectric converters can be used.
It is advisable to use combined PSC, which use several power plants with different operating principles and independent energy sources. Such combined PSCs are attractive from the point of view of ensuring high reliability, and also directly affects the safety of navigation.

Research results and discussion
The general algorithm for the operation of the automatic control system for the USV movement, taking into account the need to ensure the safety of navigation, is shown in Fig. 2. In this algorithm, in a logical condition, operations are performed with the abstract value «danger zone». This is a value that can be determined, given the speed of the object located next to the USV, and the speed of the USV itself. The necessary components for accounting are also the distance and bearing to the object or obstacles. Thus, the term «danger zone» means the area of the water area in which there is a potential threat of collision.
Shown in Fig. 2 algorithm can be simplified if to assume that threats can only be mobile. Fixed marine objects (for example, buoys, rocks, pylons) can be dangerous if the USV moves on them. Therefore, in the algorithm in Fig. 2, it is possible to exclude blocks highlighted in color. Fig. 3 presents a diagram for determining the level of danger during movement (drift, positioning) of the USV. On the diagram, concentric dashed rings reflect the distance of obstacle detection «D-1» -distant, «D-2» -middle and «D-3» -near relative to the USV. Also on the diagram are given sectors (directions) to identify obstacles relative to the USV -«right on course» or «behind the stern», which are marked «0» and «C» on the diagram, respectively. The «straight ahead» sector is divided into two subsectors with an opening angle of 10° «LO» and «RO». Two subsectors are allocated from the side projections: on the left on the course «L1», on the left on the side -«L2», on the right on the course -«R1» and on the right on the side -«R2». Sectors «L1», «R1» have an opening angle of 65°, «L2», «R2», have an opening angle of 70°, «CL» and «CR» has an opening angle of 35°. The level of threat of collision (danger level) has three gradations, which are indicated in the diagram by pink, yellow and green colors. Let's consider the hazard level to be increasing from green to pink.
The diagram shown in Fig. 3 may have a different view depending on the operating mode of the USV, its speed or other parameters (weather conditions, sea waves, the presence of currents, etc.). i. e., where R j -radius of detection of obstacles (D-1, D-2, D-3); V -USV speed; G -force of excitement (for example, on a scale developed by the World Meteorological Organization); W -wind force (for example, on the Beaufort scale). In addition, the position of the USV in the diagram of Fig. 3 can also be adjusted. According to Fig. 3, it is possible to determine the behavior of the CS, which is considered appropriate to the level of danger. In this case, the following dependencies are valid when moving the USV with respect to the obstacle: where Δl -deviation of the radius vector (distance) to the obstacle in the time interval ; l i-1 -radius vector (distance) to the obstacle -preliminary value; l i -radius vector (distance) to the obstacle -current value.
The rate of change of distance (Table 1): It is also necessary to consider bearing on the obstacle relative to its own course: where Δj -change in the angle formed by the USV motion vector V and the radius vector l (distance) to an obstacle in a period of time Δt t t ; j i-1 -previous value of the angle formed by the USV motion vector V and the radius vector l of the obstacle; j i -current value of the angle formed by the USV motion vector V and the radius vector l of the obstacle. The angular velocity of the change in direction to the obstacle ( Table 2): The corresponding graphical representation according to the Tables 1, 2 on the movement of an obstacle relative to the USV is shown in Fig. 4. The main criterion for determining the response of the CS to the danger level with certain parameters of its movement is the USV maneuver in the opposite direction with the corresponding speed. The CS response can be determined by an expert method with their refinement on mathematical models of CS, USV, the environment and obstacle models. The most suitable CSs for calculating the presented positions are systems based on fuzzy or neurofuzzy controllers [10].
The intensity of shipping on sea routes, near ports and sea bases, as well as in the fairways of rivers, canals, channels, is quite high. Therefore, a prerequisite for ensuring the safety of navigation is the CS ability to detect, accompany and respond simultaneously to several obstacles, both surface and underwater.

Conclusions
In the course of the study, it is found that combined propulsion and steering devices for unmanned vessels enhance navigation safety by duplicating the propulsive function of one propulsion system of the second. In other words, combined PSCs have increased reliability. A hazard chart is also obtained that allows to formally determine the hazard level of identified obstacles. The hazard chart allows to adjust the level of danger taking into account factors such as sea waves, wind strength, speed of USV and the like. In addition, the USV position in the diagram can also change.
It has been established that based on measurements of sensor systems, it is possible to formally determine the position, direction and speed of movement of hazardous areas, and their degree of danger with respect to USV. Based on the information received, in accordance with the laid down collision avoidance subroutine, control commands for the propulsion and steering complex can be generated.
The research results will be useful in constructing control systems based on fuzzy or neuro-fuzzy controllers.