## 1 Introduction  
Vessel Traffic Services (VTS) systems are dedicated to the monitoring and control of vessel traffic in indicated water areas [1]. The need to establish VTS systems is highlighted in the International Convention for the Safety of Life at Sea (SOLAS) [2]. Usage of these systems contributes, e.g., to the enhancement of safety of life at sea, navigation efficiency and the marine environment protection against the possible adverse effects of maritime traffic [2]. VTS systems are introduced by maritime administrations for several reasons, including [1] observed increased vessel traffic, navigation of vessels carrying dangerous cargoes, the significant impact of hydrometeorological conditions on vessel safety, navigation in narrow passages, fairways integrated with complex port structures with imposed routing patterns, the need to protect the marine environment, etc. Maritime administrations usually prioritize the issues related to navigation safety and the protection of the marine environment within the conducted activities. Maritime administrations may establish a VTS system when, in their opinion, such services are justified, considering the traffic volume or risk degree in the operated area [2]. Key benefits of implementing VTS include the following [3]:  
* identification and monitoring of vessels;
* strategic vessel traffic planning;
* vessel traffic management;
* provision of information that can influence vessel traffic and assist in decision-making;
* support of navigational safety of ships by providing relevant navigational information;
* supporting related activities such as pilotage;
* search and rescue, maritime security, law enforcement, etc. Nowadays, a great attention is paid to the development of efficient, sustainable and safe transport systems [4; 5; 6]. VTS systems are extremely important components of modern transport systems. They make it possible to not only to improve operational efficiency and reduce costs of ships service in set water areas, but also to increase safety, better manage risks and improve the quality of services provided [7; 8; 9]. According to the Act of 21 March 1991 on Sea Areas of the Republic of Poland and the Maritime Administration [10], vessel traffic monitoring and control is one of the four pillars of Polish maritime administrations' activities. Currently, three VTS systems are operated by maritime authorities in the area of Polish territorial waters (Figure 1) [7; 8; 9]:  
* VTS Zatoka Gdanska;
* VTS Lawica Shupska;
* VTS Swinoujscie-Szczecin. As a rule, VTS systems were created based on services previously provided by harbor masters' duty services for vessel traffic and order management in the port. VTS Swinoujscie-Szczecin was established in 2000 in accordance with the guidelines [1; 2], followed by VTS Zatoka Gdanska (year 2003) and VTS Lawica Slupska (year 2010). Even though the main objective of VTS activity deals with improving navigation safety and the marine environment protection, the organization of the above-mentioned systems, their architecture and realized functions are different. Polish VTS systems do not form a unified, common system. Each system performs a local function, offering services within a selected area of responsibility. As a part of the National Maritime Safety System (NMSS) project completed in 2015 [12], the technical infrastructure of VTS systems was replaced and unified (hardware-wise). The NMSS project  
Figure 1: Areas of responsibility of the VTS systems operated in Poland (our own elaboration based on [11]). was a joint venture between maritime authorities to comprehensively modernize technical infrastructure, including the VTS systems. A common network of Automatic Identification System (AIS) receivers was also created [2], ensuring the reception of data sent by vessels in the entire area of responsibility of the Polish maritime administration.