## 6 Discussion  
A review of the current state of the art in potential implementations of Global Satellite Positioning Systems in rail transportation, with a particular focus on train guidance and  
Figure 12: Principle of maintaining safe distances between train corollaries. Prepared on the basis of [100]. control systems, illustrates the widespread interest in the application of positioning of rail vehicles using satellite systems. The main direction of research and implementation projects revolves around the European Train Control System, one of the areas of development of which is the application of mobile block spacing. The potential benefits of using mobile spacing are widely reported in the literature, and in conjunction with the issue at hand, issues are being raised regarding the application of precise localization of rail vehicles on railway infrastructure. In this context, GNSSs are pointed out as one of the potential systems to be used in the guidance and control of rail traffic. The literature considered in this review describes in detail the solutions involving the use of GNSSs, indicating the need to specify the technical requirements, which are the basis for the design and construction of guidance and traffic control systems, in which the module responsible for locating rail vehicles would be based on satellite solutions. The above requires the clarification of technical standards and the definition or adaptation of the existing regulations on railway traffic guidance. The issue of GNSS application is often combined with the use of multi-sensor on-board equipment, the architecture of which is based on the use of satellite positioning along with the use of a so-called odometer, which is part of the equipment of rolling stock moving under the indications of the ETCS. The literature base of the state-of-the-art review takes into account a number of items characterizing the above solutions, which can form the basis for the implementation of satellite systems on the railway. Another issue, taken into account in part of the literature items, is the concept of creating and using a digital map in directing and controlling railway traffic. In the commercial application of satellite navigation, the map provides a reference point for locating a vehicle in space. Similarly, in the subject of the use of satellite positioning in rail transport, it is necessary to create a digital network of railway infrastructure. The authors of publications describing the above issue indicate that there is a very large gap in this area, which may translate into difficulties and limitations in the possibility of implementing GNSS in railway solutions. The above is highlighted primarily in terms of existing infrastructure, for which economic considerations may preclude the possibility of creating a digital map. Analyzing the literature base of this review, it can be observed that most of the publications do not describe issues related to the accuracy of positioning of rail vehicles and the risks of using GNSS. The most frequently cited information, indicating the potential positioning accuracy for particular GNSSs, is due to the duplication of information on positioning accuracy given as one of the parameters of a given satellite system. Another reference is the parameters derived from the characteristics of global satellite systems in relation to particular branches of commercial use, including use in transportation in the broadest sense. In a small part of the cited literature items, the above issue is addressed, and even more rarely are attempts at research in this area indicated, which is a potential basis for the development and conduct of further studies of satellite positioning accuracy of rail vehicles. In addition, it should be noted that the literature to date does not accurately describe the issue of how and what criteria are used to assess the safety of using positioning systems in rail transportation. As indicated in this article, for railway and aeronautical applications, there are different approaches to risk assessment.