## 5 GNSS Implementation
Dependency systems are traditionally trackside national systems based on national signaling rules, but they are also under discussion to harmonize their operation and interfaces, especially with ETCSs [63; 100]. The IXL operating logic described above is characterized by features that limit the possibility of optimal use of the railway infrastructure. A method of improving the efficiency of the control subsystem is to implement the so-called moving block spacing, the boundaries of which are not defined in a fixed way through reference points of a certain route kilometer distance. The boundaries of the moving interval change dynamically depending on the current traffic situation, and the distance between trains is a variable value that is the resultant of a set of current information on the position of individual trains and their speed. The movement of a rail vehicle in the interval of the braking distance, which is provided by systems that allow for continuous updates of information and transmit them to the vehicle in the form of an MA permit, which contains, in basic terms, a set of elements that determine its validity (km of the route to which the train can move) and the maximum speed granted in the MA. The differences resulting from the movement of a rail vehicle in fixed block spacing and moving block spacing are shown in Figure 11.  
In functional terms, a movable block spacing is a continuous extension of the section of track on which a train with an issued MA can travel, along with information on the permissible limiting speed at which the required distance between the end of the preceding train and the head of the train for which the MA was issued is maintained, which is not shorter than the braking distance of the train for which the MA was issued [101; 102]. The principle of mobile spacing is shown in Figure 12.  
The use of mobile spacing is possible only when the exact position of rail vehicles in a given area is known. The potential for using GNSS in the functionality of ETCS L3 offers such a possibility. However, it should be taken into account that technological changes to apply GNSS to ETCS L3 will also affect the logic rules of the dependency systems to achieve functionality in accordance with the moving distance principle [100; 103]. Figure 11: Functionality of the ETCS L2 and L3 system with respect to fixed and moving block spacing. Prepared on the basis of [100]. The implementation of satellite positioning systems will allow for the protection of runs with moving block spacing in railway traffic control systems. However, it should be borne in mind that a train, unlike an airplane, moves close to various objects that are considered obstacles to signal transmission. Consequently, various solutions have been developed to deal with these effects. One classic solution is based on an odometer and the use of GNSS positioning to calibrate the odometer error in case of discrepancies. However, most solutions in the literature are based on the fusion of heterogeneous information. Their principle is to combine heterogeneous sensors to use the advantages of each sensor to calculate position, velocity, and time. In view of the above and the complex environmental conditions along the railway alignment, positioning technology should develop towards integrated GNSS/INS. In completed studies, it has been shown that GNSS does not achieve continuous and stable positioning, especially in urban areas. It is also important to keep in mind GNSS data latency problems, which can cause further inaccuracies in rail vehicle positioning. A potential solution to the problem of low GNSS accuracy could be to use a system to combine the absolute positioning data of GNSS receivers with the relative data of sensors built on the rail vehicle, in order to realize precise vehicle positioning during GNSS outages. First, the system extracts the original data from all sensors and then requires pre-processing to convert these data into the variables necessary for the process of determining the correct location. The next step is data fusion.