## 5 GNSS Implementation  
To increase the popularity of satellite positioning in rail guidance and control, the European GNSS Agency (E-GNSS) is working with stakeholders in the rail and space industries. The deployment of GNSS within railway systems is possible in the area of the European Train Control System (ETCS), which is currently being implemented both in Europe and beyond as one component of the European Rail Traffic Management System (ERTMS). Under ETCS, train positioning is currently based on determining the position of a rail vehicle using an odometer, whose potential measurement error is corrected based on so-called balises, which are physical elements mounted at specific intervals along the railway tracks. All information is exchanged between the ETCS on-board system and the RBC (Radio Block Center) trackside system via a radio communications network. The train transmits two pieces of information to the RBC: its location and confirmation that it has not lost any cars. This information is called \"integrity\" in the rail industry and is a safety feature. The potential of using GNSS for railway applications in this case manifests itself in the ability to replace, wherever possible, physical balises with virtual balises (VBs) based on precise GNSS positioning, without any operational or safety implications for ETCS [97; 98; 99]. The ETCS, due to its functionality, is divided into several levels, which include Level 0, Level 1, and Level 2. In the review in question for the ETCS system, Level 3 additionally appears. The above is due to the fact that the review article is developed on the basis of the literature issued before the update of the so-called Technical Specifications for Interoperability (TSI) issued on 28 September 2023. Within the framework of the TSI in question for the Control-Command and Signaling Trackside Subsystem (CCS TSI), ETCS is expected to migrate to baseline 4, which does not include ETCS Level 3 in the further development process. However, due to the nature of this review article, ETCS Level 3 (ETCS L3) will be considered in the remainder of this article; Level 3's operating logic enables the implementation of GNSS functionality in train positioning and with regard to path setting, consisting of determining the destination and ensuring safe passage along routes in areas where there may be conflicts (intersection, junction, railway crossing). The above refers to the so-called dependency systems (IXLs), the purpose of which is to verify the possibility of the route along with securing the routing--in accordance with safety logic--of each relevant part of the infrastructure until all trains have passed through all sections of track reserved for the route. Thus, the system receives information on the occupancy of \begin{table}
\begin{tabular}{c c c c c c} \hline
**Reference Orbit Parameter** & **GPS** & **Galileo** & **GLONASS** & **BeiDou** & **QZSS** \\ \hline Number of satellites (current usable) & 38 & 28 & 26 & 35 & 4 \\ \hline Orbit semimajor axis [km] & \(\sim\)20,200 & \(\sim\)23,200 & \(\sim\)19,100 & \(\sim\)21,500 & \(\sim\)42,000 \\ \hline Orbit eccentricity & \(<\)0.02 & \(<\)0.16 & \(<\)0.04 & 0.00 & \(<\)0.08 \\ \hline Orbit inclination [deg] & 55.0 & 56.0 & 64.2-65.6 & 55.0 & 43.0 \\ \hline Date of origin & 1970s & 2000s & 1980s & 1990s & 2010s \\ \hline \end{tabular}
\end{table} 
Table 6: Parameters of satellite navigation systems. a section from trackside equipment on input (e.g., obtained from axle counters or track circuits), then sends commands to trackside equipment based on relay or computer devices to make automatic adjustments, and transmits free clearance information to the train. The latter information constitutes the so-called permit to run (MA). The MA is transmitted via radio messages by the Radio Block Center (RBC) to an on-board device built into the driver's cab.