## 4 Overview of GNSSs  
A system of satellites that provides autonomous geospatial positioning with global coverage is called satellite navigation. GNSS includes several existing systems, such as GPS, GLONASS, Galileo, and Beidou. These systems are supported by space-based augmentation systems (SBAS) or ground-based augmentation systems (GBAS). Examples of SBAS include the US Wide-area Augmentation System (WAAS), Europe's Geostationary Navigation Overlay System (EGNOS), and Japan's Multi-functional Transport Satellite (MTSAT). These systems augment existing constellations of satellites in medium earth orbit (MEO) with geostationary (GEO) or geosynchronous satellites [72]. These systems combine aspects of localization and navigation using satellite technology and the telecommunications medium. They are becoming increasingly interesting for emergency services, as well as in passenger service applications (emergency callouts, road navigation, information transmission, etc.). Studies have shown that applications developed for road transportation can also be implemented in a rail environment. Nevertheless, there is a growing need to develop low-cost localization systems for satellite-based applications, such as traffic guidance control, for low-density railways. For these applications, system performance and reliability depend on parameters such as positioning accuracy, availability, and integrity [73]. The first GNSSs (GPS and GLONASS) were designed to meet the military's needs for accurate navigation on land, sea, air, and space, but since satellites now transmit unencrypted, freely available civilian signals, applications in the same environments have taken root in modern life. Therefore, today, it is possible to distinguish between military and civilian user satellite positioning applications. There are significant differences between the commercial and military GNSS markets, and the size of the market is changing in a fluid fashion. The number of user receivers in maritime and rail transportation and road transport is seven times larger in 2020 than in 2006. The largest and widest commercial opportunities are in the use of satellite systems for data transmission. An example is the Japanese market, where the deployment of satellite systems in the form of QZSS was \begin{table}
\begin{tabular}{c c c} \hline
**Segment** & **Characteristics** & **Functionality** \\ \hline Space & & \(-\) & Localization of an object based on the measurement of the difference in propagation delay of radio signals from different satellites \\  & A constellation of satellites & \(-\) & Time synchronization of atomic clocks \\  & equipped with atomic clocks & \(-\) & Taking into account the error of the atomic clock \\  & & \(-\) & For three-dimensional measurements, it is necessary to use at least signals from 4 satellites \\ \hline Ground & & \(-\) & Central operational control of the entire system \\  & & \(-\) & Tracking the trajectories of individual satellites \\  & A network of ground installations & \(-\) & Monitoring navigation transmissions, analyzing ephemeris \\  & operated by MCS supported by globally distributed monitoring stations equipped with atomic clocks and GNSS receivers & \(-\) & Monitoring system parameters and the status and integrity of the satellites’ on-board systems \\  & & \(-\) & Taking corrective action in case of identified failures and deviations from the assumed parameters \\ \hline User & GNSS receivers operating in the microwave spectrum band with dedicated microprocessors and cache memory & \(-\) & Unidirectional distance measurement \\  & & \(-\) & Display of object position in relation to geodetic \\  & & & grid \\ \hline \end{tabular}
\end{table} 
Table 5: Synthetic comparison of segments. aimed at providing positioning and data transmission services in mountainous and urban environments.