## 3 The Basic of GNSS
### System Architecture  
The architecture of satellite navigation systems in the nomenclature used is divided into so-called segments, which include space segment, ground segment, and user segment. The above division defines the geographical structure of GNSSs; however, from the point of view of transmission of signals and information contained therein, GNSSs can be divided into space segment, ground segment, user terminal, and network auxiliary segment. The space segment transmits an authentication code along with authentication messages to the transmitted satellite navigation signal, the user segment authenticates the received satellite navigation signal, and the network auxiliary segment uses a communication base station (ground communication/satellite communication) to provide auxiliary network authentication information. If there is a GNSS spoofing signal in the real environment, the user segment can identify whether the current signal is a spoofing signal by authenticating the spread spectrum message/code. The architecture of satellite navigation signal authentication is shown in Figure 3[36; 37]. According to Figure 3, the architecture is dedicated to aeronautical applications, while satellite positioning systems for rail vehicles should use an architecture based on the integration of multiple sensors to avoid risks associated with potential GNSS errors, including unavailable or limited satellite signal, multi-path effects, and others. An example is inertial sensors, which appear to be a promising choice for most road transportation applications and also appear to be suitable for rail vehicles. The use of inertial sensors in the process of differential satellite positioning can provide an accuracy of about one meter, which can contribute to more efficient determination of train position, speed, and other required status descriptions. Within the framework of the global satellite navigation system scheme using inertial sensors (GNSS/INS) to supplement the odometer, the solution of the integrated satellite train positioning system architecture can be illustrated as shown in \begin{table}
\begin{tabular}{c c c} \hline \hline
**Criteria** & **RAMS** & **ICAO** \\ \hline Security & Is defined as the probability of a device and system functioning in a safe state & \\ \hline Continuity & & Describes the ability of the system to perform the localization task it has started without any interference \\ \hline Accuracy & & The degree of correspondence between the estimated or measured value and the actual value \\ \hline Availability & Determines the ability to maintain a functioning state in a given environment in which it can perform the required function, assuming that the required external resources are provided & With respect to time, determines the probability that the system is available at the beginning of a specific object positioning task; \\ \hline Integrity & Integrity of the train, that is, the fact that the train has not split & \\ \hline Reliability & Determines the ability of the device and system to perform specific functions, at specific times and conditions & \\ \hline Maintenance & Determines the ability to perform timely and easy maintenance (including servicing, inspection and control, and repair and/or modification), assuming that the service is carried out in accordance with established procedures & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of technical evaluation criteria. Figure 4, where OBC stands for on-board computer and IMU is the inertial measurement unit. In order to support the GNSS-based train control system, the presented positioning system is designed with an interface for the on-board ATP (Automatic Train Protection) device [28; 38; 39; 40]. One multi-sensor solution, which collects data from various sensors installed in the vehicle (in particular, IMU and GPS) and performs Kalman-based filtering recursion, has been studied and presented in [41].