## 3 The Basic of GNSS
### System Architecture
#### 3.5.3 User Segment  
The original premise for navigation systems such as GPS and GLONASS as global all-weather positioning systems was justified by military applications, but they were later opened up to the general public, and the advent of new systems in the form of Galileo and BeiDou have made them an indispensable part of modern life. In addition to its well-known use in vehicle navigation, the system has influenced such diverse fields as land, sea, and air navigation; surveying and geodesy; and large-scale construction projects such as mines, bridges, and tunnels. In addition, in today's railway systems, GNSS, in addition to its functionality related to monitoring moving rolling stock, shows potential applicability in directing and controlling rail traffic. This solution, of course, requires the design and development of a system with an architecture that ensures high-level reliability, the sophistication and cost of which will be commensurate with the required capabilities and accuracy. Today, professional receivers are available on the market that go well beyond the units built into handheld devices intended for general use. Many well-known companies specialize in providing advanced and professional satellite navigation equipment for aircraft and ships. Among the solution providers mentioned are names such as Garmin, Magellan, TomTom, etc., which also provide models for the general market, including drivers, hikers, boating enthusiasts, etc. The above represents the potential for future development of GNSSs in rail transportation. The GNSS receiver operates in the microwave spectrum band, with a dedicated microprocessor and memory to display its position in relation to the corresponding geodetic grid. Although the principle of the system is unidirectional distance measurement, in which the propagation times of the signals reaching the receiver from the satellites are used to measure the distance between them, it is not necessary to have an atomic clock on the equipment of an object positioned using GNSS. The above, as already mentioned, is achieved by calculating the signal propagation delay, and these calculations are called pseudo-range. The task of the control segment on the ground is to apply corrections as needed. The actual ranges must be corrected for receiver clock errors. This is where the number of satellites is critical--not only does it allow for  
Figure 8: Ground (control) segment. Prepared on the basis of [69].  
global coverage, but it also introduces redundancy of erroneous pseudo-coverage vectors, which generally do not coincide with the receiver's position, but if the clocks of the satellites are in perfect synchronization, it should require only one correction, namely, the correction of the receiver's clock to coincide. Of course, in practice, there are unavoidable errors in all measurements, and the best solution is that when taking into account the pseudo-distance to multiple satellites, a correction can be found to the receiver clock to minimize the spread in the calculated positions. One of the main functions of the receiver's microprocessor is to calculate this clock correction. It follows that just as the accuracy of a measurement can be improved by repeating it, the accuracy of a position fix in three dimensions is improved by acquiring signals from multiple satellites, but the system is designed so that at least four satellites are fully visible at all times from any point on Earth [67].  
Figure 9 shows a simplified GNNS architecture with an indication of transmission to the user segment.