## 3 The Basic of GNSS
### Criteria for Technical Evaluation of GNSS Systems (Aeronautics)  
The following technical evaluation criteria have been adopted for GNSS aeronautical applications:  
* Positioning accuracy--There are two different requirements in terms of accuracy: accuracy along the track and accuracy transverse to the track. In both cases, safety-related localization systems for traffic control systems must provide as much accuracy as possible. The level of accuracy of rail vehicle positioning depends on the accuracy of equipment and systems, which determines that the functionality of a positioning system using satellite navigation will depend on the accuracy of those systems. For some non-safety applications, such as fleet monitoring, a high level of accuracy is not required. Such applications typically require a low accuracy of 50-300 m [21; 27].
* Positioning availability (with respect to time)--The availability in question determines the probability that the system is available at the beginning of a specific object positioning task [21].  
Positioning availability (referring to space--coverage)--The term coverage describes the geographic area in which location information must be provided to the user/application [21; 27].
* Integrity--The term integrity describes the requirement that the system must independently detect and report failures. It applies only to security-related systems [21].
* Continuity--The term continuity describes the ability of the system to perform the localization task it has started without any interference. Particularly when using GNSS, this problem arises when the vehicle passes through tunnels or covered stations, for example. These obstacles prevent or hinder the reception of satellite signals. For precise positioning, a measurement using at least four satellites is necessary, and the limitation due to obstacles can make a positioning system based on satellite localization unavailable. However, even in the situation of access to a sufficient number of satellites in a limited area, it causes the accuracy of horizontal positioning information to decrease at high altitude, because the distance between several satellites is too small for a precise measurement result. In such a case, the usefulness of the information may not be sufficient [21; 27].
* Reliability--The term reliability refers to applications in safety-related systems. In safety-critical applications, knowing the position of a vehicle is very important. However, it is not only the position that is important, but also the confidence interval of the position information. Hidden errors in position information can lead to collisions and accidents. When deciding to use GNSS for positioning, it is important to consider the possibility of identifying errors in position information. The best way to do this is to use a combination of different systems to obtain position information [27]. The concept of reliability in the present context refers to solutions for using GNSS in aeronautics. At the same time, the term reliability appears in the RAMS (Reliability, Availability, Maintainability, Safety) criteria analysis, which is used in the evaluation of feasible solutions for railways. In view of the above, it is necessary to develop a consistent approach in the RAMS analysis of GNSS systems, which is discussed later in this article. A comparison of the relationship between RAMS and ICAO criteria is shown in Table 2.