## 4 Overview of GNSSs
### Other Systems  
A new and different approach is to use a constellation of satellites in low earth orbit (LEO) to dramatically reduce communication delays. The first large-scale deployment of this kind is the Starlink constellation, now supporting more than two thousand satellites. The commercial service was launched in beta in October 2020 in the United States, and from 2021 in European countries. According to the operator, it provides internet access with latency of 20 ms and speeds of 100 to 200 Mbps. Because it is a new service, its operation and performance have not yet been fully studied. One of the few works has been proposed by [93], which shows how Starlink performance changes at different benchmarks. Another paper focuses on how the performance of a single Starlink viewpoint changes when accessing globally distributed resources, under heavy network load, using TCP and QUIC transport protocols [94]. SpaceX has already launched the first satellites of its Starlink constellation, with a target number of 42,000. Earlier this year, Blue Origin announced its intention to create its own satellite internet constellation (with more than 3000 satellites), and OneWeb has already launched the first satellites in its venture. Both established companies and startups are launching large networks of small satellites. Cheaper, easier to manufacture, and cheaper to send into space, small constellations of satellites collectively cover much more of the Earth than standard satellites [95].  
The main difference between the two approaches is the distance between the satellites: while geosynchronous satellites are 35,000 kilometers apart, LEO satellites are much closer. For example, Starlink has five orbital shells, the closest of which is only 550 kilometers away. This much shorter distance allows for much lower latency, comparable to traditional broadband providers. However, this approach also requires a much larger number of satellites, since each satellite is only in sight of a ground station for a short time. This has led to the development of so-called \"mega constellations\" of satellites. Because there are many more satellites, overall network capacity is also higher, and Starlink promises much higher bandwidth than traditional satellite internet providers [96].  
Table 6 shows some of the more important parameters characterizing GNSSs. Especially noteworthy is the QZSS, which functionally provides coverage only in southeast Asia and Oceania. The above implies that this system should not be considered global; however, with regard to the issue at stake, the parallel use of more global systems in rail vehicle positioning, this system is included in the current review. The other systems provide positioning capabilities for the entire globe. Consequently, the use of any of those mentioned makes it possible to position an object regardless of its location. However, as demonstrated in this article, for the highest possible safety parameters, it is far preferable to use systems equipped with GNSS receivers using more than one satellite positioning  
system. Within the framework of this article, none of the characterized GNSSs are indicated as dedicated to railway applications, since the choice depends on a number of factors, including geopolitical factors, which may be the main basis for system selection, regardless of the technical aspects of the system in question. Another reason for choosing a particular system may be geographic location, as mentioned above with regard to the QZSS.  
The Starlink system is not included in the above table due to its completely different space segment architecture. In addition, this system is dedicated primarily to data transmission; thus, it is not included in satellite positioning applications.