## 2 Methodology for Reviewing the Current State of Knowledge  
The methodology of the state-of-the-art literature and state-of-the-art review implementation process includes a systematic approach to identifying the relevant GNSS positioning literature and areas of possible implementation. The process used for the literature review was based on the basic concepts of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) article flow [19]. The use of PRISMA can benefit many who are interested in the subject matter, as it allows for readers to assess the adequacy of the methods, and thus the reliability of the solutions, methods, and results of the projects and studies conducted. Presenting and summarizing the characteristics of the research contributing to the synthesis allows for the reader to assess the applicability of the results in the environment interested in the subject matter. Describing the solution in terms of evidence for the result and implementation can help stakeholders make appropriate recommendations for guidelines, instructions, and standards, in this case applied to rail transportation. Fully defining all elements of PRISMA also makes it easier to replicate and update the review, and to integrate systematic reviews into other reviews and guidelines, thus potentially optimizing research processes [20]. The number of articles collected from databases (Scopus, Web of Science, and Google Scholar) included a total of 1940 literature items. After removing duplicates, 1120 articles remained. According to the methodology presented below, the titles and abstracts of the articles were checked. Some articles were excluded from the process due to the following: the article did not focus on rail vehicle positioning; the full text was not available; the article did not cover GNSS. By performing the above process, the number of articles was reduced to 620. After a detailed review of the articles and an analysis of their applicability with regard primarily to the research problem, which for this review article is the identification of areas of possible application of GNSS in railway infrastructure, the number of articles was reduced to 94. In addition, due to the continuous nature of changes and modernization of GNSS systems, and for the purpose of clarifying the current data on the satellite constellations of the various systems, an additional 2 literature items were included in this article, which are technical documents of positioning system managers. In addition, this article considers the most up-to-date book literature, which includes 4 items, 2 dissertations related to the issue discussed in this article, and 1 literature item from before 2000. The final number of all literature positions was 102. The process of verifying the literature resources is shown in Figure 1.  
Figure 1: Diagram of the systematic review. For this review article, the initial step was to formulate the research question and identify keywords and phrases related to the topic. A thorough literature search was then conducted using electronic databases. In the search for articles suitable for the current review, a number of keywords and phrases were used with the aim of reducing the list of electronic database items. Of the selected articles, a relevance check was performed based on their titles and abstracts. The full texts of relevant articles were then reviewed, and information and data were extracted, including those describing research, methods, results, and conclusions in the defined area of the literature review. The extracted data were synthesized and analyzed, and the results were organized and presented in the form of the current review article. The systematics of the review is shown in Figure 1, where the steps of the methodology used to synthesize publications related to the application of satellite systems in rail transportation are illustrated.