## 2 Literature Background  
### Systematic Review Process  
For the systematic review of the literature, a formalized process was adapted from [12; 13; 14]. Once the research topics were established, a review protocol was established that encompassed the preliminary stages of the literature analysis, including: (i) designing the search strategy and the literature resources; (ii) establishing the selection criteria. Designing the inclusion and exclusion criteria to be used on certain literature resources is the initial step in selecting candidate papers that could provide valuable insights to address the research questions. The search strategy has been structured for an objective bibliographic search, although it of course preserves some level of subjectivity. Various keywords, improperly employed as synonyms for research purposes, can be chosen when querying a database (mutuallyinclusive keys), or the search might encompass mutually exclusive keys. The investigation can be limited to a timeframe that is assumed to be relevant to the scientific progression of a research topic. The query might also be associated with one or more scientific-disciplinary fields, or it may exclude certain areas. While there may be some overlap in the query of different research topics, the importance of tailoring the search on a database to the specific topic at hand. Intending to objectify the research, we made some choices in Scopus \"Advanced Query\". The search has been executed solely on: (i) the \"Title\" field (query TITLE in Scopus); (ii) both the \"Title\" and \"Abstract\" fields combined (query TITLE-ABS in Scopus). Keywords such as GLONASS, GPS, GNSS, GALILEO, and BEIDOU were used as mutually exclusive, analogously CORS and PERMANENT keywords. The search has been limited to some subject areas of physical sciences (query SUBJAREA in Scopus), namely: engineering (ENGI), earth and planetary sciences (EART), physics and astronomy (PHYS), computer science (COMP), and environmental science (ENVI). Specific keys have been added for searching specific research topics; for instance, DESIGN or PROJECT keywords were used as mutually exclusive to select candidate papers concerning network design, while ACCURACY and PRECISION mutually exclusive keywords were added for selecting the literature on accuracy assessment. Once the search keys were identified, in this manuscript, the search was conducted using the Scopus database only. After a subset of manuscripts have been selected, they can be examined based on the number of citations, the publication date, or in connection with the most prolific author in that research field. It's implicitly assumed that manuscripts with a high citation count have significantly influenced the particular research topic. However, the most groundbreaking developments might be discovered in papers that have been recently published but have yet to receive many citations. The query of a database can vary depending on the research topic. For instance, when conducting a database search on the design of GNSS networks, the search keywords employed might differ from those used when searching for applications for monitoring a structure. An initial query gave an indication of the trend in the number of publications in GNSS CORS (Figure 1). The query was based on both title and abstract, using the keywords GLONASS, GPS, GNSS, GALILEO, and BEIDOU as mutually exclusive and limiting the results to some subject areas of physical sciences (engineering, earth and planetary sciences, physics and astronomy, computer science, and environmental science). Figure 1: Percentage of GNSS CORS publications (blue bars, primary y-axis) and cumulative percentage (red line, secondary y-axis) from 1987. Results indicated a change in the trend in the number of publications in the years 2000-2003 (dashed lines). Therefore, it was chosen to limit the query to the period from the 2000s to the current year.