*1.2. Trends in Research and Scientific Publication*

In this section, the use and prominence of the above-mentioned terms or keywords (both words used synonymously) will be analyzed. The goal is to illuminate the usage of different keywords in the past and present and help the community become more aware of current developments in the field of UAM ground infrastructure. As hinted in the title of this paper, we believe "vertiport" to be the most prominent keyword and it is therefore used throughout this manuscript.

In Section 1.1, a total of 19 keywords were discussed. A search in the publication database *Scopus* (find the *Scopus* publication portal under https://www.scopus.com/; accessed on 11 July 2022) yielded that 11 of 19 keywords were used at least once in the listed scientific literature (equals database "ground infrastructure"); this means 8 keywords were not used at all. A limitation of this approach was that the keywords needed to occur in the title or abstract of the publication, as *Scopus* only searches the meta data of publications. This database was chosen as a compromise between a wide range of publications (e.g., *Web of Science* does not list conference proceedings) and quality of publications (e.g., *Google Scholar* has no transparent mechanism of selecting papers).

To gain a feeling for the trend of each keyword (see Table 1), three time spans and sub-databases were looked at in particular: the last two decades, the last 10 years, and the years after 2016 which marked a turning point due to the publication of the UAM white paper by *UBER Elevate* [27] (see Figure 1). The number of publications in each sub-database is shown as well in Table 1. The size of the database does not have to match the sum of the occurrences of all keywords for various technical reasons: for example, one paper could contain multiple keywords from the list.


**Table 1.** Prominence of "ground infrastructure" related keywords.

As the focus of this review is ground infrastructure in the context of UAM, the same search was then applied to the keyword of "urban air mobility" (equals database "urban air mobility"). The goal of this analysis is to find the best-fitting keyword for ground infrastructure in the context of UAM, which is done by comparing the two databases derived from *Scopus*. In Figure 2, the overlap of these two databases is visualized. Set A and B represent the UAM and ground infrastructure database, respectively. The comparison of two sets is conducted by looking at DOIs as unique identifier. As not all listed entries carry a DOI, these entries are removed, yielding the sets C and D representing the cleaned databases for UAM and ground infrastructure, respectively.

Entries in *Scopus* that do not carry a DOI number can be proceedings, workshop summaries or other material, but also conference papers and articles. There are other ways of comparing entries such as using the title, but this might lead to problems with consistency. Excluding all entries that do not carry a DOI number is therefore a way of dataset quality control, while we acknowledge that this might create a bias within the dataset.

The combination of both databases is labeled as set E, the papers exclusively occurring in the urban air mobility database as set F and the papers exclusively occurring in the ground infrastructure database as set G. Our set of interest are those papers shared by both databases, which are labelled as set H. In Table 2, a brief description of all sets is given, including the size of each set and their relation to one another. Comparing the databases of both searches showed that only 8 keywords (see Table 3) of the 11 keywords shown in Table 1 are used in the context of UAM throughout 49 scientific publications listed by *Scopus*.

**Figure 2.** Overlap between databases derived from the keyword "urban air mobility" and 11 keywords related to "ground infrastructure".

**Table 2.** Size of sets from database overlap analysis: 49 shared papers including keyword "urban air mobility" (UAM) and keywords related to "ground infrastructure" (GI).


**Table 3.** Keyword occurrences describing UAM ground infrastructure (set H).


Applying a document term matrix approach, the number of occurrences of each keyword in the final database can be highlighted (see Table 3). A document term matrix shows how often each keyword occurs in each publication. The number of hits shown are the sum of occurrences across all 49 publications. It can be seen that "vertiport" occurs in 40 of the total 49 publications and therefore covers over 80%. *Vertiport is the most prominently used term or keyword to describe UAM ground infrastructure*. This is in direct contrast to the wider field of aerospace research where "aerodrome" (296 occurences in the last 6 years) appears to be the more prominent keyword while "vertiport" is used less often (60 occurences in the last 6 years) as can be seen in Table 1. Yet, the keyword "aerodrome" has negligible relevance in the UAM community (1 occurence in the context of "urban air mobility", see Table 3). The process of selecting keywords describing ground infrastructure and finding the overlap with the body of research concerned with UAM is summarized in Figure 3.

**Figure 3.** Selection process of keywords used to describe "ground infrastructure" in the context of "urban air mobility".

An analysis of the full UAM database (set A displayed in Figure 2) shows, that only two papers have been published before 2016, wherefore the assumption to start our analysis with the publication of *UBER*'s whitepaper [27] in 2016 is justified. We are aware that searching for the keyword "urban air mobility" may neglect former UAM-like contributions covering intra-city air travel. The focus of this manuscript, however, is to specifically cover the recent trend of UAM addressing novel eVTOL aircraft and airspace designs as well as the concept of on-demand and multi-modal mobility.

An exponential growth in UAM related publications can be seen after the year 2018. Analyzing the vertiport database (set H displayed in Figure 2) also shows a rising trend in publications. Both trends are visualized in Figure 4. Using a data analytics approach the most frequent authors are listed in the Appendix A. Similarly, the conference proceedings and journals which published most often about the topic of vertiports are identified (see Figure A1a and Figure A1b, respectively). Finally, a list of the top ten papers with the highest impact according to number of citations is shown in the Appendix A in Table A1. This overview is supposed to give the reader an idea of which publications and authors impacted the research community; and where to search for articles and submit personal contributions to.

**Figure 4.** Trends of publication in the fields of UAM and vertiports.
