*"Ground infrastructure experts wrestle with vertiport challenges"* [197]

Urban Air Mobility needs vertiports to operate! This fact is unanimously acknowledged in the scientific community and industry, but at the same time, vertiports are not well understood and the research is scattered. This is the reason why we conducted a thorough literature review following the objective to summarize systematically the current state of the art and outline key areas where future research is needed. Due to the comprehensive collection of noteworthy UAM vertiport contributions, this manuscript provides the reader a structured setup, with each chapter concluded by a brief summary, which allows for selective reading.

Initial uncertainties in naming UAM ground infrastructure seem to be overcome since *vertiport* is now being predominantly used as the term of choice. After showing that vertiport is the most popular term for UAM ground infrastructure in Section 1.2, we continue to classify the field into eleven topics and analyze their prominence (see Section 1.3). In this manuscript, the scientific literature as well as industry and regulatory contributions such as existing vertiport and heliport design guidelines were reviewed extensively; All three bodies of publication are needed to frame the state of the art of UAM VTOL vertiports.

While searching for scientific publications in the database *Scopus* until the year 2021 (including), 49 scientific publications shared the overlap of "urban air mobility" on the one hand and "ground infrastructure" on the other hand which were used as a basis for this vertiport review manuscript. After analyzing all 49 scientific publications, it became apparent that airspace operations has been the strongest focus so far, followed by the general design of vertiports and its related considerations around throughput and capacity (see Section 4). Also the interaction between a UAM network and the choice of vertiport locations finds mention in the research as elaborated in Section 3. It was found that the majority of the vertiport network research considers U.S. UAM applications. Even German VTOL aircraft manufactures consider initial full-scale UAM applications outside Europe.

Vertiports are recognized as one of the critical elements of UAM by operating on limited spatial resources. Initial bottlenecks of a UAM network will be described by a vertiport's capacity and performance in the air and on ground. This will require thorough knowledge about the vertiport layout, dynamic behavior of airside air and ground operations and inter-dependencies of arrival, departure and passenger streams: who is responsible for coordinating arriving and departing VTOL aircraft traffic? How is a mixed VTOL aircraft fleet and multiple "UAM airlines" accommodated and managed fairly at a vertiport? What traffic densities can be processed and can UAM really reduce traffic congestion on ground?

Current vertiport designs, except of some early prototypes, are currently more describing a vision than providing a realistic and implementable proposal. And, although vertiport design and operations have been the predominant research focus, only few publications take into account non-nominal constraints and contingency incidences.

Continuing the review of current regulatory framework and design guidelines in Section 2, thorough content was virtually not existent until March 2022, when both FAA and EASA independently published a first engineering brief/prototype (respectively) covering only VFR vertiports. Discrepancies also arise when vertiport sequencing and scheduling procedures are discussed. On the one hand complex holding patterns and hover points are proposed for arriving VTOL aircraft traffic, but on the other hand UAM operations are considered using eVTOL aircraft currently providing very limited endurance characteristics. Therefore, further research is necessary to identify and quantify operating uncertainties and to evaluate the role and the limitation of strategic and tactical measures. The various UAM/vertiport design approaches are highlighted by contrasting similarities and differences of U.S., European and international standards (see Section 2.4). One crucial provider of uncertainty is described by the chosen operating environment and the prevailing weather conditions. Weather will be *the* factor constraining UAM and vertiport operational hours, consequently affecting throughput, ticket price and costumer segment. High efforts will be needed to understand urban weather behavior and phenomena in order to provide a safe but also efficient UAM operation. This review wants to highlight the importance of environmental constraints such as *weather* for future UAM and vertiport operations, since current vertiport research, except for a few publications described in Section 5, do not yet specifically focus on it.

The most underrepresented topic in the body of scientific research, but also in regulatory guidelines and vertiport design proposals is *noise* as well as *security*. None of the sighted contributions provide a distinct analysis of how noise is distributed at a vertiport considering e.g., different vertiport layouts, locations, arrival and departure paths/surfaces and VTOL aircraft designs. The same applies for the topic *security* which is mentioned rarely, and if so, only when passenger security checks are addressed. But, *security* means so much more especially when aviation eventually transitions towards a multi-connected, digitized and automated operating system. Implementing vertiports in densely populated environment will require thorough analyses in terms of noise propagation, safety and cyber-/security in order to create a business case finally being accepted by society.

Vertiport approaches and contributions considering different time horizons, maturity levels and traffic densities are currently available which need to be harmonized in order to

allow for a structured development of UAM and to finally transition from vision to reality. A European UAM road-map is necessary in order to understand the (regulatory) complexity of UAM, the role of a vertiport and to derive realistic assumptions on societal implications. This literature review gathered a considerable amount of publications to depict the state of the art of UAM VTOL vertiports. The majority of them are of theoretical nature. At some point in the future of research, realistic operational constraints and requirements have to be considered which are going to require a lot of more research, testing, failing and lessons learned until we really reach the implementation of on-demand UAM.

This review manuscript will aid the harmonization process as it summarizes all major ongoing efforts and highlights both similarities and differences. We further hope that fellow researchers will find our work helpful to position their own work well into the context of vertiport and UAM research.

**Author Contributions:** Conceptualization, K.S., L.P.; methodology, K.S., L.P.; formal analysis, L.P.; investigation, K.S., L.P.; data curation, K.S., L.P.; writing—original draft preparation, K.S. (Sections 1.1, 2.1, 2.4, 3, 4.1.1, 4.1.2, 4.3, 4.4, 5 and 6), L.P. (Sections 1.2, 1.3, 2.2–2.4 and 4.2); writing—review and editing, K.S., L.P.; visualization, K.S., L.P.; funding acquisition, K.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors thank Johannes Ernst, Franz Knabe, Reinhard Schmitz, Anna Straubinger and Antoine Habersetzer for their contribution during the internal review process, as well as MVRDV for providing us their vertiport visualizations. The authors would like to give special thanks to Tabitha Stephani for supporting this work with HorizonUAM renderings. This work was conducted under the cooperation agreement between DLR and Bauhaus Luftfahrt e.V. and contributes to the DLR project HorizonUAM.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**

The following abbreviations are used in this manuscript:



#### **Appendix A**

In Figure A1a, the top authors by number of publications in the field of vertiports are listed. Peng Wei, the number one, is an associate professor at the George Washington University in Washington, D.C. He published many papers with is co-author Priyank Pradeep. Another prominent institute is the Georgia Institute of Technology in Atlanta, Georgia: Cedric Justin, Dimitry Mavris and Brian German are associated with it. So far, it appears that the field is dominated by few strong players. In Figure A1b the top sources of publication are shown, which are both conference proceedings and journal issues. *Transportation Research Part C*, *CEAS Aeronautical* and *Aerospace Information Systems* are journals; the remaining major sources are conference proceedings. Minor sources are journals or proceedings with only one paper on vertiports. The 12 minor sources are the following with one source unknown:


The top ten individual publications in the field of vertiports according to number of citations in *Scopus* are listed in Table A1. The reference day for the number of citations was 31 December 2021.

(**a**) Top publishing authors. (**b**) Top publishing conferences and journals. **Figure A1.** Data analytics in the field of vertiports.


**Table A1.** Top 10 papers according to citations in *Scopus* (as of 31 December 2021) addressing vertiports in the context of UAM.

#### **References**

