*1.1. Taxonomy of UAM Ground Infrastructure*

One might ask the question, why is there a need to define a new class of ground infrastructure specifically for UAM when we already have a distinct set of thoroughly practiced design guidelines covering aerodromes, airports and heliports?

Assuming affordable access to UAM flights is targeted, high numbers of throughput need to be achieved which will require larger and probably more complex ground infrastructure topology and access management as it is currently available for helicopter/heliport operations [7]. This may include ground taxiing of VTOL aircraft, reduced separation, simultaneous/automatic/autonomous operations as well as steep/vertical approach and departure profiles in order to operate in densely populated and built-up urban environment. For comparison, basic flight maneuvers for rotorcraft address a typical descent profile of 8 to 12 degrees whereas a steep approach is defined by approx. 15 degrees descent angle [8]. Moreover, UAM being considered on-demand, following high dispatch frequencies and

mainly operating in urban scenery with shortly changing flight phases are characteristics of significant difference compared to current aviation operations.

As to understand with what UAM ground infrastructure is associated with and what considerations are stated in terms of classification and definition, the following Sections 1.1.1 and 1.1.2 will provide an overview of historic and current developments.

#### 1.1.1. Regulatory and Standardization Context

Both well-established and novel aircraft manufacturers, research facilities, local and public authorities, regulatory agencies, CNS providers, air navigation service providers, consulting companies and many more all around the world are currently contributing to the development of UAM. A considerable inconsistency was found in the classification of such UAM VTOL ground infrastructures throughout different (scientific) publications addressing UAM.

Starting with already familiar aviation ground infrastructure and according to ICAO, the *aerodrome*, is "a defined area on land or water (including any buildings, installations and equipment) intended to be used either wholly or in part for the arrival, departure and movement of aircraft" [9]. In the European certification specification for aerodrome design *CS-ADR-DSN*, EASA follows ICAO's guidelines but added the specification of being located "on land or water or on a fixed offshore or floating structure" [10]. This also includes small general aviation airfields, heliports, commercial airports and military airbases [11]. A distinct version for rotorcraft, the *heliport*, is defined by ICAO's *Annex 14* and EASA's *CS-HPT-DSN* as "an aerodrome or a defined area on a structure intended to be used wholly or in part for the arrival, departure and surface movement of helicopters" [9,12]. For completion, an airport has terminal(s) and car parks additional to the infrastructure used by the aircraft itself, thus the aerodrome is part of an airport [11]. Consequently, the heliport extends the characteristic of an aerodrome by the definition of an area on structure which includes the possibility of elevated areas. Also, the heliport is exclusively used by helicopters, whereas the aerodrome can be used by both vehicles. It needs to be highlighted that EASA's *CS-HPT-DSN* only provides design certification specification for heliports located at aerodromes that fall under scope of *Regulation (EU) 2018/1139*.

Transitioning from "traditional" aviation towards initial serious considerations of inter-city aerial transportation, in 1983, the National Rotorcraft Program analyzed how the national inter-urban transportation market in the U.S. can be improved [13]. Among others, the report determined that conventional helicopters did not satisfy the stated requirements due to lack of capacity, high operational costs and high noise levels. The recommendation of considering tiltrotor aircrafts offered higher speed and range and vertical take-off and landing capabilities.

Followed by this recommendation, in 1985, the FAA, NASA and the Department of Defense conducted a joint civil tiltrotor study in order to identify the potential of the commercial tiltrotor transport market [13] . Several studies followed covering the topics civil tiltrotor missions and applications, potential risk areas, market evaluations, ground infrastructure planning and development, air traffic control and public acceptance (see [13–16]).

Driven by those civil tiltrotor developments generated by industry, military and government, in 1991, the FAA developed an *AC 150/5390-3* guiding vertiport design [17]. The terminologies *vertiport* and *vertistop* were first introduced describing respectively "an identifiable ground or elevated area, including any buildings or facilities thereon, used for takeoff and landing of tiltrotor aircraft and rotorcraft" and "a vertiport intended solely for takeoff and landing of tiltrotor aircraft and rotorcraft to drop off or pick up passengers or cargo". This AC paved the way for the term vertiport and the general idea of creating classes of ground infrastructure to describe different characteristics and operational capabilities. Those considerations were never put into practice since military tiltrotor technologies were never used commercially therefore causing the cancellation of *AC 150/5390-3* in July

2010 [18]. However, years later, those former developments serve as important precedent being now adjusted and refined for modern UAM operations.

First, the generic term *UAM aerodrome* was introduced by FAA's first version of a UAM ConOps [19] addressing foundational principles, roles and responsibilities, scenarios and operational threats. It describes "a location from which UAM flight operations depart or arrive. [. . . ] UAM aerodrome is used explicitly when the context indicates functionality to support UAM operations that is not present in NAS [National Airspace System] operations" [19].

NASA is following FAA's approach by using the term *UAM aerodrome* in the first version of the published UAM Vision ConOps in 2020 [6], addressing a UAM operation of medium density and complexity. The term *UAM aerodrome* is further specified by addressing operational UAM characteristics such as VTOL capabilities and ground movement leading into the definition of a "specifically defined area that is intended for the arrival, departure, and ground movement of UAM aircraft. Because of the VTOL nature of many UAM aircraft, most UAM aerodromes look more like today's heliports with landing pads as opposed to long runways" [6]. In a follow-up ConOps addressing high-density automated vertiports [20], NASA again further specified the classification and defined the term *vertiport* in correspondence to the aircraft design (VTOL and rotorcraft) and its propulsion unit (eVTOL). Also, the physical location of a vertiport (ground-based or elevated) is now part of the definition which resulted into "an identifiable ground or elevated area, including any buildings or facilities thereon, used for the takeoff and landing of eVTOL and rotorcraft".

Responding to the rising requests claiming for a consolidated UAM ground infrastructure design guideline, in March 2022 the FAA published an engineering brief on the subject of vertiport design limited to piloted and VFR VTOL operations in order to capture early UAM VTOL operations [18]. In [18], UAM ground infrastructure is now following the initial classification of [17], but clearly stating propulsion characteristics, VTOL capabilities and the specific use of co-located buildings for passenger handling and other UAM services. Consequently, the *vertiport* is defined as "an area of land or a structure, used or intended to be used, for electric, hydrogen, and hybrid VTOL landings and takeoffs and includes associated buildings and facilities" and the *vertistop* as "an area similar to a vertiport, except that no charging, fueling, defueling, maintenance, repairs, or storage of aircraft are permitted" [18].

Transitioning to European UAM applications, EASA introduced the term *vertiport* in the first draft of the SC SC-VTOL-01 [21] in 2019. It provides an initial description naming the vertiport "an area of land, water, or structure used or intended to be used for the landing and take-off of VTOL aircraft". There is no specific requirement attached to that definition addressing the VTOL aircraft's propulsion unit, passenger handling and service facilities providing e.g., charging/refuelling and maintenance. This rather generic definition was picked-up by EASA's Prototype Technical Specification *(PTS-VPT-DSN)* for VFR Vertiports [22] published in 2022.

Since regulatory authorities are working closely together with standardization bodies, it is noteworthy mentioning them in this context. The EUROCAE, operating as a non-profit organization, is dedicated to the elaboration of aviation standards since 1963. The development of UAM operations is incorporated in working group 112 "Vertical Takeoff and Landing" which is developing several standards such as vertiport operations (ED-299 currently under development [23]), and VTOL aircraft ConOps (ED-293 [24]). Important groundwork for [22] was provided by EUROCAE. In [24], EUROCAE makes use of the term *vertiport* following the definition stated in EASA's SC-VTOL-01.

On an international standardization level, the International Organization for Standardization *ISO*, is currently developing a vertiport standard *ISO/AWI 5491* under the technical committee *ISO/TC 20/SC 17 Airport Infrastructure* [25]. A publication is still pending. Further, *ASTM International* initiated already in 2017 the work item of "New Specifications for Vertiport Design" which also indicates the usage of *vertiport* and *vertistop* and providing the following, sofar most precise definition: "Vertiport means a generic reference to the

area of land, water, or structure used, or intended to be used, for the landing and takeoff of VTOL aircraft together with associated buildings and facilities. Vertistop means a minimally developed VTOL aircraft facility for boarding and discharging passengers or cargo. The vertiport/vertistop relationship is comparable to a bus terminal-bus stop relationship with respect to the extent of services provided or expected" [26]. It is also highlighted that vertiports are expected to serve both civil VTOL aircraft and civil helicopters and the extension for electric driven VTOL aircraft should be considered carefully [26].

#### 1.1.2. Commercial and Research Context

In 2016, when *UBER Elevate* published the whitepaper "Fast-Forwarding to a Future of On-Demand Urban Air Transportation" [27], the topic short range metropolitan air transportation including the vertiport "came back to life". Several whitepapers followed addressing among others "The Roadmap towards scalable urban air mobility" [28], "The New Digital Era of Aviation" [29] and a "Concept of Operations: Autonomous UAM Aircraft Operations and Vertiport Integration" [30].

Ref. [27] picks up the terminologies introduced by [17] but focusses on layout and charging characteristics. The infrastructure which supports urban VTOL operations is defined as *vertiports*, described as "VTOL hubs with multiple takeoff and landing pads, as well as charging infrastructure" and as *vertistops* "a single TLOF pad with minimal infrastructure". This whitepaper together with the following *UBER Elevate* Summits in the years 2017, 2018 and 2019 received considerable attention and significantly pushed forward the topic of UAM. This trend is also depicted by the number of publications related to the topic UAM ground infrastructure in Figure 1. When investigating all publications listed in the online database *Scopus* from the year 2000 onwards, which are displaying a connection to the keyword UAM ground infrastructure, it appears that the number of publications is increasing explicitly with the year 2016.

**Figure 1.** Publications related to UAM ground infrastructure as listed in *Scopus* after the year 2000 and in relation to the publication of *UBER*'s whitepaper in 2016 [27].

The consulting companies *Deloitte* [31] and *McKinsey and Company* [1] both established a UAM ground infrastructure classification with multiple sub-categories addressing varying features, capabilities and local implementation. The generic term of the physical infrastructure is termed as *vertiplaces* [31] and *VTOL ports* [1], respectively. The largest archetype is defined by both as *vertihub*. Ref. [31] describes it as small airports for eVTOL aircraft, mainly located on the periphery of urban or suburban areas because of their large

footprint including the availability of MRO infrastructure, whereas, ref. [1] envisions it as a stand-alone building implemented in central and high-traffic areas providing charging/refueling capabilities for VTOL aircraft and distinct services for passenger. The second archetype is termed as *vertiport* and *vertibase*, respectively. Based on [31], the *vertiport* is located at points of interests ideally integrated with other modes of ground transportation. Multiple eVTOL aircraft can be accommodated, fast-charging, refueling and minor MRO services are provided. Security check-points, passenger waiting lounges, systems for fire safety and real-time surveillance are highlighted as well. According to [1], *vertibases* are medium size, located at medium-traffic areas and are either newly built or retro-fitted. As third archetype depicting the smallest footprint, Refs. [1,31] use the term *vertistation* and *vertipad* respectively. On the one hand a *vertistation* provides only one or two pads for which the use of existent helipads can be considered. On the other hand, *vertipads* are assigned to a "spoke" in a hub-and-spoke network. Both share the characteristic of smaller footprints and lower costs which could enable an easy implementation as peripheral infrastructure in suburban or rural locations.

Following the approach of multiple archetypes but based on aircraft performance and UAM ground infrastructure capabilities, ref. [7] uses the term *UAM aerodrome* by [19] as hypernym for UAM ground infrastructure. With regard to a UAM aircraft's performance, VTOL or STOL capabilities are distinguished resulting into different UAM aerodrome classes. The term *vertidrome* was used for VTOL operations and *stoldrome* for STOL operations only. Two additional flavors of vertidromes are used, *vertiport* and *vertistop*, in order to distinguish between operational and technical capabilities like charging, refueling, MRO and passenger handling.

Numerous terms for novel take-off and landing ground infrastructure were found by [32], such as *vertiport*, *vertipad*, *pocket airport*, *skypark*, *sky node* and *sky port*. To avoid the definition of a specific term and therefore limiting ground infrastructure to a specific characteristic, ref. [32] uses the generic term *TOLA*, take-off and landing area, for ondemand mobility operations, which describes any location an aircraft, VTOL or STOL aircraft, can depart from or arrive at. Additional terms were found such as *Verti-X* [33], *skyports* [34] and *airpark* [35] if super STOL (SSTOL) and STOL aircraft are being considered to serve metropolitan areas and intra-city operations.

But towards what terminology is the UAM community trending? The next section will run a systematic analysis of what terminologies are used in the scientific context, based on the set of *identified terms* introduced in this section.
