21 July 2021
One of the BUBBLES technical objectives is to specify safety and performance requirements (SPR) for the U-space services involved in the provision of separation management by the U-space, as well as for the systems they rely on. The ultimate goal is to limit the risk of collision between aircraft and hazards to an agreed level deemed as acceptable, in line with the ICAO conflict management concept.
BUBBLES approach to SPR at a glance.
BUBBLES project does not intend to define a set of ‘one-size-fits-all’ requirements that can be applied everywhere in a more or less straightforward way. This approach usually leads to more demanding requirements than necessary, hence putting a disproportionate burden on the implementation of the ground surveillance system with respect to performance, maintenance and cost. This statement, extracted from the draft of the EUROCAE ED-261 document, is even truer in the case of UAS, due to the huge variety of aircraft and missions. Therefore, the BUBBLES team, inspired by the generic approach proposed by the ED-261, aims at providing operationally ‘fit-for-purpose’ requirements to allow for the tailoring of U-space requirements for conflict management to actual operational needs across a specific U-space.
To this purpose, a research team in which all the project partners were involved adapted the approach described in the document EUROCAE ED-78A Guidelines for the approval of the provision of Air Traffic Services supported by data communications to the particularities of U-space services. The process consisted of four steps: (1) development of an Operational Service and Environment Description (OSED); (2) Operational Performance Assessment (OPA), leading to Performance Requirements; (3) Operational Safety Assessment (OSA), leading to Safety Requirements; and, Interoperability Assessment (IA), leading to Interoperability Requirements.
In order to develop the first step defined by the ED-78A, i.e. to define the generic OSED, the research team applied the Definition Phase of the Methodology for the U-space Safety Assessment (MEDUSA). Due to the generic approach pursued by BUBBLES, no particular scenario (including aircrafts and missions) was considered, but a set of nine ‘sector types’ was introduced instead. These sector types were defined with the purpose of being detailed enough in order to be representative of most actual local scenarios, but generic enough to not being tied to a particular one. Therefore, the ‘sector types’ are classified attending to the density of manned and unmanned aircraft and considering whether they are within controlled or uncontrolled airspace, although typical missions are also identified for each of them, based on the outcomes of BUBBLES WP3.
Then, also using the results obtained by BUBBLES in WP3 the research team involved in the development of the OSED classified the operations in ten ‘traffic classes’, with different risk levels that are related to the Air and Ground operational risk. A particular local scenario can be defined by the ‘sector type’ and by a particular mix of ‘traffic classes’. The figure of fatal injuries per flight hour defined in SORA 1.0 was adopted as a preliminary Target Level of Safety (TLS) common to all ‘sector types’ and ‘traffic classes’. However, the concept of TLS for the U-space is not straightforward, thus BUBBLES is currently exploring how to define it. The interested readers can find an outline of the BUBBLES approach to the definition of TLS here.
To illustrate the concept of ‘sector types’ and mix of ‘types of traffic’, three local scenarios were developed in detail, representative of large cities nearby an airport (APT-HD), medium size cities within the CTR of an airport, but far enough not to be considered influenced by the airport environment (C-MD), and rural, sparsely populated areas in uncontrolled airspace (UC-LD).
Traffic mixes used by BUBBLES. These figures are just indicative. They must be adapted to any particular local scenario and could be dynamically adapted to the actual demand if a Dynamic Capacity- Demand Balancing U-space service is available.
The common TLS was then expressed in terms of different conflict rates (according to the updated BUBBLES Concept of Operations for separation management described here) for each ‘sector type’ taking into account the particular ‘mix of traffic’ considered therein. To this purpose, the research team used the collision model developed in T4.1. This way, Safety Criteria (SAC) for each scenario were defined, as well as the required separation minima to guarantee that they can be attained, which are the basis for the development of the BUBBLES SPR for separation management in by the U-space.