First improvements to the BUBBLES initial concept.

21 July 2021


BUBBLES project main goal is to develop a Concept of Operations to provide separation by means of the U-space. The initial concept formulation was presented to the U-space community in a workshop organised by BUBBLES in February 2021. The open discussion held at the end of the workshop provided the research team with many relevant inputs which were summarized in a short paper published in the March issue of the BUBBLES Newsletter. The comments gathered during the workshop and from the SJU in the plenary meeting held the day before led the BUBBLES team to advance the update of the concept, originally scheduled in October 2021, to April 2021.

The first question addressed by the research team was to clarify the relation between the concepts of Remain Well Clear (RWC) introduced in the ICAO RPAS Manual, the requirement to keep well clear for manned aviation set out in the Annex 2 to the Chicago Convention on International Civil Aviation, and Collision Avoidance (CA):

  • The RWC for UAS corresponds to the provision of tactical separation by the airspace user (the UAS); therefore, separation minima for RWC are needed.
  • The requirement to keep well clear established in Annex 2, paragraph 3.2.1.0, applies to aircraft flying under Visual Flight Rules (VFR) and does not need the definition of separation minima (the distance to a hazard is safe if the pilot feels so).
  • CA consists of a set of reactive manoeuvers executed when the tactical conflict management layer fails to solve the conflict and the collision is imminent. Although the systems providing RWC and CA are being standardized in the same documents (e.g., the ACAS sXu standard which is being developed by the RTCA), they must not be confused in any way, since they clearly correspond to different layers in the ICAO conflict management model, even though they can be provided by the same Detect and Avoid (DAA) on-board system. Therefore, BUBBLES proposed to avoid using the term DAA to refer to a capability. The terms DAA-RWC and DAA-CA will be used instead.

Based on the discussion above, BUBBLES updated the collision model described in the IACO RPAS manual, as shown in the figure below. The BUBBLES model emphasizes that the separation at tactical level can be provided either by the U-space or by the on-board DAA-RWC capability.

BUBBLES collision model

BUBBLES collision model updated from the collision model in the IACO RPAS manual.

Based on this collision models, different conflicts of increasing severity were defined, as shown in the following figure (only UAS-UAS conflicts were considered at this stage; conflicts involving other hazards will be addressed in the next concept update).

conflict categories

Conflict categories considered by the BUBBLES Concept of Operations for separation management.

The updated BUBBLES concept for separation management considered that the main difference between the separation provided by the U-space tactical conflict resolution service and the self-separation provided by on-board DAA-RWC systems lies in the fact that the former can deal with wider conflict horizons and supports conflict solutions coordinated between several UAS (to prevent that solving a particular conflict between two UAS can cause a new conflict with a third one). On the other hand, DAA-RWC systems can cope with conflicts within narrower conflict horizons, typically involving just two aircraft. The latter sentence reflects the state-of-the-art of on-board sensing and V2V communication; thus, DAA-RWC conflict horizon might be extended as these technologies progress. This approach is aligned with the EUROCAE ED.267 document (OSED for DAA in VLL operations).

The choice between tactical separation provided by the U-space and the DAA-RWC based self-separation will depend on the traffic density and the risk entailed by the conflict. To represent the operational risk, the BUBBLES team defined nine ‘sector types’ and ten ‘traffic classes’ based on the outcomes of WP3 (more details here). Two options guaranteeing that any conflict has a single separator were proposed:

  1. All tactical conflicts within a particular U-space ‘sector type’ have the same separator.
  2. Tactical conflicts between high risk traffic types (manned and certified) are separated from all other traffic types by the U-space tactical conflict resolution service whereas tactical conflicts involving lower risk traffic types are self-separated.

The first option is simpler, whereas the second one can lead to a more efficient use of the airspace. Both options are currently being assessed by means of simulations and a proposal to choose between them will be included in future versions of the BUBBLES Concept of Operations for separation management. Moreover, different separation modes were proposed, depending on the separator.

A procedure to determine pairwise separation minima between aircraft considering their ‘traffic classes’ was also set up by the research team. These separation minima are obtained in a given ‘sector type’ for a particular ‘traffic mix’ in order to comply with Safety Criteria stated in terms of conflict rates so that a given TLS is attained (see here for details). The separation minima will also take into account the performance of U-space services and supporting systems needed to provide separation management.

Separation methods in the vertical and horizontal dimensions were also considered in the model. Horizontal separation methods encompass change of heading and/or speed, as well as hovering. For vertical separation, a layered airspace was proposed so that each UAS is assigned a flight layer during the strategic conflict resolution phase. Tactical vertical separation could be obtained using intermediate layers not used for regular traffic (as shown in the figure below). This vertical structure (possibly dynamic to cope with changes in demand) should be defined by the competent authority when the volume is designated as a U-space airspace and published in the Common Information Service (CIS) so that it can be applied by all U-space service providers.

Layered U-space airspace structure supporting strategic and tactical vertical separation.