A few ideas about separation provision and Remain Well Clear, and how BUBBLES will tackle them.

Conflict management is defined by the ICAO in the ATM Operational Concept (ICAO Doc. 9854) as the Air Traffic Management component in charge of “limiting, to an acceptable level, the risk of collision between aircraft and hazards”.

15 Mar 2021

Before talking about conflict management, it is worth recalling what a conflict is. According to ICAO Doc. 9854, a conflict is “any situation involving aircraft and hazards in which the applicable separation minima may be compromised”. A similar, more precise definition can be found in ICAO’s Air Traffic Services Planning Manual (ICAO Doc.  9426): a conflict is “predicted converging of aircraft in space and time which constitutes a violation of a given set of separation minima”.

According to the ICAO ATM Operational Concept (ICAO Doc. 9854), conflict management is structured in three layers that are applied sequentially to reduce the probability of a collision between aircraft and hazards to an acceptable limit:

  1. Strategic conflict management.
  2. Tactical conflict management.
  3. Collision avoidance.

The Strategic conflict management, makes use of the the Air space organisation and management, Capacity and demand and Time synchronisation components of the ATM to assign of non-conflicting trajectories during the pre-flight phase (although they can be modified during the flight). In this phase no separation minima are defined

The Tactical conflict management consists of the tactical process of keeping aircraft away from hazards by at least the appropriate separation minima. This is an iterative process consisting of:

  1. The detection of a conflict within a specific conflict horizon, based on the current position of the involved aircraft and their predicted trajectories in relation to known hazards.
  2. The formulation of a solution, which implies the selection of separation modes and minima and the designation of a separator (the airspace user or the ATC) in charge of applying them.
  3. The implementation of the solution.
  4. Monitoring the effectiveness of the solution.

This process is iteratively executed until the hazard is avoided by the appropriate separation minimum or the on-board Collision avoidance (CA) system (if available) triggers reactive measures in case it detects that a collision is imminent. Figure 1 summarises the conflict management process according to ICAO Doc. 9854.

Figure 1. Conflict management according to the ICAO Doc 9854.

In the framework of manned aviation, this is a well-established topic. Depending on whether an aircraft is flown under visual or instrumental rules and the airspace class it is perfectly clear who is responsible for the conflict management and which procedures have to be used. However, in the case of unmanned aircraft, (especially in the Very Low Level airspace) the distinction between tactical conflict management and collision avoidance has become a little bit foggy since several standards for Detect and Avoid (DAA) Systems comprising the Remain Well Clear Function (RWC) and the Collision Avoidance function are being developed. This approach can be somehow misleading because it mixes two layers of the conflict management within the same scope.

In addition to this, although manned aviation standards and guidance material contain expressions like “keep well clear from hazards”, this usually refers to the situation where the pilot is in charge of avoiding hazards when flying under Visual Flight Rules. In that context, “well clear” is a qualitative (even subjective) concept. However, in the context of unmanned aviation, the ICAO RPAS Manual (ICAO Doc. 10019) clearly states that RWC corresponds to the second layer of conflict management when the agent responsible for the separation provision it the UAS. Taking into account that in the case of UAS the RWC function is executed by an electronic systems, it requires the definition of several separation minima, as shown in Figure 2.

Figure 2. Thresholds used by the RWC and CA functions for UAS (from ICAO Doc. 10019 RPAS Manual).

Referred to Figure 2, the ICAO RPAS manual estates:

When a conflicting traffic crosses the RWC threshold of an RPA, a DAA system should alert the remote pilot to take action so as to prevent this intruder from entering the RWC volume and becoming a threat to the RPA. Likewise if a threat crosses the CA threshold of the RPA, a DAA system should alert the remote pilot to take action so as to prevent this intruder from entering the collision volume and risk colliding with the RPA.

However, Figure 2 and the paragraph above can be quite misleading, because what they are describing, strictly speaking, is the tactical separation provision process, irrespective of whether the separator is the airspace user or a ground-based separation provision service. In that sense, the terms DAA and RWC could be replaced by the term “ground based separation provision” (which in the case of the U-space should be the Tactical conflict resolution service). In the same line, it can result also misleading the possibility of allocating the RWC function to the Ground Control Station (GCS): although it actually complies with the definition of RWC in the ICAO Doc. 10019, it makes the border between RWC and a tactical conflict resolution even fuzzier. In any case, at least two relevant differences between them remain:

  1. In case of RWC, the “burden of proof” of safety is on the side of the UAS. This implies demanding requirements that can lead even to SW and HW certification. Similar requirements will apply to the U-space services responsible for providing the separation management, but their impact will be lower in this case than in the former (indeed, the high level regulatory framework for the U-space already includes the certification requirement).
  2. RWC has to apply the Rules of the Air and right-of-way rules (which are not clear at all in the case of UAS). Nothing has been said about this requirement for ground based separation management services.

In any case, even more relevant than whether the conflict management is executed by the airspace user or a U-space service is the extension of the conflict horizon. For the time being, both the RWC and the Tactical conflict resolution service consider reduced conflict horizons to tackle near-term conflicts, as shown in Figure 3(a). That approach can be valid in low traffic density environments, but as the traffic density increases, it will be necessary to extend the conflict horizon to minimise the effect of the manoeuvres executed to avoid hazards, as shown in Figure 3(b). This will be especially crucial in the VLL over densely populated areas, where traffic densities of several hundreds of UA per square kilometre are envisaged.



Figure 3. (a) Reduced conflict horizon for near term conflicts. (b) Wider conflict horizon minimises the changes to aircraft trajectories when solving a conflict.

In case of wide conflict horizons such as that of Figure 3(b), an essential difference between the RWC and the U-space separation provision arises. The former leads to a distributed conflict management system involving all the UAS within the conflict horizon (Figure 4 (a)), whereas the latter is based on a centralised system (or a few federated systems sharing a common pool of data and able to provide a seamless service to the U-space users), as shown in Figure 4(b). In addition to the intrinsic complexity of synchronising in time and space multiple agents, a purely distributed conflict management approach implies that the situational awareness has to be built from the snapshots obtained by each particular UAS. Since the on-board detection capability has a limited range (especially in complex urban environments), the UAS have to share their snapshots (e.g. using a gossip protocol approach) to get a full situational awareness within the conflict horizon. This will pose very demanding requirements to the V2V communication. These requirements could be relaxed if the situational awareness is provided by the U-space (e.g. through the Traffic information service). However, if the RWC relies on the U-space it seems more effective to entrust the separation also to the U-space (which is a simpler solution from a technical point of view).

(a) (b)

Figure 4. (a) Self- separation: extended RWC (multiagent pair-to-pair negotiation). (b) Separation provided by the U-space (centralised system).

BUBBLES has developed an operation centric, risk based, collision model that computes collision rates (number of collisions per time unit) taking into account strategic mitigation measures. Strategic conflict management is enough when the residual risk obtained by applying strategic mitigations is acceptably low. Tactical conflict management has to be implemented when the residual risk after applying strategic mitigations is still high. For strategic separation, separation minima and PBN (Performance based navigation) requirements have to be defined. For tactical separation, a conflict horizon, separation minima and methods, as well as PBCNS (performance based communication, navigation and surveillance) requirements have to be defined. In case of tactical separation, the level of automation is also crucial: increasing the automation leads to shorter response time and a more accurate estimation of the closure rate, which in turns leads to reduced separation minima. However, the higher the automation level, the more demanding the PBCNS requirements will be. The collision model developed by BUBBLES allows take into account both reaction times and the effect of performance and safety issues when deriving separation minima. BUBBLES aims at extending this model to assess the effect of the conflict horizon and the architecture of the separation provision (distributed/centralised) on the collision rate in order to identify requirements for the systems and services involved in the process.