STREAM: Strategic Trajectory de-confliction to Enable seamless Aircraft conflict Management.

Advanced Logistics Group (ALG)

Boeing Research & Technology Europe (BR&TE)

Universidad Autónoma de Barcelona (UAB)


The STREAM project aims to investigate innovative strategic trajectory-de-confliction algorithms that can reduce the conflict management automation gap between the pre-departure and flight execution phases. The underlying concept is that of extending the NOP rolling-planning concept to separation management by means of a seamless conflict-management process that would run continuously from the strategic phase (pre-departure, collaborative design of the NOP) up to the execution one (automation-assisted, controller-driven conflict resolution). The project will define new metrics to evaluate the performance of such algorithms. In particular, a metric to measure the fairness of a trajectory de-confliction solution will be defined, in order to evaluate how fairly the cost penalties associated to the deviations from the original SBTs proposed by the de-confliction algorithms are distributed among airspace users.


Introduction and problem statement


ATM trajectory conflict detection and resolution – state-of-the-art

In today’s operations, air traffic controllers resolve conflicts with little or no automation support, typically issuing tactical instructions via voice communications based mainly on surveillance information.

SESAR introduces the concept of 4D Business Trajectory, which evolves out of a collaborative layered planning through 3 main different phases: BDT, SBT and RBT.

For the planning phases SESAR anticipates pre-departure collaborative decision-making processes, also supported by automation, which will culminate in a system wide rolling plan, the NOP (Network Operations Plan).

For the execution phase SESAR anticipates the introduction of advanced decision-support tools and air-ground data link capabilities to aid controller conflict-resolution tasks.

Enables a shift from tactical reactive surveillance-based conflict resolution to strategic trajectory based separation assurance


The potential for the STREAM project to extend the SESAR state-of-the-art

The RBT constitutes the ultimate trajectory that the airspace user agrees to fly and airports and ANSPs agrees to facilitate; changes to the RBT must be kept to a minimum.

By integrating into the RBT constraints implied by conflict resolution, anticipated when it is still under its SBT status, it would be possible to enhance the overall process of conflict management and to minimize the RBT updates during the tactical phase.

SESAR investigates enhanced tools for conflict detection and resolution but focusing efforts only on a reactive type of action as provided by the Medium Term Conflict Detection (MTCD) tool used by the ATCOs.

STREAM proposes to investigate an innovative concept representing an active approach to conflict management, which starts at the pre-departure phase when the SBT modifications are negotiated among the involved actors.

STREAM tool will be compatible with SESAR tools and procedures, but represents an evolution towards a seamless realization of conflict management, starting before flight takes off (SBT modifications) and continues through the complete flight duration (tactical separation).


Project objectives and expected results


High level objective

To reduce the gap that exists nowadays between the long-term predictive part of the ATM system, represented by central flow management measures, and the short-term adaptive actions locally performed by tactical controllers.

Intermediate objectives

To develop time-efficient algorithms for the strategic detection and resolution (CD&R) of traffic conflicts:

capable of adopting a combination of different resolution strategies (route, speed and flight level modifications);

that run in linear time with respect to the number of trajectories considered;

and manage a high quantity of information describing 4D trajectories and to ensure that resolutions maintain traffic complexity under control — at local(ACC), regional (FAB) and even global (ECAC) levels

To explore the requirements on the reliability and robustness of traffic predictions in order to allow the new strategic trajectory de-confliction algorithm that would efficiently modify SBTs.

To define metrics for assessing the fairness and equity of the proposed algorithms towards an increase automation of the ATM system.

To perform simulations in a common environment to validate the proposed algorithms and metrics.

Description of the STREAM solution

STREAM solution will support Network Manager in strategically de-conflict traffic through appropriate SBT amendments.

STREAM solution is intended to be applied to 4D trajectories that:

have already been subject to the preliminary DCB (airport slots, previous negotiations)

are still published as SBT, not already agreed as RBT

prior to the take-off, when forecasts are stable and estimates become reliable

are independent from the concepts of airspace sector and sector capacity

Before reaching an agreement on the final RBT, the network manager assesses the interactions with the rest of the traffic and proposes appropriate modifications (speed, time, path) to be included in the final RBT in order to:

minimise the probability of conflicts occurring during flight execution

mitigate the estimated impact on the system

ensure that costs derived from amendments are fairly shared among users

Priority on the traffic headed to congested airspaces, with the aim of reducing traffic complexity where this eventually implies a reduction of available capacity

The global traffic picture needs to be considered in order to:

Identify critical situations of high congestion and workload

Elaborate conflict solutions avoiding complexity and conflict propagation in other sectors

Consider the user preferences when constraining the trajectories in order to achieve fairness

STREAM operational benefits

Closer integration between the global predictive part of the system, performed by ATFM, and the local reactive part, performed by tactical controllers. This will contribute by:

increasing predictability

increasing capacity (both real and declared)

Reduction of the number of conflicting aircraft to be resolved by ATCOs, since most of the traffic will be strategically de-conflicted through the inclusion of appropriate constraints into the business trajectory. This will contribute to:

reduce ATCO workload

enhance capacity

Reduction of environmental impact (gaseous emissions), thanks to the enhanced efficiency of ATM operations and the opportunity for airspace users to fly trajectories which are as close as it is possible to their optimal ones, while minimising the need for tactical modifications due to conflict resolution tasks.


Approach / methodology


Tool development approach

The strategic trajectory de-confliction tool will be developed in two successive phases, basic and advanced solution, and will be based on the following techniques:

Distributed Spatial Data Structures (SDS): To support different air space organisations (ACC, FAB’s, Sectors), a scalable distributed space database structure must be properly designed and implemented

Discrete Event Models: The management of a considerable amount of 4D trajectories will require the development of efficient models for decision making. A discrete event specification of aircraft trajectories will avoid the computational burden of a continuous approach, therefore focusing the modelling target in the causal analysis of the decision variables (i.e. route, speed and flight level modifications). The key aspects and information of SBT must be analysed during the abstraction modelling tasks in order to guarantee efficiency, user flexibility and completeness for the conflict detection algorithm

The Coloured Petri Net (CPN) formalism will be used to develop the discrete event system (DES) models. This will be done under a causal modelling approach, since the resolution of a certain conflict can spread over new potential conflicts.

State Space Analysis Tools: State Space (SS) Analysis is a rigorous approach that can be supported by causal models developed through Coloured Timed Petri Net formalism

Predictive models: time-varying regression models will be developed to complement poor information at early pre-flight phases

Scenario Design: Scenario planning will be considered as a framework of support for decision-makings based on clarifying cause-effect factors in a target business, which are mostly achieved by using a causal structural graph model. A Constraint Logic Programming approach will be implemented based on an automatic propagation of time constraints generated by the causal CPN models developed, avoiding in this way the exploration of non feasible solutions. These scenario results will allow the comparison of ATM characteristics: with or without ADS-B technology, convergent or divergent traffic, etc.



Impact assessment approach

A definition of the concepts of fairness, equity and robustness in ATM will be provided. Special focus will rely on fairness and equity, as they represent the areas where there still remains more research, development and understanding to be done.

Metrics for fairness and equity will be developed accordingly to the proposed concept definition and cost penalty models, in order to capture the impact of trajectory modifications on the flights incurred cost and, ultimately, on the airlines’ business strategy.

The robustness assessment will focus on the impact of accurate and imprecise information to the strategic trajectory de-confliction tool. Case studies with and without wind will be key for the analysis as well as the trajectory modelling tools.

The efficiency analysis will centre the study on demonstrating the usefulness and benefit of the proposed tool for strategic optimisation, whilst reducing the tactical interventions required by the ATCO.

An existing simulation infrastructure will be configured to handle the new concepts and serve as a test bed for the algorithms.

The assessment will be realized through appropriate simulations run on an event driven C++ based simulation platform that will be adapted to accommodate the necessities of the STREAM defined environment.



STREAM Deliverables