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Modeling and Optimization of Aircraft Trajectories: a review Maria Pia Fanti, Giovanni Pedroncelli, Gabriella Stecco, Walter Ukovich Maria Pia Fanti, Giovanni Pedroncelli, Gabriella Stecco, Walter Ukovich

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 2 OutlinesOutlines 1 Aim of the paper 2 Conflict Detection and Resolution (CD&R) Systems 3 Weather Models 4 Trajectory Modeling and Optimization in presence of hazardous weather 5 Conclusions and future works

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 3 Aim of the paper Airspace is more and more crowded because the air transportation flow is constantly increasing. The current Air Traffic Management (ATM) is unsuitable for the future expansion of the air traffic. A review of the main contributions about the modeling and optimization of the aircraft trajectories with two objectives: i) resolving conflicts with other aircrafts (search of the optimal path from origin point to destination point which is conflict-free) ii) avoiding hazardous weather (search of the optimal route of deviation from weather encounters). A review of the main contributions about the modeling and optimization of the aircraft trajectories with two objectives: i) resolving conflicts with other aircrafts (search of the optimal path from origin point to destination point which is conflict-free) ii) avoiding hazardous weather (search of the optimal route of deviation from weather encounters).

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 4 OutlinesOutlines 1 Aim of the paper 2 Conflict Detection and Resolution (CD&R) Systems 3 Weather Models 4 Trajectory Modeling and Optimization in presence of hazardous weather 5 Conclusions and future works

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 5 CD&R Systems The Free Flight concept has been conceived as a new and better ATM system in which every pilot can select the optimal route for the aircraft in respect of safety constraints, avoiding conflicts with the other aircrafts. In this environment, we consider each aircraft surrounded by two virtual cylindrical shapes, the Protected Zone and the Alert Zone. A conflict or loss of separation between two aircrafts occurs whenever the protected zones of the aircrafts overlap. The goal for the Conflict Detection and Resolution (CD&R) system is to predict that a conflict is going to occur in the future, communicate the detected conflict to a human operator and, in some cases, assist in the resolution of the conflict situation.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 6 CD&R Systems If the Alert Zone of an aircraft is violated, air traffic controllers will intervene to assist in conflict avoidance. To ensure safety, the Protected Zone should never be penetrated. The probability of conflict avoidance success depends on the size of the Alert Zone. If the Alert Zone of an aircraft is violated, air traffic controllers will intervene to assist in conflict avoidance. To ensure safety, the Protected Zone should never be penetrated. The probability of conflict avoidance success depends on the size of the Alert Zone. J. K. Kuchar and L.C. Yang, Prototype Conflict Alerting System for Free Flight. Journal of Guidance, Control and Dynamics; Volume 20; No. 4; 768-773, 1997.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 7 CD&R Systems: multi-agent environments Some CD&R approaches consider the airspace as a multi-agent environment in which aircrafts are considered as agents occupying zones of the space. D. Sislak, P. Volf and M. Pechoucek, Agent-Based Cooperative Decentralized Airplane-Collision Avoidance, IEEE Transactions on Intelligent Transportation Systems, Vol. 2, No. 1, pp. 36-46, 2011. S. Resmerita, M. Heymann and G. Meyer, A framework for conict resolution in Air Tra c Management. IEEE Conference on Decision and Control, pp.2035-40, 2003. Other methods (e.g. Resmerita et al.) make use of the graph theory. The airspace is discretized as a grid with each node of the graph representing a zone of the space. Usually there is no assumed cooperation: agents are greedy and there is no inter-agent communication during the conflict resolution. Some of these methods (e.g Sislak et al.) use the principles of game theory, generally assuming cooperation between aircrafts (considered as agents in negotiation). The zones of the airspace are considered as resources contended by the agents.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 8 CD&R Methods Other CD&R methods (e.g. Pallottino et al.) are based on the optimization of a cost function. This function is usually associated with a set of cost metrics (e.g. time of flight, fuel, projected separation, workload). Some other methods are based on a path planning approach. For example, Schouwenaars et al. present a cooperative, decentralized path planning method which makes use of Mixed Integer Linear Programming (MILP) problems resolution methods and receding horizon strategy. L. Pallottino and E. Feron, Conflict Resolution Problems for Air Traffic Management Systems Solved with Mixed Integer Programming, IEEE Trans. on Intelligent Transportation Systems, Vol.3 No.1, pp.3-11, 2002 T. Schouwenaars, J. How and E. Feron, Decentralized Cooperative Trajectory Planning of Multiple Aircraft with Hard Safety Guarantees, AIAA Guidance, Navigation, and Control Conference and Exhibit, Providence, Aug. 2004.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 9 OutlinesOutlines 1 Aim of the paper 2 Conflict Detection and Resolution (CD&R) Systems 3 Weather Models 4 Trajectory Modeling and Optimization in presence of hazardous weather 5 Conclusions and future works

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 10 The Weather Avoidance Problem Weather is one of the leading causes of delays for aircrafts and these delays increase as the volume of air traffic increases. Weather events cause an additional workload for the controller who has to perform tactical decisions about the weather impact reduction. Weather phenomena can also cause damages to the electronic and navigational equipment of an aircraft or danger to the passengers lives. In a Free Flight environment, weather avoidance will be more complicated as pilots will have autonomous control and efficient and robust weather avoidance algorithms will be needed to ensure safety. A weather model is the way of representing weather events in order to be avoided by the pilots: its the first step in developing a fully automated system for decisional support integrating also flight information, trajectory modeling to help the pilot performing better decisions and reducing workload.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 11 Convective Weather Avoidance Model (CWAM) CWAM is a model proposed by DeLaura et al. It generates estimates of pilot deviation probabilities in the case of weather encounters as a function of Vertical Integrated Liquid (VIL, that is the amount of liquid water detected in a vertical column of the atmosphere) and echo top (i.e. the radar observed height of a convective system). R. DeLaura, M. Robinson, M. Pawlak and J. Evans, Modeling Convective Weather Avoidance in En Route Airspace, 13 th ARAM Conference, New Orleans, LA, Jan. 2008

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 12 Convective Weather Avoidance Model (CWAM) The products of CWAM are Weather Avoidance Fields (WAFs), 3D grids which represent the estimates of pilot deviation probability in the encounter of weather. These products are used in some other weather avoidance systems: e.g. in Windhorst et al. CWAM forecasts define the contours of airspace containing convective weather, each with the probability that it will be avoided by the pilot. R. Windhorst, M. Refai and S. Karahan, Convective Weather Avoidance with Uncertain Weather Forecasts, 28th Digital Avionics Systems Conf., Oct. 2009.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 13 Weather Models In Kangapour et al. VIL measurements are quantized into 6 levels with level 3 and higher indicating a recommended no-fly zone. Then weather maps are created and each storm in the airspace is enclosed in a minimum volume bounding ellipse which is an over-approximation of the weather. M. Kangapour, V. Dadok and C. Tomlin, Trajectory Generation for Aircraft Subject to Dynamic Weather Uncertainty, 49th IEEE CDC, Atlanta, GA, USA, pp. 2063-2068, 2010.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 14 Weather Models The weather model proposed by Nilim et al. associates to each storm a 2-states Markov chain: state 0 correspods to having no storm in a particular region and timeframe; 1 corresponds to having a storm. If there are m storms there will be a 2 n Markov chain and a 2 n x2 n transition matrix as an input of the optimization algorithm. A. Nilim, L. El Ghaoui, M. Hansen and V. Duong, Trajectory-based Air Tra c Management (TB-ATM) under Weather Uncertainty, ATM 2001, Santa Fe, New Mexico, 2001.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 15 OutlinesOutlines 1 Aim of the paper 2 Conflict Detection and Resolution (CD&R) Systems 3 Weather Models 4 Trajectory Modeling and Optimization in presence of hazardous weather 5 Conclusions and future works

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 16 Trajectory models in presence of hazardous weather (I) Ng et al. make use of a DP algorithm to find the shortest path from a starting point to an ending point. The airspace is modeled as a graph with each node representing a point and edges representing the possible trajectories from one point to another. It uses CWAM forecasts. The objective cost function can be constructed by adding different components to represent obstacles or constraints: components relative to the estimated fuel cost, the cost of deviation due to weather, the cost associated with crossing a congested region. The objective cost function can be constructed by adding different components to represent obstacles or constraints: components relative to the estimated fuel cost, the cost of deviation due to weather, the cost associated with crossing a congested region. H.K. Ng, S. Grabbe and A. Mukherjee, Design and Evaluation of a Dynamic Programming Flight Routing Algorithm Using the Convective Weather Avoidance Model, AIAA Guidance, Navigation, and Control Conference, Chicago, Illinois, Aug. 2009.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 17 Trajectory models in presence of hazardous weather (II) Kangapour et al. propose a non-cooperative algorithm of trajectory optimization which makes use of a reference trajectory (planned in advance) and of a cost function that penalizes deviations from this reference trajectory. The optimization problem is solved by means of the receding horizon technique: the planning horizon is chosen as the discrete number of steps for which the trajectory planning is performed. After solving the problem in the defined horizon, it is shifted by a certain number of time units (in this case that is 5 minutes, the time for which new weather forecasts are available). Every aircraft which reaches its destination is removed from the problem and the algorithm terminates when all the aircrafts have reached their destination. M. Kangapour, V. Dadok and C. Tomlin, Trajectory Generation for Aircraft Subject to Dynamic Weather Uncertainty, 49th IEEE CDC, Atlanta, GA, USA, pp. 2063-2068, 2010.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 18 Trajectory models in presence of hazardous weather (III) Nilim et al. address trajectory optimization for a single aircraft, minimizing the expected delay facing bad weather by a DP algorithm. Even if the paper deals with the trajectory optimization of a single aircraft, the novelty is considering the weather constraints in the trajectory optimization. First, a cost is assigned to each zone of the discretized airspace; then the solution is the point which will be occupied by the aircraft for the next 15 minutes. A. Nilim, L. El Ghaoui, M. Hansen and V. Duong, Trajectory-based Air Tra c Management (TB-ATM) under Weather Uncertainty, ATM 2001, Santa Fe, New Mexico, 2001.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 19 Trajectory models in presence of hazardous weather (IV) Pannequin et al. aim to solve both the weather and conflict avoidance problems, considering also the effects of strong winds. They make use of the receding horizon technique and the cost function is obtained by solving the Hamilton-Jacobi partial differential equation (HJ PDE). The solution to HJ PDE provides a value function which contains the minimum travel time from any point of the airspace to the destination. Each destination point has a corresponding value function and the set of these value functions will serve as a look-up table for the cost function. Each deviation of the aircraft from its optimal trajectory is penalized in the objective function definition. Pannequin et al. aim to solve both the weather and conflict avoidance problems, considering also the effects of strong winds. They make use of the receding horizon technique and the cost function is obtained by solving the Hamilton-Jacobi partial differential equation (HJ PDE). The solution to HJ PDE provides a value function which contains the minimum travel time from any point of the airspace to the destination. Each destination point has a corresponding value function and the set of these value functions will serve as a look-up table for the cost function. Each deviation of the aircraft from its optimal trajectory is penalized in the objective function definition. J. Pannequin, A.M.Bayen, I.M Mitchell, H. Chung and S. Sastry, Multiple Aircraft Deconicted Path Planning with Weather Avoidance Constraints, AIAA Conf. on Guidance, Control and Dynamics, South Carolina, 2007.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 20 OutlinesOutlines 1 Aim of the paper 2 Conflict Detection and Resolution (CD&R) Systems 3 Weather Models 4 Trajectory Modeling and Optimization in presence of hazardous weather 5 Conclusions and future works

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 21 ConclusionsConclusions We summarize the proposed methods and approaches to model and optimize the flight paths in the following table, according to their most important features. MethodDiscretizationAircraftsCentralization Schouwenaars Multiple Ng Single KamgarpourMultiple Nilim Single Pannequin Multiple

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 22 Discretization: the airspace is subject to a discretization in the process of resolution of the conflict or weather encounter. The optimization is performed by solving MILP problems (like in Schouwenaars) or applying DP algorithms (like in Ng). Aircrafts: some models are conceived for a single aircraft (Single) in the airspace: in this case the possibility of a conflict with other aircrafts is not considered. Other contributions consider the presence of multiple aircrafts (Multiple) and have to find solutions for the avoidance of conflicts. Centralization: the weather or conflict problem can be solved by a centralized approach (e.g. the airspace controller) or by distributing the task among the different aircrafts in the airspace. This feature is considered only for multi-aircraft systems. In Schouwenaars the resolution of the problem is distributed among aircrafts which act in a cooperative manner. This approach leads to the optimization of the available computation resources and is one of the central ideas of the Free Flight concept. Discretization: the airspace is subject to a discretization in the process of resolution of the conflict or weather encounter. The optimization is performed by solving MILP problems (like in Schouwenaars) or applying DP algorithms (like in Ng). Aircrafts: some models are conceived for a single aircraft (Single) in the airspace: in this case the possibility of a conflict with other aircrafts is not considered. Other contributions consider the presence of multiple aircrafts (Multiple) and have to find solutions for the avoidance of conflicts. Centralization: the weather or conflict problem can be solved by a centralized approach (e.g. the airspace controller) or by distributing the task among the different aircrafts in the airspace. This feature is considered only for multi-aircraft systems. In Schouwenaars the resolution of the problem is distributed among aircrafts which act in a cooperative manner. This approach leads to the optimization of the available computation resources and is one of the central ideas of the Free Flight concept. ConclusionsConclusions

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 23 We presented how the modeling techniques and the optimization techniques are applied in order to optimize the trajectories in air traffic management. ConclusionsConclusions The problem involves potentially conflicting objectives such as minimizing deviations, weather avoidance, reducing emissions, minimizing distance travelled and hard constraints like aircraft performance. The problem is still open and future research should address, in particular, sustainability objectives such as reduction of emissions and pollution.

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7 th International Conference on System of Systems Engineering – IEEE SOSE 2012 24 This paper is a starting point for the developing of a real-time flight path optimization system. Future work The path will be described by a set of 4D waypoints: 3D points in the airspace (determined by latitude, longitude and altitude) and the time of flight over them. The objective is to compute this flight path minimizing pollutant emissions (CO 2, NO x and noise) in a non-conflict airspace (i.e. for a single aircraft). In particular, CO 2 depends on the phase of flight and NO x on weather conditions such as air pressure, temperature and relative humidity.

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Modeling and Optimization of Aircraft Trajectories: a review Thank you for your attention

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