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4 Dimension Contracts - Guidance and Control

Final Report Summary - 4DCO-GC (4 Dimension Contracts - Guidance and Control)

Executive Summary:
The 4DCo-GC European project aims at explaining and analysing the 4D contract concept (each flying aircraft must comply with a 4D trajectory associated with margins) as a possible evolution of the air transport system management, by means of simulations enabling to guide and control aircraft along those contracts. To this end, the 4D contract concept developed in the frame of the former IFATS project (FP6) was refined, taking into account the current SESAR outputs, new avionics development and the progressive change of paradigm that is anticipated by the European High Level Group on aviation research for 2050: “Automation has changed the roles of both the pilot and the air traffic controller. Their roles are now as strategic managers and hands-off supervisors, only intervening when necessary”.

The 4D contract concept is a perfect opportunity to take full benefit of the current and future aircraft guidance and control systems. The 4D contracts are generated to satisfy the airlines expectations, while aiming at optimizing the overall traffic. The execution of these contracts is based on an extensive use of the flexibility provided by the aircraft flight domain, in order to maintain the overall system performance, while limiting the need for contracts updates.

In the frame of the 4DCo-GC project, the various aspects of this futuristic air transport system (4D contracts generation, 4D FMS, contract compliance monitoring, on-line re-planning, data links, etc.) were modelled and integrated in a global traffic simulation. Two simulation campaigns were performed, enabling to simulate traffic of twice the current density, over the Benelux area. Several scenarios were simulated in order to study the system reaction against nominal and non-nominal situations (e.g. airport closure, emergency descent). Even if simplification hypotheses were used for these simulations, the system performance and robustness were found very satisfactory. Regarding the guidance and control aspects, the individual aircraft simulation showed that following a 4D contract, by computing and flying an optimized 4D trajectory, is possible with limited need for contract update – 99% of the flights were performed without re-planning. The final conclusions of the project will be proposed to the SESAR programme and to aircraft manufacturers in order to prepare what could be the far future of the air transport system.
Project Context and Objectives:
When speaking about the future of the Air Transport System (ATS), and according to the various traffic growth forecasts, there will be a need for an increased airspace capacity and more efficiency in the system. Concurrently, in order to keep the frequency of accidents at least as low as today, the safety of the ATS should be significantly enhanced. Modifications must be brought to the present system.
Pioneering the ATS of the future is possible only by looking at a distant horizon (i.e. 2050+) in order to take advantage of the design freedom and flexibility brought by a clear break with the current system. Such a way was followed by the Innovative Future Air Transport System (IFATS) project from the 6th European Framework Programme (FP6). IFATS defined a fully automated ATS based on the so-called ‘4D contract concept’. In the frame of the project, it was not possible to support the proof of concept by simulation. So, to complement this project, the small/medium size focused research project 4DCo-GC (4D Contract Guidance and Control, relating to the activity 7.1.1 “Pioneering the air transport of the future” of the Work Programme 2010 Theme 7 “Transport (including aeronautics)”) was funded by the European Commission.

The 4DCo-GC project aims at exploring the concept of “4D Contract guidance and control of the aircraft” as a step change in air transport operations by providing a more radical and environmentally efficient solution for the management of the airspace.
The 4DCo-GC brings together the expertise of 13 European and associated partners coming from 7 countries, for a total budget of 5,5 M€ and a total requested funding of 3,9 M€. Total duration of the project is 36 months.

Imagining and defining the air transport of the future is not a simple issue. Nowadays, a few years after the first flight centennial celebration, the ATS is far from being a fully developed system close to perfection.
Ambitious objectives have been defined in the ACARE Vision 2020, and metrics have been defined to assess their level of achievement: the future ATS will definitely have to be more time efficient and highly customer oriented, keeping costs low while being environmentally friendly, safe and secure; this has to be validated and proven whatever the evolution of traffic will be.
Choosing the means to reach these objectives is not so obvious; the ATS is a complex system, which integrates multiple interacting subsystems that ensure its core function: transporting passengers and goods. Huge technological progress has been made on the subsystems but man is still a major front-line actor, bringing intelligence and weaknesses into the system.

Regarding this ATS evolution, the SESAR consortium has now given a comprehensive description of the implementations to be performed up to 2020. The SESAR IP1 to IP3 show that no disruptions have to be anticipated: the short to medium term ATS has to be necessarily not dramatically different from what it is today for obvious continuity considerations.
Looking further ahead, the need to really pioneer the future allows for much more freedom and flexibility in devising new ATS concepts – yet at the expense of much more uncertainty.
Within this perspective, ideas about the future of air transport have been produced by the “Out of the Box” project.

Regarding the automation of the ATS, one conclusion of this report is that current technologies and computer science knowledge are not ready to really fulfil the needs for prediction of 4D trajectories, due to the difficulty to plan international traffic.
Indeed, preparing the long-term future of the ATS is an unchallenged opportunity to promote excellence in scientific and technological research, development and demonstration. Moreover, the international nature of the ATS calls for trans-national research and industrial cooperation to take up many of the current European challenges.

In this perspective, the central objective of the 4DCo-GC project is to address the aircraft 4D guidance and control principle. More precisely, the aim of the project is to go deeper into the definition of the “4D contract concept” that has been developed in the FP6 IFATS project and that is now mentioned in major research programmes and projects such as SESAR and NextGen.

The “4D trajectory” and “4D contract” notions deserve particular attention. These concepts are not to be confused as they are very different. The notional SESAR “Business Trajectory” is based on a 4D trajectory supplemented with additional information describing the business attributes of the flight, under the overall coordination of a network wide traffic management (SWIM – System Wide Information Management system).

The 4D trajectory concept (being at a given geographical position at a given time) already exists and was widely studied. Its main drawback is that the compliance of the actually flown 4D trajectory with the planned one has to be constantly monitored by the Air Traffic Control (ATC). Indeed, the aircraft 4D trajectory may vary due to e.g. meteorological conditions. In addition, regardless if a pilot or a controller is in charge of trying to keep the aircraft on the planned 4D trajectory, this is a challenging task.
Aircraft automation with a 4D FMS (Flight Management System) partly solves this problem. The question that still remains is: who will be in charge of maintaining separation between aircraft? Predicting the real 4D trajectory from the ground is not an easy job.

The 4D contract concept has been designed to solve the trajectory prediction problem. First, the ground segment of the system is in charge of generating conflict-free 4D trajectories according to the demand and the airspace capacity. Then, aircraft are assigned the resulting 4D contracts. This means that the aircraft are in charge of monitoring their own compliance with the contract, i.e. they have to stay inside their assigned 4D volume, or to ask for a new contract if they cannot. In doing so, the aircraft are guaranteed to fly conflict-free trajectories. In addition, there is no need to predict trajectories on the ground: trajectories will be as planned unless a modification is required by the aircraft.

A first difficulty lies in the generation of those conflict-free trajectories. To ease this task, the 4 dimensions of the airspace are fully used without the current constraints linked to procedures, navigation waypoints and airways: 4D contracts are created and have to be respected in the very large 4 dimensions airspace.

The second difficulty is to ensure that the aircraft comply with the contract despite the aerologic flight conditions that may be different from those planned.
Strongly based on the IFATS work and other projects, the objective of the 4DCo-GC project is to develop a formal and structured approach to define, analyse and optimize the 4D contract guidance and control concept. Involving collaboration and communication tools, formal multidisciplinary analysis procedures, and advanced virtual environments in conjunction with the well-established computer organisation, this approach will be concretized through the development of a set of distributed PC based computer tools, manned by a network of experts: the 4DCo-GC consortium members.
These tools will enable the analysis of various aspects of the 4D contracts generation (ground segment) and 4D contract execution (aircraft side).
For this purpose, all 4DCo-GC partners will contribute to the development of modules that will be consolidated into a consistent and comprehensive set of tools capable to comply with the relevant performance criteria and parameters defined in accordance with the SESAR Key Performance Areas (KPA) and other non-ATM (Air Traffic Management) related considerations.

What are the required changes in the ATS (ATM + aircraft) to meet the ACARE and SESAR objectives? This is a tricky question!
The Single European Sky (SES) legislation launched by the European Commission has set the political framework for actions in Europe to support the need for doubling ATM capacity by 2020 compared to 2004.The SESAR project is currently working on the definition and deployment of the tools and procedures that will be the baseline for the next generation ATM system in Europe.
Regarding the longer term vision, 2040 and beyond, and encompassing both the air and ground segments sub-systems, human actors and technologies, the 4DCo-GC project S&T objectives are as follows:
• Starting from the ATS architecture and concept of operation proposed in SESAR, to perform a closer analysis in order to define and model 4D contract concepts (on strategic, tactical and emergency levels) including a description of key 4D contract definition / negotiation / update factors, and technological issues, encompassing computer sciences;
• To develop the algorithms (strategic planning, tactical planning, etc.), process and functions (e.g. failure management, emergency separation provision) needed to construct the 4D contracts;
• To define and develop a global tool architecture aiming at demonstrating and analysing the main (or key) operating functions of the 4D contract concepts, including the role of the human (that is still paramount but may be significantly different from what it is currently);
• To elaborate an assessment methodology and metrics to qualify and quantify the 4D contact concepts for the control and guidance of aircraft: characteristics, limitations, performances;
• To perform a real time assessment of the 4D contract concept of operations using a simulator;
• To organize dissemination workshops in front of an external audience, including demonstration, in order to disseminate results and to collect feedback for completing the assessment;
• To derive, from this qualification and quantification work, recommendations for future 4D contract system development and performance standards.

Regarding the quantification issue, 4DCo-GC aims at enabling a new approach to assess the performance of 4D contract guidance and control concepts through fast-time simulations.

The developed tools provide the means to gather important data and information, providing not only far-away goals to be reached, but mainly achievable performance indicators to be expected in pre-determined scenarios. These scenarios will be based on the 2020 SESAR ATM perspective.
As an example to illustrate this capability, aiming at doubling the traffic does not speak for itself: flying a maximal number of aircraft is not the point, transporting passengers from where they want to leave to where they want to go and when they prefer to travel is the real issue.

The project will enable to get figures on the viability of the 4D contract concept: is it able to handle the expected traffic growth for the coming decades? How frequent re-planning will be required? Are there situations that the system is not able to manage?
To summarize, the project outputs will be used as documented arguments for the definition of the air transport of the future.

Project Results:
1.3.1 WP100 – Development of 4D airspace and airport models and method for planning of 4D contracts – TsAGI

a) Overall objectives of the WP for the whole project
The objective of this WP is to identify the features of future ATM and define requirements to 4-D contracts. A new airspace structure, taking the 4D capability of the aircraft into account, has to be designed. 4-D airspace and 4-D airport models for effective planning of 4-D contracts on strategic and tactical level have to be developed. Methods of 4D contracts generation and dynamic correction have to be defined and developed. Methods of automated strategic and tactical planning should be elaborated. Methods of on-board and ground 4D-contracts generation systems have to be identified and required computational resources has to be assessed.

b) Description of the methodology
This WP is divided into 5 tasks as follows. In each task, the required modules will be specified and the corresponding software will be developed:
• T110: Definition of requirements for a 4D contract management compliant with the SESAR ATM perspective;
• T120: 4D airspace digital model development;
• T130: 4D airport digital model elaboration;
• T140: Method of optimal seamless strategic and tactical planning;
• T150: Dynamical planning in case of emergency and weather hazard.

All the partners of the project have contributed to this WP. The “Task force” involved in WP100 is presented by the following partners:
• TsAGI (WP leader, specifications and modelling, strategic planning, dynamical planning for cases of emergency and weather hazard)
• MS (digital airport model and strategic planning algorithms)
• NLR (strategic and tactical planning algorithms, digital airport model)
• TECHNION (tactical 4D trajectory re-planning algorithms)
• CIRA (tactical planning and dynamical planning for case of emergency)
• ENAC (4D trajectory specifications and algorithms)
• ALA (analyse of SESAR and NextGen, requirements to 4D contract and aircraft systems and ATM)
• DLR (4D contract requirements, strategic/tactical planning and dynamical planning for case of emergency)
• IAI (requirements to 4D contract and aircraft systems and ATM)
• UPATRAS (dynamical planning for case of emergency)

c) Description of the WP results
Initially partners of WP100 analysed the results of the IFATS project and considered state of the art of the main world projects SESAR and NextGen. The results of this investigation were gathered int the D110.1 document with requirements to the ATM, on-board equipment, airspace and airport models in terms of “4D Contract” concept. These requirements come from the IFATS project. Another significant result of this stage was formulation of the requirements to the 4D Contract:
• All 4D contracts are generated on-ground (strategic planning) and could be updated on-board under certain circumstances (tactical planning);
• The 4D contract is for the whole flight, including ground operation;
• The 4D contract is conflict-free;
• The aircraft is responsible for accuracy of 4D contract execution;
• The aircraft could ask for a new 4D contract in case of impossibility to follow already accepted 4D contract;
• The 4D contract update could be calculated on-ground (regular case) or on-board of the aircraft taking into account the state of airspace and 4D-contracts of neighbouring aircrafts (off-nominal situation);
• If 4D contract update affects neighbouring aircraft, then all affected 4D contracts are updated by the ATSM (Air Transport System Management – entity in charge of 4D contracts management), potentially using CDM (collaborative decision making mechanism) – depending on the situation;
• Ground segment monitors but does not control aircraft. Ground segment could change already accepted 4D contract only in case of off-nominal situations;
• In case of “aircraft flying 4D-contract”–“non 4D-contract aircraft” conflict, the aircraft flying 4D-contract has priority except critical situations.

A more accurate definition of the 4D contract, in terms of trajectory and margins was also defined. In the 4DCo-GC project bubbles are defined around all aircraft, which move along with them. The 4D-Contact comprises a so-called 4D Bone-Trajectory together with a Contract Bubble. This contract bubble comprises at least the safety bubble, which defines the minimum separation, as well as the freedom bubble, i.e. the manoeuvre freedom an aircraft has without violating the 4Dcontract. This freedom can be used by the FMS to optimise its flight-plan, while the actual flown trajectory may divert from that. This yields 3 kinds of trajectories.

The 4D contracts are generated and updated according to a given process: this is operationally a continuous process, with no clear start or end. For the purpose of simulations, the consortium set up this process for a single day of traffic over Europe. This process is composed of 2 main stages: the 4D contracts generation and the 4D contracts execution.
The 4D contracts generation is responsible for generation of initial (default) contracts, which are considered to be a basis for all following flight management. In the 4DCo-GC concept, it is possible to do the precise planning in advance provided that qualitative weather information is available for ATSM. This latter condition defines when the strategic stage of planning (in 4DCo-GC sense) starts before actual flight execution.
The 4D contracts generation process discussed above is done in three steps:
- Individual optimization of every single flight (vertical flight profile optimization);
- Global deconfliction (producing conflict-free 4D trajectories);
- Margins addition to the 4D trajectories (producing the 4D contracts);

These modules make their job in a row. Eventually they produce together a set of contracts, which are considered to be a basis for air traffic.
The 4D contracts execution phase starts working when an aircraft is already in flight. And if the flight goes as expected, it proceeds according to the initial generated contract. Otherwise, if there are some changes in the flight conditions (e.g. unpredicted en route weather change) and a risk of contract non-compliance is detected, the tactical re-planner is triggered and a new contract is produced to adhere to current flight situation.
Besides stated above, there is one more module, i.e airport sequence management, which solves an airport problem but is very closely connected to the centralized flight planning. Apart from being optimized and deconflicted in the air, each flight should fit well its airports of departure and arrival. Otherwise a base contract of such flight will most probably be recalculated tactically which makes strategic planning meaningless. The only way for strategic planning to be useful is to produce contracts (trajectories), which will not be changed at the stage of tactical planning.
An Arrival/Departure Optimization Tool (ADOT) module was developed. An optimization of the arrival and departure sequences at Schiphol airport was produced, with an increasing density of traffic.

The following sections detail the algorithms developed in the frame of the project for 4D contracts generation (strategic planning) and execution (contract compliance monitoring and tactical re-planning).

4D CONTRACTS GENERATION
Individual 4D trajectory optimization
The trajectory for each flight of the traffic sample (see WP400 section for description) is optimized between the departure and arrival airports. This optimization is performed in the vertical plane in order to minimize the fuel consumption.
Strategic and tactical planning cover all general modes of 4D contract usage. But sometimes re-planning in emergency situation is needed. First of all, algorithms were developed, which allow aircraft rapidly recognizing the conflict with weather hazards zones or with another aircraft. This was done based on a space mesh which was developed in T120. Each node of this mesh contains the shortest distance to the obstacle. Due to the fact that there is now difference between “no-go” areas of aircraft and static/moving weather hazard zones, algorithm enables calculation of alternative trajectory for hybrid (aircraft and weather hazards) situations. The picture below depicts typical situation.
To define objects like weather hazard zones, administrative obstacles and physical fields, a mesh has been introduced. The data bought from Meteo-France was used to build weather hazards zones, as turbulence and icing zones, in the defined mesh.
Such dangerous zones are used in following calculations. This allows working with real weather data got from Meteo-France.

Strategic planning
Two different approaches for strategic planning used in 4DCo-GC project are described in this section. Both algorithms are doing global conflict resolution but using different techniques. First one (ENAC) resolves conflicts by means of solving proper optimization problem. Second one (DLR) is based on fast conflict detection algorithm in combination with trial and error method.
The role of strategic planning is to prepare a base line for the whole air traffic. It computes deconflicted and optimized trajectories (4D contracts) before corresponding flights take place. Then all flights proceed according to strategically calculated 4D contracts unless some en-route changes occur (for example unpredicted wind or weather). In the latter case tactical planning is being triggered.

Global conflict resolver – ENAC algorithm
To prevent conflict, aircraft trajectory must be separated in 3D space and in time. To separate trajectories in temporal dimension, one can adjust the departure time of each aircraft. This approach allows the aircraft to follow its initial intended trajectory, therefore does not induces more fuel consumption. However, with increasing traffic volume, large amount of time shift have to be distributed to each flight in order to separate all trajectories. To limit the amount of time shift necessary to de-conflict all trajectories, the 4D trajectories can also be de-conflicted in spatial dimension. A technique used to allocate optimal route and departure slot for vehicles in order to optimize traveling cost is called a route-slot allocation technique. In this work, we focus on using this technique to allocate optimal (conflict free) alternative route and departure time for each aircraft. Since, the resolution of this problem is implemented in a strategic level; therefore the potential conflict reduction problem can be solved simultaneously on both spatial and temporal dimensions, without having the flight that has already taken-off being a constraint of the following flight.
The second way to sole the conflict is to modify the initial trajectories shapes. To respect the given optimal altitude profile, in this de-confliction process we focus on modifying the shape of 4D trajectories in horizontal plane using a set of virtual waypoints. An alternative trajectory is constructed by placing virtual waypoints along the nominal trajectory. Then reconnect each waypoint with a straight-line segment. These virtual waypoints modify the trajectory shape only in en-route segments while respecting the nominal trajectory shape in the terminal control area (TMA).
Note that an alternative trajectory always has greater route length than its nominal one depending on the number and location of the virtual waypoints. In order to compensate this increased route length, one can extend the vertical profile at the top of descend while respecting the optimal climb and descent profile. Denote the increased route length by . The altitude profile of an alternative trajectory can be modified as the dashed line.
To detect conflict between aircraft trajectories, a pairwise comparison of aircraft positions is normally used. This approach is time consuming for a large-scale problem. Therefore, it is not suitable for a problem involves in large number of trajectory samples. In order to detect potential conflict between aircraft trajectories in a large scale, a grid-based conflict detection scheme is used.
Each 4D coordinate (x,y,z,t) of each trajectory sample is associated to a corresponding cell in the 4D grid. Then the surrounding cells of each 4D coordinate are examined. If the cell is occupied by another aircraft, horizontal distance and vertical distance between these coordinates are measured. A potential conflict is identified if the distance between two trajectory samples is less than the dimension of the contract bubbles.
The hybrid metaheuristics algorithm controls the search for a conflict free trajectory shape and departure time option. Once a candidate solution is selected, the trajectory computation module generates a new trajectory. Then the conflict detection module evaluates the number of conflicts associated to this solution. The candidate solution is then accepted or rejected according to the acceptation criteria. This iterative process repeats until a conflict free solution is found. The output data consists of a conflict free trajectory set, which will be an initial bone trajectory for the following trajectory-planning phase.

Global conflict resolver – DLR algorithm
The scenarios used during C1 and C2 simulations need to be free of conflict. Therefore, DLR performed a strategic conflict detection and resolution on the bone trajectories. Since the aircraft follow more direct routes than in the real day scenario, and flight profiles are all optimized, conflict detection yields about 29,000 conflicts for the 33,069 aircraft flying in the ECAC-wide scenario.
Solving these conflicts is very complex and computational expansive. The main means allowing conflict resolution in a reasonable time is an extremely fast conflict detection algorithm. Using a dynamic hexadecimal tree structure, the software module identifies all conflicts from one trajectory with all others in less than 6 ms. Thus, all 29,000 conflicts can be identified in less than 3.5 minutes. The conflict resolution algorithm applies the trial-and-error technique and directly benefits from fast conflict detection, allowing more than 160 probe trajectories per second.
The major objective during conflict resolution was to avoid degradation of efficiency. Therefore, it is beneficial to keep flight durations and routing constant and prefer shifting a whole flight in time. If a flight is shifted by few minutes only,
• Speed and altitude profiles can be assumed to stay unchanged, the only factor that may change slightly is wind;
• Trajectories are as efficient as the initial optimized trajectories;
• New trajectories can be calculated by decreasing/increasing times, no new profiles need to be calculated.
Applying a direct and a recursive approach, flights were shifted by a few minutes in order to reduce the overall number of conflicts. Allowing a maximum time shift of ±10 minutes, about 92% of all conflicts can be resolved for the ECAC-wide scenario. For the Benelux scenarios, this rate was even much higher because all airports outside the Benelux area did not respect the non-Benelux traffic. Thus, the nominal scenario was resolved with time-shift completely, the scenarios with increased traffic density were de-conflicted with very few (<10) lateral or vertical maneuvers.
The ECAC scenario was finally de-conflicted with lateral and vertical maneuvers to 97.1% resolved conflicts. Most of the remaining conflicts seem to exist due to insufficient conflict metrics not taking into account independent parallel runway operations. The remaining conflicts were solved by flight cancelation from the ECAC scenario. Finally, the ECAC scenario contained 32,573 de-conflicted and optimized flights.

Turning 4D trajectories into 4D contracts
The set of 4D trajectories produced by the global conflict resolvers are turned into 4D contracts by adding margins (the “bubbles”) to them. This phase is performed considering the local traffic density in order to determine the size of the bubbles at a given 4D point: large bubbles when density is low (to give more freedom to the aircraft), narrow bubbles when density is high (i.e. close to TMA or when aircraft are crossing).

4D CONTRACTS EXECUTION
Contract compliance monitoring
The problem of contract compliance monitoring with respect to pre-assigned 4-dimensional (4D) trajectories equipped with corresponding 4D margins (4D contracts) is contemplated. Two specific problems are considered and corresponding methods are developed within a unified adaptive time series probabilistic framework:
1. Present contract compliance monitoring
2. Future (predictive) contract compliance monitoring
In the first case the current trajectory is monitored in order to calculate the along-track, cross-track, and altitude aircraft deviations with respect to an assigned contract, whereas in the second one the contract compliance is predicted ahead of time (using a proper prediction horizon).
The contract compliance monitoring algorithms are used for detecting risks of 4D contracts non-compliance and then for triggering the tactical re-planner.

Tactical re-planning
In the 4D airspace concept, all the flights remain conflict free as long as all the aircraft comply with their respective 4D contracts. Occasionally, mainly due to unpredicted weather changes, equipment malfunctions and emergency situations, an aircraft may not be able to comply with its 4D contract, i.e. fly within its contract margins. In this case the aircraft will request a new flyable and properly separated 4D contract. The goal of the tactical re-planning algorithm is to generate a new well separated and flyable 4D contract for the request aircraft.
New contract computation is performed when the aircraft is airborne and therefore should be completed in a matter of minutes. Consequently the proposed algorithm consists of two main stages, aimed at attaining numerical efficiency: clustering and re-planning. The clustering stage is used to determine the neighbours of each aircraft, i.e. other flights with which separation may be lost when re-planning the contract of this particular aircraft. The main goal of re-planning is to ensure sufficient separation with the surrounding traffic. Performing the clustering step allows addressing only the relevant flights and thus increases the efficiency of the algorithm.
The main tactical re-planning algorithm will compute a new properly separated bone trajectory for the requesting aircraft. This trajectory, when complimented with the appropriate safety and contract margins supplied by an external function, will constitute the new 4D contract for the aircraft. In some cases, when changing the contract of one aircraft only leads to an unsatisfactory global solution or when no solution is found within reasonable computation time, the tactical re-planer may issue more than one new 4D contract to several aircraft in the aerospace.
During flight, each aircraft is monitoring its ability to follow its prescribed 4D contract. When a future violation of that contract is predicted, a new contract is requested. This new contract is computed efficiently while considering only aircraft that are in the cluster of the request aircraft. The re-planning results in new contracts for this and possibly additional aircraft in the cluster. These new contracts are then negotiated with the affected aircraft, which test that the proposed new contracts are flyable. Once the new contracts are accepted, the clustering data is updated efficiently by changing the clusters of the relevant traffic only.
Initially a solution is sought for updating the contract of the request aircraft only. In this case, only aircraft that are in the cluster of the request aircraft are checked for separation violations. If no solution can be found in a reasonable computation time, or if the solution requires drastic and un-realistic deviation of that single aircraft, a multi-aircraft solution is addressed. In this case, all the traffic in the clusters of the deviated aircraft is considered to ensure separation.
The trajectory re-planning process for updating a single aircraft contract is performed as follows. Initially, the 4D FMS trajectory provided by the request aircraft is checked for separation violation. If no violations are detected, as may be the case when the traffic is sparse and the aircraft deviations from the original contract are relatively small, the FMS trajectory is approved as the bone trajectory of the updated contract. However, if separation violations are detected, a new bone trajectory has to be computed by modifying the flyable FMS trajectory.
The trajectory shaping and update task is cast as an optimization problem. Instead of computing an entirely new trajectory from the current location of the aircraft to its destination airport, only part of the trajectory is modified. The trajectory deviations are set around the points where separation constraints are violated. These points will be referred to as conflict points in the sequel. To reduce trajectory deviations and thus possible interactions with surrounding traffic, the trajectory will be modified in the regions defined by 20 minutes flight time around the conflict points. If these regions exceed the current aircraft location or the destination airport, they are reduced accordingly. Consequently, the 4D coordinates of the contract points along the trajectory in those regions together with the conflict points, defined as additional points of the updated trajectory, will be modified in the optimization process to properly shape the updated trajectory to ensure sufficient separation.
The trajectory re-planning for multiple aircraft is performed in a similar way as for one aircraft. The aircraft that are re-planned are determined by the severity of the separation violations between them and the requesting aircraft. Hence, first a re-planning is performed for the request aircraft and an additional one that is closest to the request aircraft. The optimization parameters are determined for the two aircraft as before, while the separation constraints are considered for traffic in the two aircraft clusters.
The trajectories generated during the re-planning process serve as bone trajectories for the new contracts of the relevant aircraft.

AIRPORT OPERATIONS
Based on airspace digital model developed before C1, AGMP (Airport Ground Movement Planner) module was implemented which is a complete airport ground movement planner. It calculates reasonable ground traffic which has the following properties:
• Ground traffic fits to actual airspace 4D contracts;
• Ground 3D trajectories have no conflicts with each other;
• Total taxiing time is minimized/optimized;
• Ground trajectories are consistent with airport configuration and general movement rules.
To develop this model, several supplementary modules and features were implemented:
• Airspace digital model (airport database);
• Gate management;
• Pushbacks;
• Runway run profile;
• Aircraft ground performance characteristics depending on AC type;
• Simplified AC ground movement model;
• Specialized ground traffic visualization tool.
Two alternative visualization tools were also developed for ground movement.
Based on digital airspace and airport models, the algorithms for strategic and tactical planning were developed. Strategic planning includes following stages (a) calculation of optimal profiles tacking into account BADA performance characteristics, wind field, no-go areas; (b) conflict detection and resolution; (c) contract generation. In this chain (a)-(b)-(c), different advanced techniques were used:
• Optimal control based on Pontryagin's maximum (or minimum) principle, which gives better fuel consumption in comparison with BADA one’s. Average gain was 2.3% with twice the current amount of traffic.
• Usage of adaptive size of freedom bubble, taking into account surrounding traffic. This technique enables increasing the airspace capacity. At the same time only 1% of contracts were replanned.
This type of planning is performed before the flight.
Tactical planning is used for the situation when aircraft crew has enough time to coordinate and change 4D-contract. To perform this, all flight plans are processed before the flight to find all surrounding aircraft forming database (DB). Each aircraft keeps a small part of this DB, which contains only its own and neighbour’s 4D contracts. Tactical planning could be used with or without connection to ground service. If aircraft has a connection to ground service then onboard DB has actual data and all aircraft will react on the dangerous approach occurrence. In other situation (without connection to ATSM) only aircraft which is responsible for conflict will change its 4D-contract.

d) Global conclusions for the WP
Main task of WP100 was to check and then demonstrate feasibility of 4D contract concept by means of implementation of contract management service. In general this goal of WP100 was successfully achieved. Both airspace and ground segments of C2(second – and final – simulation campaign of the project) flights were covered. All modules comprising strategic and tactical planning were implemented and tested during C2.


1.3.2 WP200 –Principles of on-board aircraft Guidance and Control -NLR
a) Overall objectives of the WP for the whole project
The objective of WP200 is to identify innovative technologies in information and control scopes to provide aircraft flight in accordance with planned 4D contracts. This WP is meant for the required on-board functions. The concept of the 4D contract relies on the capability of aircraft to predict and monitor the compliance with the 4D contract. In this context, guidance and control rely also on the responsibility of each aircraft to define its own strategy, which requires the definition of procedures for cooperation between 4D trajectories. It also requires innovative technologies for aircraft avionics enabling to follow those procedures. The aim of this WP is to analyse all the aircraft guidance and control issues in order to comply with 4D contracts, and to develop algorithms that will be integrated in the simulator in order to assess their capabilities.
In the first period the objective was to define and develop the first (software) tools to be used in the first Simulation Campaign (C1) for system evaluation. The second period was used to improve functionality for the second Simulation Campaign (C2).

b) Description of the methodology
Four Tasks have been identified within WP200:
Task N° Task Name Partners involved
210 Procedures for cooperative 4D trajectories, including Guidance & Control Techniques TsAGI, CIRA, DLR, IAI, NLR, TECHNION
220 Advanced ADS-B and ASAS NLR
230 Greening of 4-D contracts ALA, ONERA, DLR, IAI, TCF, TsAGI, UPATRAS
240 Human Aspects ONERA, IAI, TsAGI, UPATRAS

Several functions have been identified per task. Initially the plan was to integrate all on-board modules, but this idea has been abandoned. Instead several functions were being integrated into separate aircraft to prevent complications. Moreover, this gained flexibility in the simulation campaigns as the different functions are needed in different scenarios and thus independent of each other.

c) Description of the WP results
To support the 4D concept, on-board functions have been identified and developed as software module except the human aspects, which is -in accordance with the DOW- a specification.
All the modules developed in this WP can be reused by the partners either as a whole or in part, for future research projects. Though 4D contracts gave special constraints to those modules, such as the size of the freedom bubble, the modules are not restricted to it. Other kind of projects may benefit from it as well.
Below those modules are described per task and the responsible partner is mentioned with those modules.

i- Results from Task 210
 4D Contract Automatic Execution 4DCAE (CIRA)
The 4DCAE module comprises an individual FMS including Guidance & Control functions.
This mmodule is the spill of the airborne part. The FMS receives the 4D-contract, as well as the trajectory adaptations from “ASAS” (T220) and “Trajectory Greening” (T230) functions.
The Guidance and Control takes those inputs into account to create a “4D FMS plan trajectory” remaining within the 4D-contract (i.e. the bubbles). Furthermore the Guidance and Control gives commands for the “Individual AP and AT” module.
FMS planning and guidance software has been developed to fulfil the special needs of the 4D contract concept.

 Individual Aircraft Motion simulation with AP and AT (TsAGI)
The Auto-pilot (AP) and Auto-thrust (AT) outputs are connected to the Flight Control System (FCS) and Engine controller of the Individual Aircraft Motion simulation module. Inputs of this function stem from “Guidance and Control” function. In this way the individual simulated aircraft flies accordingly. The flight simulator being controlled by the Individual AP and AT gives the actual aircraft state during the flight. This is used by several airborne functions.
Developments with Matlab/Simulink have delivered a simulation package based on BADA aircraft models. Real weather data is also taken into account.

 Renegotiation on 4D contract with ATSM (TECHNION)
As soon as the actual flown trajectory cannot be kept within the contract, instantaneously or in the (near) future, renegotiation for a new 4D contract is initiated with ATSM.
The process of renegotiation and regeneration of 4D contract is made of 3 steps:
1. Long term Trajectory Regeneration - on-line reshaping of the reference trajectory, starting from the nominal one provided by ATSM. This trajectory regeneration is said “long-term”, as it starts from the vehicle position and ends at the final 4D waypoint (hereafter named target waypoint), in compliance with all the assigned constraints;
2. Short term Trajectory Regeneration - on-line adjustment of the vehicle CAS (Calibrated Air Speed) reference, in compliance with the vehicle CAS envelope. This adjustment is carried out only if the aircraft is predicted to be not compliant with requested time of arrival in the “short-range” time horizon (i.e. from the current point to the next 4D waypoint);
3. Trajectory Tracking - computation of the reference signals for the autopilot, while taking into account the vehicle dynamic limitations.

 Self-generation of a 4D contract (TECHNION)
The self-generation function is used to set-up a 4D contract using the space not occupied by other traffic. It can be used to provide the ATSM with a “first guess” flyable trajectory, enabling to speed up the negotiation phase.

ii- Results from Task 220
 4D Local Contract 4DLC (NLR)
It has been decided to develop a 4DLC (Four Dimensional Local Contract) function as an additional safety net that comes into effect when an emergency manoeuvre exiting the current Contract Bubble is mandatory, without time for 4D contract renegotiation. Other than ACAS (or TCAS), which comes into effect about 60 seconds before loss of separation, 4DLC has a look ahead time of about 10 minutes. Due to that, evasive manoeuvres are much smoother than those from ACAS. Of course, as long as all aircraft remain in their 4D-contracts conflicts are not eminent even in case of a (temporary) data-link loss.
For the own-ship, 4DLC uses the its assigned 4D contract. From other aircraft the actual flown state as well as the FMS plan is available through local network information. Conflict Detection & Resolution (CD&R) algorithms are based on that. The resolution provided by the 4DLC function is sent to the Guidance and Control function as an adjustment of the “FMS-plan trajectory”.

iii- Results from Task 230
 Offline Optimizer (ALA)
The “Offline” optimizer performs an analysis of a full 4D contract by simulating the whole flight with a high fidelity performance model and evaluates if the trajectory will be flown with a good efficiency. If this is not verified, then an alternate optimized leg of the contract is suggested.

 Modules for online guidance and control optimisation (ALA, IAI)
Two different modules for online guidance and control have been developed: one performs an optimization of the trajectory without breaking the current contract; the rationale is to exploit a possible favourable wind scenario maintaining the original cruise speed using less fuel because of tail wind. The wind scenario is continuously monitored and checked on the FMS: if a favourable scenario is detected, the possible new trajectory is simulated and the possible fuel saving is estimated with a performance model. If the gain is valuable, then the alternate trajectory (within the freedom bubble) is performed. Another approach is basically a “porting” of the offline optimization in the flight phase: during contract execution, the efficiency of the angle of attack used is evaluated and, if necessary, the FMS could suggest an alternate and more efficient contract.

 Advanced model-predictive auto-throttle (UPATRAS)
An advanced model-predictive auto-throttle: rather than performing a static calculation of what should be the airspeed for fulfilling the contract, the MPC auto-throttle calculates the command after an optimization on the whole leg of the contract. This is obtained because the knowledge of aircraft dynamics enables a prediction of future trajectory. The benefit of using this technique is not only a small, but still a significant fuel saving, but also a big reduction of throttle activity, preventing the engine to work too much in a transitory phase. Reducing engine transitory states is translated into an abatement of noise emission, which is another important aspect of the greening activity, especially near airport area.

iv- Results from Task 240
 Contract compliance module (UPATRAS)
Work was also performed regarding the definition and specification of the 4D contract monitoring function. It will be an automated function based on a contract compliance module. Depending on whether or not there is a deviation from the contract, the compliance module will inform the aircrew, as well as the ground station or controllers, about the conformance outcome (UPATRAS).

 Human Aspects specification (ONERA)
First, a discussion was held within the project regarding the human roles in the 4D contract concept of operation, from which it resulted that human actors and, in particular, flight crews and air traffic controllers, will have to be involved in the 4D contract management, at least in transition phases and mixed equipage operations.
From this statement, the work focused on identification of human factors issues and impacts of the 4D contract concept, mainly from the perspective of the flight crew. A review of recent or undergoing ATM research projects involving 4D trajectory management was performed, and resulted in the identification of existing concepts of operations and associated cockpit equipment related to 4D navigation. A 'human factors case' was then initiated using a structured process proposed by Eurocontrol in order to systematically identify and mitigate Human Factors issues as early as possible in the life-cycle of a project such as 4DCo-GC (ONERA). An exhaustive review of the events and failures which may affect the nominal course of a 4D contract was also provided based on commercial pilot experience (IAI).

d) Global conclusions for the WP
Main goal of WP200 is to contribute to the Simulation Campaigns (C1 and C2) with the identified modules as on-board functionalities to validate the concept of 4D contracts. With those modules, aircraft are capable to fly 4D contracts and, when needed due to unforeseen circumstances, renegotiation or self-generation of a new contract proved to be feasible. Even a short-time local contract can be determined in case of emergency situations. Greening of the 4D contract is possible with several on-board functions, whilst a contract compliance module informs the flight crew in time to negotiate an adjusted 4D contract. Human aspects have been identified and a specification for the human role (flight crew) with HMI has been delivered. Hence all initial goals of WP200 have been achieved
Deviations in planning of the activities within WP200 have been mitigated, though limited functionality was available for the C1 campaign. Updating those functions –with lessons learnt from C1- has been performed. The modules used for C2 were fully functional.


1.3.3 WP300 – Communications Functions - TCF
a) Overall objectives of the WP for the whole project
WP300 is dedicated to the analysis and definition of requirements, specifications, development and integration in the global simulation of communication functions to sustain 4D contracting during operation.
Communication subsystem is seen as a service for the 4D contract negotiation, and as is, has to provide to the other subsystems high performances information exchanges, through secured data link and Quality of Service (QoS) transmission.
The communication system within 4DCo-GC is a step forward to the system already defined in the IFATS project, while describing further and in details the concept of global and local networking. From the data link point of view, aircrafts are considered locally in vicinity (the local network) and in a global traffic (the global network).
The WP is thus declined into three consecutive tasks to serve this general objective.
The first step provides the requirements for the project, meaning the requirements expected for the communication service for the 4D contract negotiation. This allows determining a functional architecture.
The second step translates this functional architecture into the classical OSI layer description for the communication functions.
This is the starting point for the development, integration and simulation of the system as a third step.

b) Description of the methodology
The work on this WP has been led by Thales, with contribution from the partners: TsAGI for the requirements and the functional architecture, and IAI for the subsystems definition. Co-development has been performed for the simulation step, with ONERA and DLR, to define interfaces and simulation tool specifications and developments.
The methodology used for the communication system relies on former projects conclusion to go further. IFATS conclusions were the starting point to define the communication needs for the 4D contract transmission. A similar project focused on the ad hoc network definition (NEWSKY) provided also conclusions to go further and to validate the OSI-layer like architecture.
Finally, analysis of state-of-the-art for aeronautical networks provided comparison and metrics to validate and evaluate the proposed solution.

c) Description of the WP results
The first step was to define a functional architecture to take into account the requirements inherent to the aeronautical domain. The “functional picture” lies on the following expected functionalities:
• Traffic management: transmission/reception of data after signaling and routine indications, with frames composition, including QoS strategy;
• Congestion control: management of the scarce bandwidth; this functionality is related to the following QoS management;
• Quality of Service support: strategy for priorities among several data flows; different levels of strategy are considered, gradually introducing physical effects (radio link quality);
• End-to-End data transmission: data processing including interface with data link standard;
• Mobility: management of evolving situation, in terms of coverage zone for LOS data link;
• Security: introduction of security/safety markers in the frame.

These high level functionalities will be supported through the following 4DCo-GC functions. It has to be noted that a functionality may be performed by several functions:
• Traffic management: TX/RX of data frames with signaling messages.
• Optimal allocation of radio resources: Radio Resource Management; this function covers the congestion control and the end-to-end transmission functionality, with a specific focus on interfaces with the different standards involved in the system;
• Quality of Service optimal strategy: the simplest QoS method will consist in the use of a flag for the highest priority message, to enable the maximum available bandwidth to be dedicated to it. Optimized versions will introduce radio link characteristics, gradual increase for the need in bandwidth;
• Handover framework: protocol used for mobility processing, and related to the interfaces with the different standards considered;
• Routing: this function complements the handover management one, to process mobility and ensure seamless handover;
• Mobility optimization: the choice of data links standards depends on the effort to be provided for interfaces support. Discussions are on-going both in the mobile environment (standardization of the 802.21) and in the aeronautical context (SWIM interfaces) for an optimization of access in heterogeneous environments;
• Security and safety: markers and algorithm introduced to ensure safety of transmission and to avoid intrusion in communication system.

The second step consisted in the translation of these functional requirements into an architecture dedicated to data link development domain, as an OSI-layer like architecture. The main layers under development were then decided:
The Physical layer will ensure interfaces with the standards considered for ATM services in generally, and more specifically for 4D-contract negotiation. In this layer, the main issues are related to the interaction between the radio link (the RF medium) and the aircrafts:
• Physical influence of the link budget parameters: typically coverage, power and antennas characteristics for LOS and frequency range for satellite link. The physical channel environment has also to take into account interferences in the link budget (interferences provided by one aircraft on communications to the neighboring);
• Limited resources such as bandwidth, due to spectrum regulation or cost issues lead to the implementation of strategy and share framework for optimization of a multi-users system. Access scheme depends on the standard technology and provides an optimization of resource use for a dedicated technology.
The Data link Layer is called the Radio Resource Management as it deals with the interaction of multiple terminals (multiple aircrafts and one or several Ground stations), sharing the same resources. The RRM will share QoS algorithm with other layers for this optimization.
The Network layer ensures interfaces with applications or services needed for the ATM in general, and more specifically with the 4D-Contract negotiation. The specific need will concern the access to databases, from ground and in aircrafts themselves.

For the third and last step, for development, integration and validation of the solution, new metrics related to networking protocols were introduced:
• Instantaneous connectivity: this represents the fraction of aircrafts having at least one multi-hop path to the ground infrastructure. It corresponds to the ratio between the number of connected aircrafts and the total number of aircrafts. This metric is relevant for areas where the connectivity to the ground may be suspended for certain duration. For airport density area, it has shown that thanks to terrestrial link between airports, it is possible to distribute the load of connected flights, for example for situations such as emergency;
• Flight duration connectivity: this represents the percentage of the flight duration that an aircraft can reach the ground infrastructure. It corresponds to the ratio between the connected duration and the total flight duration. Side offline simulations provide values for this metric in;
• The average number of hops: as routing involves other aircrafts to reach the airport or a ground station, this has impact on the latency, as multi-hop transmission multiplies the delay.

Real-time simulations have been performed to measure these metrics and to show how the local network can be real-time updated, based on real traffic data, with introduction of QoS strategy rules.

The major improvement provided by the work performed within the project is related to the connectivity high increase. Thanks to the implemented mechanisms, aircrafts have connectivity to the ground reaching 100%, compared to the present situation where this connectivity can fall down to 50%. The drawback with this improvement is related to the routing itself, as hops are necessary, introducing latency.

d) Global conclusions for the WP
The purpose at the beginning of the project was to fully implement the communications functions in the global simulation architecture. This integration has been replaced by a side real-time simulation, as it is not possible to introduce in the DLR and ONERA simulation tool the new routing protocols, based on present traffic systems.
Moreover, for the scenarios definition, the KPAs to prove the efficiency of the system were not adapted to the communication functions: dedicated scenario were defined and proposed, focused on the networking metrics.
Both dedicated communications parts were successfully demonstrated during the C2 campaign.



1.3.4 WP400 – Integration and validation – DLR
a) Overall objectives of the WP for the whole project
WP 400 was dedicated to the integration of project partner’s modules and validation of the proposed solutions. Integration of modules was based on Datapool on DLR’s side and HLA on ONERA’s side. An interface between Datapool and HLA enabled all partners to connect to the simulation using this backbone.
Validation was done by performing two simulation campaigns in 2012 (C1) and 2013 (C2), respectively. Whereas C1 was used for simulation runs with a subset of all modules and first findings, C2 allowed a more complete simulation with all modules involved. Four different scenarios have been simulated and multiple runs have been recorded, allowing an analysis afterwards.

b) Description of the methodology
The work in the work package has been distributed among partners. Validation strategy was led by DLR, assisted by ONERA. Specification of networked simulations was led by ONERA, assisted by DLR, where the interfacing of both Datapool and HLA to create the backbone was done. Integration of simulation components, led by DLR, was performed by all work package members in advance, using ONERA’s online system, and, more extensively, on the integrational meetings in 2012 and 2013 right before C1 and C2. Scenarios development, led by NLR, assisted by ONERA, DLR, and TCF, was achieved by processing recordings from real ECAC traffic, gathered by Eurocontrol’s Demand Data Repository. Simulation exercises, led by ONERA and DLR, were performed by all project partners during C1 and C2. Analysis, led by DLR, was performed after C2.

c) Description of the WP results
In preparatory work, validation goals and criteria were set up. Comprehensive exercise plans for both simulation campaigns were created.
Specification of networked simulations was a main task in order to have a common interface. This was achieved by creating an interface for Datapool and HLA, which proved to be beneficial. This common interface is not project specific and may be used later on.
Integration of simulation components was mainly performed right before the simulation campaigns in integrational workshops. However, an internet server, hosted by ONERA, proved to be helpful for preliminary tests. In order to have the whole simulation with all participating modules running, the larger response times via internet, compared to local networks, did not allow comprehensive simulations. The integrated system, used for simulation scenarios S1-S4 in C2. In addition, there was a setup for simulating one individual aircraft demonstrating the modules 4DCAE and Trajectory greening.

Scenario development started with the gathering of traffic samples via Eurocontrol’s Demand Data Repository. The tool chain of modules for creation of scenarios is as follows: NLR created a program for extraction of main information of all flights, like used airports, runways, departure times and aircraft type; DLR used this input for creation of a waypoints list for each flight (resembling a flight plan). This process is a quick way to create a direct-to scenario based on data that can be acquired via Eurocontrol. For future work with the need for scenarios based on real data, this process may be of use. In a subsequent step, the created waypoints lists were used by both TsAGI and DLR to create nominal trajectories. The scenario was deconflicted to 6NM/1000ft above and in FL100, and 3.2NM/1000ft below. The ADOT (Arrival/Departure Optimization Tool) tool performed an optimization for Schiphol traffic, assigning runways and sequences. Afterwards, follow-up conflicts were deconflicted in a second step. 4D contracts were generated based on these conflict-free trajectories, using varying margins along the route in order to allow maximum freedom.
Four different scenario setups were prepared for C2, as summarized in Table 1.
Scenario Traffic Event
S1 100% Benelux Traffic ~5 kts wind deviation between forecast and actual
S2 233% Benelux Traffic ~5 kts wind deviation between forecast and actual
S3 100% Benelux Traffic Airport closure Luxembourg, replan to Brussels airport
S4 100% Benelux Traffic Decompression, immediate decent, generating a onflict with another aircraft

Simulation exercises have been performed in two different simulation campaigns. The first campaign (C1) took place in Toulouse in 2012. All project partners were involved in the execution of simulation and demonstration. Since the integration work took longer than expected, no real simulations were performed in Toulouse. For C2, the simulation and demonstration was split into two weeks. The simulation week was a big success, performing all foreseen simulations as planned and proving the feasibility of the concept, even under non-nominal conditions. The demonstration in the second week was based on recordings that have been created during the simulation week.
Analysis was performed right after C2. Statistics, including number of conflicts, mean distance along route, and travel time, were gathered.

d) Global conclusions for the WP
The initial goals of the work package were achieved. The feasibility of the concept has been proven based on very successful simulations. In addition, the 4DCo-GC scenarios had a 3% higher efficiency compared to the baseline scenario. The achieved route length is very close to the shortest connection between departure and arrival airport.
The system was able to react on unexpected wind conditions and events like airport closure and decompression in an adequate way, always ensuring a safe handling of traffic.


Potential Impact:
1.4.1 Socio-economic impact
a) Impacts on the priority “Pioneering the air transport of the future”
The 4D contract concept is a step-change in the air traffic management, by proposing solutions beyond the most advanced technological improvements of SESAR (step 3). Indeed, by construction SESAR is looking at (and developing/implementing) novel technologies and procedures enabling to enhance the current air transport system. These studies are based on a smooth evolution of the current situation, and so may not be radically different from the existing systems/procedures. On the other hand, the 4D contract concept, developed by the IFATS project (FP6) and refined and simulated by the 4DCo-GC project, is based on thinking process where the objective was to imagine a fully automated air transport system, getting rid of any kind of (technological but also social and economical) constraints linked to the current system. It was indeed a pure technical exercise. Such a process was, and still is, guaranteeing the pioneering of the air transport of the future by being free from constraints limiting innovation because too much related to existing systems (i.e. not considering the possibility of technology breakthrough).
As a summary, the study of the 4D contract concept and the analysis of its feasibility and impact (and relationship) on the guidance and control of the aircraft is fully in line with the wish from the European Commission to pioneer the air transport of the future. In addition, it may provide novel ideas that will potentially enable an improvement of some of the SESAR concepts to be implemented in the next decades.

b) The greening of Air Transport
The greening of air transport is an ever increasing question to be answered by the overall aviation community, as sustainability is a central topic in all societal discussions. Reducing chemical and noise emissions is not a simple challenge: such topic involves both technical and operational aspects. On the one hand, the technical side of this challenge can be treated by improving engines technologies or by finding/using new types of fuels, for example. On the other hand, new operation modes are necessary to limit/reduce the impact of air transport on the environment, at “iso-technologies”. The 4DCo-GC project, by studying and improving the 4D contract concept, is contributing to the operational side of the greening of air transport. Such contribution comes from the use of “optimized trajectories” for all flights. Currently, flights are planned individually, with a limited knowledge of the other traffic. It results in conflicting trajectories that have to be modified by the air traffic controllers, lengthening the flight paths, leading to burn additional fuel. In addition, airport congestion creates hours of holding patterns, keeping aircraft in the air until a landing slot is free; here again, fuel and time is uselessly lost.
The operational concept studied in 4DCo-GC is based on the generation of optimized conflict-free 4D contracts: each aircraft is provided with a 4D contract it has to respect. As a counterpart, the aircraft is guaranteed that this contract is conflict-free and optimized to the better possible extent with regard to the airline initial demand. Implications are that, as long as the aircraft respect its assigned 4D contract, the flown route is as direct as possible and the landing will not require any holding pattern.
The results of the simulations performed in the frame of the project show that a reduction of 3% of the total fuel burnt can be obtained when compared to the current situation. In addition, it must be mentioned that this figure does not even include the holding patterns nor the queues before take-off, engines on, due to the lack of corresponding data. The real overall fuel savings can reasonably be estimated as even higher.

c) Improving cost efficiency safety
Based on the conclusion of the analysis of its impact on the greening of air transport, it is obvious that the 4D contract concept studied during the 4DCo-GC project will have a significant impact on the airlines Direct Operating Costs (DOC) by reducing both the flight distance and the flight duration (considering aircraft performance comparable to the ones currently flying). The impacts will be both on the quantity of fuel burnt and on the frequency of maintenance. From an airline perspective, if you add the flexibility brought by the ‘bubbles concept’ and the 4D contract update process, the cost efficiency will be significantly improved when compared to the current situation. In addition, such a system is based on a very high level of automation, leading to a situation perfectly in accordance with the statements of the High Level Group on Aviation Research: “Automation has changed the roles of both the pilot and the air traffic controller. Their roles are now as strategic managers and hands-off supervisors, only intervening when necessary”. Thus the flight crew will probably be reduced, as well as the required number of people on the ground in charge of monitoring the system. On the other hand, the necessary systems development, certification and deployment will cost a lot of money; the corresponding cost has not been estimated, but, as the overall air transport system is re-thought, many years will be needed to amortize the required investments.
Regarding the safety aspect, this was one of the main drivers for the definition of the fully automated air transport system defined in the IFATS project (FP6), from which the 4D contract concept studied in the frame of 4DCo-GC comes from. Indeed, the overall concept is based on the generation of conflict-free 4D contracts. In addition, each time a 4D contract update is necessary, the updated 4D contract is computed so that this characteristic is preserved. In addition, specific safety nets (using local situation awareness) have been developed in order to avoid conflicting situations, even in case of emergency. It means that, at any time, the overall traffic is guaranteed to be conflict-free. This statement is supported by the 4DCo-GC simulation results: in order to test the system robustness, dangerous/conflicting situations were to be created. This was a very complicated task and the system had to be specifically tweaked in order to provoke “interesting” situations, enabling the triggering of the various safety nets. Even though, the system was perfectly able to handle these very specific situations and to bring the overall traffic to a safe stable state.

d) Impacts on the “ATM Target Concept” of SESAR
The 4D contract notion is a trajectory-based operation implementing the « ATM Target Concept » that is one of the main results of SESAR. A trajectory representing the business/mission intentions of the Airspace Users and integrating ATM and airport constraints is elaborated and agreed for each flight, resulting in the trajectory that a user agrees to fly and the Air Navigation Service Provider (ANSP) and airports agree to facilitate.
The trajectory-based operations ensure that the Airspace User flies its trajectory close to its intent in the most efficient way to allow minimizing its environmental impact. The concept has been designed to minimize the changes to trajectories and to achieve the best outcome for all users. In that respect, user preferred routing will apply without the need to adhere to a fixed route structure in low/medium density area. The Airspace User owns the Business Trajectory (BT) and has primary responsibility over its operation. Where ATM constraints (including those arising from infrastructural and environmental restrictions/regulations) need to be applied, finding an alternative BT that achieves the best business/mission outcome within these constraints is left to the individual user and agreed through CDM (Collaborative Decision Making) process. The owners’ prerogatives do not affect ATC or Pilot tactical decision processes. The business/mission trajectories will be described as well as executed with the required precision in all 4 dimensions.
One of the purposes of the 4DCo-GC project was to draw a path for implementation of trajectory based operations. To this end, the definition of the 4D contract concept was refined. The various actors involved, as well as their actions/interactions, were modeled and simulated. The conclusions of the projects are that the implementation of the 4D contract concept is possible (from an operational perspective) and that it would enhance both safety, predictability, capacity and efficiency of the air transport system. To this extent, the adaptation of the SESAR concept to come closer to such a system is desirable. In addition, this brings also inputs to the definition/implementation of the SESAR concept of Dynamic Mobile Area, which can be seen as a first implementation of the ‘bubbles’ used by the 4D contract concept.

1.4.2 Wider societal implications of the project
a) Benefits to the European Community
The 4D contract concept developed and studied in the frame of the 4DCo-GC project is at the edge of the international research on the future of ATM and air transport system in general. Indeed, this concept goes beyond all the ATS research programs conducted, even in the US; all these initiatives are related to the NextGen programme, which is, like SESAR, strongly related to current technologies and operations, as it must become the “next generation” air transportation system. The 4DCo-GC project is looking at a step beyond. So, 4DCo-GC contributes to the leadership of the European Community in terms of far future research in the fields of air traffic management, aircraft operation and air transportation systems.
Besides from such leadership considerations, the benefits brought by the 4DCo-GC project in terms of air traffic sustainability are obvious as the individual optimization of each flight, associated with the suppression of take-off queues and landing holding patterns brings significant fuel savings, as consequently a large reduction of chemical emissions.

b) Increasing time efficiency
The main benefit brought to the customers by the air transport is time; indeed, no other transportation means is faster than the use of an aircraft, for sufficiently long distances. Airlines are selling time. When this statement is accepted, every improvement of the air transportation system will consequently bring an improvement of time efficiency. Even if this time efficiency increase was not part of the initial triggering topics for the study of the 4D contract concept, this is obviously one of its main outputs. Indeed, the 4DCo-GC project results showed that reduction of 3% of flight distance can be obtained by the use of 4D contracts as the basis for air transport operations. This can be broadly converted to a 3% time saving for each flight, as a mean value. In addition, as the 4D contract concept is a gate-to-gate mode of operations, the ground part of the trajectory (taxiing) is also optimized and lost time is reduced to a minimum (if not to zero). No more queuing before take-off will save several (tenths of) minutes for each single flight departing from Europe. The 4D contract concept also ensures that the arrival sequences are optimized and that no more holding patterns due to runway congestion will be used any more. Here again, direct approaches will enable the saving of tenths of minutes when landing at very congested airports.

c) Ensuring Customer satisfaction and safety
When speaking about air transport, customer satisfaction can be considered from several perspectives: punctuality, safety, comfort, services, cost, etc. On the one hand, most of these aspects are purely linked to the airlines policy (e.g. low cost often means less service) and so are not dependent on the type of Air Traffic Management (ATM), which is what proposes the 4DCo-GC project. On the other hand, other topics are related (even if not necessarily exclusively) to the performance of the ATM system. We talk here about safety, punctuality, and indirectly cost.
The safety of the air transport was the main initial issue underlying the development of the 4D contract concept, during the IFATS project (FP6). Indeed, the basic assumption is that as long as all the aircraft respect their assigned 4D contract, the overall traffic is guaranteed to be conflict-free. The implication is obvious: any incident or accident caused by air traffic control errors is not possible any more. In addition, the project defined and tested the robustness of the system versus degraded situation, by modeling and simulating safety cases, like reaction in case of depressurization. The simulation results show that the implemented procedures and safety nets are able to manage this situation, even in complex case, with high density traffic.
Punctuality, or at least predictability, of the flights is also a major outcome of the 4D contract concept. Indeed, all aircraft are flying 4D contracts. These contracts are computed gate-to-gate; it implies that the time of arrival at the gate is known, the uncertainty being very low. In addition, even in case of 4D contract update, the implication of the arrival time modification (e.g. flight connection) can be evaluated with accuracy as soon as the contract update is effective. Enough time is then provided, making the management of these situations easier.
To summarize, one can say that the 4D contract concept simulated and tested in the frame of the 4DCo-GC project can bring a significant enhancement to customer satisfaction and safety.

1.4.3 Dissemination and exploitation activities
4DCo-GC project has included a specific work package dealing with the dissemination and consultation.
The objective of this WP is to disseminate the outputs of the project in the aeronautics and scientific community and to consult with stakeholders to avoid any mismatch with the overall ATS evolution plans. A specific WP (WP500) has been fully dedicated to this matter as both dissemination and consultation are of high importance for 4DCo-GC. Consultation is made through the analysis of the feedback provided by external people (Advisory Board, Users Group) and coordination with related projects and initiatives.
The work as been divided into 3 tasks:
• Communication and dissemination: A website was created in order to provide the public with information regarding the achievements and results obtained by the consortium. Communication also included paper publication and presentations during conferences or symposia.
• Consultation through an Advisory board and a Users Group: A non-negligible part of this WP was dedicated to get feedback from an “Advisory Board”, composed of people external to the project but involved in the current air transport system. These people had a close look at the project achievements and provided the consortium with their feedback, based on their knowledge and experience on the current ATS. In addition to this advisory board, a Users Group was invited to the demonstration workshops (organized right after the 2 simulation campaigns). The detailed presentation of the Advisory Board and the Users Group is given hereafter.
• Coordination with related projects: Coordination with other projects and initiatives (like SESAR) was ensured by technical exchanges.

The first tool that has been set up to enable the dissemination was the project website (www.4dcogc-project.org). It is composed of both a public and a private parts, and so was extensively used by the consortium for internal documentation management as well as a showcase for the project progress and a useful tool for the organization of events. The website was regularly updated to ensure up-to-date information. The pretty “atypical” name of the project enabled a very efficient website referencing, making it very easy to find.
The main means for the dissemination of the project results were the two simulation campaigns. Indeed, they were planned since the beginning of the project as major opportunities to show its outputs to an external audience and to get feedback in order to enhance both the 4D contract concept definition and the simulation performance.
The demonstration part of the first simulation campaign (C1) took place on July 5th, 2012, in Onera facilities in Toulouse, France. This demonstration and dissemination session took place simultaneously with the project mid-term review; as so, the feedback was planned to be used in order to adjust the project objectives or processes for the second iteration that would end with the second simulation campaign (C2).
The C1 demonstration took place in front of 31 people, including a representative from the project Scientific Officer, 1 member of the Advisory Board and 8 members of the Users Group.
The demonstration workshop of C2 took place on June 25th, 2013, in DLR facilities in Braunschweig, Germany. This workshop gathered 30 people, including 2 members of the Advisory Board and 3 members of the Users Group.

The second means for the dissemination of the project results was the participation to conferences and the publication to technical papers. During the whole duration of the project, partners participated to a total of 16 conferences or dissemination events, presenting a total of 7 papers related to the project activities.
Among these conference participations, it must be emphasize that the “Trajectory Based Operation” session of the AIAA Guidance, Navigation and Control Conference that took place in Boston, USA, in August 2013 consisted of 3 (over 4) presentations related to the 4DCo-GC project:
• Impact of aircraft guidance and navigation on 4D contracts generation and execution - example of the 4DCo-GC project, presented by Onera;
• Tactical re-planning within the 4D contracts ATC concept, presented by Technion;
• Automatic guidance through 4D waypoints with time and spatial margins; presented by CIRA.
Thirty to forty people attended these presentations, making this conference a major dissemination means for the project.

The communication and dissemination activities were especially active in the second period of the project. This is obviously due to the fact that more mature results were available.
As a summary of the communication and dissemination activities, the highlight should be put on both the simulation campaigns of the project and their related demonstration days. Indeed, these were early planned to be major means for dissemination, and the objective was reached. In addition, the fact that almost a full session of an American conference was dedicated to the project proves that the topic is of interest and that the project results are very valuable.


As mentioned previously, consultation was considered of primary importance by the project consortium. So 2 consultation bodies were created, in order to get feedback and advices on the project achievements and orientations. Those 2 bodies are the Advisory Board (AB) and the Users Group (UG). The main difference is that the AB was created at the very beginning of the project and remained unchanged for its whole duration. It had a dedicated access to project documentation (even restricted) and participated to consortium internal discussions. Even if they were not partners in the consortium, they were “officially” included in the management structure of the project. On the other hand, the Users Group was constituted of external people, with no specific relation to the project; they had access to the public documentation via the website and participated to the demonstration days on a free-will basis. Members of the UG changed from an event to the other, as it depended on the location (as no travel fees were paid, mostly people close to the event came).
During the course of the project, the AB met the consortium 3 times: 1 technical meeting and the 2 main project events that were the C1 and C2 simulation campaigns; the UG participated to 3 events: C1, C2 and the project final meeting. The UG members were different each time.
The first meeting with the AB took place in September 2011, simultaneously with the project third Technical Review Meeting, after less than 1 year since the beginning of the project. The AB provided valuable inputs on the first project features and enabled interesting discussions by highlighting the main differences between the 4D contract concept and the current system.
The second main consultation event was the demonstration day of the first simulation campaign (C1) of the project. It took place in early July 2012, simultaneously with the project mid-term. Both the AB and UG attended this event. Their feedback was gathered mainly by reactions, comments and open discussions during the demonstration. In addition, as C1 was the last opportunity to take into account the received feedback in order to adapt the project objectives, a complementary questionnaire was sent to the AB and UG members to get a more detailed insight of their understanding and expectations. The feedback concerned the general 4D contract concept, but also the simulation means, and the expected outputs. This was used during the second phase of the project to improve the clarity of the concept by the use of more adapted demonstration means.
The third consultation event, and most likely the major one, was the demonstration day of the second simulation campaign (C2). It took place in June 2013. Both the AB and UG attended this event. As mentioned, this was the major dissemination and consultation event of the project because the final simulation results were demonstrated and presented. During this event, several slots dedicated to technical discussion about the presented results were set up. There were a lot of questions showing the interest from the UG (especially) to the 4D contract topic. This feedback was used by the consortium when drafting the final project reports and conclusions.
The consultation of the AB and UG of the project was very valuable for the consortium, as it enabled to have external views of the project orientations and achievements. Indeed, it is difficult to judge and be judged, so external comments can help rising up some critical issues that were not identified as such by the consortium. This is for example the case for the importance of flexibility, from the airline perspective. The received feedback was used to improve the 4D contract concept and the associated explanations (for more clarity). A total of around 20 different people were involved in the project, being member either of the AB or the UG.

The last objective of this WP was the coordination with related projects and activities. The main outcome of this task is the coordination, even the collaboration, with the PPlane project (FP7, www.pplane-project.org) dedicated to the definition and development of a Personal Air Transportation System (PATS). Indeed, in addition to the technical communalities between the 2 projects, the fact they were both coordinated by Onera greatly eased this association. Nevertheless, the collaboration was really needed as one of the conclusions of the PPlane project was that the development of PATS would rely on a highly automated system, and that the 4D contracts were the most adapted solution to safely manage the large amount of small automated aircraft considered.
The coordination with SESAR, started very early, as a coordination meeting was organized even before the 4DCo-GC kick-off meeting. During this meeting, it was emphasized that the possibility to have a member of the SESAR staff in the Advisory Board should be considered. In such case, a SESAR point of contact was needed. One of the action items of this meeting was: “a meeting with the SESAR technical PoC should be organized as soon as possible in order to pave the way of the coordination. SESAR should indicate to 4DCo-GC the name of this technical PoC.” Unfortunately, until the end of the 4DCo-GC project, no SESAR point of contact was appointed, and so the envisaged strong coordination was not possible. Later on, the 4DCo-GC coordinator was invited to participate to a “SESAR 4D trajectory” dedicated workshop, aiming at clarifying and uniting the concept of 4D trajectory in SESAR. This workshop was the last “official” meeting between 4DCo-GC and SESAR. Consequently, the consortium followed the SESAR progress informally to ensure consistency between both initiatives, but that was not formalized by meetings or discussions.
To summarize the coordination activities with related projects, one can say that on the one hand, coordination with other projects proved to be very efficient: the collaboration with the PPlane project lead to the use of the 4D contract concept in the definition of their system. But on the other hand, coordination with SESAR was difficult, due to the fact that no point of contact was identified. Nevertheless, the SESAR work was taken into account to keep alignment with its results.

To conclude, the progresses towards the results of the project in terms of dissemination and consultation were more than satisfactory. Indeed, two major events took place: the demonstration days of the first and second simulation campaigns (C1 and C2). The results were presented to the project Advisory Board and Users Group, and their feedback (from open discussions and questionnaires) was used to improve the concept definition and the clarification of the presentations.
In addition, a pretty high number of publications and presentations to international conferences were produced. The dissemination and consultation aspects were though of high importance, as a large audience was impacted, and valuable feedback received.
Coordination with related projects and initiatives was very fruitful (e.g. the use of 4D contracts by the PPlane project), but the coordination with SESAR was not really effective as no dedicated point of contact was appointed.So the alignment with SESAR (which was the purpose of this coordination) was ensured only by following the SESAR progress and getting feedback from various people involved in the SESAR programme.

List of Websites:
• The address of the project public website is: www.4dcogc-project.org
• The website contact is:
o Antoine Joulia
o antoine.joulia@onera.fr
o +33 5 62 25 26 37