A COMPARISON OF 4D-TRAJECTORY OPERATIONS ENVISIONED …

A COMPARISON OF 4D-TRAJECTORY OPERATIONSENVISIONED FOR NEXTGEN AND SESAR, SOME

PRELIMINARY FINDINGS

Gabriele Enea∗ , Marco Porretta∗∗∗Engility Corporation, ∗∗ At the time of writing with Ingegneria Dei Sistemi (IDS)

[email protected]; [email protected]

Keywords: 4D-Trajectory; ATC; NextGen; SESAR

Abstract

This paper presents a comparison between theconcepts underlying 4D Trajectory Based Op-erations (TBOs) from the double viewpoint ofNextGen and SESAR programs. In the proposedanalysis, motivations justifying the introductionof 4D TBOs are presented first. After that, thedifferent and similar technologies that are beingapplied to support 4D TBOs are discussed. Thisis followed by a discussion focused on the resultsobtained from different human-in-the-loop sim-ulation activities. In addition, preliminary flighttrials activities, planned and partly executed forconcepts′ refinement and final validation, are alsodiscussed. These validation activities, carried outon both NextGen and SESAR sides, aim to as-sess the impact of the aforementioned support-ing technologies on both pilots and Air TrafficControllers. This impact will actually be a keyelement for the effective implementation of 4DTBOs. Early benefits identified for SESAR andNextGen are described next. Finally, some com-parisons and preliminary conclusions are pre-sented.

1 INTRODUCTION

The global air transportation system is a cor-ner stone part of the world economy. A steadygrowth in air traffic is necessary to support theeconomic growth and produce economic wealthby itself. In order to accommodate this increased

traffic demand, the system will have to radicallychange from the current one. In the next fif-teen years the global air transportation systemwill transform more than it did in the past sixtyyears. It is also envisaged that this transforma-tion process will be led by the US and Europe,as the NextGen ([1, 3]) and SESAR ([2, 4]) pro-grams are actually the most significant initiativesthat are being developed to support the new eraof air transportation.

Specifically, both programs deal with thesame problem, as current Air Traffic Control(ATC) procedures and technologies will not beable to accommodate the increased traffic de-mand while maintaining the same levels of safety.The solution to this problem will require a rad-ical change in the whole Air Traffic Manage-ment (ATM) system. More in detail, the funda-mental shift in paradigm will be from clearance-based ATC to trajectory-based ATC operations.The key element of Trajectory Based Operations(TBOs) is actually an agreement between an air-line and the ATM system about the trajectorythat will be followed by an aircraft. The nego-tiated trajectory will satisfy many of the airlinepreferences, with particular attention to fuel con-sumption reduction. However, this trajectory willalso include additional constraints that will im-prove its predictability, thus facilitating the workof Air Traffic Controllers (ATCos). A typical ex-ample of such constraints is the fulfillment of as-signed Target Time of Arrivals (TTAs) at signifi-

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GABRIELE ENEA, & MARCO PORRETTA

cant points of the route. In this case, the term ”4DTBOs” is normally used to identify this type ofoperations. Once the aircraft is cleared to fly thenegotiated trajectory, the ATC function will beonly of trajectory management [5]. Specifically,ATCos will assist as much as possible the air-craft in the execution of the negotiated trajectory,while maintaining at the same time the separationwith other traffic. These approaches are actuallyexpected to lead to different Concept of Opera-tions (ConOps) and different operational perfor-mance for the two programs.

Differences and similarities betweenNextGen and SESAR are currently beinginvestigated to identify the most suitable ap-proach for a number of different problems orsituations. For example, operational perfor-mance for the methods proposed by NextGenand SESAR are continuously evaluated andcompared by the Performance Review Com-mission (PRC), a joint effort between the FAAand EUROCONTROL ([6, 7]). Furthermore,NextGen and SESAR′s ConOps have beenalso compared in a recent study [8]. Even theU.S. Government Accountability Office (GAO)has recommended to improve the informationdissemination exchange between NextGen andSESAR to improve the interoperability of the twoprojects [9]. In that context, this paper presentsa slightly different point of view. Focusing, infact, on the final objective, both for NextGenand SESAR, of achieving 4D TBOs, it compareshow its constituting piece, the 4D Trajectory(4DT) [10], is being defined, handled and finallyimplemented.

The paper is structured as follows: firstthe terminology adopted to define a 4D Tra-jectory (4DT) in NextGen and SESAR is dis-cussed. Then, the objectives of 4D TBOs are an-alyzed. The technology enablers are describednext. This includes Flight Management System(FMS) advanced capabilities, Data Communica-tions, Automatic Dependent Surveillance Broad-cast (ADS-B) and ATC Decision Support Tools(DSTs). After that, a discussion focused onthe results obtained from different human-in-the-loop simulation (HITL) activities is presented.

The objective of these activities is the assessmentof the impact of the aforementioned technologyenablers for 4D TBOs on practices and workingmethods of both pilots and ATCos. As such animpact will be a key element for the effective im-plementation of 4D TBOs. The metrics used toassess possible benefits are also introduced. Fi-nally, the advance of the implementation plans ispresented, together with some preliminary con-clusions.

Fig. 1 4D Trajectory Representation in Nextgen [1]

2 TERMINOLOGY

2.1 NextGen

In NextGen a 4DT is defined as a precise de-scription of an aircraft path in space and time.This description includes the ”centerline” of thepath, using Waypoints (WPs) to represent spe-cific steps along the path, together with appro-priate buffers to describe the associated positionuncertainty (See Figure 1). The path is earth-referenced (i.e. latitude and longitude specifi-cations are given for each WPs). Furthermore,the path contains altitude descriptions for eachWPs and suitable indications about the time(s) atwhich the trajectory will be executed. The re-quired level of specificity of the 4DT dependson the flight-operating environment. Some of theWPs in a 4DT path may be associated with Con-trolled Time of Arrivals (CTAs). Each CTA is

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A Comparison of 4D-Trajectory Operations

defined by a TTA requirement that must be metby the aircraft within a specified time tolerance.Therefore, CTAs actually represent time ”win-dows” for the aircraft to cross specific waypointsand are used when needed to regulate traffic flowsentering congested en route or arrival/departureairspace [1].

2.2 SESAR

In the context of SESAR, airspace users willagree a preferred trajectory with Air NavigationService Providers (ANSPs) and airport opera-tors (Figure 2). The negotiated trajectory is ex-pressed in four dimensions (three spatial dimen-sions, plus time) and takes into account all pos-sible constraints due to limited airspace and/orairport capacity. For that reason, it may be sub-ject to changes from early planning to the day ofoperations. The negotiated 4DT is called ”Refer-ence Business Trajectory” (RBT), where the term”Reference” indicates that, once the trajectory isagreed, it will become the reference trajectorywhich the airspace user agrees to fly and all theservice providers agree to facilitate with their re-spective services [2, 11].

Fig. 2 4D Trajectory Representation in SESAR [11]

2.3 Comparison

Although the basic concept is fundamentally thesame, in SESAR the term ”Business Trajectory”,

not present in NextGen, is introduced. The em-phasis that the European consortium wants to putis on the agreement between all the ATM stake-holders (ATC, Airports, Airlines, Cockpit, Mil-itary, etc.) on the 4DT to be executed gate-to-gate by the aircraft. This detailed trajectory infor-mation will be shared between all the stakehold-ers through a System Wide Information Manage-ment (SWIM) [2]. This is a network-enabledaccess where all the information relevant to 4DTBOs are shared amongst authorized users. Thesame strategy for information management is en-visioned by NextGen. Therefore, both NextGenand SESAR will have to deal with identical prob-lems in terms of communications infrastructurerequirements, protocols definition, and securityaspects.

Figure 4. FAA Data Communication ConOps Error!

Reference source not found.

Fig. 3 FAA Data Communication ConOps Rep-resentation [12]

3 DIFFERENT AND SIMILAR OBJEC-TIVES

In the FAA and EUROCONTROL′s visionsSESAR and NextGen will be both fully opera-tional by 2025. It will not be easy to meet thisschedule considering the paradigm shift that theATM environment will go through to achieve thisgoal and the slow adjustments ATM usually un-dergoes. Moreover the different nature of the

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airspace, very fragmented in Europe, and homo-geneous in the US, has to be considered as a lim-iting factor for SESAR. On the other hand capac-ity constraints in the US occur primarily at majorairports and in the terminal airspace around them,while in Europe it is the en route airspace whichposes the biggest capacity limitations [13]. Theoperational objective of NextGen is to achieveperformance-based operations in which regula-tions and procedural requirements are describedin performance terms rather than in terms of spe-cific technology or equipment [1]. Similar con-cepts are basic also for SESAR [4] to align the fu-ture ATM operations to the ICAO standards [14].The analysis of these concepts, clearly indicatesthat the utilization of reliable 4DTs is paramountfor both SESAR and NexGen [15].

4 CONCEPTS TO ACHIEVE BOTH:TECHNOLOGY ENABLERS

In order to achieve TBOs the following technolo-gies are considered necessary [3, 16]:

4.1 Advanced FMS Capabilities

The concept of 4DT incorporating the temporaldimension into operations cannot exist withoutaccurate Controlled Time of Arrival (CTA) ca-pabilities. These capabilities will be achievedutilizing the FMS with more advanced features.In [17] the results of field tests performed atthe Stockholm-Arlanda Airport for EUROCON-TROL CASSIS project proved that current gen-eration avionics can achieve CTA with 4 secondsaccuracy at the approach fix (19 nautical milesfrom the runway), and less than 15 seconds atthe runway threshold. The key factors impact-ing the accuracy of the CTAs were the wind dataavailable to the FMS, the speed and altitude con-straints, and configuration for landing. The ex-tension of the FMS′ Required Time of Arrival(RTA) capabilities, to achieve 4DT negotiationbetween cockpit and ATM, has also been studied[18]. Although the negotiation mechanism stillneeds to be refined these capabilities will be a keypart of NextGen and SESAR concepts for TBOs.

Fig. 4 ADS-B Deployment in the US in 2013 [19]

4.2 Data Communication

To make 4DT negotiation feasible the voice com-munication channel between ATC and cockpitwill not be sufficient. As a consequence, theintroduction of Data Communication (Figure 4)technologies, becomes necessary to enable 4DTBOs. The implementation of these technolo-gies will be done in three segments (short-, mid-and far-term) in Europe whereas only in twosegments (mid- and far-term) in the US [10,12, 20]. Data Communication implementationfeasibility has been studied by the FAA sincethe nineties [21, 22], proving through human-in-the-loop (HITL) simulations the benefits forcontrollers′ workload. Benefits in terms of air-port capacity have also been studied [23]. More-over an example of how Data Communication(ACARS) can be used to share 4DTs was pre-sented in [17]. The RTCA SC−214/EUROCAEWG−78 joint committee has been tasked to de-velop the requirements for the advanced datacommunication service denominated 4DTRAD.This service will enable the negotiation and syn-chronization of trajectory data between groundand air systems [20, 24].

4.3 ADS-B

In order to replace Second World War RADARtechnologies, advanced surveillance capabilities

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Fig. 5 European ADS-B Validation sites [25]

will be important parts of both SESAR (under theCASCADE program) and NextGen [19, 25]. Theimplementation of ADS-B (in and out) is alreadyat an advanced stage. The satellite-based tech-nology will be used both on the ground (ADS-B out) for surveillance and on board of the air-craft (ADS-B in) for augmented traffic situationalawareness. The RADAR-based surveillance net-work will remain only as a back-up system forthe new system. A method of safety analysis forADS-B applications, agreed upon by Americanand European’s standard bodies, was presentedin [26]. The methodology was applied to the”Enhanced Air Traffic Services in Non Radar Ar-eas using ADS-B surveillance” (ADS-B-NRA)and showed how requirements on ground and air-borne equipment for safe operations can be ac-cepted internationally. The benefit of the imple-mentation of ADS-B in en-route operations wasalso evaluated in [27]. The analysis showed howdifferent airspace environment (Gulf of Mexico,Alaska) need different analysis approaches. Oneof the biggest challenge identified will be to meetthe same system performance with visual andinstrumental conditions. Figure 4 and 5 showthe ADS-B coverage in the US (different colorfor different altitudes covered) and the EuropeanADS-B validation sites in Europe.

Fig. 6 FASTI Components Breakdown [28]

4.4 Air Traffic Control Decision SupportTools

Another fundamental piece of the puzzle to en-able 4D TBOs operations is the implementationof Decision Support Tools (DSTs) for air trafficcontrollers. In order to manage the increased traf-fic that SESAR and NextGen will accommodateit will be necessary to provide controllers withDSTs to keep their workload within acceptablelevels while at the same time maintaining the cur-rent level of safety. These DSTs are based ontrajectory prediction capabilities that will allowthem to share and negotiate 4DT data keepingtraffic safely separated. The type of data that willbe shared to achieve system interoperability isnot completely defined yet [29, 30, 31]. Nonethe-less, a detailed data definition will be necessaryto provide ATC with these new capabilities [5].

Conflict detection and resolution (CD&R) ca-pabilities to support air traffic controllers in theseparation assurance task have been studied sincethe early nineties in the US [32]. More re-cently MITRE CAASD has developed CD&R ca-pabilities to support the FAA and the en-routeDSTs that is being deployed to support NextGen(URET/ERAM) [33, 34]. In parallel researchersat NASA Ames Research Center have been de-veloping a concept to apply 4D TBOs in whichpilots configure the FMS settings from clear-ances issued by the controllers or computed bythe FMS to meet time constraints. The result-ing trajectories are then shared with the groundthrough Data Communication or recreated by a

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ground-based system to support controllers intheir CD&R tasks [35]. This concept differs fromthe European one of 4D tubes developed underthe PHARE project [36]. Some of the basic is-sues that remain in relying on DSTs, are the un-certainty that is intrinsic in the trajectory predic-tion process [37], and the utilization of intent in-formation in CD&R [38, 39]. The set of CD&Rcapabilities for SESAR are developed under theFASTI project [28] and summarized in Figure 6.It is interesting to notice that no automatic con-flict resolution capabilities are envisioned in thisproject, not even on very far term timeline. Auto-matic conflict resolution is instead part of the Ad-vanced Airspace Concept (AAC) developed byNASA Ames [40].

5 PRELIMINARY RESULTS OF HUMAN-IN-THE-LOOP SIMULATIONS ANDFLIGHT TRIALS

5.1 US Results

To evaluate the operational acceptability ofPerformance-based ATM, in which 4DTs areshared between all the ATM users, MITRECAASD supported the FAA in a series of HITLsimulations. The results of the experimentsshowed that en route and terminal area conceptsare feasible and demonstrated reduced controllersworkload. The amount of traffic safely han-dled was increased with same number or fewercontrollers [41]. Terminal area merging trafficwas evaluated in a HITL simulation performedby NASA Ames to evaluate the acceptability ofnew controllers DSTs. The simulation provedthat the DSTs enabled controllers to keep air-craft on their Area Navigation (RNAV) routes,and also achieved good schedule conformancethrough advised speed clearances. The DSTsunder evaluation did not increase the controllersworkload or decreased the throughput. The re-sults also suggested that data-linked path adjust-ments exchanging 4DTs may be useful to absorblarge delays [42].

Figure 7. SESAR Contract of Objectives Error! Reference

source not found. Fig. 7 SESAR Contract of Objectives [43]

5.2 EU Results

The concept of Contract-of-Objectives (CoO) an-alyzed by EUROCONTROL in the HITL dis-cussed in [43] has a very tight relationship with4D TBOs. The basic idea is to establish a se-quence of spatial and temporal (4D) windowswhich represent milestones to meet during theflight execution. This 4D intervals are called Tar-get Windows (TWs) and will be fundamental ele-ments of future ATM. Through the application ofTWs it will be possible to implement the ” Busi-ness Trajectory ” envisioned in SESAR (Figure7). Moreover, also with 2X traffic conditions, theefficiency of the system increased and the ATCworkload remained acceptable. As part of the ini-tial validation activities, the SESAR programmehas introduced Time Based Operations (TiBOs)as a preliminary steps toward TBOs. Specifically,in TiBOs, a TW is specified by ATC for only onewaypoint of an aircraft flight plan.

First applications of TiBOs include initialfour-dimensional (I-4D) flight operations, whereairborne computed predictions for the aircraft tra-jectory are sent to the ground systems and usedby an Arrival Manager (AMAN) to create, farin advance, a sequence for all aircraft converg-ing to a merging point in a congested area. Asa result of this coordination with the ground sys-tems, each aircraft of the sequence is allocated atime constraint at the merging point. As a com-pensation for meeting such a constraint, the air-craft is allowed to fly its optimum profile up tothat point, i.e. without any vectoring instruc-

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tion from the controller. This agreement is re-ferred to as ”4D Contract” and its compliancyis continuously checked from both the aircraftand the ATC sides using suitable monitoring andprediction tools. Possible contingencies or un-expected events may actually prevent the aircraftfrom meeting time constraint. In this case, the 4DContract can be re-negotiated; if no further agree-ment is possible, the aircraft will be vectored tothe merging point, thus losing the privilege of fly-ing its optimum profile.

In the framework of the SESAR programme,a number of activities are currently ongoing forvalidation of the I-4D concept. On 10 Febru-ary 2012, an Airbus A320 test aircraft flew fromToulouse to Copenhagen and Stockholm and thenback to Toulouse testing I-4D flight operations.The trial successfully verified that the aircraftFMS and the ground automation systems wereable to implement the I-4D technique throughdata link interoperability. The trial also aimed tovalidate how information exchanged is displayedto the controller and the pilot and which toolsshould be adapted to use this new informationto its full extent. It is underlined that system in-teroperability is actually a key element for I-4Doperations. Only if the same information is avail-able at the same time to both aircrew and con-trollers, the trajectory of a flight can be best man-aged while taking into account all existing con-straints [44].

Further validation activities for the I-4D con-cept are planned in the forthcoming months torefine the technology, adjust operational proce-dures and explore ways to further enhance thetools and the operational concept. If the valida-tions are conclusive by the end of 2013, industri-alisation could follow and some pre-operationaldeployment could start in 2018.

6 RESULTS COMPARISON

6.1 Metrics Used

The most important benefits that both the FAAand EUROCONTROL expect from the complete

implementation of 4D TBOs can be summarizedin the following Key Performance Areas (KPAs):

1. Safety

2. Efficiency/Environmental Impact

3. Capacity

4. Predictability

5. ATC Workload Acceptability

In order to assess the level of safety some ofthe metrics that have been used include:

• Aircraft separations [17, 34, 35, 42, 43, 45]

• Losses of separation [34, 40, 42, 43, 46]

• Conflict false alarms[34, 40]

To assess efficiency/environmental impact:

• Delay per aircraft [40, 47, 48]

• Number of fulfilled TWs [43]

• Planned flight time divided by flight timein the sector [43]

• Fuel consumption [47]

To assess capacity:

• Number of aircraft in the sector/hr [21, 22,43, 49]

• Instantaneous number of aircraft [43]

• ATC number of instructions [43]

• Sector capacity [47]

• Revenue Passenger Miles (RPM) [47]

To assess predictability:

• Planned flight time divided by flight timein the sector [38]

• Number of fulfilled TWs [43]

• TP accuracy [34, 40]

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Fig. 8 NextGen Implementation Timeline [50]

• Schedule conformance [42]

• CTA/RTA accuracy [17, 18]

To assess ATC workload:

• Number of ATC controllers required pertraffic level [46]

• Mental demand [35]

• Effort [35]

• Frustration [35]

• Performance [35]

• Instantaneous Self-Assessment Workload(ISA) [43]

• NASA-Task Load IndeX (NASA-TLX)[41, 43]

• Air Traffic Workload Input Technique(ATWIT) [41]

• Subjective rating scale [21, 22]

6.2 Early Estimated Benefits

In this section early benefit of the individual tech-nologies necessary to enable 4D TBOs, and of theoverall benefits of SESAR and NextGen, from re-cent studies, will be briefly discussed.

1. Automated Airspace Concept (AAC): In[49] the capacity benefit of the implemen-tation of the AAC in en-route airspaceare discussed. The most important benefitachievable is the relieve, from the separa-tion assurance responsibility, for the con-trollers that is claimed to have the biggestimpact on workload. As a result, a dou-bling of the capacity of individual sectorsappears achievable to the authors of thestudy.

2. 4DTRAD (data link): Although not pre-cisely quantified, in [30] benefits both forairborne and ground side users are ex-pected from the application of Data Com-munication technologies in Europe. Themost remarkable categories where benefits

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Fig. 9 SESAR Implementation Timeline [4]

can be achieved are: flight efficiency, bet-ter sequencing in terminal area, increasedpredictability of the trajectory that will beactually flown, reduced frequency conges-tion, improved predictability of the ATMsystem, and enabling of 4DT data ex-change.

3. Conflict resolution capability: The benefitsenvisioned by the FAA with the introduc-tion of advanced conflict resolution capa-bilities were summarized in [5]. Key areaof improvement were: reduced delays dueto increased sector capacity, reduced ma-neuvering and consequent fuel burn, re-duced altitude restrictions, and increaseduse of direct routes between city pairs.

4. Early benefits for the implementationof en-route trajectory negotiation in atrajectory-based ATC concept were pre-sented in [47]. Increased sector capacity ofup to 10% and cumulative fuel consump-

tion savings up to $ 770 millions achiev-able in 2030.

5. Overall benefits of SESAR: To ensure acommercially sustainable high quality airtransport service to the European commu-nity, the SESAR project improvements areconsidered beneficial for the European en-vironment, economic growth, and compet-itiveness [16].

6. Overall benefits of NextGen (mid-term):The FAA estimates that by 2018, NextGenwill reduce total flight delays by about 21percent while providing $22 billion in cu-mulative benefits to the traveling public,aircraft operators and the FAA. In the pro-cess, more than 1.4 billion gallons of fuelwill be saved during this period, cuttingcarbon dioxide emissions by nearly 14 mil-lion tons. These estimates assume thatflight operations will increase 19 percent at35 major U.S. airports between 2009 and

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2018, as projected in the FAA’s 2009 traf-fic forecast [3].

7 ADVANCEMENT OF IMPLEMENTA-TION PLANS

SESAR and NextGen differ in their implementa-tion plans (Figure 8-9) because they are strictlyconnected to different European and Ameri-can industry structure. NextGen tends to beclosely tied to the (unique) government frame-work. On the contrary, SESAR, given the natureof European political structure, appears to be amore collaborative approach. Therefore NextGensupports a more centralized approach whereasSESAR a more distributed one [8]. NextGen′stimeline (Figure 8) foresees complete trajectorymanagement by 2025 with expanded conflict res-olution capabilities that will rely heavily on DataCommunication technologies. SESAR′s opera-tional concept places the ”Business trajectory” atthe core of the system, with the aim to executeeach flight as close as possible to the intention ofthe user with the timeline set for 2025 (Figure 9).

Some highlights in NextGen implementationwere achieved with the complete deployment ofADS-B technologies over the Gulf of Mexicowhich allowed ATC surveillance to be offered toan area that was not covered before. Moreoverin 2010 the Collaborative Departure Queue Man-agement (CDQM) system was tested in Memphisto implement better taxi-out operations [51]. Thissystem can support better use of 4DT data onthe ground together with better surveillance thatwas already implemented at New York JFK Air-port with Airport Surface Detection Equipment,Model X (ASDE-X) system [52].

SESAR′s implementation of Data Communi-cation technologies has already advanced to ef-fective deployment in the Maastricht Upper AreaControl (MUAC) Centre where clearances areuplinked by the ATC and requests were down-linked by the aircraft crew [53]. Moreover the At-lantic Interoperability Initiative to Reduce Emiss-sions (AIRE), a joint effort between the FAAand the European Union Commission, performedfield tests with commercial aircraft to support

green operations. 4DT precision and Continu-ous Descent Approach (CDA) were tested in atransatlantic flights between USA and Europe.Key enabler such as advanced FMS capabilitiesto achieve RTAs, and ATC DSTs, were used inthese experimental flights [54]. The results of thefirst i-4D flight were already discussed in Section5.2 [44].

8 PRELIMINARY CONCLUSIONS

Preliminary results of this analysis show howvery similar concepts for SESAR and Nex-Gen are being achieved with slightly differentmethodologies and technologies. The real chal-lenge will be to make these two systems interop-erable in the last step of their implementation.

What this study has identified is a different”philosophical” approach for the implementationof 4D TBOs. The most significant difference re-gards the calculation of the initial time constraint(CTA) that the aircraft have to meet. In the I-4Dproposed for SESAR [44], the CTA negotiationis based on possible limitations associated withaircraft performance and environmental condi-tions (e.g. wind field). Specifically, such a ne-gotiation is always initiated by the aircraft whichdown-links an Extended Projected Profile (EPP)where airborne estimations for ETA, minimumETA (ETAmin) and maximum ETA (ETAmax) areprovided for each of the remaining waypoints ofthe flight plan (Figure 10). The EPP informationactually represents a summary of what is esti-mated to be feasible for the aircraft. Therefore,for a given waypoint, the ATC system will pro-pose to the aircraft only CTA constraints whichare consistent with the [ETAmin; ETAmax] inter-val.

This approach assumes that the aircraft hasthe best knowledge of his future trajectory andtherefore the trajectory predicted by the FMSis the most accurate. Once the final CTA con-straint is agreed upon, the aircraft will meet itthrough the RTA capability of the FMS. On theother hand, in the concept proposed for NextGen

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Fig. 10 I−4D Concept Representation [44]

[1, 55] the ground system calculates the firstCTA, uplinks it to the aircraft and start the negoti-ation. This because it is assumed that the groundsystem has a better knowledge of the traffic, andtherefore has a better situational awareness. Afterthe CTA has been agreed upon, the aircraft willmeet it using the RTA capabilities of the FMS.

In this context, It is interesting to report aslightly different ”philosophical” approach thatis been carried out in Australia [56]. In Fact, toimprove the predictability of the aircraft to theground system, the RTA capability of the FMS isnot used, because although it improves the accu-racy at the constraint (CTA) location, it reducesthe ability of the ground system to predict the de-scent profile. This because the cost index, amongother parameters used by the FMS, are unknownon the ground. The results of these different re-search approaches should be considered to findan optimal way to achieve 4D TBOs.

Another critical aspect that must be resolvedby the SESAR Consortium for the completeimplementation of 4D TBOs will be how toexchange the trajectory data between differentairspace sectors controlled by different ANSPs.The interoperability of ground DSTs to achievethis goal must be evaluated and tested before the2025 timeline. This problem still needs to be ad-

dressed by the FAA as well, but it can be assumedthat it will be less of an issue. Nonetheless tocompletely benefit from 4D TBOs this problemcannot be neglected.

More research is undergoing to develop thesepreliminary findings into more solid results, torefine operational concepts and to identify morespecific problems associated with human, equip-ment and procedures.

9 ACKNOWLEDGMENT

This work could not have been done withoutthe support of the Business Development Depart-ment of Engility Corporation and of the Direc-tor of the Advanced Transportation Research andEngineering Department Lynne Oliver. A specialthanks goes to Dave Knorr (FAA) for his com-ments, and to Greg McDonald (Airservices Aus-tralia) for the stimulating conversation on trajec-tory predictions.

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A COMPARISON OF 4D-TRAJECTORY OPERATIONS ENVISIONED …