WP2 3:30The Traffic Management Advisor

William NedellSan Jose State Umnversity, San Jose CA 95102

Heinz Erzberger, Frank NeumanNASA Ames Research Center, Moffett FieldCA 94035

ABSTRACT

The Traffic Management Advisor (TMA) comprisesalgorithms, ag I interface and inteactive tools forcon hlg e flow of air taffic into the temial area.The primary algorithm incorporated in it is a real-timescheduler which gentes efficient landing sequences andlanding times for afrivals within about 200 n.m. fromtouchdown. A unique feature of the TMA is its graphicalinterface that allows the affic manager to modify thecomputer generated schedules for specific aircraft whileallowing the automatic scheduler to continue geneatingschedules for all other aircraft The intfalsoprovides convenient methods for monitoring the tafficflow and changing scheuling parameters during real-timeoperation.

INTRODUCTION

Although automated decision systems for air rafficcontrol (ATC) have been investigated for at least twodecades, attempts to implement these systems in thecurrent ATC envionment have larely faild Among thereasons for this failure are the use of obsolete ATCcomputers and displays, which are preventing theimplementation of advanced concepts, and a tendency ofdevelopers to underestimate the complexity of automatingeven simple ATC functions.

Recently, the prospects for intoducing higher levels ofautomation have improved because of two concurrentdevelopments.F, a new generation of controller suitesincorporating color graphics workstation technologytogether with new ATC host computers will removemany of the limitations impeding the implementation ofautomation concepts. The new controller suites, whichwill become operational in the mid-1990s, are the keyelement of the Federal Aviation Administaton (FAA)Advanced Automation Systems (AAS). Second, recentresearch has provided new insights into the appropriaterole of automation in ATC and has yielded promisingmethods for designing such systems.

The need for an effective controller-system interfaceimposes the most critical design constraint on ATCautomation tools. To meet this constaint the interfacemakes extensive use of on-screen switches and menusselectable by manipulating a mouse or trackball. Such

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techniques improve the intface by minimng the needfor time-cnsm ng and disucting keyboard entries.Also, computer-generaed advisories are transformed,when possible, into a graphical frmat tat enhances rapidperceptio ofadvisoryifo a

Tle report begins with an overvew of the automationconcept This is followed by a dailed description of theTraffic Management Advisor. A description of theDescent Advisor can be found in reference 1, and adescription of Final Approach Spacing Tool can be foumdin reference 2.

It is strgly recommended that elements of this systembe implemented at an ATC facility for evaluation on anon-interfering basis. In view of past experience, suchoperational testing is an essential step in validatingautomation concepts for ATC. Past attempts atimplementing automation tools have failed in partbecause they only worked well under the carefullycontrolled conditions of the laboratoy. Unfortunately,such conditions rarely exist at ATC facilities. Anautomation tool which cannot handle the frequentdepaures from the quiescent design state will be quicklyabandoned by controllers. Yet these are the times whenautoaton assistance is most needed. Thus testing underrealistic conditions can only be attained at operationalATC facilities. Then, the FAA can confidently decidewhat elenents of this system warrant implementation inthe AAS.

AUTOMATION SYSTEM CONCEPT

Figure 1 gives a diagrammatic representation of theoverall system. Its key ground-based elements are theTraffic Mangement Advisor (TMA), the Dewent Advis(DA), and the Fnal Approach Spacing Tool (FAST). Thefunctions ofeah element and the relatonships betweenelemen are isued below.

The primary function of the TMA is to plan the mostefficient landing order and to assign optimally spacedlanding times to all arvals. These time schedules aregenerated while aircraft are 150 to 200 n.m. from theairport Te TMA algorithm plans these times such thattraffic approaching from all directions will merge on thefinal approach without conflicts and with optimalspacing. The TMA also assists the Air Route Traffic

Control Center (ARTCC) Traffic Manager in reroutingtraffic from an overloaded sector to a lightly loaded one, aprocess known as gate balancing. Another function of theTMA is to assist the Center Trafflc Manager in efficientlyrerouting and rescheduling traffic in response to a runwayreconfiguration or a weather disturance. In general, thefunctions of the TMA involve assisting the Center TrafficManager in coordinating and controlling the traffic flowbetween Centers, between sectors within a Center, andbetween the Center and the Terminal Radar Approach(TRACON) Facility. Moreover, the TMA must permitthe Center Traffic Manager to specify critical flow controlparameters such as runway accepance rate and to overridecomputer generated decisions manually.

Figure I.-Automation concept.

At a Center, the controller positions requing the highestskills and mental workload are those handling descenttraffic. These positions are responsible for producing anorderly flow of traffic into the TRACON. The DescentAdvisor (DA) is intended to provide controllers in thesepositions with flexible tools to implement the traffic plangenerated by the TMA.

For all aircraft entering an arrival sector, the DAimplemented at that sector computes estimated times ofarrival (ETAs) at its respective arrival gate. These ETAcomputations take into account the airspace structre andATC procedures of each arrival sector. For simplicity,only two DAs are shown in figure 1, but in general therecan be four or more, at least one for each arrival gatefeeding traffic into the TRACON. The ETAs from allarrival sectors are sent as input to the TMA which usesthem to calculate efficient, conflict free-landing schedules.These scheduled times of arrivals (STAs) at the runway

are then tansformed by the TMA to gate arrival times bysubtracting the time to fly from the gate to touchdown,and are sent to the DAs at the appropiate aival areas.

Upon receiving these STAs the DA algorithm generatescruise and descent clearances which controllers can use tokeep aircraft on schedule. For aircraft that drift off theirplaned time schedules, the controller can request revisedclearances that correct such time errors to the extentpossible. If this concept is implemented in today'senvironment, the controller would have to issue theclearances by voice, but in the near future it will be moreefficient to isse them via the proposed ground-to-air datalink.

The TRACON controllers take over control of taffic atthe feeder gates. They merge the traffic converging on thefnal approach path while making sure that aircraft areproperly spaced. If the Center controllers have deliveredaircraft at the gates on time using the DA tools, theTRACON controlers odinarily will need to make onlysmall corrections in the relative positions of aircraft toachieve the desired spacing. The FAST assists thecontroller in making these corrections with high accuracyand a miimum number of heading vectors and speedclearances. Achieving precise spacing between aircraft onfinal approach ensures that landing rtes will always beclose to the thoetical capacity of the runway.

Another qte of tool designed for the TRACON controlleris the Tactical Advisor. This tool helps the controller toreplan traffic quickly in response to several specialsitations, such as missed approaches, runway changes,and unexected conflts.

TRAFFIC MANAGEMENT ADVISOR (TMA)

This section describes elements of the TMA, withemphasis on the design of the scheduler and graphicalinterface.

Overview

The Traffic Management Advisor (TMA) comprisesalgorithms, a graphical interface, and interactive tools foruse by the Center affic manager or TRACON controllersin m g the flow of traffic within the terminal area.The primaiy algorithm incorporated in it is a real-timescheduler which genertes efficient laning sequences andlanding times for arrivals within about 200 n.m. fromtouchdown. Its graphical interface and intactive tools aredesigned to assist the taffic manager in monitoring theautomatically generated landing schedules, to overide theautomatic scheduler with manual inputs and to changescheduling parameters in real time. It has beenimplemented on a separate worstation that is interfacedwith the workstations rmning the DAs at the variousarrival areas.

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In essence, the scheduler is a real-time algorithmtransforms sequences of arrivals into sequencesof scheduled times of anival (STAs) using one of severscheduling protocols selected by the traffic manager.Operation in real time implies that the algorithmgenerates the STAs in a ail frct of the dme it tkeseach aicraft to fly from its inal position to tuchdown.This condition places important computationalconstaints on the algorithm.

Since the scheduler is the main computaional unit of theTMA, its functions and operations are described fst.Next is a description of the graphical in e and the setof tools the traffic manager uses to monitor and tointeract with the scheduler in real time.

Design Issues

The scheduling and freezeh are key parmneters indetermining the performance of the scheduler. Thescheduling horizon is a tme interval specifying when anaircraft becomes eligible for scheduling. The freezehorizon is a time iMterval specifying when an aicraft'sSTA will no longer be changed without manualintervention. The TMA gets periodic udates of theestimated times of arrival (ETAs) for all airaft. Theseupdates come from the DA at the sector currentlycontrolling the aicraft. The TMA then uses the ETAs todetermine when aircraft are eligible for scheduling orhaving their scheduled times frozen.

Although the scheduling and freeze horizns are timedependent quantities, they can also be approximatd in thespatial domain by concentric circles with the arrivalaiqxrt at the center. The circles representig the hoizonsare superimposed in figure 2 on the arrival airspacestuture of the Denver Center. The location of the fmezehorizon (in space and time) must balance two conflictingobjectives. On the one hand, it must be chosensufficiently early in the approach in order to give thearrival controllers adequate ime and arspace to meet thescheduler-generated STAs. At the very latest, the freezehorizon must be chosen before arrivals reach the areawhere descent clearances are issued. This area is about 30min to touchdown. On the other hand, the location of ftefreeze horizon should not be chosen so early that arrivalsfrom nearby airports often appear later than the freezehorizon, thus missing the scheduling window altogether.Also, an early freeze horizon increases the probability ofschedule-distubing events occuring between the freezehorizon and the TRACON boundary, such as weadwr dis-turbances. The rescheduling of frozen aircra necessitatedby such occurrences causes an undesirable increase incontroller woriload, and thus should be minimized. Theseconsiderations, as well as the results of simulation tesand experience with the current metering system at theDenver Center suggest a freeze horizon 35 minutes totouchdown, with a schedulng window 10 minutes long.Aircraft whose ETAs are in the scheduling window are

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opated upon by the scheduling algorithm to genere theoptimwn sequne.

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Figure 2.-Scheduling Regions.

When it becomes necessary to reshedule certain airat,the last and best ty to do so occws in the regionwhere the arrivals transition from the Center into theTRACON at the feeder gates. This region is identified infigure 2 as the TRACON rescheduling region. In thistransitin region, the schedulig pfocess described abovecan be repeated. Since the scheduling window is narrowand close to touchdown, rescheduling in the TRACONregion consists primarily of fine tuning the Center-determined arrival sequence. Extensive changes in theschedule for arrivals this close to the runway are neithernecessary nor feasible. Frequent reordering of arrivalsequences or large changes in STAs at this point woulddisrupt the orderliness of the arrival flow and producecomplex trajectories in the TRACON airspace, therebyincreasing controller workload. The primary reason forrescheduling aicraft in the TRACON airspace arises fomthe need to handle missed appoaches, emergency arcraf,and changes in runway.

One of the most critical aspects of designing a scheduleris definig the procedures for establishing the arrivalsequence. In today's ATC system controllers generallyattempt to maintain a first-come-first-served (FCFS)sequence when vectoring aircraft for arrival in a terninalarea. Generally first-m e refers to projected arrival timeat the runway but it is possible to define first-come inofter ways. For instan, first-come could mean the orderin which the aircraft cross a point in space such as theCenter boundary. Time based orderings can be static ordynamic. The initial estimate of arrival time for eachaircraf establishes a certain arrival order but subsequentETA updates may change the arrival order.

Controllers frequently deviate from a strict FCFS order inorder to accomplish specific objectives. When decisionsneed to be made in sequencing of a group of aircraftfactors such as wind direction, arcraft type, and the routetopology are all taken into account by the controller indeciding on a suence. This takes considerabl skill andjudgment, and different controllers may handle similarconditions quite differently. The approach taken in theTMA to address these concerns is to provide variouschoies to the controller which are selectable in r me

Design of Scheduler

Optimization of aircraft arrval schedules has been thesubject of numerous sudies in recent years (refs. 3-8).However, in the satdies cited, mization benefits havebeen difficult to quantify because they are sensitive tomany factors that are difficult to measure or estimate.Such factors include the choice of representative arrivalsequences, the distribuion of aircraft weight classes andthe selection of base line conditions against whichschedule Wtimization benefits can be accuraely gaugedIn the most recent study of this problem (ref. 8)scheduling efficiency is computed by a Monte Carlosimulation for the three types of scheduling methodsimplemented in the TMA. Results of this study will besumnmarzed aft descrtib he real time sculer.

A theory for the design of real-time schedulers capabl ofhandling the diverse conditions g in ATC has notbeen ated compnsively in the reserch litera. Inthe US.SA, the best klown implementation of a real-time scheduler is the En Route Metering (ERM) system,which has been in operation at various Centers, ludingthe Denver Center, for a number of years. ERM hasevolved, with fair success, as a tool for controlling theflow of taffic into the TRACON under capacity limitedconditions. However, it is not designed to produceconflict-free, optimum arrival schedules at the runway fora mix of aircraft weight classes, as is the objective in thisdesign. In West Germany, fte COMPAS system (ref. 3)undergoing tests at the Frnkfurt Airport also incorpoatesa real-time sceuler. The design descrbed hemrm expadson featues in ERM and COMPAS and also incorporatesnew graphical and interactive concepts that capitize onthe capabilides of high perfoman wo .

The TMA can be configured in a variety of ways. Theuser can select between time based and distance basedsequencing and also whether the sequence is static or isupdated dynamically. The user cai aLso enable or disabletime advance and optiiation. All of the oons can bechanged at any time and will take immediate effect on thecurrent schedule.

The first step in the scheduling process is to arrange theaircraft in an ordered list based on the currently selectedsequencing method. For example, if the sequence methodis set to dynamic time the airrft with thpeearliestETA

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is placed at the beginnmg of the i and the acraft withthe latestETA is placd atthe end of the list

Once the type of sequence to use has been established thescheduler next checks the ineraicaft time spacings onfinal approach. For those with less than the minimumallowed, the scheduler adds just eough time to meet theminimum distance Ion sadards required by FAAregulaions. It should be noted that the minimumsepaion dis depend on the ahiaft weight classes(heavy, large, and light) of the leaing and tailingaircaft Since the schedulr works on the basis of timeand not s , it is fi necessary to tansform thedistances into equivalent time separations usingprocedures described in reference 4. Te results of thesetransfomations are a set of time intervals which spcifythe minimum time spacings on final approach for allninepossible landing sequences of aircraft with three weightclasms. A complicating factor is that the transfoatonsdepend implicitly on the ground speed of aiaft on finalapproach. Since ground speed depends on both finalapproach air speed and wind speed, it becomes necessaryto update the time spacings in real time. Thiscumbersome and complicated procedure should beeliminated by developing new criteria specifically fortme-based minimum -tion standards.

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Figure 3.-Effect of scheduling methods on delays.

he opeation of the basic schlduling algorithm withouttime advance or optimization can be illustratedgrahically with the help of time lines drawn side by sideas in figwe 3(a). The dme line on the right shows theETAs of several large and heavy aircraft within ascheduling window. Te earliest ETA is at the bottom ofthe list and icreang future time is toward the top. Forillustrative purposes, only two minimum separationtimes are used, 2 min. for a heavy followed by a largeaircraft and 1 min. for all other sequences. Since the timeseparations between ETAs in this list are generallysmaller than the minimums, the scheduler has to delayaircraft to with the minimums. The result of thisoperaton is shown on the STA time line. Here,horizontal lines connect STAs and ETAs of the sameaircraf The original ETA order has been maintained as

indicated by the fact that none of the connecting linescross each other.

The effect of the time advane option to the basic

scheduler is ilustaed in figure 3(b). A 1-min timeadvance relative to theETA was allowed for each aiatThe effect for many ahiaft is a rdcinof delay and fuelconsumption. On the other hand, those aircraft whosetime is advanced may experience increased fuelconsmption because of higher-thanoptimal cruise and

desent speeds. Therefore, in assesng the oveall benefitof time advance, it is necessary tob time and fuelsavings for those acaft whose delays are redwed against

an increase in fuel sump for those whose time is

advanced. Nevertheless, in most situas, time advanceis iely to be advatageous.

In o d of these acle-of the dr attempt

to be intelligent in applying time advance by notadvancg aira when the benefits to be gained areminimal. The scheduler does this in two ways. Frst, theamount of advance is controlled by specifying both amaximum advance and a fction of the total advanceavailable that is to be applied. The scheduler firstdetermines the minimum time to landing for a givenaircraft as previously defined. Only a fraction of the total

advance available is used by the scheduer. This amount iscompared with the maximum allowable advance and te

smaller of the two quantities is used to arrive at theaicraft's scheduled time. ITe second technique used bythe scheduler is to advance a given aircraft only when it ispart of a closely spaced group of aicaf A closely spacedgroup is defined as a set of consecutive aircraft which arespaced at or below the minimum allowable separion.The number of consecutive ahrraft which defines a groupis an adjustable parameter, typically set to four.

A position shifting scheduler with or without timeadvance removes the constraint of presevig the ETAorder when generating the STA list Position shifng for

aicraft scheduling was studied by Dear (ref. 5) andsubsequently by others (ref 6). The schedulerimplemented here optimizes the STA list with rspect to

a user specified maximum number of position shifts.This means that the landing order of an aircraft may not

be moved more than the scified number of aaft aheadof or behind the FCFS order. The schedule produced bythe position shift scheduler with time advance isimlustrated in figure 3(c) for a single position shift. It canbe seen that position shifting has provided additionaldelay reduction. However, these reductions are highlydependent on the mix of airraft in the list There would

be no advantage in position shifting if the minimum tineseparation between all airaft were the same. Positionshifting tends to bunch aicraft of the same weight classas in figure 3(c). Although position shifting can reducedelays, it is not always feasible to implement. Forexample, position shifting of two in-trail aircraftgenerally requires one airaft to overtake the other. Thisprocedure increases controler workload, making position

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shifting undesirable. Therefore, the scheduler has beendesigned to allow position shifting only if it can be

tpleed before the posion-Shifted pair has merged on a

common route.

Estimated Performance of Scheduler

A study has been made to detmine the effect of thevarious s ng meods used in de TMA on aircraftdelay. A full discussion is available in reference 8. In

this study a taffic model was developed to simulae peakarrival taffic at the Denver Center. Traffic samples witha varying mix of heavy and large aircraft tpes were

created PSentig an hour and a half of data. Figure 4illusas the effect of using time advance and singleposition omization.

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The curves shown are cumulative probabilitydistributions for the average delay per aircrat Thedistributions are based on 2500 taffic samples each, witha taffic density of 40 aircraft per hour. The modelassumes a rectagular probability distnbution for aicraftarrival times at the Center boundary. The resultingdistibutions shown in the figue are for a 50% heavy,50% taffic mix. It can be seen that both time

advance and position shift prvide approximately equal,incremental reductions m average delay per aircraft,compared to the FCFS scheduler. Note that because thedistributions are not symmetrical the mean delay peraircraft does not fall at the 50% point These curvesshow the best possible mix of taffic in terms of reducing

delays. With a more typical traffc mix of aircraft typesthe reduction in delay per aicraft would be smaller. Theaverage delay distnbutions are very sensitive to the arrivaldistribution and winds. Assuming a triangular instead ofa rectangular distribuon neary doubles te average delay.

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A 20 knot headwind has nearly the same effect Lastly, itwas shown that decreasing the required inter-aircraftspacing by even small amounts has a large effect onreding Xt average delay.

Description of Graphical Interface and Tools

The interface for the TMA is based upon exploiting theinteraction between workstation screen and the mouse.The workstation screen is divided into severl areas orwindows. The largest area displays aircraft arrivalschedules on several reconfgurable time lines. Anotherwindow gives an overview of traffic in the Center in aminiature plan view display (PVD). Other windows givestatu infomation nd allow the modification of varousscheduling parameters such as the aiport acceptance rateand the configuration of the time lines. An additionalpop-up or ovalapping window is available for displayinginformation about the schedule such as the currentlyselected acceptance rate, avenage and peak delays, andvarious data on indiviual airraft which the controller canselect by picking the arcraft time line tag.

The time line window contains a number of time lines onwhich three types of ime schedules can be selectivelydisplayed: (1) ETAs of aircraft that have not yet enteredthe Center airspace; such ETAs are contained in flightplans sent to the Center ahead of time; (2) ETAs ofaircraft tracked by the Center radars and sent to the TMAby the DA's at various controUer ations; and (3) STAsof all aircraft which will be or have been sent to thevarious controller stains. (A line drawing of the famatof the timelines can be seen in figure 5.) These timescedules can be selectively displayed on both the left andright side of each time line. Furthrore, the display ofthese time schedules can also be segregated by arival areathrough use of toggle switches in the control panelwindow.

Aircraft time schedules move toward the bottom of theirrespective dme lines as time increases. An aircraft firstappears on the flight-plan time line at the time the Centerreceives its flight plan and planned ETA. When thearcraft becomes active in the Center airspace, its ETA isupdated and it is simultaneously removed from the flightplan time line and displayed on the ETA time line.Finally, when its ETA pentrates the scheduling horizon,its STA is computed and then displayed on the STA timeline. Color coding of aircraft IDs and graphical markersare used on the time lines to convey the aircraftscheduling status and critical scheduling parameters. Atthe time the ETA of an aircraft falls below the freezehorizon its scheduled time is sent to an appropriate PVDfor display on the PVD time line.

Interaction with time lines- Of particularimportance to the implementation of the TMA was therequirement that the traffic manager be able to interactwith the automatic scheduler.TheT MA intkfe hasbeen structured to allow traffic managers considerableflexibility in modifying the computer generated schedule

for specific aircraft, while allowing the automaticscheduler to continue generating schedules for the othereligible aircraft Also of imporance is the immediatefeedback available to the traffr manager when he or shedoes modify the computer plan. The computerimmediately modifies the scheduled aircraft display toreflect the taflic manager's input, making it easy to seethe effect of his/her actions.

Some of the ways in which the traffic manager caninterat with the auomatic scedule will now be covered.In the following explantions, frequent reference will bemade to figure 5. This figure shows two time lines. Thetime line on the right displays ETAs while the time lineon the left displays STAs for the same set of aircraft

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Manual scbeduling- The traffic manager canalter the scheduled time of any aicaft currently in thesystem (including those which have not yet beenscheduled, and those whose times are afready frozen). Tsis done by pling the moum cursor over the aircraft timeline tag, depssing the middle button of the the-buttonmouse used in Sun worknkions and dragging the tag to anew location (time) on the time line. As soon as themiddle button is released, fte computer will generate anddisplay an updated schedule. Aircraft scheduled in thisfashion am displayed in purple to highlight the fact thatthey have been manually scheduled by the controller. Inthe fiugwe both PAOOI and SP404 have been manually

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Blocked time intervals and slots- Thecontrol can block out dmes in which he does not wantaircraft to be schedule as Previously defined. Two kindsOf blocked dmes are displayed in the figure; intrvals andslot. The scduler wil not p-e any aicraft in te aeadelimited by the blocked times The figure shows aninteval which causod delays for C0409. Notice dt thescheduler has plaed airrat Ight at the limits of theblocked iMterval. Blocked slots are slightly morecomplicated. They are created by using a menu option.Tne figure shows a heavy slot just pst the 55- marUnlike the p ede for intvals, te amount ofaqcereserved by the slot d n te weight clas Of otheraircraft being scheduled, just as though a slot were anactual ai Nodce in the figure that UA134 has beenmoved behind te heavy slot even ftough it was ahead ofit on the time line.

Time line tag pop-up men- Various othersledulng options are available on a menu bught U bydepresang the right mouse button while over an airafttime line tag. These include selectively reschedulingaircaft after they have passed the freeze horizon,rebroadcasting Ue cuent scheduled time to a PVD, andreturning a manually scheduled aircraft to automaticscheduling status.

CONCLUDING REMARKS

The Traffic Management Advisr desribed n this paperdelibrately plaes Ut a tool in a subordinatdposition relative to of the-hman controller, whowill remain the coreMrone of the air afic controlprocess mn the foreseable future. The conroller selectsthe automation levels and funcdons in response tospecific taffic management problems. He or she cancombine his/her own s and decisions withcomputer generated advisories by choosing tools thatcomplement his own control techniques. At one end ofthe spectrum of computer tance, the controller canuse th tools in a passive mode to gain insight into Uteffect of the planned actions. At the other end of thespectrum, he or she can use the tools actively by issuingUt computer genrted clearas to Utela

The Mteractive graphic in s ed in th design areprobably Uhe most innovative as well as the lest provendesign featre. They build upon the user environmentincorporated in modern high mance en ngw That this w gycan be soreadily adapted to air traffic control automation isremarkable and founate forprog in this area

confidence. Such tes, which are considered an essentialstep in the development of an advanced automatonsystem, can begin as soon as access to aicaft trckngdata is obied at an en route center.

REERENCES

1. Erzberger, H.; Nedell, W.: Design of AutomatedSystem for Management of Arrival Traffic,NASA TM-102201, June 1989.

2. Davis, T.; Erzberger, H.; Bergeron, H.: Design of aFinal Apprach Spacing Tool for TRACON AirTraffic Control, NASA 'IM-102229, Sep. 1989.

3. Volckes, U.: Computer Assisted Arrival Sequencingand Scheduling with Uhe COMPAS System.Conference Proceedings, Efficient Conduwt ofIndividual Flights and Air Traffic, AGARDNo. 410.

4. Tobia, L.: Time Scling of a Mix of 4D Equippedand Unequipped Aircraft, Proc. of 22d IEEEConf. on Decisins and Control, pp. 483-488,Dec. 1983.

5. Dear, R.: neasing Capacity with Computer-AssistedDecision Making., Proc. International AirTrnspaion Conf., VoL 2, May 1979.

6. Luenburger, Robert: A Traveling-Salesman-BasedApproach to Aicraft Scheuling in the TerminalArea. NASA TM-100062, Marh 1988.

7. Magill, S.A.N.; Tyler, A.CF.; Wilkenson, E.T.;Finch, W.: Computer-Based Tools for AssistingAir Traffic Controller with Arrivals FlowManagement; Royal Signals and RadarEstablishment, Malvern, Wocsehire, UK.,Report No. 88001, Sept 1988.

8. Neuman, F.; Enzberger, H.: Analysis of Sequencingand Scheduling Methods for Arrival Trffic.NASA TM- 102795, April 1990.

Contrller accepnce of dUse inces moe than anyother issue, will determine the viability of thiscHere, real time simuaions are the main avenue forevaluating controler response, for refiing the interface,and for developing baseline controller procedures.Uldmately, however, only tests with live traffic canestablish their effectiveness with a high level of

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