NASA Technical Memorandum 4763

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A System Concept for Facilitating User

Preferences in En Route Airspace

R. A. Vivona, M. G. Ballin, S. M. Green, R. E. Bach, and B. D. McNally

November 1996

National Aeronautics and

Space Administration

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NASA Technical Memorandum 4763

A System Concept for Facilitating User

Preferences in En Route Airspace

R. A. Vivona, Sterling Software, Inc., Redwood Shores, California

M. G. Ballin, S. M. Green, R. E. Bach, and B. D. McNally

Ames Research Center, Moffett Field, California

November 1996

National Aeronautics and

Space Administration

Ames Research Center

Moffett Field, California 94035-1000

A System Concept for Facilitating User Preferences in En Route Airspace

R. A. VIVONA,* M. G. BALLIN, S. M. GREEN, R. E. BACH, AND B. D. MCNALLY

Antes Research Center

Summary

The Federal Aviation Administration is trying to make

its air traffic management system more responsive to the

needs of the aviation community by exploring the concept

of"free flight" for aircraft flying under instrument tlight

rules. In free flight, en route aircraft fly user (i.e., pilot or

airline)-defined trajectories with only minimal air traffic

control (ATC) adjustments to avoid restricted airspace

and separation loss with other aircraft. Free flight is

expected to allow airspace users more flexibility in determining optimal aircraft routing. In all current free-flight

concepts, the efficient handling of aircraft in transition

from en route to terminal airspace requires ground-based

planning and control. When aircraft transition from en

route to terminal airspace, their trajectories often must

merge subject to in-trail separation or lime-based flow

constraints necessary for ineeting airport capacity

limitations. The unconstrained nature of free flight

complicates these ground-based tasks even more. Hence,

providing controllers with effective decision support tools

that not only support free flight, but also support the

transition of free-flight aircraft into the terminal airspace,

is essential.

A logical first step toward free flight could be made

wilhout significantly altering current ATC procedures or

requiring new airborne equipment by designing a groundbased system to be highly responsive to user preference in

en route airspace while providing for an orderly transition

to terminal areas. To facilitate user preference in all en

route enviromnents, a system based on an extension of

the CenterFFRACON Automation System (CTAS) is

proposed in this report. The new system consists of two

integrated components. An airspace tool (AT) focuses on

unconstrained en route aircraft (e.g., not transitioning to

the terminal airspace), taking advantage of the relatively

unconstrained nature of their flights and using long-range

trajectory prediction to provide cost-efl'ective conllict

resolution advisories to sector controllers. A sector tool

(ST) generates efficient advisories for all aircraft, with a

lk)cus on supporting controllers in analyzing and resolving

complex, highly constrained traffic situations. When

combined, the integrated AT/ST system supports user

*Sterling Software, Inc., Redwood Shores, California.

preference in any air route traffic control center (ARTCC)

sector. The system should also be useful in evaluating

advanced free-flight concepts by serving as a test bed for

future research. This document provides an overview of

the design concept, explains its anticipated benefits, and

recommends a development strategy that leads to a

deployable system.

Introduction

The Federal Aviation Administration (FAA) is trying to

make its air traffic management system more responsive

to the needs of the aviation community by exploring the

concept of "free flight" for aircraft flying under instrument flight rules. (See refs. 1 and 2.) In free flight,

en route aircraft fly user (i.e., pilot or airline)-defined

trajectories with only minimal air traffic control (ATC)

adjustments to avoid restricted airspace and separation

loss with other aircraft. Free-flight rules begin after the

initial departure restrictions and end at the initiation of

arrival sequencing to the destination airport's terminal

airspace. Because en route aircraft in flee flight are not

required to follow ATC-prefermd jet routes, flee ['light is

expected to allow airspace users more flexibility in

determining optimal aircraft routing. Although significant

economic benefits are anticipated, very little is known

about the efl'ects of free flight across the national

airspace.

Through the FAA's National Route Program (NRP),

airlines currently can file requests to fly non-ATCpreferred routes (with some limitations). However, the

complex interaction of user-preferred routing and

dynamic changes in traffic density across the national

airspace have limited the benefits of the program. (See

ref. 3.) The problems experienced with NRP show that, if

free-flight benefits are ever to be realized, the air traffic

management (ATM) system will have to evoh, e through

an FAA strategy of increasing decision support tools

available to controllers, airlines, and pilots while

gradually decreasing route constraints. In addition, this

evolution must be accomplished safely and with no

disruption in operations.

In current flee-flight concepts, the efficient handling of

aircraft in transition from en route to terminal airspace
will continue to require ground-based planning and

control. When aircraft transition from en route to terminal

airspace, their trajectories often must merge subject to intrail separation or time-based flow constraints necessary

for meeting airport capacity limitations. The unconstrained nature of free flight complicates these groundbased tasks even more. Hence, providing controllers with

effective decision support tools that not only support free

Ilight, but also support the transition of t+rec-tlight aircraft

into the terminal airspace, is essential.

A logical first step would be to achieve as many freel]ight benefits as possible without significantly altering

current ATC procedures or requiring expensive airborne

equipment modifications. These benefits could be

achieved by a ground-based system that is highly

responsive to user preference in en route airspace while

also providing ['or an orderly transition to terminal areas.

User preference is the general term for any aircraft

operation that is explicitly requested or assumed to be

desired by the airline or pilot. Preferred aircraft operations

can vary from non-ATe-preferred mutings to the use of

airborne vertical navigation (VNAV) automation during

descents into terminal airspace. In a system that requires

positive ATe control, user preferences are facilitated (or

enabled) through verbal or procedural ATC clearances.

Developing decision support tools for controllers that

identify user preferences and their effects on the current

traffic situation would enable controllers to quickly assess

the effort of incorporating user preferences into the

current traffic plan. Furthermore, by designing automation to determine the minimum change to the user

preference required for incorporation into the traffic plan,

a large step toward free-flight benefits would he achieved.

To facilitate user preference in all en route environments,

a system concept based on an extension of the Center/

TRACON Automation System (CTAS) has been developed. (See ref. 4.) It consists of two integ,'atcd components. An airspace tool (AT) focuses on unconstrained

en route aircraft (e.g., not transitioning to the terminal

airspace), taking advantage of the unconstrained nature

of their trajectories and using long-range trajectory

prediction to maximize user efficiency by providing

cost-effective conflict resolution advisories to sector

controllers. A sector tool (ST) generates efficient

advisories for all aircraft, with a locus on supporting

controllers in analyzing and resolving complex, highly

constrained traffic situations. When combined, the integrated AT/ST system supports user preference in any air

route traffic control center (ARTCC or Center) sector.

This system should also he useful in evaluating more

advanced free-Ilight concepts by providing a test bed for

future research.

This document presents an overview of the design

concept, explains its anticipated benefits, and recommends a development strategy that leads to a deployable

system. The AT and ST are described in detail, and a

new ATM position, the "airspace coordinator," is defined.

Examples of conflict resolution for typical conflict

scenarios are also given.

Functional Design

The integrated tools concept must accommodate the wide

range of traMc environments found in the national

airspace. At one extreme, generally associated with free

flight, is an airspace consisting only of aircraft not

constrained by traffic management or other localized,

highly dynamic constraints. This environment, usually

tound in en route sectors well away from terminal

airspace, is referred to in thc following discussion as an

"unconstrained" environment. If traffic density is low,

very little ground-based ATC coordination or intervention

is required. At the other extreme, in the en route area

approaching an airport's terminal airspace, heavy traMc

management is often required. Arriving aircraft must

merge lor sequencing while simultaneously adhering to

crossing restrictions and avoiding conflicts with other

aircraft. Referred to herein as "transitional," this traffic

environment is characterized by a mix of unconstrained

aircraft with many highly constrained aircraft transitioning to terminal airspace. Any traffic situation between

these two extremes is possible in a single sector. The

environment in a sector also changes, depending on the

time of day. Adjacent sectors in a Center can have the

same environment, or several sectors may represent a

transition from one extreme to the other. Furthermore, a

normally unconstrained en route sector can actually

behave more like a transitional sector if an adjoining

Center places traffic management (e.g., miles in trail or

metering) restrictions on entering aircraft. The integrated

concept facilitates the inclusion of user preferences in all

these environments.

An overview of the integrated concept is shown in

figure I. The system is supplied with real-time radar

track and flight-plan information from the Center Host

computer or an advanced aircraft tracking system. The

AT is independent of airspace sectorization, supporting

a new "airspace coordinator" position whose objectives

are maximizing user efficiency while reducing sector

controller workload. The ST is sector controller centered,

and facilitates traffic planning and intersector coordination within and between Centers. To minimize changes to

current ATC procedures, all clearances issued to aircraft

arestillissuedbythesectorcontrollers. Clearances based

onadvisoriesfromthetwotoolsortheairspacecoordinator,however, reflectamuchhigherdegreeofuser

preferences integratedintocurrentsectortafficplanning.

Airspace Tool

The AT is designed to facilitate user preferences for en

route flights that are not inhibited by localized, highly

dynamic traffic constraints, generally for aircraft lbund

outside high-density traffic areas and/or not transitioning

to terminal airspace. The goal of the AT is to detect and

resolve conflicts with a time horizon (approximately 20 to

25 minutes) that is longer than the "decision-making time

horizon" used by sector controllers, defined as the time

normally required to realize a desired traffic plan by

issuing clearances to resolve conflicts and achieve traffic

management constraints (if any). Since the AT looks

beyond the controller's time horizon, the potential exists

to resolve predicted conflicts more efficiently than

waiting until the controller would normally detect them.

AT resolutions are initially restricted to aircraft that are

not affected by lraffic management constraints since

accurately predicting trajectories without detailed

knowledge of the sector controller's traffic plan is

difficult. The addition of these constrained aircraft to the

AT would require that the increased benefits outweigh the

negative impact of incorrectly predicting the traffic plan.

Constrained aircraft are handled by the ST as discussed in

the next section.

Cost-effective conflict resolution advisories reflecting

minimum deviations from user-preferred routes are

provided by the AT in the form of horizontal path,

altitude, and/or speed changes. A packet consisting of

the advisory and information from which-the modified

resolution trajectory can be reconstructed are sent to the

ST for display and acceptance or rejection by the sector

controller. If accepted, the advisory clearance is added to

the known trajectory constraints for the affected aircraft

(available to both the AT and ST for future trajectory

predictions). If the advisory is rejected, the AT is notified

so an alternative cost-effective resolution can be

attempted.

The AT cost-effective resolutions are based on the

concept that there is both an optimal clearance to resolve

a predicted future conflict and an optimum time to issue

that clearance (ref. 5). A resolution advisory is made up

of a suggested clearance and a time for its initiation; for a

cost-effective resolution, the time is chosen to achieve a

trade-off between maneuver efficiency and conflict

certainty. The earlier a maneuver is initiated, the more

efficient and easily executed it will be. For example, a

small speed change well in advance of a conflict is more

efficient than an altitude change close to the point of

conflict. However, resolution maneuvers made too far in

advance may often be unnecessary if conflict probabilities

are low. The later a maneuver is initiated, the less efficient it will be, but the more certain it is that the conflict

would have occurred. The optimal time to initiate a

maneuver is determined by minimizing the cost of

maneuvering as a function of time. That function depends

on the conflict probability (based on the trajectory prediction accuracy of both aircraft) and other parameters,

such as the cost of a less-efficient flightpath and the level

of controller workload required to resolve the conflict.

The AT supports a new controller position, called the

"airspace coordinator" (AC), located in the ARTCC.

This person monitors an area of airspace larger than a

single sector, interacting with the AT to enhance its

functionality. In an initial implementation of the concept,

the AT will display a conflict to the AC, who may resolve

the conflict either manually or in conjunction with

AT automatic resolution support. The AC will then

communicate the advisory directly to the appropriate

sector controller for issuance or rejection. If several AC

positions are desired within a Center, coordination among

these positions will be necessary; this scenario will be

included in a future implementation. By supporting a

controller dedicated to identifying and resolving predicted

conflicts beyond the decision-making time horizon of

sector controllers, deviations from user-desired trajectories are expected to be minimized without greatly

increasing the workload of sector controllers. Also,

because the AC analyzes and resolves multisector

conflicts while observing traffic in all affected sectors,

efficient resolutions can be obtained while reducing the

required coordination between sector controllers. The

AC should be able to direct AT resolutions to controllers

with less workload without disturbing controllers in highworkload situations. It is anticipated that many of the

initial manual AC functions will be automated as

understanding of desired solutions to typical problems

increases.

Since the AT is designed to support the evolution of

advanced concepts toward free flight, the AC position is

expected to play an increasingly important role in future

implementations of the concept. For example, the AC

may eventually apply special real-time knowledge of user

preferences (possibly after negotiation with the aircraft) to

coordinate en route flights within Center airspace. This

real-time knowledge will most likely be supplied by

datalink or some alternative form of communication with

the user (e.g., communication with the Airline Operations

Center) that does not involve the sector controller.

Because of the AC's "big picture" perspective of airspace
operations, the AC's responsibilities will eventually

include more of a traffic management role. Specifically,

in future enhancements the AT may measure and predict

en route complexity (within a sector and overall) to

ensure that sector controller workload is maintained

within acceptable bounds and if nol, develop and implement plans to constrain aircraft routing so that traffic

complexity is reduced. The AT, with the AC, represents

an extension of current ATC procedures and, when

implemented, will serve as a platform for evaluation of

advanced free-flight concepts.

Figure 2 illustrates a possible AT interface to display all

en route aircraft within a desired airspace region (e.g.,

greater than a single sector, possibly an entire area or

Center) to the airspace coordinator, who has keyboard

and trackball input devices for interacting with the tool.

The interface, referred to as a dynamic conflict display,

shows predicted conflicts and situational awareness

information as well as pending AT resolution advisories.

The display consists of a plan view representation of

the airspace, showing the position of each aircrafh

including requested data blocks. It is updated in real time

(at intervals of 12 seconds or less) as new information,

such as track updates, becomes available. Each aircraft

with a predicted cont'lict is tagged: further information

about the conflict (e.g., projected time to the conflict and

resolution advisory) can be requested. In addition, the

display provides the AC with a prediction of regions that

will have high conllict density, which may impose a

difficult traffic management workload on the sector controller. Conflict density may be represented as the sum of

probabilities for conflicts predicted to occur in any sector.

Three schemes for display of sector conflict densities are

presented in the figure: the lower right panel shows

ranges of times to conflict for each sector, along with the

worst case; the upper right and left panels show the

degree of sector conflict loading (possibly color-coded)

with time in two different formats. The AC is expected to

use the density information to prioritize the conflicts. The

figure should not be considered a final display interface;

significant efl'c_rt must be devoted to make the interface as

effective and easy to use as possible.

Sector Tool

While the AT is oriented predominantly toward

accommodating the user, the ST must accommodate the

user while also assisting the controllers. Its primary goal

is to support sector controllers in safely and effectively

managing complex traffic situations, such as those that

occur in the transitional environment to terminal airspace,

while facilitating as many user preferences as possible. Its

design is based on the research performed for the descent

advisor (DA) tool of CTAS (refs. 6 and 7). The ST serves

as a situational display and intelligent-advisory aid,

supporting the controller in devising and executing a plan

for managing traffic in all environments, even those that

are highly constrained by traffic management constraints

(e.g., metering or miles in trail). As future traffic density

increases and user preferences become more prevalent,

the ST will be needed to handle higher traffic loads while

enabling user-preferred trajectories to be extended further

into the extended terminal area.

The ST assists controllers by generating accurate,

fuel-efficient clearance advisories for the merging,

sequencing, and separation of high-density traffic while

providing automation assistance for the prediction and

resolution of conllicts between aircraft in all phases of

flight. It assists sector controllers by:

* determining efficient descent trajectories for each

aircraft, from cruise altitudes to the boundary of the

terminal airspace:

• detecting potential conflicts and providing interactive

aids for planning resolutions: and

• providing real-time information to aid controllers in

their overall traffic phmning.

One of the major ST functions is to provide descent

clearance advisories, which arc based on loutdimensional fuel-efficient trajectory predictions lbr

desccnts front cruise altitudes to terminal area fccde,

fixes. Traffic management constraints are tnet while

deviations from user-preferred descent profiles are

minimized. Through the use of rapid update cycle

weather inl'omlation (ref. 8) and aircraft models, the

ST predicts trajectories that are of the same order of

magnitude in accuracy as current airborne systems. The

trajectory solutions are translated into ATC clearance

advisories that include vector headings, cruise speed,

aircraft lop-of-descent (TOD) point, and descent speed

profile (composed of a descent Mach number and/or an

indicated airspeed). All ST advisories are continually

updated to reflect changes in aircraft states and

atmospheric predictions.

The ST uses automatic conflict detection to alert the

controller to potential separation conflicts over the

controller's entire decision-making time horizon. By

using accurate aircraft trajectory predictions that reflect

controller intent, the ST is expected to extend the current

controller's decision-making time horizon, thereby

enabling more efficient planning. The display of potential

conflicts is timed to help the controller manage the

airspace. The time of display may also be based on a

conflict probability threshold (which may be set by the
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controller). In addition to providing more advance time

for traffic planning, automatic detection should enable the

controller to resolve conflicts more efficiently. The

increased confidence provided by automatic conflict

detection may also be useful in reducing the number of

conservative clearances that controllers currently issue to

ensure separation when the5' are unsure of whether a

conflict will occur.

Automatic conflict detection at the ST level will initially

be implemented to display only conflicts between:

• any pair of aircraft "owned" by that sector controller

(independent of whether the conflict resides within

the controller's sector);

• any "unowned" aircraft within a controller-defined

distance (or time) l'rom the sector boundary and any

other such unowned aircraft or any owned aircraft; or

• any owned aircraft within a controller-defined

distance (or time) from its next sector and any

aircraft within that next sector or any aircraft within a

specified distance (or time) from and due to arrive in

that next sector.

To include probing the effects of other aircraft, the

controller can manually identify the desired aircraft to be

probed. Once an unowned aircraft has been identified, it

is probed until the ST determines that it can no longer

impact aircraft owned by the controller. Display of

potential conflicts beyond sector boundaries to all sector

controllers involved will facilitate the solution of

multisector conflicts.

The ST can aid a sector controller to quickly create and

evaluate a provisional ("what-iff') plan through the use

of provisional clearance feedback, which allows the

controller to determine the effect of issuing a clearance

without affecting other controllers or other parts of the

automation system. In addition to provisional clearance

feedback, controllers are expected to use the ST to

provide information for maneuver feedback. For example,

a controller may absorb required delay by turning an

aircraft away from its intended metering fix, while

observing a countdown in remaining delay to be

absorbed. After the required amount of delay has been

absorbed, the controller would turn the aircraft back on a

path to the metering fix.

The ST also helps the controller to monitor aircraft

spacing by displaying separation information at points

along predicted trajectories. In-trail spacing can be shown

at a defined location (e.g., at a Center or terminal airspace

boundary) for selected streams of aircraft (e.g., aircraft

exiting into a specified, adjacent Center or assigned to a

defined meter fix). Alternatively, minimum separation

locations can be shown for two or more aircraft to identify dynamic "choke points" for aircraft not on standard

routes. The display includes options for showing all

minimum separations, or just separations that are less

than a specified value. The spacing function is expected

to support the merging of traffic without requiring the

traffic to conform to specific routes, altitudes, or speeds, a

procedure often done to simplify predicting aircraft interactions. Spacing, provisional clearance feedback, and

conflict detection are expected to greatly enhance the

provisional planning capability of the sector controller.

To a sector controller, AT resolutions represent either

solutions to problems beyond their normal planning

horizon or very efficient solutions for conflicts between

nontransitioning, unconstrained aircraft. When AT

resolution advisories are received, the ST automatically

checks the affected sector controllers to see if they are

configured to accept AT resolutions. Sector controllers

can configure the ST to automatically reject AT

resolutions to their sector if they deem their traffic load

too heavy m analyze resolutions beyond their current

decision-making time horizons. If any controller is not

accepting AT resolutions, a rejection is automatically

sent to the AT without any notification to the sector

controllers. If all affected sector controllers are accepting

AT resolutions, then the ST checks the advisories lk_r

possible conflicts with all en route aircraft, using its

controller-intent (i.e., provisional planning) inlk)rmation.

It should he recalled that the AT is primarily concerned

with unconstrained aircraft and that it has a limited

knowledge of the often rapidly changing sector controller

environment, so AT resolutions are not guaranteed, at this

stage, to be conflict free. If a conflict is detected, the ST

checks to see if the affected sector controllers are

configured to accept AT resolutions that have potential

conflicts. Again, sector controllers can configure the ST

to automatically reject AT resolutions that are not conflict

free if they deem their traffic load to be too heavy to

analyze this information. If all affected controllers are

configured to accept the AT resolution, the ST displays

the AT conflict information, any conflict information

generated at the ST level, and the suggested resolution

to the appropriate sector controller(s). If the resolution

can be worked into the sector controller's plan, the

controller accepts the AT advisory and issues the

clearance(s) to the aircraft; otherwise, the AT is notified

of the resolution being rejected.

Future objectives for the ST will continue to locus on

sector controller traffic planning. The addition of

automatic or semi-automatic conflict resolutions will

further reduce controller workload and facilitate intersector coordination. Automatic resolution techniques
designedlbrtheCTASDA(ref.9)aredirectlyapplicable

toSTtrafficproblems. Integrationwithimproved

schedulingautomationwillallowtheSTtoautomatically

taketime-basedtrafficmanagement constraintsinto

account. Finally,exchangeofinformationbetweenthe

ST and airborne flight management systems (FMSs) has

the potential to greatly reduce frequency traffic and

clearance adherence errors (ref. 7).

As in the current plan-view display (PVD), the ST

displays to each sector controller all the aircraft tracks

visible within a selected area. Superimposed on this

display is any appropriate ST and/or AT advisory

information being considered by that controller. The

controller always has the option of removing all AT

and/or ST advisories from the display. The ST display

interface is based on the interface used in the DA; it

provides keyboard-trackball input, and is updated in

real time (typically every 12 seconds or less) as new

intbrmation (e.g., track updates and controller inputs)

becomes available.

All display features will ultimately be integrated into an

advanced display (such as the display system replacement, or DSR), but some features may appear in early

development phases on an auxiliary display interface,

as shown in figure 3, where an example of controller

interaction with the ST is shown. In the figure, flight

UAL001 must be delayed to meet a desired crossing time

(13:01) at the TOMSN metering fix while avoiding a

conflict with overtlight UAL0(12. Through interacting

with the ST's provisional planning tools, the controller

has determined that a horizontal path stretch with

turnback directly to TOMSN is a workable solution. In

this case, the controller issues the path stretch clearances

to UAL001 at the turnout and turnback points, followed

by a descent clearance approximately 30 n. mi. prior to

the TOD point. The AT-detected conflict between

UALI00 and UAL200 is also shown with supporting

information. The AT resolution advisory of a 20-knot

indicated air speed (KIAS) reduction is shown on the

fourth line of the data tag for UAL200. (Note that the

Iburth line is used for illustrative purposes only.) The AT

and ST advisories will be dcsigned so the controller can

easily distinguish between them, possibly through colorcoding or blinking. The figure should not be considered a

final display interface; significant effort will be devoted

to make the interface as effective and easy to use as

possible.

Integrated AT/ST Tools

The needs of any en route traffic environment can be met

through integration of the AT and ST. In a completely

unconstrained en route environment, a high percentage of

AT resolution advisories are expected to be accepted by

the sector controllers and issued as clearances to the

aircraft, thereby facilitating user preferences. Traffic

management planning in this environment is generally

low, so the controller is able to treat the AT resolutions

like current user requests. In a transitional enviromnent

with numerous aircraft arriving to the terminal airspace,

the sector controller is concerned primarily with devising

and executing a plan for managing traffic flow. Therefore,

the cost-effective resolutions provided by the AT will

likely conflict with the controller's traffic plan and will

often be replaced by the provisional planning solutions of

the ST. For a highly congested, unconstrained, en route

environment with many randomly scattered conflicts, the

AT and ST each contributes to the traffic management

solution: the AT provides a dynamic display of highconflict areas to aid in airspace coordination, and the ST

aids the controller in executing a traffic management

plan. Since most traffic environments are a mixture of

constrained and unconstrained aircraft, it is expected that,

in general, both the AT and the ST will contribute to

facilitating user preferences in all en route sectors.

Table I summarizes the conflict detection and resolution

capability ditfcrences between the two tools for an initial

implementation. The AT considers unconstrained.

nontransitioning, en route aircraft only since it is not

integrated with the traffic planning provisional solutions

necessary to handle flights lransitioning to the terminal

area. For transitioning aircraft, required traffic manageunent constraints are achieved while facilitating as many

user preferences as possible; the ST handles this environment. The AT resolutions are dcsigned to handle conflicts

beyond the controller decision-making time horizon,

assuming no changes in either the aircraft's current path

or its altitude. By definition, aircraft transitioning to/from

the terminal area require large speed and altitude changes

within the controller's decision-making time horizon.

Departing aircraft can be handled by the AT outside of

terminal airspace if the climb segment is not constrained

by traffic management restrictions. (Note: Satellite

departures are considered transitioning aircraft if they are

transitioning to a nearby airport.) Because the AT is

intended to provide coordination over all en route

airspace, its conflict detection responsibilities include the

entire Center. For each sector, the ST is responsible for

conflict detection over each sector controller's planning

horizon, which typically includes the controller's sector

and neighboring sectors.
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Table I. Initial AT/ST conflict probing characteristics

Airspace tool Sector tool

Aircraft probed Unconstrained en route traffic only All en route traffic

Trajectory constraints None Traffic management

Detection responsibility Entire Center airspace For each sector, all aircraft within sector

and individual aircraft in neighboring

sectors

Conflict displayed to AC a for resolution SC(s)b

Conflict resolution AC resolves conflicts with AT and sends to ST SC manual resolution (aided by ST

(AC can negotiate resolutions with sector through provisional planning aids)

controllers)

SC issues clearances

Resolution type Cost-effective trajectory with provisional

planning aids

ST provisional planning aids

aAC = Airspace Coordinator

bSC = Sector Controller

In order for the cost-effective AT resolutions to have an

impact in an environment in which the AT and ST are

working together to facilitate user preferences for a

mixture of both unconstrained and constrained aircraft,

the resolutions must be available to the sector controller

before the conflict is within the controller's decisionmaking time horizon. Work based on current prediction

accuracy suggests that these cost-effective resolutions

would generate advisories approximately 10 to

14 minutes in advance of the conflict (ref. 5), a time

that is expected to be within the prediction horizon

needed for the ST. However, reference 5 indicates that

increasing trajectory prediction accuracy increases the

advance time of the minimum-cost point. One prediction

error source is the existing FAA radar tracking algorithm

of the Center Host computer. Using an advanced radar

tracking system (ref. 10) is expected to improve prediction accuracy, thereby enabling minimum-cost resolutions

20 or more minutes in advance. The system is now in

place for testing at the Denver ARTCC, and its accuracy

is being verified through analysis of flight-test data.

Conflict Resolution Scenarios

Since much of the concept relies on providing increased

flexibility (e.g., routing) for users, considerable attention

has been given to determining how the integrated tools

will help resolve conflicts. In this section, the anticipated

operation of the tools is described in some detail tot

several conflict sccnarios. The expected benefits of the

integrated tools in each traffic environment arc also

discussed. Four typical separation conflict scenarios are

identified in figure 4. Consider the numbered areas to be

sectors within a Center. In the figure, the sectors are

shown to be horizontally adjacent, but the scenarios also

hold for vertically adjacent sectors. They even hold for

adjacent sectors in different Centers, although the

coordination between facilities will be more complicated.

For convenicncc in the discussions that follow, all the

conflicts are shown to occur in sector 2.

Figure 4(a) shows an example of an "intrasector" conflict,

the situation where both aircraft and the predicted point

of conflict (i.e., initial loss of minimum separation

requiremcnts) are within a single sector. This scenario

should yield the most straightforward resolution since

only one controllcr is involved. A somewhat more

complicated scenario, an "external" conflict, is shown in

figure 4(b): both aircraft are in one sector, and the point

of conflict is in anothcr. Figure 4(c) shows an "external

intruder" scenario: one aircraft and the predicted conflict

point are in one sector, and the other aircraft is in another

sector. Figure 4(d) shows an "intersector" conflict, where

the two aircraft are in different sectors and the predicted

conflict point is in a third sector. The latter two scenarios

generally require the greatest amount of coordination

between controllers.

l0
i

i ',,i i ',,i

L ,, _ L...... ___J

(a) "Intrascctor" conflict. (b) "External" conflict.

% I I

I I _ I

#

I \ I I \ I

I % I I \ I

L _ i L.... --___J

(c) "External Intruder" conflict. (d) "Intersector" conflict.

Figure 4. Typical conflict scenarios in a multisoctor environment.

11
Three of these scenarios (b, c, d) may involve the

coordination of two or more sector controllers. The ST

facilitates efficient intersector coordination by identifying

which controllers need to be made aware of conflicts and

then displaying conflict information accordingly. In some

cases where aircraft owned by different controllers are

involved, the ST may notify both affected controllers or

only the controller that would be expected to resolve the

conflict. By displaying the conflict to the controller who

can most efficiently resolve a conflict before displaying to

other affected controllers, eMcient resolutions can be

facilitated. In other cases, the ST may notify a controller

to contact another sector to negotiate a solution. It may

also be possible to use the ST automation and display aids

to facilitate efficient negotiation between sectors. In this

case, controllers will have the capability to share their

provisional planning information during negotiation,

greatly simplifying the coordination process currently

done by phone.

The ST uses "controller awareness" boundaries to define

the horizon for displaying conflict detection results within

adjacent sectors. Each adjacent sector pair (e.g, sectors 1

and 2) is represented by four awareness boundaries, two

lor traffic l'lowing in each direction. Figure 5 illustrates

the two boundaries for traffic flowing from sector 1

(upstream sector) to sector 2 (downstream sector):

• Upstream controller's awareness I?oundat3':

a specified distance or time from the downstream

sector within which the upstream controller becomes

aware of conflicts between upstream aircraft within

the horizon and aircraft in the downstream sector

• Downstream controller's awareness boundat3':

a specified distance or time from the downstream

sector within which the downstream controller

becomes aware of conflicts between upstream aircraft

within the horizon and other upstream aircraft within

the horizon or aircraft in the downstream sector

The distance or time that defines a boundary may be

defined as a function of aircraft type (e.g., jet versus

turboprop or unconstrained versus transitioning). Recall

that ST-detected conflicts between aircraft in the same

sector are displayed to the controller who owns the

aircraft as long as the conflict is within the controller's

decision-making time horizon, independent of the sector

in which the conflict resides, so no awareness boundary is

required tbr this case. Downstream controller awareness

boundaries are used for display of both ST and AT

conflict intbrmation. The downstream controller may

configure display criteria lbr AT intormation separately

from ST information. Thc upstream controllcr boundary

is necessary only for ST conflict information since AT

resolutions are assumed to be for conflicts beyond the

upstream controller's decision-making time horizon.

When an aircraft crosses a controller's awareness

boundary, the ST displays infbrmation to that controller

about conflicts between the aircraft and any other aircraft

in the downstream sector or any aircraft that has also

crossed that awareness boundary. In figure 5, aircraft A

is predicted to have a conflict in the downstream sector.

When aircraft A crosses the awareness boundary for the

upstream controller (SC1), the ST displays the conflict

information to SC 1; when the aircraft crosses the awareness boundary for the downstream controller (SC2), the

conflict information is displayed to SC2. The positions of

the boundaries are configurable (and interchangeable);

they are expected to bc set by mutual agreement between

coordinating controllers. By interchanging the order that

an aircraft crosses the boundaries, the adjacent sector

controllers can define, on a sector-by-sector basis, which

controller (upstream or downstream) will be aware of the

conflict first (i.e., who will have the first attempt at

resolving the conflict). It is unclear at this time whether

two or just one combined boundary will be necessary in

all or some environments. During development, human

factors research will be performed to determine the

number and best sequence of awareness boundaries to

most effectively resolve multiple controller resolutions.

The use of awareness boundaries is more fully described

in the scenarios that follow; one combined awareness

boundary is assumed in all references to intersector

coordination unless otherwise noted.

In the following analysis, operation of the integrated

tools is described for each conflict scenario in two traffic

environments: a completely unconstrained en route

environment and a transitional environment with many

aircraft transitioning to a terminal airspace. An overview

of the expected integrated system behavior is presented

in table 2. As previously discussed, in a completely

unconstrained en route environment, the AT is expected

to detect most conflicts and provide cost-effective

resolutions. In a transition to terminal airspace environment, the ST is expected to be the primary tool, and AT

resolutions will be used only if they do not adversely

impact the controller's traffic plan. Although these factors

might affect the display options a controller would select

or how a controller might respond to an AT advisory, the

logic for each scenario prcsented in figures 6(a) through

6(d) and discussed in the following sections should be

considered independent of the trafl'ic environment.

12
Upstream Controller's Awareness Boundary

Downstream conflict will be displayed to

upstream controller

.-.-.....-----"- / . _ Downstream

_ .-------"-" 7 , / _ sector

I'gl Upstream t / I I

\ Ill sector t f ! f"_

\\ _I.• i f I 1

\ A , l

/

\ / I

\ 1 I I 1

Downstream Controller's Awareness Boundary

Downstream conflict will be displayed to

downstream controller

Figure 5. Intersector coordination boundaries.

13
Table 2. Expected conflict resolution behavior of the AT/ST system

Conflict type Unconstrained environment

AT resolutions predominant; ideally AT resolves

before SC "sees" conflict

Transitional environment

ST resolutions predominant since many

aircraft are transitioning (AT resolutions are

likely to conflict with traffic plan)

lntrasector SC accepts AT resolution when conflict identified

earlier by AC; cost-effective solution desirable

AT resolutions used only if they do not

interfere or conflict with arrival plan

External Aircraft owner resolves conflict

Option: alert conflict owner to negotiate resolution

Display of conflict to conflict owner facilitates

early handoffs

Aircraft owner "sees" conflict and resolves it,

even if not probing other aircraft within

conflict sector

Display of conflict to conflict owner

facilitates early handoffs

External intruder Either SC (or both) resolves conflict, selected by

AC/AT based on situation

If one SC rejects AT resolution, AC/AT can try

other SC

Conflict detected by ST when intruder is

within probing range of conflict sector

Either SC (or both) resolves conflict

Intersector Either SC (or both) resolves conflict, selected by

AC/AT based on situation

If one SC rejects AT resolution, AC/AT can try

other SC

Option: alert conflict owner to negotiate resolution

Display of conflict to conflict owner facilitates

early handoffs

Conflict displayed by ST when both aircraft

are within awareness boundaries of conflict

sector

Either SC (or both) resolves conflict

Display of conflict to conflict owner

facilitates early handoffs

14
...... t

/

i_ _ /

f \ f

f \ I

i \ f

Intrasector Conflict:

Aircraft A owned by SC2

Aircraft B owned by SC2

Conflict in sector 2

SC Sector Controller

ST Sector Tool

AC Airspace Coordinator

AT Airspace Tool

When AT detects a conflict:

AT displays conflict to AC;

AC/AT resolves one/both trajectories;

AT sends conflict/resolution to ST.

If SC2 has configured ST to accept:

ST probes resolution for conflicts,

displays probe results and the resolution

to SC2.

If SC2 accepts resolution:

SC2 issues clearance(s)

or:

Persistent conflict will be handled by

ST.

SC2/ST notifies AT/AC of decision.

When ST detects a conflict:

ST displays conflict to SC2;

SC2/ST resolves one/both trajectories;

SC2 issues clearance(s).

(a) "Intrasector" scenario.

Figure 6. Conflict resolution.

15
i ili External Conflict:

Aircraft A owned by SC4

Aircraft B owned by SC4

Conflict in sector 2

SC Sector Controller

ST Sector Tool

AC Airspace Coordinator

AT Airspace Tool

When AT detects a conflict:

AT displays conflict to AC;

AC/AT resolves one/both trajectories;

AT sends conflict/resolution to ST.

If SC4 has configured ST to accept:

ST probes resolution for conflicts,

displays probe results and the resolution to

SC4;

ST may display conflict and message to

SC2.

If SC4 accepts resolution:

SC4 issues clearance(s)

or:

Persistent conflict will be handled by ST.

SC4/ST notifies AT/AC of decision.

When ST detects a conflict:

ST displays conflict to SC4;

ST may display conflict and message to SC2.

If SC2 obtains handoffs of both aircraft:

SC2/ST resolves one/both trajectories;

SC2 issues clearance(s)

or:

SC4/ST resolves one/both trajectories;

SC4 issues clearance(s).

(b) "External" scenario.

Figure 6. Continued.

16
// /t

@ //El 1

\ / /d" 1

/ \ boundary/

External Intruder Conflict:

Aircraft A owned by SC2

Aircraft B owned by SC4

Conflict in sector 2

SC Sector Controller

ST Sector Tool

AC Airspace Coordinator

AT Airspace Tool

When AT detects a conflict:

AT displays conflict to AC;

AC/AT resolves one trajectory (aircraft A);

AT sends conflict/resolution to ST.

If SC2 has configured ST to accept:

ST probes resolution for conflicts,

displays probe results and the resolution to

SC2;

If SC4 has configured ST to accept:

ST displays conflict and message to SC4.

If SC2 accepts resolution:

SC2 issues clearance to aircraft A.

SC2/ST notifies AT/AC of decision.

If resolution for aircraft A is rejected:

AC/AT resolves trajectory for aircraft B

or persistent conflict is handled by ST.

When ST detects a conflict:

ST displays conflict to both SC2 and SC4

when within awareness boundary;

SC2/ST resolves trajectory for A, and/or

negotiates with SC4 to resolve B

(SC2 may request early handoff from SC4);

SC2 and/or SC4 issue clearance(s).

(c) "External Intruder" scenario.

Figure 6. Continued.

17
//1 1//D

U_ Awareness' ! 1

\\ boun_. Iit . /

\ A , t

....2-t.-" f

f \--- \ I

l , Awareness t

\' boundary /

Intersector Conflict:

Aircraft A owned by SC 1

Aircraft B owned by SC4

Conflict in sector 2

SC Sector Controller

ST Sector Tool

AC Airspace Coordinator

AT Airspace Tool

When AT detects a conflict:

AT displays conflict to AC;

AC/AT resolves one trajectory (aircraft A);

AT sends conflict/resolution to ST;

If SC1 has configured ST to accept:

ST probes resolution for conflicts,

displays probe results and the resolution

to SC ! ;

If SC4 has configured ST to accept:

ST displays conflict and message to

SC4;

If SC2 has configured ST to see conflicts:

ST displays conflict and message to

SC2.

If SC 1 accepts resolution:

SCI issues clearance to aircraft A.

SC1/ST notifies AT/AC of decision.

If resolution for aircraft A is rejected:

AC/AT resolves trajectory for aircraft B

or persistent conflict is handled by ST.

When ST detects a conflict:

ST displays conflict to SC1 and SC4

when within awareness boundaries;

may display conflict and message to SC2.

If SC2 obtains handoffs of both aircraft:

SC2/ST resolves one/both trajectories;

SC2 issues clearance(s).

or:

SC 1 and SC4 negotiate a resolution;

SCI and/or SC4 issue clearance(s).

(d) "Intersector" scenario.

Figure 6. Concluded.

18
Intrasector Conflict

If the AT detects a conflict between two aircraft

(fig. 6(a)), a cost-effective resolution for one or both

trajectories is determined and sent to the ST. If the

sector 2 controller (SC2) is configured to be notified of

AT advisories, the ST checks the trajectories associated

with this resolution against all controller-intent trajectory

predictions (including all provisional plans). If the AT

resolution is in conflict with any of these trajectories, and

SC2 is configured to not allow conflicting AT resolutions,

the ST notifies the AT that the advisory cannot be used. If

the AT resolution is not in conflict or SC2 is configured

to allow conflicting AT solutions to be displayed, the ST

displays both ST conflict probe results (if any) and thc

AT conflict and resolution information to the controller,

who then has the option to issue a clearance or reject the

advisory. If the ST detects conflicts with the AT resolution, SC2 will have to develop additional clearances

through provisional planning to resolve these conflicts.

Finally, SC2 notifies the AT of acceptance or rejection

through the ST intcrfacc. If the advisory is rejected, the

AT updates a resolution constraints list and if feasible,

AC/AT generates a new resolution for the conflict. If

accepted, the advisories are anticipated to reduce overall

controller workload by solving conflicts well in advance

of potential conflicts while providing cost-effective

resolutions based on user preferences.

When the ST detects a conflict within the decisionmaking time horizon of the controller, it is displayed to

SC2. The controller then resolves the conflict using the

provisional planning aids provided by the ST. When a

satisfactory resolution has been obtained, SC2 issues a

clearance to the aircraft. Of course, if one or both of the

conflicting aircraft are transitioning, the AT will not have

detected the conflict. If both are unconstrained, however,

situations may occur where the ST displays a conflict

when the AT does not. A possible scenario occurs if the

AT resolution advisory has already been rejected or it

will not be cost effective until the aircraft is within the

controller's decision-making time horizon. If the controller has previously rejected the AT advisory, then the

ST resolution is utilized. If an AT resolution is still

possible, it is at the discretion of the controller to decide

if waiting lora cost-effective AT resolution is appropriate

under the current situation. It is expected that a controller

would wait for an AT resolution only if traffic conditions

were relatively light. Human factors research will be

necessary to determine the best display options if both

AT and ST resolutions arc available.

External Conflict

If both aircraft are unconstrained, en route aircraft

(fig. 6(b)), the AT detects a conflict and provides a

resolution to the ST, as in the intrasector scenario. If the

resolution passes the SC4/ST display-configuration logic

(sec intrasector scenario case), the conflict information

and resolution are displayed to SC4. If both aircraft are

within the awareness boundary lor SC2 (downstream

controller's awareness boundary), the ST displays the

conflict to SC2 along with a message that a resolution is

pending in sector 4. In this way, SC2 is made aware of

upstream decision making that may be of interest; voice

communication between the two controllers is not

required. SC4 then has the option to either issue a

clearance or reject the advisory. As in the previous

scenario, the AT is notified of acceptance or rejection of

the advisory, and if rejected, the AC/AT may attempt

another solution.

If the ST detects a conflict, it displays the conflict

inlbrmation to SC4, even though the conflict occurs

outside sector 4. Recall that the conflict is displayed to

SC4 even if one or both aircraft are outside SC4's

awareness boundary since the upstream controller's

awareness boundary is for displaying conflicts with

aircraft in scctor 2 (conflicts between aircraft owned by

the same controller are always detected by the ST,

independent of the sector in which the conflict exists).

SC4 thcn uses the provisional planning capability of the

ST to resolve the conflict and issue a clearance to the

aircraft. The ST also displays the conflict to SC2 if both

aircraft are within SC2's awareness boundary (i.e., the

downstream controller's awareness boundary). The

display of conflict intbrmation to the downstream sector

allows for SC2 to request an early handoff if desired. If

SC2 does not request and obtain handoffs for aircraft A

and B, the resolution is pcrlbrmed by SC4 as described.

In general, for conflict resolution involving more than one

controller, there is a potential to lower workload through

the SC/ST display logic discussed previously. For typical

choke-point sectors (such as a low-altitude sector containing a feeder fix), a benefit in providing an upstream

controller with an opportunity to resolve the conflict is

also possible, thereby redistributing the overall workload.

In addition, if an upstream solution is not desired, SC2

has the option to resolve the conflict by requesting early

handoffs from SC4. These workload benefits are expected

to apply to many traffic situations, so the controllers will

have a strong incentive to take full advantage of the

integrated tools.

19
External Intruder Conflict

For the external intruder conflict case (fig. 6(c)), assume

that the AT has chosen to modify the trajectory of aircraft

A in order to resolve the conflict predicted to occur in

sector 2. When the ST receives the conflict resolution

packet that contains the advisory for aircraft A, it checks

the display logic for SC2. If SC2 is configured to accept

AT resolutions, the ST probes for conflicts. Finding none

(or if SC2 is accepting AT resolutions with ST conflicts),

the ST displays the conflict information and resolution to

SC2. If SC4 has also been configured to accept display of

AT advisories, the conflict information is displayed with

a message that a resolution is pending in sector 2. This

message is important to alert SC4 that any clearance

issued to aircraft B at this time may cause the AT

resolution to be invalidated and that sector 2 should be

notified if verbal negotiation is desired. SC4 will also

be aware that, if SC2 rejects the advisory, the AT may

resolve the conflict by modifying the trajectory for

aircraft B. Again, all situational awareness is accomplished without need for voice communication between

controllers.

If the conflict has not already been resolved by the AT,

the ST detects the conflict and displays the conflict

information to SC2 and SC4 after aircraft B crosses the

awareness boundary and when the conflict is within the

decision-time horizon for each controller. The resolution

may be performed by SC2 for aircraft A, by SC4 for

aircraft B, or for both aircraft in a negotiated solution. In

early implementations, the decision of who will resolve

the conflict is made verbally (or procedurally) between

the two affected controllers. ST provisional planning aids

are expected to be used by the controllers for resolution.

Display of conflict information to both controllers may

also facilitate an early handoff of aircraft B to SC2 if

desired.

Early implementations of the integrated tools may require

the AC to perform the role of selecting which aircraft

receives the AT advisory. In this example, the AC may

know that the traffic situation in sector 2 makes aircraft B

more appropriate than aircraft A for receiving a resolution

advisory. By placing the integrated tools in operation with

functions such as this performed manually, data can be

generated that will serve as a basis for a heuristics-based

set of aircraft selection criteria, which could later be

automated. In addition, the external intruder scenario

would benefit greatly from automatic conflict resolution

in the ST, which will also be developed for an advanced

implementation. ST automatic resolution logic would

parallel the logic of the AT; it would remove much of the

need lor negotiation between controllers for scenarios that

require resolutions based upon controller intent.

Intersector Conflict

Again assume that aircraft A is chosen by the AT to

resolve the conflict. When the ST receives the AT

advisory for aircraft A (fig. 6(d)), it checks the display

logic for SCI, and ifSCl is configured to accept, the ST

displays the resolution advisory to SC 1. If the resolution

is acceptable, SC1 then issues the clearance. If SC4 is

configured to display AT advisories, the conflict information is displayed with a message that a resolution that

involves aircraft B is pending in sector I. Again, SC4

knows that any clearance issued to aircraft B at this time

may invalidate the AT advisory. In addition, SC2 may be

informed about the conflict situation by display of the

conflict and a message that a resolution involving aircraft

A and B is pending in sector 1. SC2 is informed of the

conflict if both aircraft are within their respective awareness boundaries, and SC2 has all information needed to

know the AT resolution plan. If the AT advisory interfetes with SC2 traffic planning, SC2 can ask SC 1 to reject

the AT advisory and/or negotiate with both controllers for

early handoffs.

Assume that the ST detects a conflict between aircraft A

and B. SCI and SC4 have the conflict displayed when

both aircraft have crossed the awareness boundaries

with respect to sector 2 and are within the controllers"

decision-making time horizon. The two controllers then

use the ST provisional planning aids and work together

to negotiate a solution. SC2 also sees the conflict and

aircraft information if SC2's display is configured to

show all potential conflicts within sector 2. Again, if SC2

wishes to resolve the conflict, SC2 may ask for early

handoffs of both aircraft.

Because the interscctor scenario occurs when aircraft in

separate sectors have a conflict predicted at a point

outside either sector, it is the most complicated situation

for sector controller coordination; therefore, it offers the

greatest potential for increased efficiency and reduced

workload. As in the external intruder scenario, large

benefits are expected by using the AT to coordinate

conflict resolutions that involve two or more controllers.

The manual negotiations currently required for solving

multisector conflicts can be reduced significantly, so

controllers should have an incentive to use the tool. Early

implementation of the integrated tools may require the

AC to perform the coordination role tbr AT advisories.

As in the previous scenario, ST automatic conflict

resolution (to be implemented in an advanced development) is expected to further reduce the need for verbal

controller coordination.

20
Development Strategy

A phased development approach is proposed that focuses

on obtaining benefits as early as possible, validating the

concept under real-world conditions, and using operational experience to expand tool capabilities. Table 3

summarizes a three-phase development strategy. Phase 1

concentrates on demonstration of the core capabilities

of the individual AT and ST tools and develops the

functionality necessary to perlbrm concept validation.

Phase 2 provides an initial integrated tools capability,

with many of the tool functions performed manually.

Both simulation experiments and field testing are an

integral part of Phases 1 and 2 development. Phase 3

provides the fully developed integrated system described

in this document, including its use as a research platlbrm

for advanced concepts. Simulations and field evaluations

will be used during this phase to automate many of the

manual functions developed in earlier phases. With

adequate staffing, the development could be completed

in about four years.

Table 3. Development strategy

Phase Capability Function

AT automatic conflict detection advisories AT

AC manual notification of potential conflicts to

area through voice communication

SC/ST conflict detection and provisional ST

planning through an auxiliary display

1

Concept feasibility

demonstration

Automatic conflict detection

Dynamic conflict display

Auxiliary display

Descent advisory aids

Manual and limited automatic conflict

detection

Spacing advisory aids

Provisional planning aids

2

Initial operating

capability

AT automatic conflict detection advisories

AT cost-effective resolution advisories displayed AT

to AC

AC/AT provisional planning

AT advisories passed to ST (approved by AC) ST

SC/ST automatic conflict detection, provisional

planning, and spacing aids display to an auxiliary

controller through a fully developed interface

All Phase 1 functions

Provisional planning aids

Cost-effective resolutions

Mature display interface

Full automatic conflict detection

3

Full operating

capability

AT conflict detection and resolution advisories

AT advisories passed to ST (monitored by AC) AT

SC/ST automatic conflict detection, provisional

planning, and spacing aids display at sector via

an advanced display interface

Fully developed logic for ST probing of AT ST

resolutions and display to sector controller

Fully developed logic for AT response to

resolution rejection

All Phase 2 functions

Cost-effective resolutions enhanced to

include AC response to AT resolution

rejection

AT resolutions probing logic

Display configuration logic

Deployable display interface (such as

DSR)

21
Phase 1

In the first phase, the AT and the ST will be developed

and evaluated as independent decision support aids. All

integration between the AT and the ST is performed

manually. A strong emphasis will be placed on development and evaluation of the core capabilities of each tool,

and on validation of the fundamental concepts. In

addition to laboratory development and evaluation, the

tools will undergo operational evaluation in a limited

area of en route airspace, involving a few representative

sectors (representing both traffic environment extremes)

that are chosen based on development and evaluation

goals. All display interfaces will be developed only to a

level that permits concept evaluation and human-factorsrelated research.

The AT will provide automated detection of potential

conflicts and the probabilities associated with these

predictions for en route aircraft in the Center airspace.

The dynamic conflict display will be used to provide this

information to the AC, who may then notify the appropriate area supervisor of projected areas of high congestion through voice communication. The sector controllers

will use the ST to probe for predicted spacing and

conflicts between specified aircraft (manual or limited

automatic conflict detection), resolve predicted conflicts

through provisional planning, and support aircraft in

making efficient descents. An advanced radar tracker

will be used to make accurate conflict predictions up to

20 minutes in advance.

Research for Phase I will concentrate on validating

concept feasibility. It will be designed to answer fundamental questions regarding benefits to controllers and

users. The research will also focus on defining needed

operational procedures (e.g., intersector coordination) and

the key elements of a mature display interface through

controller evaluations. Some of the concept feasibility

issues to be explored are:

• the effectiveness of the dynamic conllict display in

assisting the AC in managing the airspace;

• the extent of the assistance the ST provides to the

controller in devising and executing traffic plans,

especially for managing transitioning aircraft;

the appropriate sector controller decision-making

time horizon for aircraft trajectories with differing

traffic management constraints;

whether the controller considers the benefits received

from the ST to outweigh the additional workload

required to interface with the tool;

the expected time horizon tor AT cost-effective

resolutions based on advanced radar trackcr data, and

the timeliness of these resolutions in accommodating

user preferences and not infringing upon controller

intentions;

the sensitivity of AT cost-effective resolutions to

sector controller issuance timing and its effect on

resolution effectiveness; and

the appropriate probability threshold for display of an

ST-detected conflict to a controller.

Phase 2

In the second phase, most of the capabilities of the fully

developed integrated tools will be achieved by allowing

some of the tasks to be performed manually. The AT will

provide cost-effective resolutions to the AC, who will

then use experience-based judgment to determine whether

to accept the solutions or modify them using provisional

planning techniques. The AC will then request the AT to

send the resolutions to the ST for display to the controller

through a fully developed display interface. The controller will provide an input to the ST to notify the AC

(through the AT display interlace) whether the advisory is

accepted or rejected. The ST will provide full automatic

conflict detection in addition to the tools provided in

Phase I. Automatic detection should allow the controller

to devote more attention to other tasks required in this

phase, such as acceptance or rejection of AT advisories.

Extensive human factors development is expected during

this phase. A limited operational deployment could

possibly be achieved after Phase 2; if so, the deployment

is expected to be limited to a set of sectors chosen on the

basis of benefits and cost.

Phases 1 and 2 will require the use of an auxiliary display

and an additional controller to be located at each sector

position. To maintain all current radar controller operations, the additional controller will interact with the ST

and then will transfer advisory clearances to the radar

position (R-side). The display should provide a plan-view

graphical interface and a keyboard lor input. In addition

to the proposed Phase 2 functions, this workstation should

have all capabilities currently used to perform sector

controller duties, such as accepting handoffs, displaying

trend vectors, and providing tools for aiding separation

maintenance.

Sector-certified radar controllers will probably be

required to interface with the auxiliary display. The

handoff position could be responsible for monitoring the

additional display for sectors with heavy traffic, and the

interphone or flight-data (D-side) position could assume

this responsibility for sectors with light traffic. When the

ST is fully developed and approved for direct use by both

the R- and D-side sector controllers, its functions will be

22
integrated into the sector controller display and the

additional controller will no longer be needed.

Research for Phase 2 will include evaluating the utility of

AT cost-effective resolutions, evaluating various types of

resolutions (e _, single- or multiaircraft), developing

automation algorithms for selecting the best resolution

alternative, designing manual switching between AT

resolution alternatives, and evaluating appropriate levels

of advisories between AC/AT and the sector controller.

The mature ST display interface will also be used to

identify and resolve tin)' remaining sector controller

workload issues. ST filtering of AT conflict resolutions

R_r display to the controller (display configuration logic)

will he refined in Phase 2, based on the evaluations and

preferences of sector controllers. Some of the

development tasks are as follows:

• Develop an automated procedure for selecting an

appropriate AT conflict resolution from a set of

alternatives, based on awareness of the traffic

situation at the sectors involved. For example,

selection between two aircraft could be based on the

estimated workload (based on factors such as conflict

density) of each owning controller.

• Determine appropriate AT followup procedures R_r

rejection of conl]ict resolutions, based on awareness

of the traffic situation at the sectors involved. For

example, ira controller rejects an advisory, s(he) may

not want AT to attempt another resolution for that

aircraft.

• Determine whether duplicating the necessary, AC

awareness of a sector traffic situation in the AT

automation system requires controller (AC and/or

SC) inputs that have a negative impact on tool utility.

• Determine, based on controller evaluations, whether

the ST display information can be added to radar

controller displays, or if it must continue to be

displayed separately.

• Determine the effectiveness of an auxiliary display,

strip replacement, and/or integrated sector controller

display/interlace.

Phase 3

The full operating capability of the AT/ST concept will

be attained in Phase 3. Development will focus on implementation of functionality for deployment and for a

research platfoml for advanced functionality. After

Phase 3, the tools will be available ['or deployment over

the entire Center airspace and in all sectors. An advanced

display system (such as the DSR) will be used to display

all AT/ST advisories directly to the sector controller. It is

expected that most controllers will have incentive to use

the tools, although some in light traffic areas may not

require them.

In Phase 3, the AT conllict resolution functions should be

fully automated, so the AC will no longer be needed to

direct the cost-effective conflict resolutions. The AC

position will be freed to handle high-level planning tasks

in addition to monitoring AT operation. The AC may be

able to evaluate Center weather and traffic conditions and

make recommendations to users for efficient routing. This

scenario may require nonintrusive communication with

the aircraft, such as datalink. The communication will

also allow the AC to have real-time knowledge of user

preferences that can be applied to manage the airspace.

Future enhancements may also allow the AC to coordinate user preferences across Center boundaries or to

facilitate dynamic resectorization based on actual traffic

patterns. The fully operational tool will enable study of

these advanced concepts, as well as the evaluation of new

airspace management planning tools. Advanced ST

research will concentrate on enhancing controller highlevel planning under special conditions, such as enabling

a transitioning aircraft to meet meter fix crossing restrictions obtained from the CTAS Traffic Management

Advisor (TMA) and using datalink to specify user preference or negotiate a trajectory with an airborne FMS.

These tasks may require high levels of intersector

coordination, so ST automatic conflict resolution will also

be studied for deployment in an advanced AT/ST system.

Concluding Remarks

An automated advisory system has been described that

achieves many of the benefits of free flight without

radically modifying the procedures of ground-based air

traffic control. Because it is based on the foundation

provided by CTAS, significant operational capability

can be achieved in the near future. It also provides a

platform for exploring advanced concepts in air traffic

management.

The major features of the integrated AT/ST tools are as

follows:

• Many of the benefits anticipated for free flight are

achieved with only small extensions to current ATC

operations and procedures. All current sector controller responsibilities and authority are preserved.

All aircraft types can benefit because no new

airborne equipment is required.

• The needs of the entire Center airspace are addressed.

In en route environments, the integrated tool anticipates and facilitates user preferences, while providing

23
advisory aids to help the controller solve complex

traffic management problems.

• An operational system can be placed in the field

quickly, where it can serve as a testbed for new

technology. Planned technology exploration includes

trajectory negotiation with airborne flight management systems, integration with traffic schedulers

such as TMA, and free-flight concepts that transfer

responsibility for maintaining separation to the user.

• Sector controllers will have an incentive to use the

proposed system because it will improve their

capability and reduce their workload. The system is

not intended to be a replacement for controllers, but

an aid to increase productivity.

• The design leads to a logical and systematic

evolution. It will be implemented as a series of

new functions that will gradually increase system

capability. Manual tasks will be automated based on

the semi-automatic operation of early deployments,

thereby freeing controllers to give attention to more

advanced tasks as the design ew)lves.

• The system is not dependent on planned hardware

upgrades, such as DSR, to be successful. It will,

however, take advantage of such upgrades.

Although development of this system faces many

challenges, no unresolvable implementation issues are

anticipated.

References

1. Report of the RTCA Board of Directors' Select

Committee on Free Flight. RTCA Inc.,

Washington D.C., Jan. 1995.

2. Hart, D. C.: Free Flight: An Evolutionary Revolution.

Avionics, Apr. 1995, pp. 50-51.

3. Smith, P. J., et al.: Interactions of Airline Operations

Control Centers with the Air Traffic Management System. AIAA Paper 96-3854, presented at

the Guidance, Navigation, and Control

Conference, San Diego, Calif., July 1996.

4. Erzberger, H.; and Nedell, W.: Design of an

Automated System for Management of Arrival

Traffic. NASA TM-102201, June 1989. Also,

Engle, L.: Conflict Detection Tool. Addendum to

TM- 102201, Oct. 1989.

5. Erzberger, H.: Conflict Probing and Resolution

Algorithms. Presented to FAA/AUA-500

management, NASA Ames Research Center,

May 2, 1996.

6. Green, S.: and Vivona, R.: Field Evaluation of

Descent Advisor Trajectory Prediction

Accuracy. A1AA Paper 96-3764, presented at the

Guidance, Navigation, and Control Conference,

San Diego, Calif., July 1996.

7. Green, S.; den Braven, W.; and Williams, D.:

Development and Evaluation of a Profile

Negotiation Process for Integrating Aircraft

and Air Traffic Control Automation. NASA

TM-4360, Apr. 1993.

8. Benjamin, S. G.: Brundage, K. J.; and Morone, L. L.:

The Rapid Update Cycle. Part I: Analysis Model

Description. Technical Procedures Bulletin

No. 416, NOAA/NWS, 1994. [National Weather

Service, Office of Meteorology, 1325 East-West

Highway, Silver Spring, MD 20910. Also

available on the World Wide Web at

http://www.fsl.noaa.gov/frd-bin/tpbruc.cgi.]

Slattery, R.; and Green, S. M.: Conflict Free

Trajectory Planning for Air Traffic Control

Automation. NASA TM- 108790, Jan. 1994.

Interlace Requirements Specification for the Sensor

Data Acquisition and Processing System

(SDAPS). Contract No. DCA 100-94-D-0015,

prepared for NASA Ames Research Center

by BDM Federal, Inc., Boulder, CO 80301,

Mar. 1996.

9.

10.

24

Form Approved

REPORT DOCUMENTATION PAGE ou8 No o7o4-o188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

November 1996 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

A System Concept for Facilitating User Preferences in gn Route

Airspace

6. AUTHOR(S)

R. A. Vivona,* M. G. Ballin, S. M. Green, R. E. Bach,

and B. D. McNally

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Ames Research Center

Moffett Field, CA 94035- 1000

SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

505-64-36

8. PERFORMING ORGANIZATION

REPORT NUMBER

A-962788

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM-4763

11, SUPPLEMENTARY NOTES

Point of Contact: R.A. Vivona, Ames Research Center, MS 210-9, Moffett Field, CA 94035-1000

(415) 604-0723

*Sterlinl_ Software, Inc., Redwood Shores, California

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified -- Unlimited

Subject Category 03

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The Federal Aviation Administration is trying to make its air traffic management system more responsive to the

needs of the aviation community by exploring the concept of "free flight" for aircraft flying under instrument flight

rules. A logical first step toward free flight could be made without significantly altering current air traffic control (ATe)

procedures or requiring new airborne equipment by designing a ground-based system to be highly responsive to "user

preference" in en route airspace while providing for an orderly transition to the terminal area. To facilitate user preference in all en route environments, a system based on an extension of the Center/TRACON Automation System (CTAS)

is proposed in this document. The new system would consist of two integrated components. An airspace tool (AT)

focuses on unconstrained en route aircraft (e.g., not transitioning to the terminal airspace), taking advantage of the

relatively unconstrained nature of their flights and using long-range trajectory prediction to provide cost-effective

conflict resolution advisories [o sector controllers. A sector tool (ST) generates efficient advisories tor all aircraft,

with a focus on supporting controllers in analyzing and resolving complex, highly constrained traffic situations. When

combined, the integrated AT/ST system supports user preference in any air route traffic control center sector. The

system should also be useful in evaluating advanced free-flight concepts by serving as a test bed for future research.

This document provides an overview of the design concept, explains its anticipated benefits, and recommends a

development strategy that leads to a deployable system.

14. SUBJECT TERMS

Air traffic control, Free flight, User-preferred routing, UPR

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-2B0-5500

18, SECURITY CLASSIFICATION

OFTHIS PAGE

Unclassified

19. SECURITY CLASSIFICATION

OF ABSTRACT

15. NUMBER OF PAGES

27

16. PRICE CODE

A03

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std Z39-18