NASA Langley and NLR Research of Distributed Air/Ground Traffic Management

13 Pages·2002·1.2 MB·English

by  Ballin, Mark G.

NASA LANGLEY AND NLR RESEARCH
OF DISTRIBUTED AIR/GROUND TRAFFIC MANAGEMENT
Mark G. Ballin
NASA Langley Research Center, Hampton, Virginia
Jacco M. Hoekstra
National Aerospace Laboratory -NLR, Amsterdam, The Netherlands
David J. Wing
Gary W. Lohr
NASA Langley Research Center, Hampton, Virginia
Abstract
Project (AATT) began developing and exploring the
concept of Distributed Air/Ground Traffic
Distributed Air/Ground Traffic Management (DAG-
Management (DAG-TM). NASA experience in
TM) is a concept of future air traffic operations that
developing airborne and ground-based decision support
proposes to distribute information, decision-making
systems was used to provide a detailed definition of the
authority, and responsibility among flight crews, the air
broad vision of free flight. DAG-TM is based on the
traffic service provider, and aeronautical operational
premise that large improvements in system capacity as
control organizations. This paper provides an overview
well as flexibility and efficiency for the airspace user
and status of DAG-TM research at NASA Langley
will be enabled through
Research Center and the National Aerospace
Laboratory of The Netherlands. Specific objectives of • sharing information related to flight intent, traffic,
the research are to evaluate the technical and
and the airspace environment,
operational feasibility of the autonomous airborne
• collaborative decision making among all involved
component of DAG-TM, which is founded on the
operational paradigm of free flight. The paper includes system participants, and
an overview of research approaches, the airborne • distributing decision authority to the most
technologies under development, and a summary of appropriate decision maker.2
experimental investigations and findings to date.
Although research is not yet complete, these findings Flight crews, the air traffic service provider (ATSP),
indicate that free flight is feasible and will significantly and aeronautical operational control organizations
enhance system capacity and safety. While free flight interact as both information suppliers and users,
cannot alone resolve the complex issues faced by those thereby enabling collaboration and cooperation in all
modernizing the global airspace, it should be levels of traffic management decision making.
considered an essential part of a comprehensive air Distributing decision-making authority may be the key
traffic management modernization activity. enabler in multiplying the capacity of National
Airspace System by minimizing the occurrence of
Introduction human workload bottlenecks. It offers the potential of a
linearly scalable system that accommodates an increase
The aviation user community has identified a need for
in demand through a proportional increase in
significantly increasing airspace capacity and the
infrastructure and human decision-making capability,
flexibility of aircraft operations. This need and the
whereby each additional aircraft contributes actively to
introduction of new surveillance concepts have led to a
the traffic management solution. System-wide
new operational paradigm, _free flight," in which
reliability and safety improvements may also result
reliance on centralized air traffic management is
from the increased redundancy of traffic management
reduced in favor of distributed management. In 1995,
capability.
RTCA Task Force 3 defined free flight as a safe and
efficient flight operating capability under instrument Under the sponsorship of AATT, NASA Langley
flight rules in which operators have the freedom to Research Center (Langley), NASA Ames Research
select their path and speed in real time. 1In 1997, the Center (Ames), and the National Aerospace Laboratory
NASA Advanced Air Transportation Technologies of the Netherlands (NLR) have collaborated in research
and development to explore the feasibility of DAG-
*Associate Fellow-,AIAA
TM. Two of the fifteen DAG-TM concept elements
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American Institute of Aeronautics and Astronautics
focusontheoriginavl isionof freeflightasa
contradictions may have been the result of
distributeadirbornterafficmanagemceonntcepwthere
compartmentalized perspectives and expertise, leading
theflightcrewhasindependeanutthorittyoperform to uncoordinated assessments and inconsistent use of
taskcsurrentlayssociatweidthanairtrafficcontroller.
enabling technology.
DAG-TMConcepEt lement5 envisionsaircraft
operatinagutonomouisnlyconstraineednrouteand To resolve these ambiguities, a balanced research
unconstrainetedrminal-arrivaelnvironmentas,nd approach was chosen that focuses primarily on
ConceptElement11 envisionsflight crews determining the limits of feasibility and the technology
contributinagctivelyin maximizinagirportarrival requirements of the concept elements. Under this
throughpwutheninstrumeanpt proachaersein use. approach, the following steps are taken concurrently
Bothconcepetlemenatsrebasedonthepremisoef and iteratively: a set of hypotheses is developed, the
Referenc1ethatflightcrewsshouldhavecapability future airspace system is modeled in simulation,
andauthorittyoself-separafrtoemotheraircrafbty technology prototypes and operational procedures are
adjustinthgeirself-selecttreadjectoriethse,rebfyreeing developed, and evaluation is performed. The
airtrafficservicperovidertsoconcentraotnetraffic hypothesis set includes nominal operations, rare-
flowmanagement. nominal operations (events or conditions that stress the
concept or define the limits of feasibility), and failure-
Thispapeprrovideasnovervieawndstatuosfongoing
mode operations. If feasibility is established for
researachtLangleayndtheNLRintheareadsescribed
airborne operations, further activities will focus on the
bythesetwoconcepetlementSsp.ecifiocbjectiveosf
issues involved in integrating the airborne components
the researcahreto evaluatethe technicaal nd
with ATSP components. Benefits and safety aspects,
operationfaelasibilityof theautonomouasirborne
while important, have only been characterized at ahigh
componeonftDAG-TMandtodeveloapndvalidate
level until feasibility is established. High-level benefits
theenablinagirbornteechnologaiensdprocedurtehsat
assessments are in progress, but are not discussed in
willberequireTd.hepapeinrcludeasnovervieowfthe
this paper.
researcahpproachtehse,airbornteechnologuiensder
developmenatn, d a summaryof experimentalBecause decision support automation technology is an
investigatioannsdfindingtsodateI.nthepaperth,e integral part of the DAG-TM concept, it is necessary to
term"freeflight"isusedtorefertothecapabilitoyf develop prototype technology and procedures to make
aircrafttooperataeutonomouasslyacomponeonfta a valid determination of feasibility. Airborne decision
largeDr AG-TMconcepratthetrhanasthecomplete support technology is therefore under development. 7'8
conceipntitself. Free flight operations may require flight deck systems
that support the flight crew in making trajectory
Research and Development Approach management decisions. Requirements may include the
Published position statements and pre-existing research capability to collect, fuse, and present relevant
provide an ambiguous assessment of free flight information; analyze surveillance and constraint data
feasibility. The Task Force 3 report states that, in for potential airspace or traffic conflicts; calculate
addition to autonomous airborne capability for conflict resolution options that optimize specified
separation, ground-based automation and restrictions parameters, such as fuel burn; provide tactical
will be used to resolve tactical conflicts, to manage information for conflict-free maneuvering; and analyze
traffic flows in congested airspace, to prevent over-constrained problems for viable solutions. These
unauthorized entry into special use airspace, and to and other capabilities that may be critical for free flight
assure safety of flight, 1which suggests that feasibility in complex traffic and airspace environments do not
and safety depend on ground capabilities. The task exist in current flight deck systems.
force stipulated that free flight must be at least as safe
While the airborne decision-aiding systems described
as today's operations and implied that ground
above may improve crew planning capabilities and
involvement and restrictions are adequate to provide
reduce workload, they may also increase the cost of
this. To alleviate concerns, the National Academy of
equipping aircraft for free flight operations and be
Science's National Research Council coordinated an
difficult to retrofit. In addition, they may place an
activity to gather expert opinion regarding safety
increased burden on the future communication,
implications of the task force's vision. The resulting
navigation, and surveillance (CNS) infrastructure that
opinion stated that the safe distribution of traffic
will enable aircraft to exchange needed information.
management responsibility is very difficult, if not
Both of these factors may delay the transition to future
impossible. 3'4 However, other research indicates that
free-flight operations. Therefore, it was deemed
free flight is not difficult and that it may in fact be safer
appropriate to also determine the minimum airborne
than ground-based separation assurance. 5'6 These
requirements necessary for free flight.
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American Institute of Aeronautics and Astronautics
LangleayndtheNLRhavechosetnocollaboraintea
intruder utilizes the same rules, it will maneuver to
complemenataprpyroacthhatinvestigattehsisrangeof
further increase the separation distance. Using
capabilitieLsa.ngleiysprimarilyinvestigatiinsgsues
rules rather than coordination messages is called
thatconcerandvanceadirbornteechnologays,maybe
implicit coordination, and is important in reducing
requireodrpreferreindamaturseystemthatintegrates
requirements for data exchange between aircraft.
autonomoauirscrafatndmanageadircrafitnthesame
airspacein,tegratewsithground-basseydstemas,nd • 100% autonomous during cruise flight
performlosng-terpmlannintghatmayaidtrafficflow.
All aircraft were assumed to have free flight
TheNLRis primarilyinvestigatintegchnologaynd
procedurtehsatrequireminimumchangetos flight capability, and no active role for the ground was
decksystemasndtheCNSinfrastructuTreh.eNLR included in the operational concept. This
corresponds to the European concept of
resultasrepresentiendthesectiotnhatfollowsa,ndthe
segregating free flight operations from managed
NASAresultsarepresenteindtheremainintgwo
sections. operations into 'free flight airspace' and 'managed
airspace.'
Tactical Free Flight Operations Conflict Resolution Method
Off-line traffic simulations comprising up to 400
One research and development alternative attempts to
aircraft were used to validate several methods for
determine what is minimally required to enable
conflict detection and resolution. These simulated
airborne separation assurance. Led by the NLR, the
traffic densities are equivalent to ten times today's
approach considers a tactical concept that resolves a
average western European density. The resolution
predicted loss of separation with traffic (referred to as a
method that proved to be most effective was based on
"conflict"), but does not include a recovery of the
the method described in Reference 9.
aircraft's flight plan as part of the predetermined
resolution maneuver. Since the concept assumes that The conflict resolution algorithm uses the geometry of
the separation assurance function can be accomplished the closest point of approach to prevent counteracting
independently from strategic functions such as maneuvers. Assuming both aircraft use the same
conformance to flow management constraints, this system, both aircraft maneuver cooperatively, which
assumption was investigated as part of the research. provides implicit coordination. The calculated
positions at the closest point 'repel' each other, similar
Operational Concept
to the way charged particles repel each other. As
As a first step, an operational concept for the
shown in Figure 1, the 'repelling force' is converted to
experiments was defined. The advanced CNS
a displacement of this predicted position in such a way
capability assumed available is Automatic Dependent
that the minimum distance will be equal to the required
Surveillance-Broadcast (ADS-B) or a comparable
separation. This avoidance vector is converted into
capability that provides, at a minimum, basic aircraft
advised heading and speed changes. The same
state data at intervals on the order of one second. The
principle is used in the vertical plane, resulting in an
operational concept has the following characteristics:
advised vertical speed and target altitude.
• State-based conflict detection
When both aircraft are able to maneuver cooperatively,
a redundant element is introduced. If both aircraft do
Only state data, i.e., position (latitude, longitude
and altitude) and velocity (ground speed, track and
vertical speed), are exchanged between aircraft. ... • Intruder's
Heading Minimum F .
The conflict detection function uses no flight plan distance distance _ protectea zone
information. This was used as a null hypothesis to
determine whether flight plan information would
be fundamentally required for concept feasibility.
A look-ahead time of five minutes was provided
by the detection function.
e\
• Cooperative conflict resolution
Rules, implemented in the conflict resolution
function that advises the pilot, use the geometry of Intruder
the detected conflict to determine the direction in
which to maneuver. The system does not assume Figure 1. Graphic representation of conflict resolution
the intruder aircraft will maneuver, but if the method.
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maneuvethr,econflicatlerwt illdisappewahreneach
aircrafhtasexecutepdartofthemaneuveinrd, icating
thatno furtheravoidingactionis requiredT.he
resultingmaneuvearsretypicallya fewdegreeosf
.............................ii.i.i.i.ii.i.i.i.ii.i.i.i.i .i.i.i.i.ii.i.i.i.ii.i.i.i.ii.i.i.i.ii.i.i.i.ii.i.i.i.ii.i.i.i.ii.................
headincghangoerabou2t00ft/minofverticaslpeed
changeP.assengceormforitsnotaffectebdythese
.......................i.i. .ii.i.i.i.ii.i.i.i.ii.i.i.i.ii.i.i.i.ii...............................
shallowmaneuvers.
Complegxeometriaensdrestrictiownsereusedtotest
therobustneosfsthemethodT.hesgeeometriinecslude
circularradiaclonflictasndseveratylpesofopposing
100Ni;i;i;i;i;i;i;i;i;i;iii;i;i;i;i;i;i;i;i;i; i;i;i;i;i;i;i;i;i;i;i_
wallsofaircraftT.hougthheydonotrepreseancttual
trafficconflicstcenariothse,ssecenariwoseredesigned
Single Double Tnple
todetermintehepowerandlimitsof theconflict
resolutiomnethobdyprovidinmgorecompletxraffic
constrainthtsanwouldeveorccuirnoperation. Figure 2. Workload assessments of a simulation of en
route cruise tactical free flight.
It wasfoundthateventhoughtheaircrafftollow
independerenstolutioandvisorieasg,lobaslolutiotno Conflict Prevention System
theproblemarisesI.n the 'wall' scenariofso,r A conflict prevention system called "Predictive ASAS"
exampleso,meaircrafitnthewalldecelerastleightly was also developed. The system prevents short-term
whileotherascceleratthee,reboypeninagspacfeoran conflicts as a result of turns and vertical maneuvers.
opposinagircraftto fly throughD. istributintghe For safety, a conflict prevention rule was added that
cooperatirveesolutioanlgorithmamonagllinvolved forbids flight crews from maneuvering to create a
aircrafrtesultisnanefficienatndglobaslolutione,ven conflict. This rule and the Predictive ASAS display
for situationwshereno solutionexistedpriorto were hypothesized to be an alternative to the exchange
resolutioTnh.ealgorithmprovedcapabolefresolving of flight plan information as a method to prevent
conflicstituationfosrwhichaprioritysystemw,hich conflicts.
forcesoneaircrafotf apairtoperformtheentire
maneuvwero,uldnothaveobtaineadsolution. Because of the simplicity of the architecture and the
resolution method, the conflict prevention system was
Cruise Human-in-the-loop trials transparent to the crew, allowing a display design as
shown in Figure 3. The display shows both ahorizontal
The conflict resolution algorithm used in the traffic
and vertical resolution advisory to the pilot. Either
simulation has been developed further into an airborne
advisory will completely resolve the conflict. The
separation assurance system (ASAS) for a research
conflict prevention system draws amber and red no-go
flight simulator at the NLR. The ASAS includes a
human-machine interface that has been tested in zones ('conflict prevention bands') on the heading,
vertical speed, and speed scales of the displays, even if
several flight simulator trials. Airline pilots have been
there are no conflicts for the current heading. This
exposed to scenarios replicating current densities
system was tested in a second human-in-the-loop
('single') up to three times the Western European
experiment. It was found that pilots used the
density ('triple'). Only a few hours of training was
dynamically changing conflict prevention bands as
found necessary.
precursors to a conflict alert and were able to exploit
No significant increase in workload was found during this to lower the number of conflict alerts. This
the cruise phase. The acceptability was surprisingly indicates that a conflict prevention system can
high and, further, the subjective safety was equal or contribute to the traffic situation awareness of the
better than today's situation, l° Figure 2 shows crew.
workload assessments for three levels of traffic density.
Cruise findings
The Rating Scale for Mental Effort (RSME) system of
Reference 11 was used as a measure. The total range of None of the studies refuted the feasibility of airborne
the RSME measure is 0 to 130. The single density separation during cruise, even under extremely dense
session shows a workload rating very close to a rating and constrained traffic situations. The findings indicate
of 27 found for a comparable ATe situation. These that decentralizing today's ATM system can
results were obtained using no flight plan information, significantly increase the capacity in the en-route
explicit coordination procedures, priority rules, or domain, even with minimal flight deck technology.
ground-based systems.
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Workload for flight phase and procedure
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[]CDTI
[]FF
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0 iiiiiiiiiiiiii
Cruise Descent Arrival
Figure 4. Workload ratings for three flight phases and
three levels of airborne capability.
the flight crew was responsible for the separation until
established on the localizer, and the controller
established only the arrival sequence..
At the end of each run, pilots were asked to rate the
mental workload of the descent and the arrival
Figure 3. Co-planar traffic display as used in the segments. These ratings and the earlier results for
study. The symbology indicates a conflict (amber) and cruise are compared in Figure 4. Three categories of
the resolution advisory (green). operational procedures were compared: free flight
Contrary to what was believed when the project (FF); managed with a CDTI (CDTI); and managed
started, there appears to be no fundamental requirement without aCDTI (ATC).
for exchanging intent (flight plan or mode control
It has often been suggested that a higher workload
panel) information, although possible benefits of doing
during flae descent might inhibit free flight during flaat
so are still acknowledged.
phase of operations. As can be seen in the figure, the
It was also found that the conflict resolution maneuvers FF descent rating is not significantly different from the
had a negligible impact on flight efficiency or the FF cruise rating, at least for the initial traffic scenarios
ability of the aircraft to meet a time constraint. When studied. During the FF arrival, the workload was found
the crews were trained to nominally use a vertical to be significantly higher than for the other phases of
resolution maneuver instead of a horizontal one, there flight, but not high on the absolute scale, which
was no noticeable impact. This finding is important in extends to a value of 130. Although flae limits of its
establishing a transition procedure from free flight feasibility are not fully established, evidence suggests
airspace to managed airspace. Because the planning there is a role for airborne separation assurance in the
terminal area.
problem and the separation problem appear to be
independent, there is no fundamental difference
In general, the arrival phase of flight is characterized
between this transition and today's hand-over from one
by much higher workload than the cruise phase. 12In
ground-based controller to another. This alleviates the
Figure 4, the workload rating for the CDTI arrival
need for complex hand-over procedures that use huge
shows a dramatic impact of providing traffic
transition zones around entry points.
information to the crew. The arrival rating is even
lower than the ATC cruise scenario without CDTI.
Other flight phases
However, during merging maneuvers required in the
As anext step, the basic ASAS was tested in a piloted
high traffic density terminal area, the crews were
simulation experiment for different flight phases that
occasionally not able to maintain separation. Some
are more constrained than the cruise phase. Each
pilots commented flaat more training would be
aircraft started in cruise and proceeded through
sufficient, but this is not supported by the objective
descent, arrival, and approach phases of flight. In half
data, which show no training effect of separation
the runs, the terminal area was managed airspace with
effectiveness during merging. Therefore, the basic
a basic hand-over as the transition procedure, and the
ASAS retrofit will not be sufficient for complex high-
crew was provided with a cockpit display of traffic
density terminal airspace. Either more sophisticated
information (CDTI) tomonitor traffic. In the other half,
tools and procedures are required, or the terminal area
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American Institute of Aeronautics and Astronautics
should remain managed airspace with the addition of For conflict detection and alerting, the approach in
the CDTI to reduce crew workload. As described later, strategic free flight is to use the best information
approach spacing with limited delegation may also available for determining whether a loss of separation
offer solutions for capacity-constrained terminal with another aircraft or an encroachment on hazardous
environments. airspace is threatened and whether the flight crew
should or should not take action in each particular
Strategic Free Flight Operations and Air/Ground situation. Trajectory information on traffic aircraft may
Integration include only state data for some aircraft (for instance if
that aircraft is currently being maneuvered manually),
En-Route Strategic Airborne Operations
or it may also include varying degrees of intent ranging
Complementary to the tactical airborne operations in from target states to FMS-level intent if the autoflight
free flight investigated by the NLR are strategic system is engaged. The determination of which
airborne operations investigated by Langley. The main trajectory information is best suited for detecting
purpose for a strategic mode in free flight is to allow conflicts relates to several conditions, such as traffic
the flight crew to be proactively engaged in the aircraft conformance to its broadcast intent and
dynamic airspace operational environment. This may proximity to the ownship. Performing conflict
require an increase of the planning horizon beyond detection using both state and intent information has
what tactical mode enables and a capability to replan the potential for reducing both false and missed alerts
trajectories to gain operational advantage under a while increasing detection accuracy over a larger look-
complex set of constraints. For cost effectiveness, ahead time horizon. These effects, in turn, can reduce
strategic free flight capabilities would make maximum unnecessary and excessive maneuvering, thereby
use of automation systems already present on most freeing the crew to better manage their other tasks.
modem aircraft, such as the flight management system
(FMS) and the FMS-coupled autoflight system. Just as With the increased look-ahead horizon for conflict
these systems assist the crew in managing the strategic detection, strategic free flight allows problems to be
intent of the flight, i.e., specifying and enacting higher- solved with strategic solutions. A strategic solution is a
level goals than simply aircraft state changes, strategic set of complete resolution maneuvers including return
free flight capabilities focus on the use of intent-based to course that can be calculated and evaluated by the
trajectories and longer look-ahead time horizons for crew before the first maneuver is initiated. This
strategic flight replarming, conflict detection and complete solution can also be integrated into the FMS
resolution, and constraint management. flight plan, and the maneuver can be accomplished
while remaining in FMS guidance mode during the
In addition to that of the tactical mode, a strategic free entire event. An important community benefit of doing
flight capability provides functionality in the areas of so is that the modified FMS flight plan is broadcast as
information management (gathering and processing), new ownship intent, thereby increasing the
conflict detection and alerting, and conflict resolution. predictability of the ownship to other airborne and
Each area will be briefly described. ground-based observers. An aircraft that resolves
conflicts tactically by exiting FMS guidance mode
The objective for information management in strategic
would not be as predictable, since the intent of that
free flight is
aircraft is not shared. An additional benefit to strategic
• to gather and integrate sufficient predictive resolutions is the ability to simultaneously incorporate
information on the operating environment to all operational constraints into the solution, including
enable increased trajectory prediction accuracy those unrelated to the specific conflict. Examples of
and replarming time before action is required such constraints are required time of arrival (RTA)
flow constraints issued by the ATSP, ownship
• to make available more options for solving a given
performance limitations, and company-specific flight
problem, and
optimization strategies. Derived user benefits from
• to allow replarmed trajectories to be valid (i.e., incorporating these operational constraints into
constraints to be met) over a longer-term portion resolution maneuvers may include improved schedule
of the flight. conformance and reduced fuel burn.
Information needed to support these goals includes A hypothetical example of strategic free flight problem
traffic surveillance with trajectory intent, airspace solving is shown in Figure 5. The trajectories of two
system status, weather products, and assigned
aircraft are in conflict in en-route airspace. Assuming a
operational constraints to facilitate traffic flow
priority system is in effect and the aircraft to the right
management (TFM).
(Aircraft _B') has priority, aircraft CA' must replan its
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American Institute of Aeronautics and Astronautics
the capability for strategic free flight. The development
activity involves defining and creating a software
prototype of a flight crew decision support tool that
facilitates the strategic operations described in the
previous section. This tool, referred to as the
Autonomous Operations Planner (AOP), provides for
management of operational constraints and user
preferences. It processes surveillance and airspace
status information, and searches for conflicts among
several relevant trajectory combinations between
ownship and traffic aircraft. These combinations
include state and several forms of intent, as well as
conflicts with airspace hazards. Crew alerting is
provided based on threat priority and required crew
action. Strategic and tactical conflict resolution
advisories are provided to the crew, with options from
Figure 5. Hypothetical example scenario for strategic which to chose. The strategic resolutions are iterated
planning. through the FMS to ensure the trajectory
recommendations are within aircraft performance
trajectory such that separation does not decrease below limits and that operational constraints such as an RTA
the accepted regulatory standard for safety (accounting can be met. See reference 7 for a more complete
for applicable navigation and surveillance description of AOP capabilities.
uncertainties). However, several other constraints on
the aircraft 'A' trajectory are also present. The aircraft An example of a highly constrained conflict scenario
is approaching the terminal environment, and the with a strategic AOP resolution is shown in Figure 6.
ATSP has issued a TFM constraint to aircraft 'A' in the In the scenario, the ownship flight plan passes between
form of an RTA crossing restriction at the arrival- two SUA regions flaat constrain the available solution
metering fix. An SUA is active to the north of the fix, space. The broadcast intent from a traffic aircraft is in
and convective weather cells are forecast for the conflict with the ownship flight plan, and the
vicinity. Furthermore, additional crossing traffic below conflicting portion of the flight plan is shown to flae
and to the left may impact the decision on whether an pilot. In this example, AOP only considered lateral
early descent is advisable. path-stretch solutions to flae conflict. An AOP-
generated conflict resolution trajectory involving a
Assuming these constraints are input to the flight path-stretch maneuver to the right is shown for pilot
crew's decision-aiding automation, the FMS flight plan review. This trajectory has been determined to be free
can be modified by computing an RNAV path between of any traffic conflicts, to adhere to the airspace
present position and the destination that adheres to constraints, and to be flyable by the aircraft. Note flaat
each of these constraints. This computation can be the outboard waypoint is a fly-by waypoint, and flaat
initiated as soon as the conflict is identified and
given the aircraft's turn radius, flae aircraft itself will
provided to the flight crew, either automatically or on not pass into the SUA. Future builds of the AOP
request. In the example, a resolution trajectory is software will also consider the vertical and speed
calculated that diverts the aircraft to the right and degrees of freedom.
behind the conflicting aircraft, and then the return path
is calculated to miss the SUA and weather cell while Some aspects of strategic free flight were recently
minimizing excess air miles flown. Airspeed and top of evaluated in a human-in-the-loop simulation conducted
descent are adjusted as necessary to meet the crossing in the Air Traffic Operations Laboratory at
restriction. A diversion to the south is not offered Langley. 13,14 Operations were conducted under
because of the size of the weather cell. A climb is not conditions of low and high operational complexity
recommended because it would not be consistent with (traffic and airspace hazard density) with operational
the impending arrival, and an early descent would constraints including RTA adherence and airspace
create a new traffic conflict. The flight crew is able to hazard avoidance.
review the strategic plan and modify it if necessary. If
Strategic free flight was found to be effective in
it is deemed acceptable, the crew activates the route in
reducing unnecessary maneuvering during conflict
the FMS, and the route is broadcast as new intent.
situations where the intruder's intended maneuvers
Developing and Evaluating Strategic Free Flight would resolve the conflict. Scenario case studies
Researchers at Langley are developing and evaluating illustrated the need for flight restrictions to prevent the
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American Institute of Aeronautics and Astronautics
throughout the airspace. As more aircraft operate in the
autonomous mode and exercise the freedom to modify
their trajectories as needed, flae need for these aircraft
to contribute to system predictability by operating
strategically increases, which includes broadcasting
intent and making fewer but more strategic intent
changes.
Strategic free flight also benefits air-ground integration
in conflict situations and managed-aircraft trajectory
control. For non-segregated operations to be feasible, it
is likely that free flight operations (for which the
controller is not responsible) must have the appearance
to the controllers of occurring largely on a non-
interference basis. This has several implications. First,
conflicts between autonomous aircraft should generally
be resolved before the controller (or monitoring
automation) sees the conflict. Second, changes in intent
or maneuvers by autonomous aircraft must not create
new conflicts with managed aircraft. For both
conditions to be met requires autonomous aircraft to
Figure 6. Example conflict scenario with a strategic
use a longer look-ahead horizon than the controller
AOP conflict resolution (on an MD-11 navigation
team, which often plans ahead many minutes. The
display).
second consideration is particularly important in that
creation of new conflicts through maneuvering, as the noninterference operations provide some protection
discussed in the previous section. The case studies to the controller from the maneuvering activities of
have also shown the need for an improved user autonomous aircraft. This protection is likely a key
interface design that appropriately focuses the pilot's feasibility requirement because it allows the controller
attention on conflict prevention information. Pilot real- to effectively and strategically plan the managed
time assessment of maximum workload indicated aircraft trajectories.
minimal sensitivity to operational complexity,
providing further evidence that pilot workload is not An important aspect of air-ground integration is
the limiting factor for feasibility of an en-route handling situations for which an autonomous aircraft
distributed traffic management system, even under can no longer operate autonomously and must
highly constrained conditions. transition to managed status. Assuming that the need
for the transition may occur unexpectedly and possibly
Strategic Air-Ground Integration at an inopportune moment for flae receiving controller,
The discussion thus far has focused primarily on the the operational concept must allow for a finite and non-
characteristics and advantages of strategic free flight instantaneous transition time before the controller can
from an airborne perspective. When considering assume responsibility. An analogy would be a VFR
integration of the airborne and ground components in aircraft requesting a pop-up IFR clearance; until the
air traffic operations, the strategic mode for free flight clearance can be issued, the pilot must continue to
provides some additional key benefits. These benefits operate under VFR and provide for its own separation.
would be particularly important where autonomous and Strategic free flight can facilitate this transition in that
managed aircraft are intermixed rather than segregated the autonomous aircraft, before the failure, would be
into 'free flight airspace' and 'managed airspace'. established on a trajectory that was determined to be
conflict free potentially for tens of minutes into the
As stated earlier, the strategic free flight broadcast of future. The increased look-ahead and traffic-aircraft
intent information provides increased predictability of
intent information of strategic free flight would provide
the operational environment to airborne recipients of a significant amount of protected time to accomplish
that information. This predictability is also of great the transition.
importance to the ATSP in its role of providing for
orderly flow through the airspace and into and out of Feasibility
high-demand terminal environments. Metering would The research and development performed thus far
be a principal technique for controlling the flow; support the feasibility of strategic autonomous aircraft
however the ability to meter may be highly dependent operations. It has been learned that autonomous aircraft
on the ability to dynamically predict the demand operations scale well with traffic density. En route
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American Institute of Aeronautics and Astronautics
workloadin nominaslituationiss nota feasibility
Increased positioning capability obtained by providing
issue,evenin veryhightrafficdensitiesG.iven
crews with limited maneuvering authority also results
appropriadteecision-suppaourttomatiopni,lotscan in fewer missed arrival slots over time. A further
strategicarlelysolvceonflictisncompleexnvironments
advantage is the growth potential provided for today's
that includeTFM, airspacea,nd performance
underutilized airports that will see increased demand in
constraints.
the future. Because each aircraft brings with it a
significant portion of the needed CNS infrastructure
Combineudseofstateandintentprovidevsaluable
and human-decision-making capability, minimal
operationbaelnefitsa,lthoughintentmaynotbea
ground infrastructure additions will be required for
fundamentraelquiremefnotr feasibilityE. vidence
these terminal areas as demand increases.
indicatethsatautonomooupserationcasnbereliably
performewdithouttheneedforcontrolleinrtervention.
As for en route operations, the advanced CNS
capability assumed for the terminal area research is
Whileadditioncahl allengetosfeasibilitryemainand
ADS-B or its equivalent. Additional ADS-B
haveyetto befully exploredn,oinsurmountable
information content such as planned final approach
impedimenhtasveyetbeendiscoverefodrfreeflight
speeds, wind data, and intent information may be
aircrafotperatioinnsallenvironmeenxtscephtighly-
constrainteedrminaalreaMs.oreresearcishneedeodn required depending on the mode of operation.
air-grounindtegratiotondetermintheefeasibilitoyf
Concept Overview
integratinmgixed-equipaogpeerationins thesame
Distributed air/ground terminal area operations are
airspaceto, studyfailure-modtreansitionbsetween
being investigated through the definition of three
equipagsetatulsevelsa,ndtoidentifythesensitiviotyf
operational modes: tin-trail spacing,' _merging
controllewrorkloatdolargeincreasienstrafficdensity.
operations,' and _maneuvering.' For all modes,
procedures involve limited delegation of specific
Capacity-Constrained Terminal Area Operations responsibilities by the ATSP to each participating
aircraft. Each of these modes embodies characteristics
The inherent dynamic and highly constrained nature of
that allow for implementation independent of the
terminal area operations requires an approach different
others or in combination, depending on the specific
from that employed for the en route domain.
operational needs of the environment. Figure 7 depicts
Operations in the busy terminal area are characterized
a general overview of the three operational modes,
by:
which are defined as follows:
• constraints on the volume and configuration of
• In-Trail Spacing
airspace caused by number and geometry of
runways, surface topology, and environmental The in-trail spacing mode is based on the ATSP
considerations such as noise providing a spacing interval to the following
• dynamically changing operations caused by aircraft and issuing a clearance for the flight crew
changes in wind direction, meteorological to adhere to algorithm-generated speed cues to
conditions, availability of individual runways, and achieve the spacing. Spacing is time-based to
the relative demand for arrival, departure, and account for differing final approach speeds and
wind environments. The aircraft follows the same
surface operations
flight path as the aircraft immediately preceding it,
• varying aircraft performance and equipage levels following either a defined route or a flight path
The high-density airports in the current system require history (i.e., following the ground track of the lead
solutions that will ultimately increase capacity. If aircraft, which is provided on an onboard map
maximization of airport throughput dominates other display).. In-trail spacing can be applied at any
operational needs, a distributed air/ground traffic point in the terminal area, and should be applied as
management approach offers several advantages. far in advance as possible to maximize throughput
Increasing airborne responsibility for trajectory benefits. In-trail spacing may also have
management can provide increased conformance with applications in the en route and oceanic domains.
constraints that maximize throughput. Flight crews are
capable of high precision in managing their own • Merging
trajectory. Increased precision leads to a reduction in Aircraft arriving on different routes that merge at a
spacing buffers and hence higher throughput. A common point are appropriately spaced at that
ground-based controller cannot achieve such precision, point, based on an RTA that is either assigned or
and he must simultaneously manage numerous aircraft. computed based on the lead aircraft's estimated
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American Institute of Aeronautics and Astronautics
ATSP-defined
Metering
maneuvering
boundary
corridor
Unequ'_iippedAircraft
Terminal airspace
Figure Z DAG-TM terminal ardval concept element.
The investigations began with a complete definition
time of arrival at the point. This mode assumes a
the concept and procedures, including a checklist for
pre-defined route environment where a sufficient
the flight crew and phraseologies specific to the new
number of routes exist to allow aircraft to be
approach spacing operation. The concept includes use
scheduled ahead or behind traffic on separate
of an airborne decision support tool, which is made
routes and arrive properly spaced at the merge
up of two components:
point. Under a less rigid application, aircraft could
pass common boundaries, as opposed to points, • A specialized algorithm, referred to as Advanced
with appropriate time separations. Terminal Area Approach Spacing (ATAAS),
generates speed guidance to achieve a desired
• Maneuvering
spacing interval at the runway threshold.
To enable a greater spacing dynamic range than is Described in Reference 17, ATAAS has been
possible with speed adjustments alone, aircraft are refined through extensive Monte Carlo analysis.
given the flexibility to define their own routes
• Supporting displays provide a crew interface.
within prescribed airspace, provided that system-
imposed constraints are met. Although the term A nominal speed profile is provided that reflects
'maneuvering' might suggest last minute changes speeds typically used in arrival operations, and is
included as part of a charted arrival procedure. This
in heading or speed, it is envisioned that advanced
procedure can be used by all arriving aircraft to
planning (prior to terminal area entry) would result
follow the nominal speed schedule, regardless of
in a near-optimized, stable flight path through the
terminal area. Assuming an adequate conflict their ability to perform an approach spacing
detection and resolution capability is available, operation. Since the ultimate goal is to provide
more dynamic maneuvers could potentially also be maximum achievable system throughput in a stable
executed. Future research will determine if this is and acceptable manner, the ATAAS speed guidance
is limited to +10% of the nominal profile so that
feasible and provides benefits.
system stability and pilot acceptability are
In-Trail Spacing Operations maintained.
Langley research of time-based in-trail spacing dates
Figure 8 provides an example of some of the display
back to the 1970s. The results of several simulator
symbology. The Navigation Display provided
studies indicated that the potential existed for capacity
information on the ATAAS guidance and aircraft
increases 15'16although some issues related to displays
spacing status. The information included a data block
and supporting data links remained to be addressed in
containing currently entered ATAAS data and lead
the development of an acceptable operation.
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American Institute of Aeronautics and Astronautics