A Simulation Testbed for
Airborne Merging and Spacing
Michel Santos*, Vikram Manikonda†, Art Feinberg‡
Intelligent Automation, Inc.
Rockville, Maryland, USA
Gary Lohr§
National Aeronautics and Space Administration
Langley Research Center
Hampton, Virginia, USA
The key innovation in this effort is the development of a simulation testbed for airborne
merging and spacing (AM&S). We focus on concepts related to airports with Super Dense
Operations where new airport runway configurations (e.g. parallel runways), sequencing,
merging, and spacing are some of the concepts considered. We focus on modeling and
simulating a complementary airborne and ground system for AM&S to increase efficiency
and capacity of these high density terminal areas. From a ground systems perspective, a
scheduling decision support tool generates arrival sequences and spacing requirements that
are fed to the AM&S system operating on the flight deck. We enhanced NASA's Airspace
Concept Evaluation Systems (ACES) software to model and simulate AM&S concepts and
algorithms.
Nomenclature
ACES
= Airspace Concept Evaluation System
AM&S
= airborne merging and spacing
ft.
= feet
IAS
= indicated airspeed
n.mi.
= nautical mile
NAS
= national airspace system
STA
= scheduled time of arrival
t spacing
= duration of time-based spacing between aircraft
I. Introduction
Air traffic demand over the next few years is expected to increase significantly with demand in 2025 reaching
almost three times current demand1. Traffic management flow efficiency, airport and terminal throughput and
controller workload remain some of the foremost limitations of the air traffic system. To address this issue the Joint
Planning and Development Office (JPDO) is currently working to define the Next Generation Air Transportation
System (NGATS) with the objective of determining what changes are necessary to meet future demand. NASA’s
NGATS ATM-Airportal project currently supports this vision by focusing research efforts to develop, demonstrate,
and validate operational concepts, proof-of-concept systems, algorithms, technologies, tools, and operational
procedures for use in maximizing capacity and throughput in the Airportal environment.
*
Research Scientist, Intelligent Automation, 15400 Calhoun Drive, Suite 400, Rockville, MD 20855, AIAA Member
Vice President, Intelligent Automation, 15400 Calhoun Drive, Suite 400, Rockville, MD 20855, AIAA Member
‡
Program Manager, Intelligent Automation, 15400 Calhoun Drive, Suite 400, Rockville, MD 20855
§
Aerospace Technologist, NASA Langley Research Center, M/S 156A, Hampton, VA 23681
†
1
American Institute of Aeronautics and Astronautics
This research effort directly supports the objectives of the NASA Airportal Project. AM&S is envisioned as
one of the key concepts that will result in achieving increased efficiency in the Airportal environment. This effort
has resulted in the development of an operational concept, modeling and simulation capability that will enable the
evaluation of technologies related to AM&S. As discussed in the Federal Aviation Administration (FAA)/JPDO
Industry Day Briefing on Surveillance and Broadcast Services¶, with the equipage of all aircraft with Automatic
Dependent Surveillance Broadcast (ADS-B), it is anticipated that significant capacity and safety gains will be
realized by the delegation of responsibilities related to self-separation, merging, and spacing to the flight deck.
Some examples of expected operational improvements include:
•
Reduced arrival spacing (with altitude offset) for very closely spaced parallel runways at Operational
Evaluation Plan (OEP) airports (Super Dense Airports Concepts)
•
High density en route corridors (tubes) characterized by parallel tracks and delegation of separation
responsibility to the flight deck via CDTI and ADS-B
•
Self-spacing with CDTI/ADS-B coupled with sequencing automation use at non-towered airports
Airborne separation and assurance systems (ASAS) are being researched under the guiding principle that “Air
Traffic Services can be enhanced through greater involvement of flight crews and aircraft systems in cooperation
with controllers and the Air Traffic Management system.” 2 Section II briefly surveys the research on ASAS which
includes research on AM&S. Section III identifies the AM&S concepts used for this paper. Section IV reviews how
the AM&S concepts are implemented within ACES3. Section V presents the results from some AM&S simulation
studies within ACES.
II. Literature Survey of Airborne Separation Assurance Systems
Airborne separation and assurance systems have been categorized into four applications2: airborne traffic
situational awareness, airborne spacing applications, airborne separation applications, and airborne self-separation.
Airborne traffic situational awareness is aimed at enhancing the flight crews' knowledge of the surrounding traffic
situation both in the air and on the airport surface. In airborne spacing applications, the controller instructs flight
crews to achieve and maintain a given spacing with designated aircraft. In airborne separation applications, the
controller instructs flight crews to maintain separation from designated aircraft. In airborne self-separation
applications, flight crews bear the responsibility of separating their own aircraft from all surrounding traffic.
Research on airborne traffic situational awareness has partly focused on presenting additional air traffic
information to flight crews. Some information, such as the position and velocity of surrounding aircraft, can be
obtained either through Automatic Dependent Surveillance - Broadcast (ADS-B)4, which is broadcast by the
surrounding aircraft themselves, or through Traffic Information Service - Broadcast (TIS-B) 5, which can be
broadcast by radar-equipped ground stations on behalf of transponder-equipped aircraft that are incapable of
broadcasting ADS-B data. Once obtained, this information can be displayed on cockpit displays of traffic
information (CDTI)6. Several human factor studies related to CDTI have been undertaken by NASA7,8,9.
Airborne spacing applications have been researched from multiple perspectives. Some work has investigated
in-trail spacing by using automated control laws without humans-in-the-loop. This is done partly to isolate the
dynamics between spaced aircraft10,11,12 and partly to identify critical characteristics of the data link from the aircraft
being followed13,14. The use of human flight crews in spacing applications has studied pilot effectiveness, workload,
and trust15,16,17. Similar studies have also been undertaken from the controller perspective18,19,20. Interestingly, the
controllers studied in Ref. 19 preferred issuing maneuvering instructions to flight crews rather than delegating the
spacing responsibility to them.
Airborne separation has been investigated to determine whether it can increase controller availability and
enhance flight crew situational. Some research has focused on the communications procedures to be used to enact
controller-designated separation21,22. Controller-designated separation instructions sometimes consist of two
instructions: an explicit maneuvering instruction that redirects the aircraft around another aircraft, and an instruction
to resume course afterwards22,23. In another case, the separation instruction includes the aircraft to avoid yet leaves
the maneuver selection to the flight deck21.
Airborne self-separation applications have previously been studied under the names of “free flight”24,25 and
“distributed air/ground traffic separation and management (DAG-TM)”26,27. The issues identified have ranged from:
making aircraft situationally aware through data links28; identifying conflicts based on trajectory predictions29 and/or
intent information30; and resolving trajectory conflicts by independent maneuvering of individual aircraft31,32,
maneuvering of pairs of aircraft through trajectory negotiations33, maneuvering of pairs of aircraft by standard
maneuvers33,34, and maneuvering as instructed by controllers who exercise positive control33.
¶
FAA Industry Day Briefing on Surveillance and Broadcast Service, June 19, 2006
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American Institute of Aeronautics and Astronautics
This work is covered under airborne situational awareness and airborne spacing applications.
III. Approach to Airborne Merging and Spacing
The approach taken in this work is to divide the area surrounding an airport into two regions: an outer,
cylindrical region, and an inner, cylindrical region (see Fig. 1). Arriving aircraft pass through the outer region
before passing through the inner region. An “arrival scheduler” is responsible for directing all arriving aircraft to the
airport by issuing airborne merging and spacing (AM&S) instructions to the aircraft. Aircraft are then responsible
for executing the AM&S instructions.
Outer Region
Inner Region
Merging
Spacing
Figure 1.
The area around an airport is
divided into two regions. Multiple flight
streams are merged within the outer region.
The merged stream of aircraft are then
spaced within the inner region.
A. Arrival Scheduling
The arrival scheduler instructs aircraft to approach the airport via merge points, which are located on the
boundary of the inner region, at specific times. These times are referred to as scheduled times of arrival (STA). It is
assumed that if aircraft properly execute these merge instructions then the arrival scheduler will have created a
desired sequencing and spacing of aircraft as they enter the inner region. Upon entering the inner region, each
aircraft will space itself behind another aircraft that has been assigned by the arrival scheduler. This arrangement is
intended to ensure a stream of safely-separated aircraft all the way to the airport. In summary, the instruction from
the arrival scheduler to the aircraft includes a STA to a specific merge point, the identification of the lead aircraft to
follow after the merge, and the type of spacing to maintain with the lead aircraft within the inner region.
The arrival scheduler operates under a set of constraints which are configurable. These constraints are:
•
the radius of the outer region;
•
the maximum permissible groundspeed within the outer region;
•
the radius of the inner region;
•
the maximum permissible groundspeed within the inner region;
•
the number and location of merge points along the inner region boundary;
•
the approach paths from the merge points to the airport;
•
the maximum arrival rate to each merge point; and,
•
the type of spacing to assign to aircraft passing through the inner region.
The arrival scheduler maintains a list of arrival times to each merge point. The maximum arrival rate to each
merge point effectively determines the minimum amount of time-separation expected at each merge point. When
arriving aircraft enter the outer region, the arrival scheduler selects a merge point for the aircraft. The arrival
scheduler then estimates the travel time for an aircraft to fly a direct path to the merge point based on the maximum
permissible groundspeed within the outer region. If the estimated time of arrival to the merge point does not violate
the minimum amount of time-separation from the preceding aircraft's merging time then the aircraft is instructed to
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arrive at the merge point at the estimated time. Otherwise, the aircraft is instructed to arrive at the preceding
aircraft's arrival time plus the minimum time-separation duration. These instructions, if properly executed by the
aircraft, ensure aircraft sequencing and a maximum arrival rate at the inner region boundary. After merging, aircraft
initiate spacing behind whichever lead aircraft was assigned by the arrival scheduler.
B. Spacing
The simulated aircraft execute spacing instructions by employing a speed-control law similar to those described
in Refs. 10, 11, 12, and 13. This control-law alters the aircraft speed in order to properly space itself behind the
designated lead aircraft. This control-law is a function of the difference between the aircraft's current, longitudinal
position and velocity versus the lead aircraft's longitudinal position and velocity as it was a fixed amount of time in
the past, t spacing . The difference in longitudinal position, y t , at time t is calculated by
yt= ylead t− tspacing − ytrail t
(1)
where ytrail t is the position of the trailing aircraft at time t , and y lead t−t spacing is the position of the lead
aircraft at time t−t spacing . The difference in longitudinal speed, V t , at time t is calculated by
Vt =V lead t−t spacing −Vtrail t
(2)
where Vtrail t is the velocity of the trailing aircraft at time t , and Vlead t −t spacing is the position of the lead
aircraft at time t−t spacing . (Note that the positions and velocities are measured relative to a one-dimensional, inertial
reference frame centered on the trailing aircraft. Within the three-dimensional world of ACES, positions and speeds
must be transformed into this frame in order to be able to use this formulation.)
The commanded speed, V CMD , that the trailing aircraft should follow in order to space itself t spacing behind the
lead aircraft is defined by
VCMD t=Vtrail t V rel when yt≥ ythreshold
(3a)
max
VCMD t=Vtrail t yt V t when y threshold∣ yt∣≥0 n.mi.
(3b)
VCMD t=max {Vtrail t−Vrel , V min } when yt≤− ythreshold
(3c)
max
where
=
Vrel
max
ythreshold
;
ythreshold is a threshold value used to switch between the two speed-control laws and distinguishes when an aircraft
is “close to” versus when it “far from” the lead aircraft; Vrel is the prescribed maximum longitudinal relative
speed; V min is the prescribed minimum groundspeed for the aircraft type; and, is a dimensionless parameter. In
this work: the value of ythreshold is set to 5 n.mi.; the value for Vrel is set to 100 knots; the value for Vmin is set
to 150 knots IAS for jet aircraft; and, the value of is set to 1. As a result of being set to 1, the trailing
aircraft's commanded speed, VCMD , is effectively set to match the lead aircraft's speed with a speed correction
proportional to the difference in longitudinal position, yt .
max
max
Flight A
10:00
10:01
10:02
10:03
10:04
10:05
Flight B
10:02
10:03
10:04
10:05
10:06
10:07
2 minutes
behind
Figure 2.
For time-based airborne spacing, following aircraft track the historical position of the lead aircraft.
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C. Aircraft situational awareness
In order for an aircraft to execute this speed-control law, it must have knowledge about the lead aircraft's
position for some time, t spacing , in the past. The position of surrounding aircraft can be periodically updated when
those aircraft broadcast their state information via ADS-B4, or when ground stations transmit this information via
TIS-B5. Storing this information for a duration exceeding t spacing will then assure that the trailing aircraft has
sufficient data to create a historical trajectory of the lead aircraft and to execute the speed-control law described in
Section III.B.
Flight A
10:00
10:01
10:02
10:03
10:04
10:05
Figure 3.
Situational awareness achieved through simulated broadcasts of Automatic Dependent Surveillance
Broadcast (ADS-B) messages. Surrounding aircraft receive the ADS-B messages and only process and/or retain
those messages from flights of interest.
D. Merging
Upon receipt of a merge instruction, each aircraft estimates the average groundspeed that would be required to
directly fly to the merge point. This groundspeed is then calculated as an indicated airspeed (IAS) appropriate for
the aircraft's current altitude35. (In this work, merging is defined to a vertical line at a particular longitude and
latitude.) If the average IAS is greater than a prescribed minimum IAS then the aircraft plans a direct route from the
current location to the merge point. If, on the other hand, the average IAS is less than the prescribed minimum
speed then a delaying maneuver, in the form of a right-hand holding pattern, is prefixed to the direct route to the
merge point such that the average IAS along the combined, longer route equals the prescribed minimum IAS.
Periodically selected waypoints along the planned route are then time-shifted and stored in the same state queue
that is used to store a lead aircraft's trajectory. The result is that the merging is accomplished by having the aircraft
space itself behind a virtual lead aircraft that is executing its own nominal trajectory some time, t spacing , in the past.
Virtual
Flight A
10:00
10:01
10:02
10:03
10:04
10:05
Flight B
10:02
10:03
10:04
10:05
10:06
10:07
2 minutes
behind
Virtual Flight A
Figure 4.
For airborne merging, an aircraft plots a route to the merge point. The planned route is used to
generate a virtual lead aircraft to follow.
IV. Implementation of Airborne Merging and Spacing in ACES
ACES v.4.63 was modified to implement the AM&S activities described above. ACES is a non-real-time,
computer simulation of local, regional, and nationwide factors covering aircraft operations from gate departure to
gate arrival. The overarching objective of ACES is to provide a flexible simulation and modeling environment for
the national airspace system (NAS) that can assess the impact of new tools, concepts, and architectures, including
those that represent a significant departure from the existing NAS operational paradigm. To meet this objective,
ACES utilizes a distributed architecture and agent-based modeling to create the large scale, distributed simulation
framework necessary to support NAS-wide simulations. The foundational core of ACES models the physics and
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structure of the NAS. This includes models of 1)
flight physics, 2) airspace configuration such as
airways and various air traffic control regions, 3)
airport configurations such as arrival/departure
rates, runway configurations and surface
configurations, 4) weather and environmental
factors such as winds, and the impact of weather
on en-route and airport capacities, 5) flight
demand and schedules. In addition to this core
functionality, ACES is also intended to model
command and control entities in the NAS and
how they communicate and interact with the
physical and structural model of the NAS and
with other command and control entities. For
this capability, ACES explicitly models
communications and information flow, thereby
ACES simulation lab at Intelligent
allowing ACES to be used to study the dynamic Figure 5.
interactions between agents in the NAS and to Automation Inc.
assess how local disruptions may propagate system-wide. This capability of ACES can also be used to evaluate
alternative roles and activities for command and control agents in the NAS. An important feature of ACES is the
ability to represent the forecasting ability that command and control agents use to make decisions.
ACES is built on IAI's CybelePro, which is an an agent-based modeling and simulation infrastructure. At the
lowest level of the ACES architecture is CybelePro, which provides a set of services related to communication,
event management, time management, and data distribution required to handle aspects related to distributed
simulation. CybelePro also provides the modeling framework and a software layer between the models and the
underlying distributed simulation framework. The applications layer of the architecture consists of applications built
on this common core infrastructure. This includes simulation containers as well as utilities such as simulation
control, visualization, local data collection, centralized logging, and the profile analysis tools. Simulation
configuration and set-up is through the Multiple Run user interface. The Multiple Run system is a support tool for
ACES that provides the capability to perform multiple simulation executions without user intervention. A user can
specify a single or series of runs to be executed on a set of machines. The MultipleRun system has a graphical user
interface (GUI) that allows the user to configure and schedule ACES simulation runs.
ACES development has been ongoing for several years, with the first version of ACES, Build 1, being released
in March 2003. The most recent version of ACES, Build 5, was released in October 2007. In an ongoing effort the
architecture of ACES is currently being modified to support development of Next Generation Air Transportation
System (NGATS) related concepts.
A. Limitations of ACES 4.6 in Implementing Airborne Merging and Spacing
ACES 4.6 has a couple of limitations that conflict with airborne Merging and Spacing near the terminal area.
The first limitation is a result of ACES 4.6 terminating the flight simulation once the aircraft reaches an arrival fix
that is situated 40 n.mi. away from the arrival airport at 10,000 feet. The remainder of the of the flight is coarsely
represented by some amount of time spent descending within the terminal area, and some amount of time moving on
the airport's surface. This limitation is problematic since we are interested in modifying aircraft spacing within 40
n.mi. of the airport and down to the airport surface.
The second limitation pertains to how ACES 4.6 constrains aircraft descents from the cruising altitude to the
arrival fix situated at 10,000 feet. First, aircraft descents are performed as idle thrust descents from cruise altitude to
the arrival fix altitude. Second, aircraft speeds are constrained to less than 250 knots once an aircraft descends
below 10,000 feet (which is consistent with U.S. Federal Aviation Regulations 91.117 regarding aircraft speed).
These limitations may be problematic since some portion of the flight will likely need to be powered and speedvaried to accommodate timed spacings and timed arrivals along waypoints within the terminal area.
B. Changes to ACES 4.6 to Accommodate Airborne Merging and Spacing
Several changes and additions were made to ACES in order to accommodate these new merging and spacing
activities.
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1. Arrival scheduling
A special agent was created around every airport executing AM&S operations. This agent effectively created a
unique airspace and ATC around the airport that was responsible for executing the duties described for the “arrival
scheduler” in Section III.A. This agent sends a single merging and spacing instruction to each arriving aircraft. Due
to certain settings in ACES, rerouting instructions are delayed between one to two minutes after an aircraft crosses
into the outer region. This delay effectively reduces that amount of distance available to an aircraft to reroute itself.
2. Navigation planning
An airborne “navigational planner” was added to each aircraft to receive and process the AM&S instruction
from the arrival scheduler. The planner would then prepare a route to the merge point using the logic described in
Section III.D. Two assumptions/constraints were made during these calculations: (a) merging would be performed
at each aircraft's cruise altitude; and, (b) a minimum permissible IAS would be specified sufficiently high so as to
avoid an aircraft stall. This minimum permissible IAS is an important value since it is the threshold speed below
which a delaying maneuver is inserted into the route planning to ensure timely arrival at the merge point.
The planner was also responsible for maintaining situation awareness by broadcasting and receiving and ADS-B
messages, as described in Section III.C.
3. Speed control
Finally, a new “speed controller” has been added to each aircraft in order to calculate the commanded
groundspeed as described in Section III.B. The speed controller also has a safety mechanism that prohibits
commanded speeds that will reduce the IAS below the aircraft's stall speed.
V. Simulation Studies
A. Spacing of a Single Arrival Stream
1. Scenario Description
The sample scenario, which is simulated in ACES 4.6, has four flights departing New York City area airports
(i.e. LaGuardia, JFK, and Newark) and arriving in Atlanta (see Fig. 6). The flights are of different aircraft types
(e.g. Boeing 737, Airbus 319, Airbus 320), all departing a few minutes apart, and flying at different altitudes
(e.g. 29,000 to 39,000 feet). The flights' trajectories are checked at a point situated 12 n.mi. northeast of Atlanta at
12,000 feet (Point A). Figure 7 shows the location of these flights as they approach Point A. No winds were
simulated in this scenario.
To test time-based spacing, the previously described speed-control law was applied to the same set of flights.
When using the speed-control law described above, the aircraft speeds are adjusted such that the aircraft acquire and
maintain two-minute spacings, t spacing , throughout the entire flight. To test this speed control-law, Flight B is
instructed to follow Flight A; Flight C is instructed to follow Flight B; Flight D is instructed to follow Flight C; and
Flight A is simply instructed to follow its flight plan.
2. Scenario Results
Figure 8 shows the location of the speed-controlled flights as they approach Point A. The arrival times at
Point A, both with and without time-based spacing, are shown in the Table 1. Figure 9 shows the spacing of the
Flights B, C, and D (trailing flights) relative to Flight A during the last hour of the flights. At the beginning of the
hour, the trailing flights have just begun acquiring the two-minute spacings. By 22:09, the spacings have been
acquired. Shortly after 22:09, Flight A begins its descent from cruise altitude to the arrival fix altitude. In this
scenario, Flight A was not given a speed limit during its descent, so its speed was allowed to increase significantly.
As a result, the spacings of the flights behind it initially increase like a wave that first affects Flight B then Flight C
and, finally, Flight D. Despite this initial increase in spacing, the other aircraft eventually speed up to re-acquire
two-minute spacings just as they arrive at Point A.
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Table 1: Arrival times of the four flights at Point A
Flight
Without Time-Based
Spacing
With Time-Based Spacing
A
22:43:41
22:43:41
B
22:45:03
22:45:42
C
22:47:41
22:47:44
D
22:45:14
22:49:46
New York City
C
B,D
A
Atlanta
Figure 6.
Overview of flight routes from New
York City area to Atlanta.
B
C
Figure 7.
Aircraft locations arriving at Atlanta
without sequencing or spacing. Although this image
is showing four flights, only three are apparent. This
is because two of the flights, Flights B and D, are
situated nearly at the same longitude and latitude at
this moment in time.
D
A
Figure 8.
Aircraft locations arriving at Atlanta
with sequencing and time-based spacing.
Figure 9.
Spacing of Flights B, C, and D relative
to Flight A during one-hour of flight.
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B. Merging and Spacing of Two Arrival Streams
1. Scenario Description
This scenario demonstrates AM&S activities for a set of 13 aircraft arriving at Louisville, KY (KSDF). Nine of
the aircraft depart from Reno, NV (KRNO) while following the same flight plans with the one difference being that
their departure times differ by one minute (see Fig. 11). Two aircraft depart from Albuquerque, NM (KABQ)
following the same flight plans (see Fig. 13) with the one difference being that their departure times differ by three
minutes. One aircraft departs from San Diego, CA (KSAN), and another departs from Santa Ana, CA (KSNA) (see
Fig. 12). All the aircraft in this scenario are being modeled within ACES as Boeing 757 aircraft. As is visible in
Fig. 14, the two streams of aircraft from southern California and New Mexico converge prior to entering the outer
region. As a result, two streams of aircraft arrive at Louisville, KY. No winds are simulated in this scenario.
The AM&S properties for this scenario are shown in Table 2. Upon entering the outer region, the arrival
scheduler will merge the aircraft to a point located due west of the airport along the inner region boundary. Their
arrival sequence to the merge point is assigned on the order of entry into the outer region (i.e. first come, first
served.) The STA to the merge point for the first aircraft is calculated assuming that the aircraft will travel at the
maximum groundspeed allowed within the outer region, 400 knots. All subsequent aircraft will be scheduled to
arrive at the merge point no sooner than two minutes later. (This minimum interval, t spacing , of two minutes is
calculated directly from the maximum arrival rate permitted at the inner region merge point of 30 aircraft per hour.)
Upon entering the inner region, all flights, except the first (KRNO1), execute time-based spacing until arrival at
the airport. Time-based spacing is assigned by the arrival scheduler upon entering the outer region but only initiated
after merging. Simultaneous with executing airborne spacing, flights descend from their corresponding cruise
altitude to 8,000 feet so as to prepare for landing. Simulation of the flights are stopped when aircraft are within
5 n.mi. of the airport.
Table 2: AM&S properties for Section V.B
Outer Region Radius
400 n.mi.
Maximum Groundspeed in Outer Region
400 knots
Minimum IAS in Outer Region
200 knots
Inner Region Radius
200 n.mi.
Groundspeed in Inner Region
300 knots
Arrival Rate at Inner Region Merge Point
30 aircraft per hour
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KRNO5
KRNO6
KRNO7
KRNO9
Reno
KRNO4
KRNO3
KRNO2
KRNO1
KRNO8
Louisville
Reno
Santa Ana
Albuquerque
San Diego
Figure 10. Overview of flight routes from
Reno, NV, San Diego, CA, Santa Ana, CA, and
Albuquerque, NM to Louisville, KY.
Figure 11.
Reno, NV.
Aircraft locations departing from
KSNA1
KSAN1
KABQ1
KABQ2
Santa Ana
KSNA1
Albuquerque
San Diego
KSAN1
Figure 12. Aircraft locations departing from
San Diego, CA and Santa Ana, CA.
Figure 13. Aircraft locations departing from
Albuquerque, NM.
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KRNO5
KRNO6
KRNO4
KRNO7
KRNO3
KRNO2
KRNO8
KRNO9
KRNO1
KABQ1
KSAN1
KSNA1 KABQ2
Figure 14. Aircraft routes before entering the outer
region. Note that the routes for the four southern
flights converge to a common route before entering
the outer region.
2. Scenario Results
Table 3 shows various times for the simulated flights. These times include the time of takeoff; the time of
entering the outer region; the STA to the inner merge point as issued by the arrival scheduler; the actual time of
arrival to the inner merge point as executed by the aircraft, and whether a delaying maneuvering was inserted by the
aircraft in order to achieve that arrival time. Note that some of the flights inserted a delaying maneuver upon
receiving a STA from the arrival scheduler. These delaying maneuvers took the form of a holding pattern on the
right-side of the original route (see Fig. 15).
Figure 15 shows the aircraft locations prior to the first flight (KRNO1) arriving at the merge point. The figure
clearly shows how the stream of nine flights from Reno, NV create gaps along their route that will be filled in by the
four other flights at the merge point.
Figure 16 shows the aircraft locations after all the flights have merged to a single stream within the inner region.
Note how the aircraft are equally spaced. Figure 18 shows the time-based spacing of each flight relative to the first
flight (KRNO1) while passing through the inner region. Note how the aircraft are spaced two-minutes apart.
Figure 17 shows the groundspeed of each aircraft upon entering the outer region. There are two speed
transitions for each flight. The first speed transition occurs when the flight enters the outer region as it alters its
speed such that it arrives at its STA to the merge point. The second speed transition occurs when the flight enters
the inner region (after merging) and executes time-based spacing relative to the lead aircraft assigned by the arrival
scheduler. Note the second speed transitions are marked by spikes in the speeds when airborne spacing is initially
engaged. These speeds spikes are a result of the initial relative speeds of the lead aircraft, which is coming from one
direction, relative to the trailing aircraft, which is coming from another direction. This behavior is not exhibited by
the first aircraft to the merge point, KRNO1, since it has no lead aircraft to follow. Neither is it exhibited by the
second aircraft to the merge point, KABQ1, because its original flight direction to the east is largely maintained as it
moves in behind KRNO1. This speed spike behavior begins when the third aircraft to the merge point, KRNO2,
moves behind KABQ1. This is a result of the ability of KRNO1 to maintain its easterly flight while KRNO2 must
turn from the southeast to the east. During this turn, KABQ1 is relatively speeding away. Since the speed control is
partly a function of the relative speed, a higher speed is commanded. The aircraft surges to the east, then overshoots
its location behind KABQ1, and finally settles in behind it. This behavior repeats for all subsequent flights from the
northwest (i.e. all those from KRNO). The remaining flights from the west (i.e.KABQ1, KSNA1, and KSAN1)
exhibit the opposite behavior in that they initially get too close to their lead aircraft. Again, those lead aircraft,
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which are coming from the northwest, are initially moving too slowly to the east because they are still turning left
towards the east.
Table 3: Data for Section V.B. Flights are sorted by arrival times to the merge point.
Flight
Takeoff Time
Outer Region
Entry Time
Scheduled Time
of Arrival to
Inner Region
Merge Point
Delay
Maneuver
Inserted by
Flight
Actual Time of
Arrival to Inner
Region Merge
Point
KRNO1
01:21:00 EDT
03:54:00 EDT
04:28:12 EDT
No
04:28:20 EDT
KABQ1
02:28:00 EDT
03:55:00 EDT
04:30:12 EDT
Yes
04:30:15 EDT
KRNO2
01:22:18 EDT
03:56:00 EDT
04:32:12 EDT
No
04:32:20 EDT
KRNO3
01:23:36 EDT
03:57:00 EDT
04:34:12 EDT
No
04:34:20 EDT
KABQ2
02:31:00 EDT
03:58:00 EDT
04:36:12 EDT
Yes
04:36:15 EDT
KRNO3
01:24:54 EDT
03:59:00 EDT
04:38:12 EDT
No
04:38:20 EDT
KRNO3
01:26:13 EDT
04:01:00 EDT
04:40:12 EDT
No
04:40:20 EDT
KRNO4
01:27:31 EDT
04:02:00 EDT
04:42:12 EDT
No
04:42:20 EDT
KSNA1
01:28:00 EDT
04:03:00 EDT
04:44:12 EDT
Yes
04:44:10 EDT
KRNO5
01:28:49 EDT
04:04:00 EDT
04:46:12 EDT
Yes
04:46:10 EDT
KRNO6
01:30:07 EDT
04:05:00 EDT
04:48:12 EDT
Yes
04:48:10 EDT
KSAN1
01:34:00 EDT
04:05:00 EDT
04:50:12 EDT
Yes
04:50:10 EDT
KRNO7
01:31:26 EDT
04:07:00 EDT
04:52:12 EDT
Yes
04:52:10 EDT
KRNO4
KABQ2
KRNO3
KRNO2
KABQ1
KRNO9
KRNO8
KRNO7
KRNO6
KRNO5
KRNO4
KRNO1
KRNO3
KRNO2
KSAN1 KSNA1
KABQ2 KABQ1
KRNO1
KRNO9
KSAN1
KRNO8
KRNO7
KSNA1
KRNO6
KRNO5
Figure 15. Aircraft locations prior to the first
aircraft (KRNO1) arriving at the merge point.
Figure 16. Aircraft locations prior to the first
aircraft (KRNO1) arriving at the merge point.
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American Institute of Aeronautics and Astronautics
Figure 17. Groundspeed profiles [knots] for each flight in the scenario upon entering the
outer region. Note that there are two speed transitions for each of the flights. The first
speed transition occurs when the flight enters the outer region to prepare for merging. The
second speed transition occurs when the flight enters the inner region (after merging) to
execute time-based spacing.
Figure 18. Time-based spacing [min] of each aircraft relative to the first flight to enter
the inner region (i.e. Flight KRNO1). Time-based spacing shown only while the aircraft is
within the inner region which is when time-based spacing relative to the preceding aircraft
is performed.
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American Institute of Aeronautics and Astronautics
Figure 19. Altitude profiles [ft] for each flight in the scenario upon entering the outer
region. Note that some of the flights are initially cruising at 36,000 ft. whereas others are
initially cruising at 33,000 ft.
Figure 20. Distance-to-go profiles [n.mi] of each flight in the scenario upon entering the
outer region. All of the distances are initially decreasing as the aircraft approach the
arrival airport. Note, however, that some of the aircraft's distances experience a
temporary increase in distance-to-go at approximately 380 n.mi. followed by a monotonic
decrease to zero. This temporary increase is incurred by the aircraft that execute a
delaying maneuver in the form of a holding pattern. Also note how the sequence of the
aircraft remains unchanged from 200 n.mi. out (i.e. the boundary of the inner radius) down
to 0 n.mi. (i.e. airport.)
C. Merging and Spacing of Multiple Arrival Streams
3. Scenario Description
This scenario investigates what effect that the outer region radius has on the number of delaying maneuvers that
an aircraft must execute in order to arrive at either merge point at the time specified by the arrival scheduler. This
14
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will be investigated by simulating a set of 61 aircraft arriving at Louisville International Airport (KSDF) and
departing from airports across the U.S. The flight routes used were the actual routes and cruise altitudes flown on
May 17, 2002 to Louisville, KY although shifted in time in order to increase traffic density during a few hours of the
day. All the aircraft in this scenario are being modeled within ACES as Boeing 727 Stage 3 aircraft. Finally, no
winds are simulated in this scenario.
The AM&S properties used for this scenario are shown in Table 2. Note that the outer region radius is being
used as a parameter for each run of the simulation. The number of flights that implement a delaying maneuver is
then reported for each outer region radius that is simulated.
Upon entering the outer region, the arrival scheduler instructs all aircraft to merge to either of two points along
the inner region boundary: due west of the airport, or due east of the airport. Flights arriving from the east are
assigned to the eastern merge point, and flights arriving from the west are assigned to the western merge point. In
effect, there are two outer regions: an eastern half and a western half. Their arrival sequence to each merge point is
assigned on the order of entry into either the eastern or western outer region (i.e. first-come, first-served.)
Table 4: AM&S properties for Section V.C
Outer Region Radius
See Table 5
Maximum Groundspeed in Outer Region
400 knots
Minimum IAS in Outer Region
200 knots
Inner Region Radius
60 n.mi.
Groundspeed in Inner Region
300 knots
Arrival Rate at Inner Region Merge Point
30 aircraft per hour
Outer Region
Boundary
Inner Region
Boundary
Eastern
Merge Point
Routes
Louisville
Holding
Patterns
Figure 21. Aircraft routes around Louisville International Airport
with a 20 n.mi. inner region radius and a 40 n.mi. outer region radius.
Note the holding patterns that must be performed by some flights in
order to arrive at the eastern merge point at the time issued by the
arrival scheduler.
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American Institute of Aeronautics and Astronautics
4. Scenario Results
Table 5 displays the number of flights that implement a delaying maneuver as a function of the outer region
radius. As the outer region radius increases, the number of delaying maneuvers is observed to decrease. This was
expected because the larger outer radius results in longer distances between all points along the outer region
boundary and the merge point located on the inner region. Longer distances require linearly more time to traverse if
speed remains constant. Despite the longer distances, the delaying maneuvers did not decrease to zero as the outer
region radius increased to 200 n.mi.
Table 5: Number of Delaying Maneuvers as a Function of the Outer Region Radius
Outer Region Radius [n.mi.]
Number of Delaying Maneuvers
80
15
100
11
120
10
140
7
160
5
180
4
200
4
An explanation for the persistence of delaying maneuvers can be found by inspecting the scenario when the
outer radius was set to 80 n.mi. A summary of the data for flights assigned to the western merge point can be found
in Table 7. All aircraft entered the outer region along different points of the outer region. Consequently, the
distance to the merge point was different for each aircraft. The arrival scheduler scheduled times-of-arrivals such
that all flights were separated in time at the merge point by at least 2 minutes. (This minimum interval of two
minutes was calculated directly from the maximum arrival rate permitted at the inner region merge point of 30
aircraft per hour.) The difference between the STA and the rerouting time is the scheduled travel duration allotted to
the aircraft. The quotient of the distance to the merge point with the scheduled travel duration is the average
scheduled groundspeed for a direct route between the aircraft's current location to the merge point.
In order to determine whether a delaying maneuver is required, the scheduled groundspeed must be compared
with the minimum permissible IAS of 200 knots. However, for proper comparison, these speeds must be expressed
in a common speed frame. The common speed frame used in Table 7 is that of groundspeed. The conversion of
indicated airspeed to groundspeed takes into account four factors: (a) calibration errors of the speed sensor aboard
the aircraft; (b) variations of air density with temperature; (c) variations of air density with pressure; and, (d) winds.
In this work, the following assumptions were made: (a) aircraft have perfectly calibrated speed sensors; (b) aircraft
fly within a standard atmosphere; and (c) there are no winds. Therefore, the indicated airspeed was influenced
solely by the variation of air density with respect to temperature and pressure as a function of altitude. This
variation is well defined by the International Standard Atmosphere. With these assumptions, Table 6 displays the
the equivalent groundspeed of 200 knots IAS at different altitudes above mean sea level.
Table 6: Groundspeed as a function of altitude for aircraft traveling at 200 knots IAS
0 [feet]
10,000 [feet]
20,000 [feet]
30,000 [feet]
40,000 [feet]
200 knots
232 knots
271 knots
319 knots
385 knots
The data in Table 7 indicates that the aircraft requiring delaying maneuvers had scheduled groundspeeds less
than the minimum permissible groundspeed for the aircraft's cruise altitude. (This is to be expected because, as
defined in Section III.D, this was the trigger for requiring a delaying maneuver.) Similarly, the aircraft that did not
require delaying maneuvers had scheduled groundspeeds greater than the minimum permissible groundspeed. These
observations held for all of the flights in all of the scenarios. The reader is reminded that the fastest aircraft within
the outer region will have an average groundspeed of 400 knots. All other aircraft that need to delay must travel
slower than this speed. In this scheme of merging and spacing, an aircraft traveling at 40,000 feet can only slow
down by 15 knots (see Table 6) in order to avoid a delaying maneuver.
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Table 7: Data for flights directed to the western merge point for Section V.C
Flight
Outer Region
Entry Time
35
36
42
37
50
44
49
75
47
73
39
54
87
84
86
88
89
91
76
92
59
85
52
65
23:35:00
0:24:00
0:54:00
0:56:00
1:31:00
1:42:00
2:00:00
2:12:00
2:16:00
2:19:00
2:29:00
2:34:00
2:49:00
3:05:00
3:14:00
3:47:00
3:50:00
3:50:00
4:19:00
4:25:00
4:35:00
4:38:00
4:41:00
4:47:00
Flight to
Follow
within
Inner
Region
49
75
88
89
76
59
52
Scheduled
Scheduled
Minimum Permissible
Delay
Time of
Travel
Distance to
Cruise Groundspeed at Cruise
Scheduled
Maneuver
Arrival to
Duration to Merge Point
Altitude
Altitude [knots]
Groundspeed [knots]
Inserted
[n.mi.]
[feet]
(Converted from
Inner Region Merge Point
by Flight
[s]
200 knots IAS)
Merge Point
23:36:33
93
10.35
400
29,000
314
No
0:30:20
380
42.22
400
29,000
314
No
1:00:12
372
41.34
400
23,000
284
No
1:08:46
766
85.15
400
33,000
336
No
1:37:21
381
42.38
400
29,000
314
No
1:43:20
80
8.92
400
33,000
336
No
2:14:02
842
93.61
400
14,000
246
No
2:16:02
242
12.08
179
25,000
293
Yes
2:18:02
122
8.77
258
33,000
336
Yes
2:26:51
471
52.34
400
25,000
293
No
2:30:50
110
12.28
400
37,000
361
No
2:35:13
73
8.16
400
33,000
336
No
2:50:55
115
12.78
400
27,000
303
No
3:14:08
548
60.94
400
33,000
336
No
3:22:09
489
54.38
400
37,000
361
No
3:59:11
731
81.21
400
37,000
361
No
4:01:11
671
7.44
40
33,000
336
Yes
4:03:11
791
79.93
364
33,000
336
No
4:27:08
488
54.27
400
33,000
336
No
4:29:08
248
8.42
122
37,000
361
Yes
4:42:57
477
53.03
400
33,000
336
No
4:44:57
417
11.87
102
33,000
336
Yes
4:47:20
380
42.27
400
37,000
361
No
4:49:20
140
8.91
228
37,000
361
Yes
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American Institute of Aeronautics and Astronautics
69
66
79
78
82
83
4:49:00
5:00:00
5:04:00
5:09:00
5:24:00
5:24:00
65
66
79
4:51:20
5:07:54
5:09:54
5:11:54
5:25:23
5:30:19
140
474
354
174
83
379
12.49
52.70
12.18
8.05
9.29
42.12
320
400
124
166
400
400
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American Institute of Aeronautics and Astronautics
33,000
33,000
33,000
33,000
33,000
33,000
336
336
336
336
336
336
Yes
No
Yes
Yes
No
No
VI. Conclusion
In this effort, we successfully developed, prototyped and demonstrated a simulation testbed for merging and
spacing in NASA’s Airspace Concept Evaluation System (ACES) software to support NASA’s Airportal research.
The merging and spacing modules consists of two components: a ground-based scheduling tool that generates arrival
sequences and spacing requirements for aircraft entering a terminal airspace; and, an airborne component that
merges aircraft, as necessary, and then maintains the requisite spacing within the merged stream. We demonstrated
the feasibility of using the simulation testbed to investigate airborne merging and spacing, and the effect of control
horizons on the ability of aircraft to absorb delays.
It is interesting to consider the available options for delaying an aircraft's arrival. Two approaches evaluated in
this work and demonstrated in these scenarios were to slow an aircraft at its current altitude, and to insert delaying
maneuvers at an aircraft's current altitude. An alternative that might be considered in future work is for an aircraft to
descend to lower altitudes where slower groundspeeds are feasible with less risk of stalling. Interestingly, this is a
side effect of what occurs when ATC directs aircraft to perform stepped descents for arrival.
Acknowledgments
We would like to thank Yingchuan Zhang, a Research Scientist at Intelligent Automation Inc., for her assistance
and insight with investigating the root cause of the delaying maneuvers in Section V.C.
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