ENRI Int. Workshop on ATM/CNS. Tokyo, Japan (EIWAC2010)[EN-102] Study on Traffic Synchronization+

Claus Gwiggner

ATM Department
Electronic Navigation Research Institute (ENRI)
Tokyo, Japanclaus@enri.go.jp
Abstract We give a short overview of recent activities in the field of traffic synchronization. Traffic synchronizationis concerned with the tactical management of queues, both on the ground and in the air, in order to establish andmaintain a safe, orderly and efficient flow of air traffic. We describe the major applications for traffic synchronizationin Japanese airspace: arrival synchronization to Haneda airport and coordination between several departure airports.
Finally, we briefly summarize our own results, namely the quantification and interpretation of metering delays bystochastic queueing models and the trade-off between low altitude (fuel inefficient) high altitude (fuel efficient) delay
absorption.
Keywords Traffic Synchronization, Queue Management, Delay Propagation, Stochastic Model1

INTRODUCTION

1

There is worldwide activity in research and development of modern Air Traffic Management (ATM)systems [1, 2]. The reasons include that airspace demand keeps growing, technology is improving, andsociety raises new expectations, such as the environmentally sustainable air transportation.
The International Civil Aviation Organization (ICAO)
proposes several components of a modern ATM system that are based on expectations of human capabilities and the ATM infrastructure [3]. One of thesecomponents is called ‘Traffic Synchronization’. It is
described as the ‘tactical establishment and maintenance of a safe, orderly and efficient flow of air traffic’. Thus, traffic synchronization takes place at the
day of operations. According to the ATM MasterPlan [4] (electronic version, frequently updated), it
contains the following major functions:• Departure Synchronization: the departure timeof aircraft is adjusted. Optimizing departure sequences under uncertainties of push-back, startup and taxi times and in coordination with surrounding airports and airspace structure will use
the available capacity more efficiently.
• Arrival Synchronization: the en-route trajectories are re-optimized. Precise sequencing is performed by setting time constraints on arrivaland en-route merging points.
• Interactions between arrivals and departures: Enabling complementary and optimized spacingof arrivals and departures, minimizing airborne
delays and also minimizing departure queuing.
1 This is a reprint of the paper published at ENRI’s Annual Re-

search Meeting (Happyo Kai 2010)149

As can be seen, traffic synchronization includes the
management and provision of queues both on the groundand in the air. It is interrelated with two other components, namely conflict management and demand andcapacity balancing, and may in the future be indistinguishable from them [3].
The main enablers for traffic synchronization are
improvements in 4D-trajectory prediction and control,
and a collaborative way of distributing congested airspaceresources. Future flight management systems will provide a required time of arrival (RTA) function, allowing the aircraft to navigate with higher time precision through the airspace than today. Based on this,flow managers can compute time-windows, in whichaircraft shall pass designated way-points. Moreover,in the framework of Collaborative Decision Making(CDM), future operations such as slot swapping or
trajectory negotiations, are also expected to smooth
the airspace demand.
The main expected benefit of traffic synchronization is a better usage of the available capacity. This
leads to more fuel and workload efficiency, because it
will create smoother flows.
The remainder of the article is organized as follows: in the next section we describe on-going activity in the creation of concepts of operations for traffic synchronization. Then, we explain the possibilities, where traffic synchronization might be useful inJapanese airspace. This is followed by a summary of
our own research results. Finally, we summarize thearticle and point out some ideas for future work.

2

CONCEPT OF OPERATIONSA Concept of Operations (ConOps) is a document
describing the characteristics of a proposed systemfrom the viewpoint of an individual who will use that

Claus Gwiggner

speed to meet a CTA. As a comparison, MD80 aircraft, which do not have the RTA functionality, werealso involved in the trials to investigate how non-equipped
aircraft could be handled in CTA operations. Of alltrials, 90% met their assigned time with 30 seconds
accuracy and 97% met their CTA with 60 seconds accuracy (see Figure 1). Moreover, some flights took
place from a nearby airport, where delays were assigned prior to takeoff. When there was a peak situation, these flights were able to bypass the holding
stack. Pilots were satisfied with the larger flexibilityof planning the arrival phase, and with fewer times
spent inside holding stacks. On the other hand, theamount of communication between ATC and the aircraft, and the workload inside the cockpit increased.
The CASSIS project recommended that further work
(trials, simulations and concept discussions) have to
be carried out to quantify the benefits to be obtained
from CTA use.

Figure 2 Target Windows (Source: [9]).

Figure 1 CTA error distribution (Source: [7]).

system. There are many projects related to traffic synchronization that build on new technology or innovative concepts and produce concepts of operations(see for example the Queue-Management category in
SESAR’s Joint-Undertaking web-page [5] or the Time
Based Flow Management and extensions in the U.S.[6]).
The following two examples give a short backgroundon the research behind the ConOps in traffic synchronization.

CATS Target Windows (TW), proposed by the Contractbased Air Transportation System project (CATS), arespatial and temporal constraints placed between sectors at different parts of the flight plan of each aircraft (Fig. 2), whose position and interval size reflectsconstraints imposed by downstream components suchas punctuality at destination, congested en-route areas, or safety requirements. They represent the commitment of each actor (air traffic controllers, airports,
airlines, air navigation service providers) to delivera particular aircraft within a specific constraint set.
The major aim of the project is to increase punctuality. The major technical steps for the feasibility of theconcept include the quantification of the maneuverabilityfreedom of each aircraft constrained by a specific target window. To this end, simulation and modelingstudies are currently on-going [8].
As mentioned above, these and other projects arein progress [5]. Their target is to identify the conditions, under which the new technologies and paradigms
will prove most beneficial. Based on such results, newConcepts of Operations will be produced.

3• Accuracy of the RTA function (CASSIS project)

Traffic Synchronization in Japan

In Japan, the traffic flows are already pre-scheduled
in order to respect the airspace capacities. But operational uncertainties (e.g. weather, tactical maneuvering) and competition for punctual arrivals may lead totemporary capacity excesses, especially in the vicinityof the airports. This leads to high controller workload
and fuel-inefficiencies.
The specialists in the Air Traffic Management Center (ATMC) in Fukuoka already introduce tools forpredicting, sequencing and optimizing traffic flows,
such as traffic monitors, slot swapping terminals, andso on. Two typical problems in future traffic synchronization are the following:• Target-window concept (CATS project)
CASSIS In the CTA/ATC system integration studies (CASSIS), a large number of flight trials were carried out in Stockholm, Sweden [7]. These trials investigated how well aircraft could meet a controlled
time of arrival (CTA) to a feeder fix, and how theaircraft’s ability to meet this time could be used to
improve arrival management. The trials were carriedout using technologically advanced aircraft including
Boeing 737, Airbus A321 and A330. The RTA function was used to allow aircraft to self manage their150

ENRI Int. Workshop on ATM/CNS. Tokyo, Japan (EIWAC2010)

the change of runway capacity or configuration(due to weather conditions) and the aircraft types
in the sequence.
• Arrival phase: the calculated information is combined with the ATC tools, such as arrival management software.
One expects from the new control techniques amore regular flow, which can be adapted to changing
environmental conditions more efficiently and moresafely.
3.2

Departure synchronization

As mentioned in the introduction, a source of flow
inefficiency is the missing coordination between several airports. For example, sectors sometimes requirelarger spacing than the minimum separations (minutesin-trail separation). The reasons are temporary capacity/demand imbalances, for example due to weather
conditions. For the flow managers, this requires thecoordination of departure times at the airports connected with these sectors. Today, such a task is often carried out by hand. In the future, more efficient
strategies are sought. They also have to be flexible
with respect to airline priorities (e.g passenger flights
vs. cargo flights) and delayed departures (e.g. groundcongestion).
In both projects, one expects workload and fuelreductions through the use of better strategies to coordinate and synchronize traffic flows.

Figure 3 Major Japanese traffic flows.

Figure 4 Lateral inefficiencies of the East-bound flow.

4
3.1

CURRENT RESULTS

At ENRI, we are working on both traffic synchronization projects mentioned above. The work involves
the analysis of radar data and the construction of mathematical models that are helpful for decision-making
in flow control. In particular, we are interested inefficient sequencing/merging strategies under the impact of trajectory-uncertainties and in de-centralized
concepts for the flow control. In this section, resultsfrom our research are briefly summarized. More information can be found in [12, 13, 14], the references
therein or by contacting the authors.

Arrival synchronization

The major source of Japanese air-traffic congestion is the arrival flow to Haneda airport [10]. The
future concept of synchronization of the main arrival
flow to Haneda airport concerns the east-bound arrival
flow, as can be seen in Figures 3 and 4. The traffic
synchronization will be divided into four phases [11]:• Take-off phase: updated departure clearancetimes for all aircraft are calculated, taking into
account the airspace capacities and current traffic predictions.

4.1• Early cruise phase: flight plan predictions are
updated, based on shortcuts, and more accurate
traffic predictions.
• Feeder phase (Osaka/Nagoya airspace): the scheduled time of arrival at the runway threshold isdetermined. Sequencing and spacing information is deduced. Contributing information is

151

Radar data analysis

We analyzed patterns in metering delay from arrivals to Haneda airport [12, 13]. Metering delays occur due to spacing aircraft more than the the minimum
separation at the gate to the terminal area. A major
factor for the metering delays was the traffic density.
Traffic ‘peaks’ exceeding the capacity are handled byvectoring or speed control. These peaks occur spontaneously and are difficult to predict.

Claus Gwiggner

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

freq

Delay per region: radar

Central
South
Int’l
~
dM G 1~
dM D 1

0

1

2

3

4

5

6

7

d (min)n= 2794

Figure 5 Observed and theoretical frequencies of metering delays.

4.2

Queueing-analysis

We modeled the metering delays by classical stochastic queueing theory [12, 13]. Two models were considered, a very simple one (with constant metering
separations), and a more realistic one (with the empirically observed separations). The correspondence
between data and models can be seen in Figure 5. The
horizontal axis depicts the metering delay (unit: minutes). The vertical axis shows the frequency of theoccurrence of these delays. The colored bars are extracted from the radar data. They were classified into
three groups of origin airport (red: central Japan, yellow: south Japan, green: International). The straightlines are the predictions from our queueing models.
Both models produce almost indistinguishable delay
predictions. And both models correspond well to theempirical data. This means that the controllers currently solve the metering problem efficiently, because
the theoretical models represent the minimum delays,
given the first-come-first-served rule. An explanation
for the good results is given in [15]. Based on a more
realistic arrival flow model (with pre-scheduled arrivals), it is found that the congestion properties inmedium traffic densities are similar to the ones of classical queueing theory.
4.3

Delay distribution analysis

We currently analyze efficient strategies for enroute metering delay absorption [13, 14]. In delay absorption, the following trade-off between individual152

and total fuel-efficiency in the presence of trajectory
uncertainty is known from the literature [16]: whenmetering delays are absorbed in high altitudes, fuelburn is minimized for individual flights. But due torandom delay, there is a risk of under-usage of the
runway capacity. Lost landing slots may propagate
back to the remaining aircraft, which increases the total delay, and as a consequence the total fuel burnt.
This means that metering delays have to be distributed
between the low altitudes (fuel inefficient) and highaltitudes (fuel efficient), even when the objective is tominimize fuel consumption.
We analyzed the scheduling process in the presence of trajectory prediction errors (see Figure 6). It
is a complicated process, consisting of initial schedules, absorption buffers, prediction errors and schedule updates (please see [14] for more details).
Our main result is in agreement with Erzberger’s
simulation study [16] and can be seen in Figure 7. The
horizontal axis is the fraction of metering delay thatis absorbed on low altitudes, designed by α ∈ [0, 1].
The vertical axis has two units: additional delays andfuel consumption (both are normalized in our illustration). The green line is the additional delay that occurs due to trajectory prediction errors. It decreases
sharply with increasing fraction of absorbed delay inlow altitude. The intuition is that the low altitude
serves as a buffer for late arrivals. The blue line is thefuel consumption in the case that no trajectory prediction errors occur. In this case, the most fuel-efficient
strategy is to absorb all metering delays in high altitude (α = 0). The red line is the fuel consumption
under the effect of delay propagation. The trade-off
between the low altitude (fuel inefficient) and highaltitude (fuel efficient) delay absorption can be seenas the minimum value of the curve. Currently, ourresearch identifies the conditions under which suchminimum values exist, such as the traffic densities andthe magnitude of trajectory prediction uncertainty.
The expected output of our research is improvement in human-centered decision making in flow control. Understanding better the mechanisms of congestion is a good basis for traffic management coordinators. Moreover, future automation tools (e.g. sequencing/merging) can incorporate such knowledge
in their design and in their parameter settings.

5

Conclusions and Future WorkIn this article, we gave a short overview of recent activities in the field of traffic synchronization.
We stressed that traffic synchronization is concernedwith the tactical management of queues, both on the
ground and in the air, in order to establish and maintain a safe, orderly and efficient flow of air traffic. We
then looked at activities in the creation of concepts ofoperations based on the two major enablers for traffic synchronization: better technology to predict andcontrol aircraft trajectories, and better tools to share

ENRI Int. Workshop on ATM/CNS. Tokyo, Japan (EIWAC2010)

Figure 6 Real-time arrival scheduling, time-control errors and delay propagation.

fuel
cost

prop.
delay

minimum
fuel

1.0

0.8

0.6

strategies under the impact of trajectory-uncertainties
and in de-centralized concepts for the flow control.
This is just the beginning of the journey, and for
future work we propose to continue the fundamental
approach to sequencing under uncertainty as well as astudy on the impact of traffic growth on the Japanesecongestion patterns.

total fuel0.4

6

propagated delay
baseline fuel

0.2

0.2

0.4

0.6

0.8

1.0

ACKNOWLEDGMENTS

The authors are grateful to the Japan Civil Aviation Bureau of the Ministry of Land, Infrastructure,
Transport and Tourism for their cooperation in theprovision of radar data.

Copyright

Figure 7 Trade-off in delay distribution.

flight plan information. We concluded that more experiments and research are necessary before creating
a concept of operations. Then, we described the major applications for traffic synchronization in Japanese
airspace: arrival synchronization to Haneda airportand coordination between several departure airports.
Finally, we briefly summarized our own results, namely
the quantification and interpretation of metering delays by stochastic queueing models and the trade-off
between low altitude (fuel inefficient) high altitude(fuel efficient) delay absorption.
The expected output of our research is improvement in human-centered decision making in flow control. For this, we focus on the analysis of radar data
and the construction of mathematical models. In particular, we are interested in efficient sequencing/merging

153

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have obtained permission, from the copyright holder
of any third party material included in their paper, topublish it as part of their paper. The authors grantfull permission for the publication and distribution of
their paper as part of the EIWAC2010 proceedings or
as individual off-prints from the proceedings.

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Claus Gwiggner[3] ICAO. Global Air Traffic Management Operational Concept. International Civil Aviation Organization, 2005.

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