**En route Spacing Tool (ErST)**

# General Description

# January 31, 2002

# NAS2-98005

# Technical Research in Advanced Air Transportation Technologies

Final

General Description

En route Spacing Tool (ErST)

CDRL Item 5e

# Prepared under

# RTO - 62

# AATT Operational Concept for ATM -- Year 2002 Update

#### January 31, 2002

#### Prepared For: NASA Ames Research Center

#### Moffett Field, CA 94035-1000

#### Prepared By: Titan Systems Corporation -- SRC Division

#### Billerica, MA 01821-4134

![](media/image1.jpeg){width="5.416666666666667in" height="0.75in"}

**PREFACE**

The purpose of this document, and of the descriptions of the other AATT
decision support tools (DST), is to provide a current description of the
tools in a consistent format for the AATT Project Office, the tools
developers, the NASA AATT contractors, the FAA, and other interested
parties. The objective is to document available information, not to
invent new information. The exception to this rule is the generation of
a functional flow chart for each tool. These were needed to support the
AATT DST Safety Assessment activity that is also being conducted for the
AATT Project Office.

**ACKNOWLEDGEMENT**

The majority of the descriptions presented in this document have been
taken directly from NASA documents shown in the bibliography. Minor
modifications have been made to the text to provide readability. The En
route Spacing Tool (ErST) flow chart has however been independently
developed for this description. This approach to the development of this
document was taken in order to remain faithful to the efforts that are
presently being undertaken by the NASA AATT Project Office, the ErST
tool developers and the associated NASA AATT contractors. The graphics
for the ErST System Overview and for the ErST Functional Flow are new
additions to the previous documentation.

Thanks go to Steve Green of NASA Ames Research Center for his help in
understanding the ErST concept and in his careful review of the drafts
of this document.

**Table of Contents**

1.  **Description 1**

2.  **Operational Concept 5**

3.  **Functional Flow 19**

**List of Figures**

1.  **Generalized Spacing - Fix Algorithm 2**

2.  **Generalized Spacing - Arc Algorithm 3**

3.  **En route Coordination of Spacing Delays 7**

4.  **User-preferred Routes 8**

5.  **Reroutes to form Spacing Trail 9**

6.  **Spacing Impacts on CP Accuracy 10**

7.  **ErST System Overview 11**

8.  **Spacing with Minimum Deviation 12**

9.  **Path-independent Spacing 13**

10. **Convergence of Synchronous Streams 14**

11. **En route Spacing Example: CP without spacing conformance 16**

12. **En route Spacing Example: CP with spacing conformance 16**

13. **En route Spacing Example: Conflict-free spacing conformance 17**

14. **ErST Functional Flow 20**

**Appendices**

### A. Bibliography 21 

## AATT EN ROUTE SPACING TOOL (ErST)

## GENERAL DESCRIPTION

**1. DESCRIPTION**

**Purpose**

A fundamental goal for en route decision support tool (DST) automation
is to assist the controller in providing better air traffic control
(ATC) service (i.e., greater flexibility to airspace users and fewer
ATC-related deviations to user's preferred trajectories) while
increasing safety and productivity (i.e., reductions or shifts in
controller workload that enable additional productivity). The economic
benefits to airspace users come in the form of increased
capacity/throughput, reduced restrictions and deviations (time and fuel
consumption), and increased flexibility to plan and to fly aircraft.
Specifically, the En route Spacing Tool (ErST) has the following
objectives:

-   Provide en route controllers a system and method to efficiently
    conform to miles-in-trail (MIT) spacing restrictions and to provide
    appropriate advisories to the controller;

-   Provide en route controllers with a system and method that is
    integrated with conflict probe to reduce its false-alarm and
    missed-alert rates; and

-   Minimize aircraft deviations needed to comply with an en route MIT
    spacing requirement imposed during air traffic control operations.

**Users**

-   En route air traffic controllers.

**Potential Benefits**

National statistics for the U.S. indicate that en route spacing
restrictions are applied by Centers for approximately 5,000 hours per
month. Approximately 45,000 flights per month are impacted by dynamic
MIT-spacing restrictions throughout U.S. airspace. Significant potential
exists for reducing user deviations, fuel burned, and the controller
workload associated with today's procedures for spacing conformance.
Analysis indicates potential airspace user benefits of at least \$45
million per year in fuel savings alone. Furthermore, the integration of
the en route spacing system with conflict probe will significantly
reduce the probe's false-alarm and missed-alert rates during spacing
operations. Integration will further reduce workload and fuel
consumption by reducing the number of corrective clearances needed to
achieve flow-rate conformance while avoiding conflicts. These potential
benefits are of particular value because they are achieved during
flow-rate constrained operations, precisely the time when airspace users
are impacted by deviations from their preferred trajectories. Additional
benefits consist of:

-   Reduced workload and fuel consumption by decreasing the number of
    corrective clearances needed to achieve flow-rate conformance while
    avoiding conflicts; and

-   More efficient distribution of spacing workload upstream and across
    sectors.

**Overview**

En route miles-in-trail (MIT) spacing restrictions are often used to
distribute arrival delays upstream of destination airports and to
mitigate local areas of en route airspace congestion. Current day
practices for MIT spacing increase controller workload, concentrate
traffic unnecessarily, and degrade the performance of conflict probe
decision support. Today\'s procedures also result in inefficient
conformance actions that directly impact the airspace user. The En route
Spacing tool is being developed to help en route controllers efficiently
conform to miles-in-trail (MIT) spacing restrictions.

An en route spacing function was previously developed within the CTAS
baseline in 1994 in preparation for field tests of the Descent Advisor
(DA)\[2\]. This function allowed a controller to identify a stream of
traffic and a spacing reference fix within or beyond the boundaries of
their sector. The reference fix may be an arbitrary position, defined by
the controller, independent of the airway or routing for any one flight.
Streams may be defined to include flights on independent paths (i.e.,
paths that are not constrained to any one airway, routing, or common
fix). The algorithm allows for a stream to be comprised of aircraft in
the climb, cruise, and/or descent phase of flight. This enables the same
tool to be applied to problems involving en route spacing, arrival
spacing, and the merging of departures into an en route stream. The
subtle variations in along-path predictions may be accounted for within
the supporting trajectory prediction functions (i.e., variations in
ground speed due to winds and lateral path, true airspeed profile, and
aircraft performance in the case of climb/descent segments).

Figure 1 illustrates the spacing computation based on a reference fix. A
spacing prediction is made for each flight in the stream when the first
flight (or next flight) is predicted to cross

![](media/image2.wmf){width="4.5in"
height="4.013194444444444in"}**Figure 1 - Generalized Spacing - Fix
Algorithm**

abeam the spacing-reference fix. A corresponding spacing marker shows
the predicted spacing position of each flight when the first flight in
the stream passes abeam the reference fix. If a controller vectors or
assigns a new speed/altitude to a flight, this predicted spacing
position is updated to reflect the changes to that flight\'s predicted
trajectory. The equivalent in-trail spacing is computed for each flight
based on the along-track distance from its predicted spacing position to
its future position abeam the control fix. In this case, the figure
illustrates a spacing merge of a departure (flight C) into an en route
stream comprised of flights A, B, and D. Alternatively, the spacing
computation may be based on any one of several reference geometries: an
airspace/sector boundary, a fixed line, or a fixed arc from a reference
fix/airport. Figure 2 illustrates an algorithmic implementation for a
reference-arc based computation.

![](media/image3.wmf){width="4.5680555555555555in"
height="4.022222222222222in"}**Figure 2 - Generalized Spacing - Arc
Algorithm**

For delay planning, the tool interface includes \"quick\" keyboard
inputs for speed and altitude changes, and a graphical interface for
route changes. Quick inputs allow the controller to enter speed/altitude
changes with a minimum of keyboard entries and head-down time. The speed
changes could be entered as a function of phase of flight (climb,
cruise, and/or descent), and speed, route, and altitude changes can be
combined in one plan. The trajectory prediction accuracy, associated
with speed/altitude/route changes in cruise and descent, was validated
in a field test \[2\]. The graphical display automatically updates the
predicted spacing while simultaneously displaying any conflicts
predicted by the conflict-probe function. The controller may then use
the CTAS trial-planning capability to plan actions for spacing
conformance while simultaneously resolving any predicted conflicts. This
integration allows the controller to create and implement a
conflict-free plan for spacing conformance. This capability also
provides the controller with a flexible tool for managing complex merge
problems even if MIT-spacing restrictions are not in effect.

The En route Spacing Tool (ErST) consists of an en route spacing system
and method to help en route controllers efficiently conform to MIT
spacing restrictions. Integration with conflict probe reduces the
probe's false-alarm and missed-alert rates due to better knowledge of
the controller's intended actions for spacing conformance. Integration
further reduces workload and fuel consumption by reducing the number of
corrective clearances needed to achieve flow-rate conformance while
avoiding conflicts.

ErST has the capability to generate automatic "meet-spacing" advisories
to reduce controller workload in manually trial planning spacing
conformance solutions. CTAS currently can provide controllers with
automation advisories to meet a time for any one aircraft (using speed,
altitude changes, and path-stretch vectoring) and automated speed
advisories for the spacing of arrival flights. ErST introduces automated
advisories (similar to EDA Milestone 5.10 techniques\[4\]) to advise
combined speed, altitude, and/or path-stretch vectors to achieve spacing
conformance, applying the "meet-time" capability developed for EDA
"meet-time" to the en route spacing problem. The capabilities of ErST
include:

1.  Speed advisories for which the automation calculates adjustments in
    climb, cruise, and/or descent speed (as appropriate) to meet the
    spacing requirement;

2.  Graphical advisory display indicating the speed-control envelope
    (i.e., a graphical display showing the range of spacing that can be
    achieved with speed for each flight (which depends on the
    path/distance, speed, and performance capability of each flight);

3.  Automated path-stretch advisory to compute the "added" path to
    absorb delay needed (above that absorbable with speed and altitude)
    to conform with spacing;

4.  Semi-automated altitude advisories to determine what new altitude
    will bring a flight into spacing conformance, and if altitude change
    is not enough, to determine how much spacing delay is achieved with
    an altitude change (and thus provide information for the controller
    to combine altitude changes with speed and path control advisories;
    and

5.  Manual (trial plan) capability to direct/constrain the set of
    speed/altitude/path advisories to be consistent with controller
    desires (essentially this gives the controller the ability to adapt
    the advisories, on a per flight basis, to their individual
    preferences and practices).

Another important feature of ErST is the ability to specify the streams
of aircraft that are being monitored by a controller. In current
practice, MIT spacing restrictions are delegated to individual control
sectors by the ATC facilities traffic management personnel. The traffic
manager either establishes the restriction (to address a problem within
their facility's airspace), or simply facilitates the implementation of
a restriction that is delegated to them (i.e., a "passback") from a
neighboring ATC facility or the FAA's ATC System Command Center
(ATCSCC)). In either case, the local traffic manager defines the stream
to be restricted in terms of the flights to be impacted, a reference fix
for spacing, and the time/distance horizon within which specific sectors
must begin maneuvering aircraft. Typically, the flights are identified
by a combination of noting specific call-signs and/or a stream (e.g.,
all flights with a common flight plan element such as a common
destination or routing).

ErST helps to automate the stream/aircraft identification process, by
permitting an appropriate traffic manager to input the stream
characteristics. The impacted flights are identified at the traffic
manager level, permitting distribution of the appropriate data for
display at each impacted sector. Preferably, the set of all controlled
traffic is continually analyzed (such as via standard ATC host computer
all-flight-all-track (AFAT) interface communications) and flights
belonging to a stream defined by the traffic manager are continually
updated. Additional flights outside the envelope of a traffic manager's
stream definition can also be added by direct identification of flights
via keyboard input of flight identification and/or graphical selection
on a plan view display of traffic.

Operational considerations may require the system to support controller
inputs to manually add or remove a specific flight from a spacing
stream. Preferably, ErST also accesses the ATC host computer's
designation of the controlling sector that "owns" each restricted flight
(again, such as via standard host AFAT interface). ErST outputs the
spacing advisory data to each sector position via network connection to
each sector's display suite (currently referred to as the Display System
Replacement (DSR)). Depending on the preferences of each sector
controller, they may configure the data to display only data for
restricted flights under their control and/or all data for all flights
within a stream (when displayed as a list) in order to visualize the
relative positions of the flights within their sector compared to
sequentially neighboring flights in other sectors. As each controller
uses ErST\'s capability to plan and implement their spacing-conformance
maneuvers, ErST updates the traffic manager's display to indicate
(either graphically or alphanumerically) the current state of spacing
conformance within each restricted stream.

**Future Research**

An advanced \"automatic\" version of the Spacing Tool was also developed
within CTAS to provide arrival spacing support for high-density airports
that are not served by the CTAS Traffic Management Advisor (TMA).
However, due to the FAA\'s near-term focus on initial conflict probe and
TMA capabilities, development of the CTAS spacing applications was put
on hold. In the mean time, the basic DA tool (now referred to as the En
route/Descent Advisor (EDA)) has undergone many refinements to its
controller interface, trajectory planning, and conflict-probe capability
\[8- 10\]. The current plan is to re-implement the Spacing Tool
capability within the CTAS baseline and to conduct research activities
in support of Free Flight Phase 2.

**2. OPERATIONAL CONCEPT**

There are many factors that impact air traffic operations, but primary
factors include conflicts and traffic flow management (TFM) flow-rate
restrictions. Conflicts relate directly to safety while flow-rate
restrictions relate directly to the efficient management of
capacity-constrained resources (e.g., runways and sectors). Certainly
the safety considerations alone warrant the community's past emphasis on
conflict probe technology. However, in terms of mitigating user
deviations, particularly in light of the projected rate of traffic
growth, it is the flow-rate restriction that is at the core of unlocking
user benefits. Although flow restrictions only impact a percentage of
flights, the resulting deviations are significant compared to those
required for maintaining basic radar separation. Furthermore, the lack
of ATC-sector decision support for flow-rate conformance planning and
execution results in a significant degradation in the performance of
conflict probe. Conflict probe lacks the trajectory "intent" of the
controller's plan for flow-rate conformance leading to the "conflict
probing" of the "wrong" trajectories (thus increasing the probe's rate
of false alarms and missed alerts). This degradation occurs in just the
sort of "problem" airspace where the air transport industry needs
automation assistance such as conflict probe.

In general, two types of flow-rate restrictions must be considered.
These include time-based arrival metering and en route miles-in-trail
(MIT) spacing. Arrival metering tools include the CTAS Traffic
Management Advisor (TMA), COMPASS, and MAESTRO with future developments
including Multi-Center TMA (U.S.) and Arrival Manager (Eurocontrol).
Where operational, arrival metering is generally performed in en route
airspace within the last 20 min. of flight prior to entering terminal
airspace. Even with arrival metering operations, many flights will still
be subject to MIT-spacing restrictions. MIT-spacing procedures can be
expected to play a predominant role for several reasons. The first is
the ATC-operational need to merge departures with en route traffic that
is "spaced" for downstream capacity limitations. Second, the limited
number of arrival-metering sites (i.e., CTAS-TMA-adapted airports)
leaves the remaining airports to depend on MIT-spacing procedures.
Third, there is a need to occasionally propagate delays upstream of
terminal airspace prior to the arrival-metering horizon. As traffic
growth outpaces capacity, more flights will be affected by dynamic
flow-rate initiatives including MIT-spacing restrictions.

Traffic management coordinators (TMCs) within each ATC facility are
responsible for coordinating MIT-spacing initiatives within their
facility when needed. Dynamic initiatives are either generated within
the facility (e.g., local arrival spacing to a non-metered airport),
received from neighboring facilities, or coordinated through the ATCSCC.

MIT-spacing restrictions are defined in terms of a stream of flights,
spacing-reference fix, active period, and a spacing requirement (e.g.,
20 nm in trail). Restrictions may also segregate streams by altitude
stratum and/or arrival routing. Once a MIT-spacing restriction is
initiated, local TMCs identify the flights within their facility that
are affected by the restriction. TMCs then coordinate re-routes to form
"freeways in the sky" that allow sector controllers to visualize the
stream and determine the maneuvers necessary for conformance.
Controllers primarily use vectors to establish and maintain the desired
spacing. The "path-dependent" nature of this process makes MIT-spacing
restrictions operationally feasible to implement, monitor, and control
across sector boundaries, with little or no automation assistance.

TMCs assess each MIT-spacing situation and determine the appropriate
sectors, upstream of the spacing-reference fix, to begin coordinating
controller actions for conformance. This effective range (or time
horizon) for controller conformance depends on the available airspace
and the magnitude of delays. Traffic streams nominally have a natural
spacing: the greater the difference between the nominal and required
spacing, the greater the delay resulting from conformance. Depending on
the magnitude of the delays and available airspace, it may be necessary
to propagate MIT-spacing restrictions to upstream facilities via
\"pass-back\" restrictions (with coordination facilitated by the
ATCSCC).

![](media/image4.wmf){width="5.711805555555555in"
height="3.5833333333333335in"}Figure 3 illustrates an example scenario
for Chicago's O'Hare Airport where it is not uncommon for delays to
propagate upwards of 1,000 nm upstream. The "delay ability" of a flight
(i.e., the operationally acceptable amount of delay that can be
absorbed) grows with the range-to-go and airspace capacity. As
terminal-area delays grow, Chicago Center must throttle the arrival
flow. Even with airborne holding, the backup of arrival traffic can
saturate the airspace. Chicago Center then coordinates a restriction
with Minneapolis Center to space incoming arrivals (e.g., 10 MIT by Fort
Dodge (FOD)). Depending on the situation, Minneapolis may in turn need
to slow the rate of incoming traffic from Denver Center (e.g., 20 MIT by
Oneil (ONL)).

**Figure 3 - En route Coordination of Spacing Delays**

Even if high-density terminal areas (such as Chicago) convert to
time-based arrival metering, MIT-spacing initiatives still provide TMCs
with an effective means for dynamically distributing excess delay
upstream. MIT initiatives have a significant operational advantage in
that they are relatively straightforward to delegate (within and between
ATC facilities), implement, and monitor. When flights are formed into
in-trail streams, controllers are able to visualize and control spacing
at the sector without automation assistance.

Although today's "manual" MIT-spacing techniques are straightforward to
implement, there are several disadvantages related to their
path-dependent nature. From the airspace-user's point of view,
deviations from their preferred trajectory come in three forms:

● TMC-initiated re-routes to establish a stream;

● controller vectors to establish spacing; and

● controller vectors for conflict resolution.

![](media/image5.wmf){width="5.527777777777778in"
height="4.7340277777777775in"}Figures 4 and 5 illustrate the problem.
Three flights are initially on user-preferred eastbound routes. The
circles indicate the relative sequence of the un-delayed flights when
the first flight crosses the boundary. The natural order of arrival at
the boundary is B, C, and A. Consider the situation where the downstream
center (ARTCC 2) imposes a MIT-spacing restriction at the boundary.
Without automation assistance, it would be difficult for sector
controllers to visualize and space their flights relative to flights in
other sectors that are orthogonal to the flow. Referring to Figure 4,
the controller in sector 2 would have difficulty in spacing B relative
to A or C. To overcome this problem, TMCs coordinate the re-routing of A
and C (Figure 5) to form a stream that can be visualized and controlled
by sectors 2 and 5. Depending on the natural distribution of flight
paths, these re-route actions add a significant penalty.

**Figure 4 - User-preferred Routes**

Once streams are formed, spacing adjustments typically involve vectors.
Although speed control can help fine-tune spacing under current
procedures, it is often too little to establish spacing because of
performance mismatches and limited range within a sector (for speed
changes to take effect). In-trail flows also reduce the opportunity for
faster aircraft to pass slower ones when the

![](media/image6.wmf){width="5.527777777777778in"
height="4.725694444444445in"}

**Figure 5 - Re-routes to form Spacing Trail**

faster aircraft would naturally arrive first at the spacing-reference
fix. Once spacing is established within a stream, additional deviations
may result from conflicts with crossing traffic.

From the ATM point of view, current-day spacing procedures present
several disadvantages. First is the workload required to establish the
stream. Second, controllers must rely on tactical techniques to
establish spacing based on experience and trial and error. Third,
in-trail techniques force flights into streams that concentrate traffic
density and workload in the "spacing" sectors as opposed to distributing
flights across sectors. Finally, the spacing sectors are impacted in
terms of conflict detection and resolution because the tactical nature
of current-day spacing techniques negatively impacts the operational use
of Conflict Probe (CP) tools.

Regarding conflict detection, consider the situation illustrated in
Figure 6. The two eastbound flights are subject to a spacing restriction
while the other two flights represent crossing traffic. The solid lines
indicate the path used by CP. The spacing-conformance path for the first
eastbound flight is also shown in a dashed line. CP has no knowledge of
the controller's plan for spacing conformance until the conformance
maneuvers are completed. More often than not, such plans are not updated
or reflected in the ATC Host computer. This is due to several factors
including the controller workload associated with flight plan amendments
and the difficulty controllers would have in reflecting today's
relatively tactical spacing techniques in a flight-plan

![](media/image7.wmf){width="5.995833333333334in"
height="3.7729166666666667in"}

**Figure 6 - Spacing Impacts on CP Accuracy**

amendment. As a result, CP may experience a greater rate of false alarms
(due to the lack of spacing-conformance intent) and missed alerts (if
the controller's conformance actions result in a new conflict).

Two near-term enhancements to CP technology provided by ErST can go far
in reducing user deviations from their preferred trajectories. First, a
tool is provided to help en route controllers efficiently conform to
flow-rate restrictions. This will enable controllers to strategically
plan conformance actions resulting in reduced workload, flight
deviations and fuel consumption. The second enhancement to CP involves
the integration of conflict detection and resolution capability with
flow-rate conformance. Integration will further reduce fuel consumption
and workload by reducing the conflict-probe false alarm and missed-alert
rates. This improved accuracy, due to better knowledge of the
controller's intended conformance actions, will reduce the number of
corrective clearances needed to achieve flow-rate conformance while
avoiding conflicts. As a first operational step, there should be a large
return on investment in applying CP technology (conflict detection and
trial planning) to flow-rate conformance. CP technology lends itself
well to en route spacing operations, as demonstrated by ErST. Although
trial planning could be performed for aircraft approaching an airport,
the manual trial-planning approach is too cumbersome for arrival
metering. This is based on the high percentage of flights that are
typically deviated in each sector. Downstream sectors, closest to the
congestion, have the greatest number of aircraft that need be spaced;
the upstream sectors have fewer aircraft per sector to delay, and
therefore that environment is more appropriate to conduct trial plans.

![](media/image8.wmf){width="6.5625in"
height="4.825694444444444in"}Another enhancement provided by ErST is the
addition of automatic "meet-spacing" advisory capabilities to reduce
controller work in manually trial planning spacing conformance
solutions. ErST provides for automated advisories (a la EDA milestone
5.10 techniques) to advise combined speed, altitude, and/or path-stretch
vectors to achieve spacing conformance. Figure 7 illustrates the system
overview of ErST. The ErST system uses aircraft flight plans and
position data from the host computer through the Display System
Replacement (DSR) to produce advisories to assist controllers in
managing and controlling traffic. An alternative configuration is a
direct interface between the DSR and ErST and between the Host and ErST.

**Figure 7. ErST System Overview**

In ErST, the disadvantages of today's MIT-spacing procedures are
overcome by a simple application of the 4D trajectory-prediction and
trial-planning capability associated with CP technology. Such
application may be applied to any baseline ATM Decision Support System,
including CTAS, URET, or systems of a similar nature (URET is a conflict
probe tool originally developed at Mitre-CAASD and implemented by the
FAA as part of the Free Flight Phase 1 program). Figure 8 illustrates
the desired situation, assuming that the downstream "receiving" facility
will still require an in-trail stream at the hand off to their facility.
As long as the tools and procedures result in conformance prior to the
spacing-reference fix, each of the cross-stream sectors may work their
flights independently and thus delay the merge until the
spacing-reference fix.

![](media/image9.wmf){width="5.519444444444445in"
height="4.677083333333333in"}

**Figure 8 - Spacing with Minimum Deviation**

Additional benefits can be achieved if the downstream "receiving"
facility relaxed the requirement for an in-trail flow at the hand off.
At the theoretical extreme, the automation could help controllers
deliver an "equivalent" spacing across a "wide" stream of flights
(Figure 9) with the absolute minimum deviation from each user's
preferred route. Of course, depending on the amount of delay required
(i.e., relative to the aircraft's performance and speed envelope), a
certain amount of vectoring may be necessary to space each flight.
Figure 9 approaches the user-desired concept of "free routing" where
flow-restrictions are implemented, as needed, with required
time-of-arrival (RTA) assignments. In fact, spacing solutions could be
used to determine RTA assignments for equipped aircraft.

Figures 8 and 9 illustrate several of the advantages to the spacing
approach. First, the degree of route deviations required for spacing
conformance is minimized. Second, the traffic density and spacing
workload is distributed across more sectors. This distribution of
flights reduces the impact of dissimilar speeds among sequential flights
in a stream, thus allowing more opportunity for natural overtakes. It
also provides for a more equitable distribution of delays based on the
nominal performance of the aircraft. In addition, the integration of CP
and spacing-conformance tools will result in more efficient trajectories
with fewer false alarms and missed alerts.

![](media/image10.wmf){width="5.527777777777778in"
height="4.7340277777777775in"}

**Figure 9 - Path - independent Spacing**

One of the benefits of applying CP technology to the conflict-free
planning of MIT-spacing conformance is the reduction of path deviations
for both stream formation and spacing adjustment. By allowing flights to
remain on independent paths (delaying any merge until the
spacing-reference fix), speed control may be exercised more effectively
and to an economic advantage. For example, consider Figure 10 which
illustrates two west-bound streams. In anticipation of a later merge
with a net spacing of 10 nm, each stream is restricted to a 20 nm
spacing. If the streams happen to be synchronized (coincidentally),
there will be little downstream effort needed to achieve a single flow
with 10 nm spacing. However, if the flows are not synchronized,
controllers will be forced to delay flights to merge the streams. Since
the Spacing Tool provides guidance for spacing conformance independent
of routing, it enables the controllers to synchronize the 10 nm spacing
up front.

Aside from direct fuel savings, ErST reduces the uncertainty associated
with today's methods for monitoring and control of critical traffic
streams. Improvements to the ability to monitor and control flow rates
provide TMCs with the confidence to reduce the frequency and extent of
MIT-spacing restrictions. Although difficult to measure, there is
additional value associated with the tool's ability to increase the
conflict-probe performance and lower traffic densities across sectors.

![](media/image11.wmf){width="4.729166666666667in"
height="4.151388888888889in"}**Figure 10 - Convergence of Synchronous
Streams**

Initially, CP technology (in the form of the User Request Evaluation
Tool (URET)) is being deployed as a "D-side" tool under the FAA's Free
Flight Phase 1 program. Each en route sector has two primary controller
positions/roles: the R-side and D-side. The R-side monitors the plan
view radar display and issues all clearances to the aircraft in the
sector. In general, the D-side complements the R-side by analyzing the
flight plans of incoming traffic, coordinating upstream changes to
protect the sector (R-side) from high workload situations, and other
duties to allow the R-side to focus on the tactical situation. During
light traffic periods, one controller performs both positions; during
heavy periods, additional controllers may help the sector team to handle
the workload.

Initial CP problem-resolution capability is based on a "manual"
trial-planning process. The controller uses a graphical user interface
to trial plan changes in route, altitude, and speed. Problems include
the predicted loss of separation between two flights (i.e., a conflict)
and penetration of special use airspace. Compared to the manual process
that D-side controllers perform with flight plans, CP represents a
significant improvement to the operational system. However, the time
consuming nature of the trial-planning process may reduce its usability
during high-workload-density-traffic situations.

There are three dimensions of "options" needed to provide ATC facilities
with the flexibility to adapt the automation to specific airspace
applications and site preferences for the display of advisories. One of
the three dimensions is the "spacing reference" calculation (i.e.,
reference fix of Figure 1 vs. reference arc of Figure 2). The other two
dimensions have to do with the nature of the spacing computation.

The first of the two remaining dimensions has to do with whether the
spacing calculation for a stream is "rolling" or "fixed." "Rolling"
means that the spacing computation for each flight in a "stream" is
based on the first aircraft in the stream. As each flight passes the
spacing fix, the designation of the "first" flight "rolls" to the next
flight in sequence (so if an aircraft arrives out of conformance, any
"residual" spacing error is dropped and the next flight in sequence sets
the new reference for all following flights). "Fixed" means that the
spacing for all aircraft is defined by the flight that was the first in
the stream. In other words, as the first flight crosses the spacing
reference, any residual spacing "non-conformance" is not dropped; the
spacing computation for each sequential flight is based on the crossing
of the original first flight (all sequential spacing calculations are
corrected to reflect the actual crossing of the original lead flight).

The second of the two remaining dimensions has to do with whether the
spacing calculation for a stream is "absolute" or "relative." "Absolute"
refers to the "spacing-distance" calculation and means that the spacing
calculation reflects the along-path distance from the aircraft to the
spacing reference. For example, a perfect 20-mile-spaced stream would
show the first aircraft to be "X" miles from the spacing reference, the
second aircraft X+20 miles, and the third X+40 miles (and so on). The
"relative" spacing between any two flights is simply the difference
between their "absolute" spacing.

The initial demonstration version of ErST uses a "rolling-absolute"
spacing calculation with a spacing reference based on either a fix or an
arc. However, considerations for any particular implementation or
location may merit different choices for the three dimensions.

The following example scenario is presented to illustrate the
integration of MIT-spacing conformance with conflict detection and
resolution (Figures 11-13). The figures represent a simplified depiction
of the tool's graphical interface from a 1996 version of CTAS. Note that
the figures illustrate the spacing data in a tabular list, but the same
data could also or instead be displayed on the aircraft's data tag.
Providing an option to a controller permits the controller to see all
the data in one location even if aircraft are graphically located all
over the display (or are off the display), or lets the controller see
the data for each aircraft on the aircraft's tag which is located
graphically where the aircraft position is.

To reiterate, the display of spacing-conformance analysis may be
accomplished in a variety of ways depending on the operational
considerations of the ATC facilities and controllers using the tool (or
the operational limitations of the ATC computer/display infrastructure
capabilities). One display alternative is illustrated in the Figures
11-13. This alternative places the spacing-conformance data in a spacing
list and via graphical spacing markers. Alternative display methods
include, but are not limited to, the display of the alphanumeric spacing
data on:

(1) the primary traffic ("R"-side) display on the flight data tags (as
    an optional field within one of the 3 standard data-tag lines, or an
    optional 4th line);

(2) the primary traffic ("R"-side) display on or near the aircraft
    target;

(3) flight-progress strips (either current-art paper strips and/or
    future-art "electronic" strips;

(4) URET CCLD displays ("graphic" or "plans" displays); or

(5) similar display implementations on controller display features
    developed in the future.

![](media/image12.wmf){width="5.992361111111111in"
height="3.777083333333333in"}

![](media/image13.wmf){width="5.999305555555556in"
height="3.763888888888889in"}**Figure 11 - En route spacing example: CP
without spacing conformance**

**Figure 12 - En route spacing example: CP with spacing conformance**

![](media/image14.wmf){width="5.993055555555555in"
height="3.6444444444444444in"}

**Figure 13: En route spacing example: conflict-free spacing
conformance**

Spacing-conformance data may be displayed in terms of each flight's
projected spacing position (absolute or relative spacing) and/or
projected spacing-conformance error (i.e., the difference between the
desired spacing, according to the flow restriction, and the projected
spacing of each aircraft). For example, the fourth flight in a stream
restricted to 20 nm spacing must either be "absolutely" spaced at 60 nm
(with the first flight establishing the "0" nm spacing position) or
"relatively" spaced 20 nm behind the preceding third aircraft. If the
fourth flight was perfectly spaced, it would have a spacing error of 0
nm. If the flight were projected to have a spacing position of 55 nm
(absolute) or 15 nm (relative), the spacing error would be +5 nm (5 nm
ahead of the conformance position. Display of this projected
spacing-error state provides the controller with a direct indication of
the "residual" spacing error that remains to be addressed.

The example involves the northern portion of the Denver Center airspace
centered on sector 33, a sort of cross roads for transcontinental
traffic. The scenario focuses on a simulated traffic problem involving
the five flights depicted in Figure 11. Four of the flights are destined
for the Northern California Bay Area (San Francisco, San Jose, and
Oakland airports). A fifth flight, DAL 357, is destined for Seattle
along a route that crosses the paths of the westbound traffic.

DAL 357 is a conventionally-equipped B-727 that is navigating with
ground-based navigational aids along jet airways (hence the slight
zigzag in its routing). NWA52 (B-757) and UAL1029 (B-737-300) are
equipped with flight management systems (FMS) and are navigating along
NRP flight plans comprised of a series of direct segments along a
"best-wind" path. AAL96 is a DC10, with area navigation (RNAV)
capability, flying direct on a NRP flight plan. N57MB is a
conventionally-equipped Citation Jet. The data block for each flight
indicates the flight's call sign, flight level (line 2), and ground
speed in knots (line 3). The following scenario is based on standard
atmosphere and zero-wind conditions.

The scenario begins with all five flights progressing along their
flight-plan routes. Figure 11 depicts a conflict probe of the situation.
The conflict-probe list indicates that the separation between AAL96 and
DAL357 is predicted to fall below minimums in 15 min. The
minimum-separation distance is predicted to be 2.9 nm. This
conflict-probe alert is based on the current flight plan and track data
for each flight.

However, the scenario is far more interesting when a MIT-spacing
initiative is considered for the Bay Area arrivals. For the purposes of
this illustration, it is assumed that terminal area delays (due to fog)
have propagated upstream and forced Salt Lake City Center to place a
restriction on Denver Center. The restriction requires that a spacing of
20 MIT be established on all Bay Area landing traffic before the hand
off at the Salt Lake boundary.

For this situation, the spacing function is invoked for the four
westbound flights. The Cherokee navigational aid (CKW), just inside
Denver airspace, is selected as the spacing-reference fix by the TMC.
Results from the spacing analysis are depicted graphically (Figure 11)
with spacing markers. The markers indicate the predicted position of
each restricted flight when the lead flight is predicted to pass abeam
the reference fix. As the lead flight crosses the reference fix, the
next flight in the sequence becomes the lead.

A precise representation of the spacing analysis is also presented in
the flow-restriction list (upper right corner of the figure). The list
displays each flight in the order of its arrival time, abeam the
reference fix, along with a prediction of its equivalent "in-trail"
spacing and arrival time. The spacing is displayed here in terms of the
"total" spacing for each flight relative to the lead flight. The total
spacing represents the predicted along-track range to go to the
reference fix when the lead flight is predicted to cross the reference
fix. An alternative approach is to display the relative spacing between
each succeeding flight based on the difference between the "total"
spacing of each succeeding flight. An additional option (not shown here)
is to display the spacing error in terms of the difference between the
predicted and desired spacing values for each flight.

The flow-restriction list indicates that the first flight, NWA52, is
predicted to cross CKW at 38 min and 22 sec after the hour. The
following flights are all predicted to arrive early relative to the 20
nm spacing restriction. UAL1029 is predicted to have an equivalent
in-trail spacing of 9.5 nm with the lead flight and is therefore 10.5 nm
"early." AAL96 is predicted to be 25.2 nm early, based on a total
spacing of 14.8 nm (5.3 nm behind UAL1029), while N57MB is predicted to
be 13.1 nm early, based on a total spacing of 46.9 nm (22.1 nm behind
AAL96). Clearly, the flight plans used for the conflict probe do not
reflect the future actions necessary to bring UAL1029, AAL96, and N57MB
into MIT-spacing conformance.

Figure 12 shows the same traffic situation after initial trial planning
for spacing conformance. The trial plan calls for UAL1029 to reduce
speed to 255 knots indicated airspeed (KIAS). This action, if
implemented, would reduce UAL1029's ground speed by 21 knots (resulting
in a 20.5 nm spacing without deviating from the user's preferred path).
The tool also indicates that a speed reduction to 250 KIAS (400 knots
ground speed) would bring N57MB into MIT-spacing conformance. That
action would result in a total spacing of 58.7 nm also while keeping
N57MB on its preferred path.

For AAL96 however, only part of the delay will be absorbed by a speed
reduction. For the purposes of this example, the speed reduction will be
limited to 260 KIAS (443 knots ground speed) to illustrate the use of
vectors. Such a speed reduction would result in a total spacing of 27.5
nm or 12.5 nm early for AAL96. For the remainder of the spacing, the
controller would use the graphical user interface to generate a combined
vector and speed solution. As the controller "stretches" the path
graphically, the spacing feedback helps the controller zero in on a
conformance solution. The resulting plan for AAL96 calls for a turn to a
heading of 300 degrees (for 8 min and 50 sec), followed by a turn to 254
degrees to rejoin the user's preferred route.

With the tool-based spacing-conformance plans generated, the conflict
probe will have an accurate model of intent upon which to base any
conflict predictions. In this case (Figure 12), the automation still
predicts a conflict between AAL96 and DAL357, albeit at a later time (19
min). For a complete solution, the controller could use the trial
planner while combining the feedback from the spacing and separation
predictions.

Figure 13 illustrates the controller's final solution. AAL96's
path-stretch vector was adjusted to achieve separation with DAL357. This
plan calls for AAL96 fly a heading of 300 degrees (for 4 min 30 sec),
followed by a turn to 240 degrees to rejoin the user's preferred route.
The final plan achieves spacing conformance while resolving the conflict
between AAL96 and DAL357 with a minimum separation of 10.2 nm. The
automation feedback helps the controller minimize the extent of the
deviations to get the job done.

3.  **FUNCTIONAL FLOW**

Figure 14 illustrates the detailed functional flow of ErST operating in
the ARTCC and depicts the functional interfaces with external data
sources. The input/output sources, appearing in double boxes, are shown
in the figure. The major functions within ErST are:

-   Data Acquisition and Processing

-   Data Formatting and Distribution

-   Trajectory Generation

-   Spacing Planning

-   Advisory Generation

-   Conflict Detection

-   Display Message Processing

![](media/image15.wmf){width="7.5in" height="5.357638888888889in"}

**Figure 14. ErST Functional Flow**

The interfaces required for ErST include:

-   Display System Replacement (DSR) - To provide data to/from the Host
    Computer and to provide Erst Display to the controller

-   Host Computer System - This interface is through the DSR and is
    required for proper operation of ErST. This interface provides ErST
    with flight data, radar tracks, TM Constraints, and Airspace and
    Sector configurations from the ARTCC\'s HCS via the DSR. An
    alternative is a direct interface with the Host.

-   Weather data

-   Controller and TMC I/O

Functionally, ErST collects information from the Host and en route
weather sources using the CTAS displays, trial planning, trajectory
generation, and conflict probe capabilities to interactively assist the
controller in solving en route MIT spacing problems. Speed, path
stretching, and altitude advisories are also generated by ErST and
displayed to the controller.

**APPENDIX A. BIBLIOGRAPHY**

1.  **Erzberger, H., Davis, T.J., and Green, S.M.,** \"Design of
    Center-TRACON Automation System,\" AGARD Guidance and Control
    Symposium on Machine Intelligence in Air Traffic Management, Berlin,
    Germany, May 1993.

2.  **Green, S.M., Vivona, R., Grace, M.P., and Fang, T.C.,** Field
    Evaluation of Descent Advisor Trajectory Prediction Accuracy for En
    route Clearance Advisories, AIAA-98-4479, AIAA Guidance, Navigation,
    and Control Conference, Boston MA, August, 1998.

3.  **Slattery, R. and Green, S.M**., Conflict-Free Trajectory Planning
    for Air Traffic Control Automation, NASA TM-108790, January 1994.

4.  **Green, S.M., and Vivona, R.,** En route Descent Advisor (EDA)
    Concept, Advanced Air Transportation Technologies Project Milestone
    5.10 Report, September 1999, M/S 262-4, NASA Ames Research Center,
    Moffett Field, CA, U.S.A., 94035.

5.  **Swenson, H.N. et al,** Design & Operational Evaluation of the
    Traffic Management Advisor at the Fort Worth Air Route Traffic
    Control Center,. 1st USA / Europe Air Traffic Management R&D
    Seminar, Saclay, France, June 1997.

6.  Quarterly Restriction Report, Third Quarter 1998,.Federal Aviation
    Administration David J. Hurley Air Traffic Control System Command
    Center (ATO-200), Reston, VA

7.  **Klopfenstein, M., et. al**., \"En route User Deviation
    Assessment,\" RTO-37 Final Report, Contract \# NAS2-98005, NASA AATT
    Project Office, NASA Ames Research Center, Moffett Field, CA 94035,
    November, 1999.

8.  **McNally, B. D., Erzberger, H., Bach, R., and Chan, W.,**
    Controller Tools for Transition Airspace, AIAA-99-4298, AIAA
    Guidance, Navigation, and Control Conference, Portland OR, August,
    1999.

9.  **Erzberger, H., Paielli, R.A., Isaacson, D.R., and Eshow, M.,
    Conflict** Detection and Resolution In the Presence of Prediction
    Error, 1st USA / Europe Air Traffic Management R&D Seminar, Saclay,
    France, June 1997.

10. **Laudeman, I.V., Brasil, C.L., and Stassart, P.,** .An Evaluation
    and Redesign of the Conflict Prediction and Trial Planning Graphical
    User Interface, NASA TM-112227, April, 1998.

11. **Green, Steven M.,** \"En route Spacing Tool: Efficient
    Conflict-Free Spacing to Flow-Restricted Airspace\", 3^rd^
    USA/Europe Air Traffic Management R&D Seminar, Naples Italy, June
    13-16, 2000

12. **Green, Steven M.,** \"Patent Application: En Route Spacing System
    and Method\", ARC-14418-1