###### **00188.41--02**

NAS2-98005 RTO-41

Technical Research in Advanced Air Transportation Technologies

Detailed Description for CE-11

Terminal Arrival: Self Spacing for Merging and In-trail Separation

John A. Sorensen

###### 

November 2000

Prepared For:

NASA Ames Research Center

Moffett Field, CA 94035-1000

and

NASA Langley Research Center

Hampton, VA 23681-2199

Prepared By: Seagull Technology, Inc.

Los Gatos, CA 95032-2547

![](media/image1.png)

Preface

This report is the first version of a detailed description for the
Distributed Air/Ground Traffic Management (DAG-TM) Concept Element (CE)
11, Terminal Arrival: Self Spacing for Merging and In-trail Separation.
The ideas presented here are preliminary.

NASA is soliciting review of this report and welcomes comments. Comments
should be sent to:

-   Del Weathers, Manager, AATT ATM Concept Definition Sub-element, NASA
    Ames Research Center -- dweathers@mail.arc.nasa.gov

-   Richard Mogford, Manager, AATT Human Factors Sub-element, NASA Ames
    Research Center -- rmogford@mail.arc.nasa.gov

Acknowledgement

This subtask of preparing this initial detailed description of the DAG
TM Concept Element 11 (CE-11), Terminal Area Arrival: Self Spacing for
Merging and In-Trail Separation, was performed by John Sorensen, of
Seagull Technology, via subcontract to System Resources Corporation
(SRC) under Research Task Order 41 of the Advanced Air Transportation
Technologies (AATT) project's NASA Research Agreement Contract
NAS2-98005 with SRC. Charlie Phillips served and overall contract
manager, and he provided the overall outline from which this report is
organized. He further made valuable suggestion for section layout and
content level.

The author wishes to acknowledge Richard Mogford, of NASA Ames, and
David Wing, of NASA Langley, who provided astute input and overall
editing suggestions for revising the first draft of this document. Gary
Lohr, Terry Abbott, and Sheila Conway, of NASA Langley, provided answers
to detailed questions aimed at defining the technical and operational
environment for CE-11. Anand Mundra and Oscar Olmos, of Mitre CAASD,
gave a good summary of on-going research to support CE-11-related
traffic display experiments being conducted by the FAA and Cargo Airline
Association. In addition, Vernal Battiste, Walter Johnson, and Everett
Palmer, of NASA Ames, summarized their past traffic display research and
demonstrated their laboratory facilities for future investigation of
CE-11 research issues.

Table of Contents

# 

1\. Introduction [1](#introduction)

1.1 Background [1](#background)

1.2 Objectives [4](#objectives)

1.3 Scope [4](#scope)

2\. Problem Description [6](#problem-description)

3\. Approach [8](#approach)

3.1 Concept Overview [8](#concept-overview)

3.2 Summary of the CE-11 Operational Modes
[10](#summary-of-the-ce-11-operational-modes)

4\. Operational Requirements [15](#operational-requirements)

5\. Operational Environment [15](#operational-environment)

5.1 Airspace Structure and Route Constraints
[15](#airspace-structure-and-route-constraints)

5.2 Traffic Mix and Equipage [17](#traffic-mix-and-equipage)

5.3 CNS Infrastructure [20](#cns-infrastructure)

5.4 ATM Environment [21](#atm-environment)

6\. Operational Characteristics [22](#operational-characteristics)

6.1 ATSP View [22](#atsp-view)

6.2 Pilot View [23](#pilot-view)

7\. NAS Functional Impacts [24](#nas-functional-impacts)

7.1 Functional Requirements [24](#functional-requirements)

7.2 Functional Design [26](#functional-design)

8\. User/Operator Roles and Responsibilities.
[28](#useroperator-roles-and-responsibilities.)

8.1 ATSP Roles and Responsibilities
[28](#atsp-roles-and-responsibilities)

8.2 Flight Crew Roles and Responsibilities
[30](#flight-crew-roles-and-responsibilities)

9\. Operational Conditions and Scenarios
[32](#operational-conditions-and-scenarios)

9.1 Normal or Nominal Condition [32](#normal-or-nominal-condition)

9.2 Off-Nominal Condition [32](#off-nominal-condition)

9.3 Failure Condition [33](#failure-condition)

10\. Operational Process/Operational Sequence Diagrams
[35](#operational-processoperational-sequence-diagrams)

10.1 ATSP: Nominal In-Trail Spacing Process
[35](#atsp-nominal-in-trail-spacing-process)

10.2 Flight Crew: Nominal In-Trail SPACING Process
[36](#flight-crew-nominal-in-trail-spacing-process)

11\. Benefits [38](#benefits)

12\. Issues and Key Decisions [41](#issues-and-key-decisions)

12.1 Issues Summary [41](#issues-summary)

12.2 Key Decision Points [42](#key-decision-points)

Appendix A. Operational Requirements Table
[43](#appendix-a.-operational-requirements-table)

Operational Requirements -- Traffic Management, Synchronization Area
[43](#operational-requirements-traffic-management-synchronization-area)

Appendix B. Previous CDTI and Related Research and Development
[45](#appendix-b.-previous-cdti-and-related-research-and-development)

B.1 Pre-1990 CDTI Research [45](#b.1-pre-1990-cdti-research)

B.2 Post-1990 CDTI Developments [50](#b.2-post-1990-cdti-developments)

B.3 On-going CDTI Research and Development
[51](#b.3-on-going-cdti-research-and-development)

Appendix C. Variations to the Baseline System for CE-11
[53](#appendix-c.-variations-to-the-baseline-system-for-ce-11)

Moving Slots [53](#moving-slots)

Merging [53](#merging)

References [54](#references)

# 1. Introduction

Concept Element 11 (CE-11) is focused on bringing greater flight
efficiency and runway throughput to busy terminal areas and runways
through flight crew (FC) use of flight management system (FMS) and
cockpit display of traffic information (CDTI) technology. The general
idea is that by implementing a distributed control system via
integrating FMS and CDTI avionics with the air traffic management (ATM)
system would enable the flight crew (FC) to provide tighter control of
the merging and spacing processes. The excess spacing buffers that
exists between consecutive aircraft during approach could be reduced.
This spacing buffer reduction could increase runway throughput. In
addition, by enabling the aircraft to fly more direct or efficient
routes within the terminal airspace, additional flight efficiencies
could be realized.

This concept is based on the general hypothesis that by enabling
distributed approach control conducted by the individual participating
FCs would provide greater flight efficiency and other benefits and would
be more cost effective than providing the air traffic service provider
(ATSP) with more automation tools to pursue the same benefits. Future
research experiments are to be conducted to prove or disprove this
hypothesis.

This report is aimed at providing an initial detailed description of
CE-11. It provides a resource for devising the CE-11 research plan.
Because there are still unknown elements and details of the technology,
operating procedures, scenarios and assumed operating environments that
characterize the eventual mechanization of CE-11, this report should be
considered a living document. As the research plan unfolds and further
input is obtained from the NAS users, service providers, and other stake
holders, this description will be updated to encompass these new inputs.

In this section, background material is first summarized concerning
previous and on-going research on applications of CDTI and its perceived
place within the distributed air-ground traffic management project (DAG
TM) within NASA's AATT project. Then, the objectives of CE-11 are
presented along with the scope of this document.

## 1.1 Background

### 1.1.1 Previous and On-going Related Research and Development

In pursuit of Concept Element 11, it is important to review the
research, development, and testing that have previously been conducted
relative to use of CDTI and related technology developments such as
Threat Alert and Collision Avoidance System (TCAS) and Broadcast
Automatic Dependent Surveillance (ADS-B). This is so that (a) previously
well established technical results can be factored into defining the
flight deck avionics requirements; and (b) documented previous research
is not repeated; this saves both project development resources and time.

A review of this previous work is presented in Appendix B. The following
summarizes the review findings.

#### 1.1.1.1 Pre-1990 CDTI Research

The CTDI concept has been suggested and studied since sometime in the
1940's \[3\]. Airborne station-keeping equipment has been employed
successfully by the military services for many years to maintain safe
air-to-air separation in formation flying.

CDTI studies were pursued in the 1970's and 1980's by NASA to
investigate potential applications that could increase airport capacity,
reduce controller stress and workload, and enhance safety of flight.
These studies used simulations of the TCAS, Mode S radar, and other data
link systems to provide a prototype CDTI. Based on these studies,
traffic displays were postulated and tested under simulated traffic
conditions. In particular, strings of aircraft on approach to landing
were set up with pilot instructions to establish and maintain specified
spacing by using the CDTI for spacing cues. Pilots and controllers
participated in these tests, and much was accomplished in understanding
the relative vehicle dynamics, the human factors of traffic displays,
and the potential of CDTI to provide throughput benefits. The studies
also revealed potential problems such as increased pilot and controller
workload and possibilities of traffic flow instability, secondary
conflicts, and pilot distraction. \[3-16\]

Installation of TCAS II began about 1990 for the large air carriers. The
requirement to carry and use a TCAS II was extended to cover all
aircraft carrying more than 30 passengers. Later, aircraft carrying
10-30 passengers were required to carry the TCAS I. In all cases,
installations have included some form of traffic display. Thus, via the
TCAS program, the inclusion of a cockpit display of adjacent traffic
became a reality.

Three elements of the TCAS or other traffic display design are of
importance to the CDTI applications:

1.  Surveillance -- The accuracy, reliability, and volume of spatial
    coverage of the surveillance and the associated accuracy of the
    tracking algorithm govern to what extent TCAS/CDTI can be used for
    merging and spacing applications. The TCAS II surveillance system
    design was primarily the results of initial work performed at MIT
    Lincoln Laboratory.

2.  Logic design -- This is the conflict detection and resolution (CD&R)
    logic used to determine which aircraft to display or to indicate
    which aircraft may pose a threat. This must be interfaced with
    merging and spacing cues for other CDTI applications. The TCAS II
    threat detection and collision avoidance logic was primarily
    developed by the Mitre Corporation. Seven revisions of this software
    design have been released to the TCAS manufacturers..

3.  Pilot interface -- The human factors aspects of the display and
    other interface mechanisms used by the pilot are critical for
    adapting the system to merging and spacing. The TCAS II display
    design format and other aspects of the flight crew interface were
    investigated and perfected at both NASA Ames and Langley Research
    Centers.

During the 1980-1984 time period, NASA Ames and Langley Research Centers
both sponsored analytical studies and conducted a series of cockpit
simulator experiments to determine:

(a) What were the important elements that allowed pilots to use the CDTI
    for in-trail following?

(b) How could the CDTI be mechanized?

\(c\) What benefits might be realized from CDTI implementation?

At least six different cockpit simulator studies of multiple following
aircraft in approach strings were made at NASA Langley and Ames to
produce data to analyze in-trail dynamics to address these questions.

#### 1.1.1.2 Post-1990 CDTI Developments

After TCAS was mandated for commercial air carriers, and TCAS I and TCAS
II systems with their traffic displays became commonplace, pilots soon
began to use these displays for other "unofficial" purposes than
collision avoidance. The traffic display could help the pilot/flight
crew with situational awareness of other traffic. Pilots started to use
the display for in-trail following when cleared for unconstrained
transcontinental routes. It became apparent that the TCAS/CDTI would
provide many useful applications and that these applications should be
identified, documented, and standardized so that operational use could
be orderly. These applications have been pursued by the activity of the
RTCA SC 186 and SAE G10 committees.

The first "official" use of the TCAS/CDTI for in-trail following control
was for oceanic en route flight \[17\]. Currently, the United States has
authorized the use of the TCAS II traffic display for in-trail climb
(ITC) and in-trail descent (ITD) procedures when following other
aircraft on oceanic routes. The constraints of this operation are that
the trailing aircraft FC must see the Lead aircraft on the traffic
display and there must be enough initial separation so that the ITC or
ITD can be completed without violating acceptable separation as the
trailing aircraft passes through the Lead's altitude. In an enhanced ITC
and ITD procedure, the CDTI would provide flight identification, speed,
altitude, and range information directly to the FC thereby reducing or
eliminating coordination with the Lead aircraft \[18, 19\].

#### 1.1.1.3 On-going CDTI Research and Development

Two on-going technical developments are further enabling the use of
CDTI:

(a) the broadcast of automatic dependent surveillance (ADS-B) where the
    aircraft broadcasts its precise state and intent based typically on
    GPS navigation data; and

(b) the broadcast of traffic information service (TIS-B) where the
    ground radar system determines states of aircraft and broadcasts
    these to those adjacent aircraft as a supplement to ADS-B or TCAS.

The minimum aviation system performance standards (MASPS) for ADS-B
describe nearly 80 potential applications of CDTI based on ADS-B
information \[18\]. These technologies have spurred activity by RTCA to
define further acceptable applications, to develop requirements and
operational procedures for these applications, and to develop and
document minimum operating standards (MOPS) for the CDTI equipment
\[19\].

Safe Flight 21 (SF21) is a current FAA sponsored cooperative
government/industry effort to evaluate enhanced capabilities for Free
Flight based on evolving communications, navigation, and surveillance
(CNS) technologies \[20\]. SF21 will demonstrate the cockpit display of
traffic, weather, and terrain information for FCs and will provide
improved information for controllers. Under SF21, a cooperative
government-industry team is conducting a series of operational
evaluations (OpEvals) of various ADS-B applications in conjunction with
the Cargo Airline Association plans to equip their fleets with advanced
TCAS/CDTI based upon ADS-B. In 1999, enhanced visual approaches and
see-and-avoid enhancements were the subject of the OpEval conducted by
24 participating aircraft in the Ohio River Valley (Airborne Express
facility at Wilmington ILN). This was preceded by extensive cockpit
simulation studies conducted to prepare for the OpEval by Mitre CAASD
\[21, 22\]. In 2000, the OpEval was continued to examine approach
spacing concepts using Constant Range and Constant Time Delay spacing
cue criteria at the UPS facility at Louisville SDF airport. (See
Appendix B for definition of the different spacing cues).

The goal of Capstone, a related project under SF21, is to implement and
test traffic, terrain, and weather display technologies on general
aviation aircraft flying out of Bethel, Alaska. This project is
pertinent in that it is bringing the benefits of the cockpit display
technology to the low end general aviation (GA) user.

Recent NASA and Mitre research has addressed the related use of ADS-B
and CDTI to facilitate dual approaches to closely spaced parallel
runways in instrument meteorological conditions (IMC) \[24-28\]. The
objective is to maintain throughput and capacity as under visual
conditions (VMC). Here the CDTI is used by the trailing aircraft to
maintain a constrained longitudinal spacing relative to the Lead
aircraft approaching the parallel runway. The concept is to provide
adequate separation for both wake avoidance and blunder
protection/collision avoidance purposes.

#### 1.1.1.4 Initial Concept of CDTI within DAG TM

DAG TM is based upon the free flight premise that the pilot/FC can be
more actively engaged in the problem of air traffic management which
will provide cost-effective benefits that cannot be matched by
implementing more sophisticated ground control. This inherently assumes
that the FC has good situational awareness of the surrounding traffic
and can use that information to conduct the processes of self separation
with respect to interacting trajectories such as crossing paths,
overtaking, merging, and station-keeping. This awareness is provided by
the CDTI and the various cues presented to the FC on that display. Thus,
the CDTI becomes a vital link between the FC, the FMS, digital data
link, collaborative maneuvering with other aircraft, and collaborative
decision making with the ATM controller/air traffic service provider
(ATSP) and the airline operations control (AOC)/dispatcher.

Concept Element 11 addresses a practical first set of applications for
the CDTI that will build off of the previous and on-going CDTI research
summarized previously.

## 1.2 Objectives

This detailed Concept Element 11 description has the following
objectives:

-   It provides for technical transfer and sharing of information within
    the NASA research community concerning this concept. It is intended
    to capture the current thinking of NASA researchers concerning the
    future ATM environments and capabilities that may be created by this
    concept. It is intended to be a living document that is continually
    revised as the CE-11 project unfolds.

-   It is a guide and resource for a planned program of research for
    CE-11 through 2004.

-   It is consistent with and a subset of the AATT objectives as
    described in the AATT Air Traffic Management Operations Concept
    (ATM/OPSCON)\[31\].

## 1.3 Scope

This description of CE-11 is intended to provide enough detail to form a
basis for further research into the concept. It is not, however, a
research plan. The research plan is a separate document being developed
by NASA \[2\] that describes how CE-11 presented here will be
investigated, and how statements presented elsewhere as hypotheses will
be tested.

The detailed description has a focus of operational and system
requirements, and deliberately avoids design information to the extent
possible. NASA Ames and Langley Research Centers are in the process of
planning for and designing simulations and facilities to test the CE-11
premises, including the integration of the CDTI with the FMS and an
"Autonomous Operations Planner" (AOP) which will function on board free
maneuvering aircraft. This detailed description is consistent with, and
provides additional guidance to these planning and design efforts.

Specifications for the mechanization of CE-11 concept are omitted from
this document, since requirements for capabilities to support the CE-11
concept should evolve as a result of the research to be conducted. To
avoid confusion with widely discussed tools and mechanisms such as
flight information service (FIS) or Center-TRACON Automation System
(CTAS) whose specifications are being developed elsewhere, this
description typically uses general terms to describe the capabilities
necessary to support this concept.

# 2. Problem Description

As stated in Ref. 1, the problem addressed by CE-11 is:

*Excessive in-trail spacing buffers in arrival streams reduce runway
throughput and airport capacity, especially in conditions of poor
visibility and/or low ceilings.*

In terminal area environments for which arrival demand approaches or
exceeds capacity, aircraft landing rates are significantly lower under
instrument meteorological conditions (IMC) than under visual
meteorological conditions (VMC). In order to compensate for
uncertainties in aircraft performance and position, the air traffic
service provider (ATSP) applies in-trail spacing buffers to arrival
streams under IMC in order to ensure that minimum separation
requirements between successive aircraft are met. The resulting generous
arrival spacing reduces runway throughput below its capacity to accept
aircraft.

Thus, a primary problem that application of CE-11 services is directed
at is the existence of excessive spacing buffers and spacing gaps
between consecutive aircraft during the approach-to-landing phase of
flight and how to reduce this excess using CDTI and FMS technology.
Alternately, this problem can be stated as:

*Determine how the use of FMS and CDTI avionics, associated procedures,
and additional responsibilities by the flight crews of aircraft in the
approach phase of flight can be used to reduce the amount of excess
spacing buffer and inefficient spacing gaps between successive aircraft
in order to improve the runway throughput.*

A corollary problem is to determine how best to enhance the Decision
Support Tool (DST)-generated clearances given by the ATSP to the
approaching aircraft to facilitate improved runway throughput by taking
advantage of the fact that the aircraft are CDTI equipped. For example,
one DST - the CTAS Final Approach Spacing Tool (FAST) - determines best
sequences and schedules to balance runway use and maximize throughput.
Active FAST (aFAST) computes and displays to the controller cues of
where to slow the aircraft down and where to turn the aircraft to
achieve the best flow. It is then up to the controllers to issue
clearances to the individual flight crews to realize the correct
sequence and schedule to achieve these flows. Intuitively, it seems that
if the flight crews could see these DST-generated clearances combined
with traffic directly on the aircraft electronic navigation chart (i.e.,
CDTI) display, without needing verbal clearance by the controller, the
flight crews could do a better (more timely and exact) job in executing
the clearances. Combining DST-generated preferred approach trajectories
with CDTI self spacing activity needs further investigation.

A related problem is the need to evolve away from using rigid, fixed
routes within the Terminal Radar Approach Control (TRACON) airspace so
that aircraft can fly more direct-operating-cost (DOC)-efficient paths
from the entry boundary to the final approach fix (FAF). This freedom
would also facilitate having flexibility to maneuver around storm cells
while still using DST-generated information to provide efficient traffic
flow. Here the issue is to determine how to use the CDTI for merging and
separation while simultaneously each aircraft is using its FMS in
conjunction with the DST information to determine individual optimum
paths to the FAF.

This problem description includes a number of inherent assumptions that
need to be verified by experimental research. These assumptions include:

1.  After applying ATSP DSTs to controlling the terminal area approach
    traffic, there remains significant excess spacing buffers between
    consecutive aircraft that warrant investigation into methods of
    further reduction -- including FC participation in spacing reduction
    via enhanced flight deck technology.

2.  Within the TRACON airspace, the common route segments used by
    approaching aircraft are of sufficient lengths to allow the FC's to
    capture and maintain specified spacing at a net reduction in overall
    ATSP and FC workload.

3.  ATSP and FC personnel will accept the CE-11 concept as operationally
    viable and aircraft operators will accept the concept as
    economically beneficial, given that (a) responsibility for
    longitudinal spacing between consecutive aircraft will be turned
    over to the FC's, and (b) this concept is shown to be technically
    feasible.

CE-11 addresses these and many related problems, assumptions, and issues
regarding the mechanization of and human factors associated with using
the CDTI, FMS, and related avionics during the approach phase of flight.

# 3. Approach

In this section, a FC-directed free maneuvering, merging and spacing
concept is first summarized to set the context for the more detailed
description. This is followed by a summary of the three modes of flight
being addressed. In all cases, the objective is to determine how
enhanced flight deck technology and FC procedures working in conjunction
with the ATSP can be used to facilitate safe, orderly, and more
expeditious traffic in the terminal area.

## 3.1 Concept Overview

In VMC, aircraft are often able to maintain closer spacing during the
terminal approach phase of flight, thereby increasing the capacity of
the terminal area and the runway acceptance rate. In the current system,
the FC's are often requested to accept responsibility for visual
self-separation once they acknowledge they can see the immediately
leading aircraft. In this situation, the FC is responsible for
determining and then maintaining a safe separation from the immediate
Lead aircraft, and is therefore not subject to the ATSP's minimum
separation requirements. CE-11 addresses providing similar spacing
during IMC via use of the CDTI.

Figure 1 is a bubble chart of the different aspects of CE-11. It shows
the conceptual sequential roles of the ATSP and FC (that is, properly
Equipped Aircraft) in using both ground system and flight deck
technology to improve the approach phase of flight, beginning outside of
the TRACON and ending at the FAF. Note that the ATSP continues to have
extensive involvement in this concept in defining and managing the
traffic approach scenario and in conducting the ATSP procedures that
enable this concept to work. The roles and responsibilities of ATSP and
FC are discussed further in Section 8.

![](media/image2.wmf)

Figure 1. Sequential stages of the CE-11 processes.

Self separation will enable the FC's of equipped aircraft to merge
autonomously with another arrival stream and/or maintain in-trail
separation relative to a designated Lead aircraft under IMC as they
would under VMC, thus potentially increasing arrival throughput. In this
investigation, self merging and spacing applies to aircraft that are
subject to spacing requirements during arrival, extending from the
terminal area feeder fix (FF) or TRACON boundary to the FAF.

Anticipated procedures for self merging and spacing involve the ATSP
transferring responsibility for in-trail separation to FCs of properly
equipped aircraft, while retaining responsibility for separating these
aircraft from crossing and non-equipped traffic. Once the FC receives
clearance to merge and maintain spacing relative to a designated Lead
aircraft, the FC establishes and maintains a relative position of their
aircraft with frequent monitoring and speed/course adjustments.

Under some conditions, information such as required time of arrival
(RTA) at the FAF may be provided by an appropriate ATSP-based DST,
thereby enabling accurate inter-arrival spacing that accounts for
differing final approach speeds or wake vortex avoidance. Similarly,
RTAs may be used at each traffic stream merge point so that aircraft FMS
guidance generates trajectories that are smoothly merged by meeting the
associated RTAs.

Self merging and spacing will make use of data link capabilities to
provide traffic position information. The CDTI and/or advanced flight
director/heads up display (HUD) will provide guidance technology as the
source of spatial and temporal situation awareness to the FC. Cues
within the traffic display will provide information to the FC to enable
either manual merging followed by station-keeping or monitoring of
automatic 4D trajectory management by the FMS.

## 3.2 Summary of the CE-11 Operational Modes

For the purposes of addressing the phases of flight being considered
within CE-11, the flight approach process is divided into three
operational modes. Each of these modes has different operational
complexity, technical capability, and potential benefits:

1.  FREE MANEUVERING Mode: The self guidance and separation of each
    equipped aircraft within unstructured arrival corridors, or zones.
    Here, the aircraft FC is cleared to design their own direct path
    within a defined approach corridor during the initial arrival phase
    of flight. During this process, the FC or FMS defines its own route
    leading to the future merge point. (Some aircraft will remain on
    fixed routes within this corridor.) This mode of operation precedes
    the process of MERGING onto a common route. In the MANEUVERING mode,
    the FC responsibility for self-separation is longitudinal only. The
    hypothesis is that ATSP precludes lateral separation issues by 1)
    metering aircraft in the stream across an arrival boundary
    ("pre-organizing" the stream to have the right sequence and to not
    have lateral conflicts), 2) assigning spacing or RTA clearances that
    match the sequence coming into the TRACON such that aircraft will
    not be passing each other with inadequate separation, 3) keeping
    streams adequately separated from other streams, and 4) being
    responsible for over-flight separation from the streams.

2.  MERGING Mode: The MERGING of multiple routes, or streams of
    aircraft. Each aircraft FC is responsible for adjusting in-trail
    position consistent with proper MERGING and then SPACING behind the
    designated Lead aircraft approaching from another stream (and
    arrival zone).

3.  SPACING Mode: The in-trail temporal SPACING mode of flight along
    either a flexible or a specific structured arrival route. Here,
    aircraft are in a common stream, or flying along a common path
    (e.g., post MERGING) that crosses the FAF and leads to the
    designated runway for that string. Each equipped aircraft FC is
    responsible for maintaining a specified temporal separation from a
    designated Lead aircraft in the same string or stream.

Note that the SPACING mode can occur anywhere throughout the arrival
process, not just after the final merge. However, the MANEUVERING and
MERGING modes are currently envisioned to be segregated. Each of the
three modes is now discussed further.

### 3.2.1 In-trail temporal SPACING mode

The spacing mode is directed to in- trail spacing control of multiple
aircraft that either form a string along a fixed route or form a stream
within an approach zone. Such a string is depicted in Figure 2 that
shows traffic approaching an airport under a south flow configuration.
In Figure 2, Aircraft B is tracking Lead Aircraft A on the extended
final approach to Runway 18R.

In this mode of flight, the FC is given the authority to implement
reduced spacing between their aircraft and the preceding Lead aircraft
while in a single arrival string or stream ending at a stabilized
approach point (e.g., FAF). Optimal arrival spacing is defined not in
the spatial sense such as a fixed distance between aircraft, but rather
in the temporal sense, whereby geometric spacing is continually
tightened as the aircraft reduce speed. This results in the achievement
of a tighter desired separation time between aircraft once the trailing
aircraft reaches the stabilized approach point. In this way, all
participating aircraft are able to maintain the higher speed profiles
that support maximum flow across the runway threshold. The target
separation is based on an accurate ETA for the preceding Lead aircraft
at the runway threshold that is derived from knowledge of the final
approach threshold crossing speeds of both aircraft.

In order to implement this spacing management, the trailing aircraft
would require certain information. First, the target separation time
between it and the designated Lead aircraft would be calculated by the
ATSP and transmitted via addressed datalink or voice to the trailing
aircraft FC. This target separation time calculation could be based on
either

1.  wake vortex behavior predictions taking into account the Lead
    aircraft type and configuration and current local weather conditions
    (a la AVOSS \[29\]);

2.  runway occupancy time estimate; or

3.  regulatory separation requirements between the particular aircraft
    types.

Additionally, the target separation time between the two preceding
aircraft (Lead and Lead + 1) may be required for more accurate
prediction of the speed profile of the immediately preceding aircraft.
Regular frequent updates of the Lead aircraft state and trajectory
information through broadcast datalink may be required to generate the
dynamic temporal-spacing guidance cues.

The spacing guidance cues could be provided to the FC on the CDTI for
manual control, or the guidance requirements could be provided to the
autopilot/autothrottle system through the FMS for hands-off active
automatic control. In the absence of broadcast trajectory information
for the Lead aircraft (e.g., non-ADS-B equipage such as for Aircraft C
in Fig. 2), an alternate trajectory prediction would be required,
possibly supplied by an ATSP DST.

![](media/image3.wmf)

Figure 2. Depiction of approaching aircraft on common route segments.
Aircraft B is self spacing relative to Aircraft A. (Diagram courtesy of
Gary Lohr of NASA Langley Research Center.)

3.2.2 MERGING onto a common arrival route mode

The concept of merging aircraft from several streams, or routes, onto a
common route is also depicted in Figure 2. Here, Own Aircraft E on the
northeast diagonal leg is to merge behind Lead Aircraft D which is
approaching from the northwest to the extended base leg. The merge will
take place when Aircraft E turns from its base leg onto the extended
final approach leg behind D which has previously turned onto this leg.

During this mode of operation, the processes of merging aircraft onto a
common arrival route and time-spacing management along such routes are
implemented simultaneously. Before or upon entering the terminal area,
participating aircraft on both free maneuvering and structured arrival
routes are provided sequence assignment instructions that may include
time spacing behind another aircraft on a separate arrival route. The
multiple routes later merge onto a common approach route. Each aircraft
would follow its route but adjust speed and vertical profile while
merging with another arrival stream to position itself in terms of
meeting minimum geometric separation requirements while following
temporal-spacing guidance cues to fall in behind the assigned Lead
aircraft. Additionally, the aircraft may undergo multiple stream merges
in this way before ending up in the final arrival stream prior to
landing.

There are two ways each equipped aircraft may be guided before it merges
behind the assigned Lead aircraft:

1.  It may be given a "ghost" image of the Lead aircraft as computed and
    projected on its own route before the merge point; or

2.  It may be given an RTA at the merge point that is computed based
    upon the Lead aircraft crossing that point at the appropriate time
    separation before the RTA.

### 3.2.3. Free MANEUVERING in unstructured arrival corridors

The concept of aircraft flying in flexible, free maneuvering, arrival
zones is depicted in Figure 3, again for a generic TRACON. The flexible
zones are shown in green. Aircraft G which is to merge behind Lead
Aircraft F is on a different route than F, but it maintains proper
spacing relative to F as both aircraft head toward a point where they
will be on a common structured route.

In this mode, structured arrival routes are replaced with or
conceptually broadened into arrival regions or zones. Instead of
entering the terminal area via an arrival fix and following fixed
structured routes thereafter, participating aircraft cross an "arrival
zone boundary" and are provided authority to maneuver laterally within
designated arrival zones that are segregated from departure corridors.
The sizes of the arrival and departure zones could be static or could be
dynamic to optimize terminal operations for inbound and outbound pushes
or for weather considerations. Aircraft would be responsible for
separation assurance, for remaining inside the arrival zones, and for
merging into close-in arrival streams based on sequence assignments
provided by the ATSP.

In this mode of flight, aircraft would have the authority to maneuver
tactically for weather avoidance, separation, spacing, or descent
profile management without clearance from the ATSP. Non-participating
(e.g., non-equipped) aircraft, such as Aircraft E in Fig. 3, would
remain on structured arrival routes (or vectored paths) and receive all
clearances from the ATSP. Although CE-11 does not specifically address
the management of departures, the arrival operations would be designed
in such a way as to not unduly impact departure operations.

![](media/image4.wmf)Figure 3. Sketch of Free Maneuvering Zones and
Aircraft (D, F, G) in free maneuvering mode on approach (Courtesy of
Gary Lohr, NASA Langley Research Center).

#  

# 4. Operational Requirements

This section refers to a description of the Operational Needs Statements
(ONS) which apply to CE-11. These ONS have been created to support the
development and on-going revision of the AATT ATM/OPSCON.

See Appendix A for the operational requirements, presented as a table.
CE-11, Self Spacing for Merging and In-Trail Separation, applies to the
Traffic Management -- Synchronization service area as defined in the
AATT ATM/OPSCON.

# 5. Operational Environment 

This section describes the assumed operational conditions under which
CE-11 will be applied. The operational environment includes the airspace
structure, routes within this structure, and their constraints; the mix
of aircraft types, their equipment, and performance limits; the weather
and visibility conditions; the CNS infrastructure within the airspace
that enable connecting the FC with ATSP; and the ATM capabilities of the
environment. Each of these elements is discussed from the point of view
of representing the range of environments that need to be considered.

## 5.1 Airspace Structure and Route Constraints

The CE-11 operating airspace environment consists of the approach
sectors, areas, or "zones" within the TRACON ranging from the entry
feeder fixes at the TRACON boundary, and the boundary itself, to the
final approach fixes (FAF) and possibly to the runway thresholds. During
the in-trail spacing mode, flight will be constrained to be along fixed
routes within these zones starting from specific entry point feeder
fixes. These routes would be part of the normal aircraft flight plan and
be specified by the Standard Approach Route (STAR) and runway approach
plates for the given airport. During the free maneuvering mode, the
aircraft can fly along any non-conflicting path from entry to exit point
within its approach zone. For this mode, the entry point can be anywhere
along the boundary between the zone and the adjacent Center airspace; it
is not restricted to specific entry points.

For in-trail spacing, the aircraft are initialized in a single string
along a fixed route where no further merging is required before reaching
the FAF. Therefore, a long route beginning at zone entry point and
having down wind, base, and final approach legs represents a suitable
baseline test case. Such routes are depicted in Figure 2. An example
would be a corner post entry approach to Runway 18L from the BYP feeder
fix at Dallas-Ft Worth (DFW) TRACON. A variation would be a long
non-corner post route such as the path from Big Sur feeder fix in the
Bay Area TRACON from the Oakland Center and Southern California into San
Francisco (SFO) Runway 28L. Initially, the typical four-cornerpost
TRACON with flow to a single runway would be the nominal case for
defining the route structure and surrounding airspace. This nominal case
can be chosen by survey of several terminal areas to determine where
CE-11 promises the most benefit.

For the merging mode, the aircraft are initialized into two or more
strings that merge at waypoints before reaching the FAF. This mode
extends the in-trail spacing mode in that it includes the process of
merging two or more strings of aircraft. Means must be provided for
showing spacing cues on the CDTI to account for aircraft that will
become the designated Lead after the merge takes place. That is, each
equipped aircraft is guided such that it will successfully merge behind
and self space with a Lead aircraft that is initially on a different
approach route. This is illustrated in Figure 2 where Aircraft E is on
the northeast diagonal leg and will merge behind Aircraft D which is on
the northwest diagonal leg. The merge occurs as Aircraft E turns onto
the final leg behind Aircraft D.

In the free maneuvering mode, each of the aircraft can use the airspace
within a defined corridor for the initial segment of their approach. In
this phase of flight, the aircraft FMS or FC computes the desired
approach route within the zone before the aircraft enters the zone. This
route may be picked to (a) minimize direct operating cost (DOC); (b)
minimize fuel while achieving a required or assigned time of arrival
(RTA) at the FAF or other waypoints; or (c) weave through a series of
storm cells representing inclement convective weather that are blocking
the zone. In each case, the intended route may be linked to the ATSP to
aid the ATSP in monitoring the in- trail spacing process.

Each aircraft that enters the approach zone may have unique entry points
and unique approach routes thereafter. They may not merge until close to
the FAF. However, they will merge eventually, and the CDTI is used to
give each FC the merging and spacing cues along the specific route so
that the future spacing after the merge is as desired for safety and
throughput maximization.

One way the free maneuvering self separation process can be visualized
and mechanized is to project each of the various approaching routes onto
a common route passing through the zone and to compute the spacing cues
to facilitate spacing control along that route. This process would allow
two different flexible routes to be merged at any range to the FAF.

An alternate way is to constrain in-trail spacing to only after the
common merge point and to compute the RTA for each aircraft at the merge
point so that proper spacing occurs thereafter. This allows each
aircraft to follow a more flexible speed profile along their specific
route leading up to the common merge point. It requires that the FMS
have RTA capability.

Some of the airspace and route geometry issues that need to be
investigated to define the CE-11 operational environment, especially
during the free maneuvering mode, are the following:

1.  Definition of the approach zone geometry:

> a\. How are the lateral and vertical dimensions and boundaries of the
> approach zone defined and communicated to the FMS/FC before the
> aircraft reaches the TRACON?
>
> b\. What are the lateral and vertical spacing constraints relative to
> the zone boundaries?
>
> c\. How are the common merge points and FAF defined?

2.  Definition of approach zone weather and its effect on approach route
    computation -- By definition, CE-11 is directed at control of flight
    in IMC under instrument flight rules (IFR). The goal is to achieve
    or exceed the same flight efficiencies as seen under VMC. The base
    case is that the approach zones are free of storm cells. The
    additional weather cases have increasingly dense weather cells that
    restrict more and more of the approach airspace. Issues are:

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a.  How many different weather cell situations need to be examined?

b.  How do the free maneuvering mode approach routes and associated
    trajectory computation process change as the available airspace
    becomes more restricted with weather cells?

c.  At what point does the approach through the zone become closed due
    to weather?

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```
3.  Route structure (or absence thereof) -- A variation of the free
    maneuvering mode is the case where some of the aircraft have the
    equipment to allow computation and use of flexible routes within the
    approach zone and others have older equipment that only allows
    flight along fixed routes. This mixed equipment environment may
    require that fixed route aircraft be segregated from those capable
    of more flexibility if the approach that an RTA at the merge point
    is used to guide the aircraft on the flexible path.

4.  Dynamic re-sectorization -- This is a longer term case where the
    definition of the approach zone airspace and its boundaries are
    changed from time to time to accommodate variations in traffic flow
    and direction or changing weather conditions. The issues are:

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```
a.  How do the new zone boundaries get computed by the ATSP and linked
    to the incoming aircraft?

b.  What information is needed to make these computations?

c.  How soon before the aircraft reaches the TRACON boundary does the up
    link of new zone boundaries need to occur?

Within the design of the airspace for CE-11, other assumptions are made
regarding the nature of the structure:

1.  There may be separate routes and entry points for slower aircraft,
    such as turboprops. These routes would be merged with those of the
    faster aircraft at waypoints close to the FAF.

2.  The effects of weather cells affect all three modes of flight and
    will be brought into the scenarios tested as appropriate.

3.  Each approach zone is defined by a centerline that may represent the
    structured route that would be followed by unequipped aircraft.

4.  Special Use Airspace is not considered as part of the airspace
    constraints for CE-11.

5.  Runway closure and re-configuration because of wind shifts are
    off-nominal cases that are dealt with at later stages of the
    research.

6.  TRACON airspace sectorization and handoff between controllers are
    part of the ATSP-FC procedures of CE-11.

## 5.2 Traffic Mix and Equipage

It is assumed that some or all aircraft in the CE-11 scenarios will be
equipped with traffic displays with common information and spacing cues
presented to the flight crews. Those aircraft without CDTI equipment
will be vectored and cleared manually by the controller, as is done
today. Two parameters that will affect the use of and operational
procedures for the CDTI during approach are the variations in (a) the
types of aircraft within the approach, and (b) the avionics equipment
(data link and FMS/CDTI capability) that is on the aircraft.

Users of the approach airspace range from piston engine to turboprop to
turbojet aircraft, each with a different speed range. At certain
TRACONs, there are different approach routes used to segregate the
slower turboprops from the turbojets. These different route
possibilities need to be preserved as a test case for the three modes of
operation. The flexible route free maneuvering mode accommodates
different speed envelopes of each aircraft if the
RTA-at-common-merge-point methodology is used to space the aircraft at
the merge point. This assumes that after the merge point each aircraft
can fly at compatible speed with respect to the other aircraft in the
approach string. Also, within this mode, different speed classes can be
segregated onto different non-interfering routes.

Another aircraft type factor is the different type-dependent nominal
separations that are used to maintain safety due to wake vortex
considerations. These nominal separations are usually defined in spatial
terms at the point where the Lead aircraft crosses the runway threshold,
and they are a function of the weight of the aircraft -- light, large,
or heavy. For CE-11 purposes, these nominal separations are converted to
time separations and projected back to the FAF. Thus, for example, if
two consecutive B737 aircraft each have V~ref~ speeds of 120 kt as they
cross the runway threshold, with a minimum of 2.5 nmi separation, this
would represent a minimum time separation of 75 sec. If each aircraft
crosses the FAF at 170 kt, this 75 sec separation would represent a
minimum spatial separation at the FAF of 3.54 nmi. Some nominal buffer
spacing is added to these separations to account for the uncertainty in
the spacing that will occur between two consecutive aircraft to ensure
wake vortex separation safety. Likewise, if two consecutive B747
aircraft must be greater than 4.0 nmi apart as they cross the threshold
with V~ref~ of 140 kt, this represents a minimum 103 sec separation.
Again, if 170 kt is the nominal FAF crossing speed, this translates into
4.85 nmi minimum separation at the FAF point.

The approach airspace can be used by air carrier, air taxi, corporate,
and private aircraft, each of which will have FCs with different levels
of experience and proficiency as pilots. This will affect how well a
given pilot maintains the desired spacing with respect to the Lead
aircraft in a string. This is largely a function of training, flight
currency, and motivation on the part of the different FCs to maintain
tight separation tolerances.

It is assumed that a certain minimum set of avionics equipment is
required for an aircraft FC to participate in the three modes of flight
of CE-11. Candidate equipment or capability includes:

1.  A suitable traffic display with appropriate cues to allow the FC to
    guide the aircraft during free maneuvering, merging, and in-trail
    spacing.

2.  A navigation system with required navigation performance (RNP)
    rating that supports (a) broadcast of aircraft state with sufficient
    accuracy to support the traffic display requirements; and (b)
    adequate adherence to the intended route to provide safe separation
    assurance.

3.  An ability to sense accurately the state of adjacent traffic and the
    designated Lead aircraft. This may be via ADS-B, existing TCAS
    mechanization, or TIS-B.

4.  An FMS that can be used to compute preferred routes for free
    maneuvering, achieve RTA in-trail position control, and possibly for
    automatic merging and in-trail spacing control to the FAF.

5.  An autonomous operations planner (AOP) that the FC can use to
    organize and manage the three modes of CE-11. The AOP is considered
    an extension and enhancement to the FMS.

6.  A two-way data link to allow exchange of digital information between
    FC and ATSP. This may also be used for data exchange between Lead
    and following aircraft.

Aircraft that don't have at least some subset of these capabilities
cannot participate actively in CE-11 processes. Aircraft that don't have
these capabilities are termed "unequipped", and they could represent
targets that could serve as Lead.

The equipped aircraft may have the FMS with longitudinal autopilot/
autothrottle so that relative spacing can be automatic after the Lead
aircraft has been designated. However, each aircraft can also be flown
manually where the pilot uses the CDTI as a flight director to control
airspeed and relative spacing to the designated Lead.

In the merge mode, it is assumed that the traffic display cues are
extended to depict an indicated location/state of a designated Lead that
may be on another route segment that will merge with the current
aircraft route at a later point. This indicated Lead location could be
the projection of the actual Lead location onto the current route with
the same range to go to the common merge point of the two routes. This
location projection is often referred to as a "ghost" and it could be
used by both FCs and ATSPs for spacing \[30\].This type of projection
would have to take into account the difference in winds along the route
segments that would manifest in differences in ground speeds which would
affect the associated desired spatial distance between consecutive
aircraft.

For the free maneuvering mode, it is desirable for aircraft that follow
flexible routes to have the following capabilities:

1.  An FMS that can provide lateral and vertical position, and
    longitudinal speed guidance along a curved approach reference route
    or trajectory, and can store and use approach zone and restricted
    airspace boundaries;

2.  FMS ability to compute an optimum DOC or fuel burn curved trajectory
    from present position to the FAF within ATSP constraints. This
    includes ability to use an RTA at some future desired merge point or
    FAF. This path is stored as the nominal reference trajectory;

3.  Data link ability to link flight path intent to the ATSP for
    monitoring purposes. In the far term, this may include the ability
    to receive and use up-linked ATSP modifications;

4.  FMS ability to modify a nominal route to take into account and avoid
    restricted airspace defined by the approach zone boundaries and
    restricted convective storm cell locations within the zone;

5.  Data link, FMS, and CDTI ability to receive, display in graphical
    form, and use for trajectory computation and steering guidance storm
    cell volume boundaries or other restricted airspace boundaries; and

6.  A CDTI that can depict the curved reference path, boundaries of
    restricted airspace, and spacing cues to ghost projections, as
    appropriate. This includes ability to project the relative position
    of the designated Lead (that is on a different route) onto the
    aircraft route in the form of a ghost aircraft.

Some additional traffic and equipment mix questions that need to be
investigated to define the CE-11 operational environment are the
following:

1.  What different mixes of equipage should be investigated in the
    different experimental scenarios (both simulator and flight test)?
    How many different combinations are necessary to understand the
    dynamics of a mixed-equipage environment in terms of the transition
    to a fully equipped fleet?

2.  What mix of turboprop and turbojet aircraft should be examined?
    Should these aircraft be isolated onto different approach routes or
    into different approach zones? Which TRACONS have representative
    mixes of aircraft types that would be good test cases?

## 5.3 CNS Infrastructure

It is assumed that the aircraft states are very accurately measured (via
GPS WAAS / LAAS) and available to each flight deck and ground ATSP
through some suitable form of data exchange (e.g., ADS-B) or
surveillance (e.g., TIS-B). Furthermore, it is assumed that digital data
link is available to send digital clearances, flight information, and
traffic information if needed to the flight deck. It is assumed that
each aircraft can be flown manually or automatically via FMS autopilot /
autothrottle.

For communications, required data and information are assumed to be
available to provide in digital form, controller advisories and
clearances to the FC, graphical information concerning weather cells and
zone boundaries, and adjacent traffic states not available through
ADS-B. However, voice clearances for merging and in-trail spacing
represents viable means for the ATSP to clear the FC to enact these
modes of flight. Also, the ATSP DST may be able to up-link necessary
information to define the desired profile for a Lead aircraft that
defines the beginning of a string. Alternately, each aircraft FC could
have a standard speed profile per aircraft type that is automatically
followed if the aircraft is the string leader.

In terms of navigation, differential GPS provides adequate means for
CE-11 applications. The FMS will use this information for precise
guidance, and to compute state and intent messages for ADS-B. Aircraft
without FMS will be assumed to have area navigation (RNAV) to follow
routes defined by sequences of waypoints. Aircraft without autopilot or
autothrottle will have flight director information on the CDTI to enable
the FC to capture and track desired spacing relative to the Lead
aircraft.

ADS-B is assumed to be the standard for aircraft data
exchange/surveillance both from the CDTI perspective and the ability of
the ATSP / DST to monitor progress of each of the aircraft. Ground
broadcast (TIS-B) traffic information can be used by subject aircraft to
track those aircraft without ADS-B but with transponders tracked by the
ground radar.

Some of the CNS infrastructure questions that need to be answered
further to define the CE-11 operational environment are the following:

1\. Communications - For the free maneuvering mode, can we assume that
the ATSP or other service provider sends graphical weather data to the
flight deck to show to the FC as a layer of information on the
multi-function display (MFD)/CDTI? Can we assume that ATSP and FC have
access to the same graphical weather picture?

2\. Navigation - What variations in FMS equipment, autopilot /
autothrottle, and RNP should be investigated in terms of ability of the
aircraft to hold tightly in altitude, lateral deviation, and speed
variation from the route represented by the flight plan?

3\. Surveillance - Is the period of transition from radar to ADS-B
surveillance of interest here? Do we need to consider the situation
where some aircraft are ADS-B equipped, some are tracked by a secondary
surveillance radar (SSR), and the two sets of data are fused for a
common surveillance picture? How about a mixture of CDTI systems with
sources of traffic information from either ADS-B, TIS-B, or TCAS II/Mode
S?

## 5.4 ATM Environment

Three different ATM environments exist in which CE-11 merging and
spacing modes may take place:

1.  No special DSTs beyond today's environment are used (e.g., such as
    being tested by the Safe Flight 21 Cargo Airline Association
    Operational Evaluation of ADS-B for approach spacing).Here, the
    controller/ATSP determines the desired string position of each
    approaching aircraft. The controller advises the pilot which
    aircraft is the immediate Lead, the controller clears the pilot to
    use the CDTI to maintain a certain spacing relative to the Lead, and
    the pilot uses the CDTI to capture and track a specific spacing
    relative to that aircraft.

2.  Extension of CTAS Active FAST (or equivalent DST) where the desired
    approach trajectory (including waypoints of where to decelerate and
    where to turn) is communicated to at least the String Leader
    aircraft in the string. This ensures that the reference speed
    profile for starting each string is efficient. The DST would also be
    extended to allow the controller to monitor the progress of aircraft
    in the string to ensure compliance with the planned arrival
    schedule. Furthermore, the DST would be used to determine the
    positions of aircraft on merging routes, possibly compute the ghost
    positions of these aircraft as projected onto other routes (as a
    function of range to go to the next merge point), and provide the
    up-link message to communicate these positions to the flight
    deck/CDTI.

3\. Special non-CTAS TRACONS where the ATSP provides information and
capability that facilitates use of CDTI for merging and spacing by the
aircraft and monitoring of the process by the controller. This would
include the features of the first two environments, but it might not
have all the features of the FAST DST. This scenario represents a point
somewhere in the range of capabilities defined by the first two
environments.

# 6. Operational Characteristics

A basic premise of CE-11 is that a designated "string leader" aircraft
follows a desired speed profile from TRACON entry to the FAF or
threshold. The next arriving aircraft is cleared by ATM to merge behind
the immediate Lead and then to self space according to some accepted
spacing criterion. This second aircraft then becomes the Lead aircraft
for the next (third) arrival aircraft in the string, etc. Various
specified spacing gaps are used to account for different wake vortex
spacing constraints based upon aircraft type, and allowances for
departing aircraft on the runway. Also, natural spacing gaps will occur
because of the distribution of arrival aircraft over time. Thus, there
will be need to re-start the strings from time to time.

The desired spacing of the aircraft behind the designated Lead may be
based upon one of the following cues (discussed further in Appendix B):

1.  History trail of the Lead (e.g., where the Lead was 90 sec ago); or

2.  Constant time predictor with acceleration cue (e.g., where the
    CDTI-equipped aircraft will be 90 sec from now).

## 6.1 ATSP View

It is assumed that the ATSP/DST determines the desired sequence and
spacing of arrival aircraft and this information is provided to the
flight crew by voice or data link. The ATSP transfers responsibility for
merging and spacing to the flight deck until the aircraft crosses the
designated end point. (The ATSP maintains responsibility of protecting
the arrival aircraft from crossing intruder aircraft and of monitoring
the performance of each aircraft along the string to ensure that each
aircraft maintains safe separation limits.

The primary roles of the ATSP are indicated in Fig. 1. These include the
following activity:

1.  calculate and communicate the assigned separation (spacing) time
    with respect to the designated Lead for each participating aircraft;

2.  estimate and possibly uplink trajectory predictions of non-FMS,
    non-state broadcast equipped aircraft as needed;

3.  provide separation assurance between streams and for
    non-participating aircraft;

4.  monitor the arrival merging, flow rate, and conformance to the
    assigned spacing time;

5.  using some kind of alerting mechanism, advise participating aircraft
    of predicted deviations from the designated separation value; and

6.  provide means to terminate the CE-11 process because of abnormal
    situations such as airport closure, equipment failure, or pilot
    request for route deviation.

Some questions that need to be addressed to further define the ATSP
operational role are as follows:

1\. What information is needed on the ATM display to enable the
controller to monitor merging and spacing performance? What is the
minimum ATSP equipment and capability required?

2\. What monitoring and alerting criteria should be used whereby the
controller re-takes responsibility of approach control?

3\. What operational conditions besides lack of equipage should prevent
the controller from transferring merging and spacing responsibility to
the flight crew?

## 6.2 Pilot View

For the in-trail spacing mode, it is assumed that the succession of Own
aircraft are positioned such that their FCs can quickly capture and
maintain spacing relative to the designated Lead aircraft.

For the merging mode, it is assumed that each equipped aircraft is
positioned and is given necessary information to merge behind the
designated Lead or Lead Ghost.

For the free maneuvering mode, it is assumed that each aircraft FMS or
FC computes the desired path from entry point to exit point, and guides
the aircraft along this path. Own either (a) merges and spaces with a
Lead Ghost projected onto this path; or (b) uses an RTA to control speed
to arrive at the next merge point at a time that is consistent with
merging with the Lead aircraft arriving on the other route.

# 7. NAS Functional Impacts

This section discusses the NAS impacts, including planned NAS
architecture components, of the Concept Element 11. Section 7.1
describes functional requirements, and Section 7.2 provides an overview
of the functional design resulting from these requirements.

## 7.1 Functional Requirements

The following functional changes from the NAS 4.0 mature baseline,
expressed in terms of technology and infrastructure, are needed to
support the concept. These are described in the area of Communications,
Navigation, Surveillance, Automation, Weather, and Traffic Management.

### 7.1.1 Communications

Within the NAS 4.0 baseline, ground-to-air communications with
participating aircraft within the approach zones of the TRACON are both
by datalink and voice. Datalink communications are both broadcast and
addressed. The ATSP broadcasts or provides before flight advisories and
information on winds aloft, graphical weather cell location and
intensity, and for free maneuvering, the geometric boundaries of the
approach zones. For participating aircraft without ADS-B, the ATSP up
links state information of adjacent aircraft.

To mechanize CE-11, aircraft specific advisories and clearances, sent
via addressed data link include the assigned Lead aircraft and the
assigned time spacing the following aircraft applies to self spacing
behind the Lead. For environments with ATM automation and DSTs, the
desired speed profile may be computed and linked up to the first Leader
Aircraft in a given string. For the merging mode, either the ghost image
or actual position of the Lead aircraft on a merging route may be linked
up to be projected onto the following aircraft's route. If the RTA
capability of the FMS is used to facilitate merging, that RTA is linked
up to the aircraft.

Air-to-ground communications include the participating aircraft
accepting assignment of following the specified Lead or ghost,
acceptance of assigned RTA if appropriate, and acceptance of approach
speed profile if up linked from ATM automation.

Air-to-air communications occurs through ADS-B discussed in Section
7.1.3.

Voice communications are procedurally used when the ATSP takes over
control of specific aircraft because of emergency or non-compliance with
intended merging and spacing assignments. In an airport environment
where a DST is not used, the controller may use voice communications to
vector aircraft for proper merging followed by assignment of Lead
aircraft and desired spacing to a CDTI-equipped aircraft.

CE-11 requires that special messages be created for ground-to-aircraft
and aircraft-to-ground communication, as just described. Special
avionics software will be required to facilitate implementation of the
CDTI with proper presentation of other aircraft, spacing cues, and
airspace constraints. Similarly, DST software will be required to extend
planned NAS capabilities to include mechanization of CE-11. However, no
new communications systems or hardware should be required for flight
deck or ATSP implementation.

### 7.1.2 Navigation

There are no new functional navigation requirements imposed on the ATSP
by CE-11. GPS and suitable backup (i.e., DME-DME) are used as means of
navigation.

In terms of the aircraft avionics, the CDTI is an additional layer of
information on the multi-function display; its design is primarily one
of software. It is assumed that the on-board navigation system has a
Required Navigation Performance (RNP) level with sufficient accuracy to
support merging and in-trail spacing applications. It is also assumed
that this information is used for state and intent broadcast.

### 7.1.3 Surveillance 

Participating aircraft within CE-11 will be equipped to broadcast their
state and possibly their intent information computed in the FMS and to
receive this information from adjacent aircraft. State and intent
information are broadcast at 1 Hz. Intent information extends to the
runway threshold, and contains the FAF as a waypoint. Participants
without FMS will not broadcast intent.

The ATSP surveillance function will fuse broadcast state information
with that obtained from area RADAR. The intent information provides the
ATSP with a forward look of the intended path the participating aircraft
will fly.

The only new surveillance function beyond current plans for NAS is the
added capability of the ATSP to monitor the progress and relative states
of the participating aircraft during the three flight modes. This
includes the capability to assess actual and future separations, to
determine when the aircraft is outside of acceptable separation
assurance conformance limits, and to provide alerts to the controller
for immediate response.

### 7.1.4 Automation 

The ATSP functions within CE-11 are presented in Figure 1. As described
previously, new ATSP automation within the DST beyond current NAS plans
includes capabilities to compute and display:

(a) the desired sequence and pair-wise spacing between consecutive
    aircraft in the sequence strings;

(b) the RTA for each participating aircraft to facilitate smooth
    merging;

(c) possible means to change approach zone boundaries to adapt to
    convective weather cell locations;

(d) means to monitoring the merging and in trail spacing processes for
    desired separation conformance; and

(e) alerting mechanism when conformance is not met to point of affecting
    safety.

These automation functions are supported by appropriate two-way data
link between aircraft and ATSP DST.

Automation functions required on the flight deck are summarized in
Section 5.2.

### 7.1.5 Weather

The ATSP or commercial weather service provides accurate winds aloft,
atmospheric conditions, and graphical weather information to aircraft
and ATSP DST. These data are updated regularly by down linking of
measured wind and temperature by participating aircraft. This function
is expected to be part of the future NAS architecture.

### 7.1.6 Traffic Management

There are no changes required for strategic traffic management, that is
at the Command Center level. Local traffic management participates in
setting the arrival acceptance rate the airport can handle which in turn
affects metering up to the TRACON. It also sets up the arrival sequence
as manifested by the sequence and schedule of aircraft crossing into the
TRACON. If CE-11 is in use, the runway throughput should increase which
will be reflected in the acceptance rate used for traffic management.
Thus, no new traffic management functions within the projected NAS
architecture are required.

## 7.2 Functional Design

Figure 4 is a high level functional design diagram showing those NAS
systems and services that are essential for supporting CE-11. Current
and future air traffic systems and services which are general to ATM but
not specifically utilized in CE-11 are not shown.

![](media/image5.wmf){width="6.4in" height="4.413888888888889in"}

Figure 4. High Level Depiction of CE-11 Functional Design (Modification
to original layout provided by courtesy of Charlie Phillips, Titan SRC.)

The two aircraft (Lead A and Follower B) shown are members of a formed
string. Each maintains accurate state information and trajectory
conformance using GPS as the primary navigation input to the FMS. Each
aircraft broadcasts state and intent information to aircraft and ground
receivers. The ATSP uses secondary surveillance as a backup to the
aircraft broadcast information, and the SSR measurements are fused with
the state broadcast data for improved total situation awareness.

Two possible additional uplink messages come from the ATSP to the
participating aircraft. One is the ATSP flight information including
weather, winds, atmospheric conditions, zone boundaries, and regular
ATIS information. The weather data may be generated by either the ATSP
or a commercial weather service. The other message is the ATM clearance
and advisory information such as assigned Lead aircraft, assigned
spacing (time) interval, RTA for merges, and general clearances for
setting up the string and turning over self spacing responsibility to
the FC of each aircraft.

#  

# 8. User/Operator Roles and Responsibilities.

In this section, the roles and responsibilities of the ATSP/controller
and the participating aircraft/flight deck/FC for CE-11 operations are
summarized.

## 8.1 ATSP Roles and Responsibilities

Here, we first discuss the full responsibility of the ATSP, meaning
automation, air traffic manager, and air traffic controller. We then
discuss issues related to the controller interfacing with both the
automation and the FCs. These functions are depicted in Figure 1.

The ATSP responsibility for enacting the CE-11 methodology begins well
before the participating aircraft enter the TRACON. There is a
collective activity between airport, TRACON, and surrounding Centers
that determines the acceptance rate of the runways, the airspace
capacity as affected by weather and other non-approach-to-landing
aircraft, and the demand for airport landing operations. (This activity
is aided by knowing the flight plans of each aircraft that desires to
land; that is, there is an aircraft operator/airline role involved,
too.) The runway acceptance rate, airspace capacity, and runway
throughput demand set up the traffic management process for the greater
terminal area (i.e., out to beyond top-of-descent). This process
determines the desired sequence and schedule of aircraft entering the
TRACON with intention of landing. In turn, the desired schedule sets up
the metering processes to bring aircraft from the surrounding Centers to
the TRACON approach zone boundaries and feeder fixes.

Within the TRACON, the ATSP defines the nominal routes and route
segments leading to the runway, the merge points for those routes, the
FAF, and the approach zone boundaries. If the boundaries are changed
because of dynamic density or weather restriction considerations, that
also is the responsibility of the ATSP. For the free maneuvering mode,
the approach zone boundaries are published in a navigation data base and
are static for nominal operations. These geometric definitions are done
well in advance of the particular flights involved. In a far term
scenario, dynamic zone definition could be done during the flight
process, but before the aircraft crosses into the zone.

The TRACON ATSP is also responsible for the traffic management process
governing the aircraft sequence and schedule. That is, the TRACON ATSP
takes over sequence management of the aircraft from the Center ATSP.
Landing schedules are accepted or modified depending upon the dynamics
of the TRACON, airports, and runways involved. The ATSP determines which
runways are in use using information on flight plans, winds, weather,
and departing traffic. It determines the desired schedule of landing
operations taking into consideration departing traffic, conditions of
the runway (e.g., wet surface, icing), weight and class of the aircraft,
wake vortex and runway occupancy factors.

Inherent in the landing schedule are the safety reasons which
established relative minimum-plus-a-buffer acceptable spacing between
successive aircraft at or just before the time points that each Lead of
each aircraft pair crosses the runway threshold. The ATSP sets these
threshold crossing time points in the schedule to maintain minimum
spacing (a spatial distance) and to allocate enough additional buffer to
account for variations in the final approach trajectory of each aircraft
because of differing speed profiles and winds. This variation in spacing
is illustrated in Figure 5. The role of ATSP controlled landing
scheduling is to remove unnecessary "gaps" between aircraft and to
reduce the "buffers" within the landing schedule by reducing the
trajectory uncertainties. (The premise of CE-11 is that the FCs can
control the relative spacing between Own and Lead aircraft better than
ground ATSP thereby allowing for reduced buffers. Never the less, it is
still the ATSP responsibility to set the minimum plus buffer spacing
parameters based upon safety considerations.) The desired spacing is
converted to an equivalent time spacing between consecutive aircraft,
which is built into the landing schedule.

![](media/image6.wmf)Figure 5. Illustration of minimum spacing, spacing
(uncertainty) buffer, and excess spacing gap between consecutive
aircraft in an approach string.

The ATSP takes the landing schedule and backs it upstream along each
route and route segment leading to the particular runway. This process
then establishes the schedules for crossing the FAF and the merge points
(route waypoints in the flight plan) for each aircraft approaching the
given runway.

The ATSP uses the waypoint schedules to assign required time-of-arrival
(RTA) to each FMS equipped aircraft to be used by the aircraft FC to
facilitate safe merging at particular merge waypoints. The ATSP also
uses the landing sequence and relative time spacing between aircraft to
assign first the Lead aircraft for each participating aircraft to follow
and then the incremental time spacing that the following aircraft is to
maintain relative to the Lead.

The ATSP is responsible for determining if conditions warrant allowing
the properly equipped aircraft and FCs to choose their own flexible
approach route, to merge with other participating aircraft, and to self
space thereafter. The ATSP issues the clearances to each participating
FC to designate its Lead aircraft, to set its spacing parameter and
RTAs, and to take over responsibility for merging and self spacing until
the ATSP takes that responsibility back.

Thereafter, it remains the responsibility of the ATSP to monitor the
progress of the self merging and in trail spacing process to ensure that
this is done with some pre-established threshold of spacing compliance.
The ATSP is responsible for issuing warning if compliance is not being
met and to take over the separation control if safety is being
compromised. At the same time, the ATSP remains responsible for
protecting the approaching aircraft from other traffic such as
departures, over flights, and pop-ups.

Even in busy terminal areas and during rush periods there will be
necessary gaps in the approach traffic because of the somewhat random
nature of arrivals from the Centers or deliberate gaps to allow for
departures or crossing traffic. Thus, each string that is formed by the
ATSP is of specific duration, and new strings must be started throughout
a day's operations. The String Leader aircraft sets the reference
approach trajectory for subsequent aircraft in each string, and so it is
important for good traffic flow that these String Leader reference
trajectories are fast and efficient. It is the ATSP responsibility to
ensure that the String Leader follows such an efficient trajectory,
either by FC reference to one published in the airport approach charts
or one in which the ATSP clears the String Leader FC along an efficient
speed profile.

The ATSP maintains the ability to take over responsibility of the
approach process at any time to account for non-normal events such as
runway change, missed approach, emergency operation, non-compliance to
spacing standards of one or more aircraft in a string, or some other
dynamic situation. The ATSP is responsible for making the call -- employ
the CE-11 applications or not.

The ATSP continues to be responsible for those non-equipped or unable to
participate aircraft involved in the approach process. This includes
determining if such aircraft need to be segregated from participating
aircraft or mixed into the strings but still controlled by the ATSP.

Much of the ATSP responsibility for the CE-11 service pre-supposes a
certain level of automation on the part of the ATSP. For terminal areas
without this automation, many of the previously mentioned ATSP roles and
responsibilities can still be executed, but without the precision that
automation provides. The minimum requirements in ATSP automation to
mechanize the CE-11 service remains as a research topic.

## 8.2 Flight Crew Roles and Responsibilities

Here, we discuss the combined roles of the flight crew interacting with
their on-board avionics (e.g., FMS, CDTI, data link interface) in terms
of mechanizing and utilizing the CE-11 processes and associated ATSP
services.

Up to crossing into the TRACON, the FC responds to clearances given by
Center ATSPs to arrive at the approach zone boundary or entry fix
according to a metering schedule. The FC receives and evaluates from the
ATSP the conditions of the operational environment within the approach
zone in terms of boundaries of the approach zones, any restricted
airspace defined by weather cells or other traffic, airspace and surface
winds, availability of flexible maneuvering, and conditions of the
assigned runway. The FC determines their desired reference speed Vref
for runway threshold crossing. The FC evaluates the airspace ahead based
on up linked graphic weather and that sensed by aircraft weather radar.

If a flexible route is to be used within the approach zone, the FC may
interact with the FMS in computing that reference trajectory.
Alternately, the FC manually guides the aircraft within the space
allocated for free maneuvering.

At the time the participating aircraft crosses into the TRACON approach
zone, the FC is assigned its position in an arrival string, the
immediate Lead aircraft, and the desired spacing parameter via up link
or voice clearance from the ATSP. If one or more RTAs are to be used for
merging control, these are also up linked to the FC. The FC enters or
approves entry of these parameters into the FMS and CDTI logic. The FMS
regulates speed along the approach path if RTAs are used. The logic
computes and displays the appropriate spacing cue with respect to the
Lead or Lead's ghost on the CDTI.

The ATSP clears the FC to use their avionics to enact flight along the
flexible reference path, to use RTAs or a ghost Lead to control merging,
and to use the spacing cues on the CDTI to control subsequent in-trail
spacing. This transfers the in-trail spacing responsibility to the FC.
This FC responsibility remains in effect until either the aircraft has
crossed the FAF or landed, or the ATSP/controller has resumed control.

The FC steers their aircraft along the reference (lateral and vertical)
path and controls speed to maintain tight spacing within a pre-specified
tolerance. The FC informs the ATSP if they are unable to accept this
responsibility or they are unable to continue self spacing after the
process has begun. At the appropriate time after crossing the FAF, the
FC transitions to the normal landing process.

# 9. Operational Conditions and Scenarios

In the following discussion, we illustrate three conditions of CE-11
operation -- nominal, off-nominal, and failure. Each is defined, and
narrative scenarios are used to describe the interplay between FC and
ATSP during the operational process.

## 9.1 Normal or Nominal Condition

The normal, or nominal, condition for CE-11 is where all air and ground
systems function as expected under normal conditions, traffic is in a
steady state condition in terms of approach airspace used, routes and
zones are not blocked by weather cells, and the runway in use is not
changing.

##### Scenario 1

Delta 452 (DA452), a B757, is CDTI and FMS equipped and is about to
enter the DFW TRACON over the boundary whose center point is the Bonham
BYP feeder fix after travel from Atlanta. DA452 links down to Approach
Control that its Vref speed is 137 kt and that it wishes to use the free
MANEUVERING and IN-TRAIL FOLLOWING procedures to expedite the approach
process. Approach Control links up acknowledgement and activates
computation of landing sequence and desired temporal spacing at landing
to meet wake vortex constraint for a B757 following the appropriate Lead
(which happens to be an MD80). Seconds later Approach Control assigns
DA452 the No. 23 position in the landing sequence behind the previously
assigned No. 22 Lead aircraft and assigns the standard separation
parameter as 75 sec. No. 22 is American 1088 (AA1088), an MD80 inbound
from Chicago on the same STAR. Approach Control clears DA452 to MERGE
with and maintain 75 sec behind AA1088 to the Runway 18L FAF.

DA452 FC identifies, designates and enters AA1088 as the Lead and 75 sec
as the distance parameter on the flight deck CDTI. DA452 FC chooses to
let the FMS define the direct route to the waypoint defining the
intersection with the base leg; thereafter the FMS steers the aircraft
along this direct path while maintaining appropriates separation with
respect to the Lead. Simultaneously, the FMS automatically closes and
maintains the specified distance with respect to AA1088, as represented
by AA1088's projected position on DA452's route. Thereafter, the FC
monitors the separation throughout the approach phase of flight leading
to the FAF. Likewise, Approach Control monitors this process to ensure
that nominal progress is made.

After DA452 crosses the FAF, the DA452 FMS automatically switches to
Final Approach Control frequency, and the FC is cleared to land. DA452
FC resumes manual control of the aircraft, disengages the FMS and CDTI
spacing cue, and proceeds to a normal landing behind AA1088. Because
tight spacing control was maintained between the two aircraft up through
FAF crossing, an average time savings of 8 sec was gained in the
inter-arrival spacing.

## 9.2 Off-Nominal Condition

The off-nominal conditions for CE-11 exist when unusual weather (such as
a preponderance of storm cells blocking nominal approach routes or
zones) occurs, a runway change takes place, or there is a disruption of
a string because of a missed approach or a FC that is not maintaining
proper positioning. In the following scenario, a major weather cell
blocks the nominal approach zone so that the participating aircraft must
be diverted and merged with traffic within a different approach zone.

##### Scenario 2

AA401, an MD80, is in-bound from DEN to DFW via the BOWIE3 STAR passing
through the northwest approach zone defined by the Bowie (UKW) feeder
fix. A major summer convective weather storm is passing through the
area. At 100 nmi to go to DFW and during the initial part of AA401's
descent, Bowie Approach Control communicates with Ft. Worth Center and
AA401 that the Bowie approach zone is closed because of a large weather
cell. All Bowie traffic is being diverted to the northeast approach zone
defined by the Bonham (BYP) feeder fix.

AA401 is FMS and CDTI equipped and so the AA401 FC requests use of a
flexible route and RTA to MANEUVER along the direct path to and then
MERGE with Bonham traffic. Ft. Worth Center and Bonham Approach Control
cooperate to compute an RTA of 0952 GMT at the KARLA intersection within
the Bonham zone. This also includes assignment of AA401's sequence
number and nominal spacing behind UA55, a B737, approaching from ORD.

The Ft. Worth Center/Bonham Approach ATSP automation links the KARLA
waypoint, RTA, designated Lead (UA55), and spacing parameter (65 sec)
for use after crossing KARLA to AA401. The ATSP also up-links the latest
graphical depiction of weather cells in the immediate area. AA401 FC
acknowledges, enters these parameters in the FMS via the CDTI, and
activates the FMS to MANEUVER to these parameters. The FMS makes local
adjustments to the route leading to KARLA to bypass small cells north of
DFW. Thereafter, the FC monitors the flight with respect to on-board
weather radar supplementing the up-linked weather data and the CDTI
depiction of adjacent traffic leading to the Bonham zone. The ATSP
monitors the flights of AA401 and UA55 to ensure that the MERGE at KARLA
is smooth and within acceptable spacing tolerances for the subsequent
traffic string leading to DFW.

## 9.3 Failure Condition

The failure conditions for CE-11 are those events where equipment fails,
human errors disrupt normal operation, or operational conditions
abruptly change so that nominal or off-nominal operation cannot
continue. Each of these conditions needs to be defined and analyzed so
that safe recovery processes can be developed which revert to a more
manual traffic management process. In the following scenario, a
surveillance failure causes a major disruption.

##### Scenario 3

It is in the middle of an arrival rush during a normal busy day at DFW.
Arrivals are coming from all four approach zones leading to the "south
flow" operations on Runways 18R and 18L. Even though visibility is
limited, because of the density of arrivals, CDTI MERGING and IN-TRAIL
SPACING procedures are being used to increase landing rates and runway
throughput.

Suddenly in the midst of this process, the ATSP surveillance function
that fuses radar tracking data with ADS-B state messages broadcast from
each participating aircraft fails. The ATSP is no longer able to monitor
compliance of spacing constraints between consecutive aircraft in the
approach strings. DFW Approach Control automation notifies Ft. Worth
Center to shut off all entering traffic. Ft. Worth Center controllers
subsequently put approaching aircraft in holding patterns until the DFW
problem is resolved.

The Glen Rose Approach controller takes over manual control of each of
the seven aircraft in the CDTI-driven approach string within the Glen
Rose zone. The controller continues to observe relative positions of the
aircraft via radar tracking (today's technology). Starting with the No.
2 aircraft, the controller vectors each aircraft to the right or left of
the nominal route and then back to the route to open more space between
the aircraft. No. 2 is vectored to the right and then back. No. 3 is
vectored to the left and then back, etc. CDTI driven spacings of 60, 75,
and 90 sec are opened to the equivalent of 75, 90, and 105 sec in
spatial distance terms.

Each participating FC is fully aware of the problem and can optionally
cooperates to open the spacing via the CDTI. That is, upon controller
clearance, the FC can enter the diversion MANEUVER and new temporal
spacing parameter on the CDTI, and then follow the maneuver guidance
cues to path stretch to the desired new spacing.

After the last aircraft in the string is re-spaced, Glen Rose Approach
notifies Ft. Worth Center that the southwest approach flow can be
continued but with a lower acceptance rate than before the equipment
failure. This lower rate and manual approach control is continued until
the surveillance equipment is repaired or restored to nominal operation.

# 10. Operational Process/Operational Sequence Diagrams

This section describes and diagrams at a high level, the processes to be
followed during the solution created by the CE-11. The processes are
based upon the description of roles and responsibilities (Section 8) and
operational conditions (Section 9).

In the following, the nominal IN-TRAIL SPACING process is illustrated
from both the ATSP and FC points of view.

## 10.1 ATSP: Nominal In-Trail Spacing Process

Figure 6 shows the operational process that the ATSP approach controller
goes through when clearing and then monitoring the actions of a flight
crew that is using CDTI for in trail SPACING. The ATSP first determines
if the aircraft is appropriately equipped. If not, the aircraft is
vectored as is currently done during the approach. If equipped, then the
ATSP determines if the aircraft is positioned properly to easily capture
the designated spacing behind the Lead for subsequent following. If not,
the controller vectors the aircraft into a position that facilitates
ease of capture. Then the ATSP clears the aircraft in terms of issuing
the identity of the Lead and the desired spacing that Own aircraft is to
maintain behind the lead; these quantities come from the DST.

After the Own aircraft is cleared for in-trail SPACING, the ATSP
continues to monitor the spacing process as the aircraft transitions
through the approach zone. If Own's spacing falls outside of some
acceptable tolerance with respect to the Lead, the ATSP issues a
corrective advisory to Own. If the resultant action on Own's part is not
satisfactory, the controller takes over spacing responsibility and
vectors the aircraft to the FAF. If Own does an adequate job of spacing
relative to Lead, the ATSP continues to monitor the progress until Own
crosses the FAF. Thereafter, Own is cleared to land.

![](media/image7.wmf)

Figure 6. Operational Sequence Diagram for ATSP: Normal In-Trail SPACING
Process

## 10.2 Flight Crew: Nominal In-Trail SPACING Process

Figure 7 shows the operational process that the flight crew goes through
when cleared to use their CDTI to capture and then maintain a designated
SPACING behind a designated Lead aircraft. On or before the FC enters
the TRACON, the request in-trail SPACING permission. ATSP determines if
airspace conditions warrant and if the aircraft is properly equipped. If
not, the ATSP denies the request and continues to vector the aircraft
through the approach.

If the FC and aircraft are trained and equipped, and conditions warrant,
the ATSP clears the FC to execute the in-trail SPACING procedure. This
includes issuing the designated Lead aircraft and the desired spacing.
These parameters are entered into the CDTI. Thereafter, the FC capture
and maintain desired spacing behind the Lead until FAF is reached. They
are then cleared to land.

![](media/image8.wmf)

Figure 7. Operational Sequence Diagram for Flight Crew: Normal In-Trail
SPACING Process

#  

# 11. Benefits

Concept Element 11 is directed at increasing terminal airspace and
runway capacity (throughput), flight efficiency, and airspace
flexibility. These primary benefits occur through use of CDTI and FMS
technology for in-trail merging and spacing control as well as being
enabled to fly along flexible routes within the approach zones.
Additional benefits regarding trajectory predictability, airspace
access, and scalability of the controller work force to increased
traffic volume also exist. These benefit mechanisms are now described.

Capacity

The capacity-related potential benefits of CE-11 service are:

1.  CDTI-equipped aircraft provide the FC increased situation awareness
    and the ability to maintain tighter in-trail SPACING control. Thus,
    excess safety-related separation buffers used by controllers today
    can be reduced, increasing operational densities and runway
    throughput.

2.  An increased volume of airspace can be utilized by FMS-equipped
    aircraft in that they are not constrained to follow a fixed route
    structure. Flexible approach route MANEUVERING capability allows
    efficient by passing of weather cells and more direct routes from
    approach zone entry to the FAF.

3.  Close trajectory management by FMS-equipped aircraft flight crews
    allows increased RTA conformance, which leads to increased
    throughput along a system of merging routes.

Efficiency

The following efficiency-related potential benefits for CE-11 have been
identified. These are separated into benefits to Users/FCs and to the
ATSP.

##### Users and Flight Crews

Users should experience reduced operating costs (time and fuel) and
reduced delays, due to:

1.  Increased predictability of operations;

2.  Capability for MANEUVERING along optimized routing;

3.  Reduced excess spacing buffers; and

4.  Reduced excessive MERGING and SPACING maneuvers.

##### Air Traffic Service Provider

The ATSP should experience more efficient operations (reduced workload)
due to:

1.  The ATSP has decision support for ATC clearance advisories.

2.  The ATSP has reduced voice communications since there is little
    voice contact with equipped aircraft after their FCs have been given
    FMS-CDTI-based MANEUVERING, MERGING and SPACING clearances.

3.  Because many aircraft will have MERGING, in-trail SPACING, and
    self-separation control capability via FMS / CDTI, the ATSP can
    focus more on aircraft that do not have these capabilities.
    Therefore, the curve of workload as a function of traffic density
    will be below that experienced by today's ATC system.

ATSP can focus on traffic management and less on traffic control.

Flexibility

The following flexibility-related potential benefits of CE-11 have been
identified.

1.  FC preferences of flexible routes for FMS-equipped aircraft are
    implemented directly by the FC and may not require ATSP approval.

2.  The ability to free MANEUVER within designated approach zones
    increases the FC available and realizable routing options to
    convective weather cell problems.

3.  The option of not needing to adhere to a fixed route structure and
    ability to use the entire approach zone airspace allows more
    efficient flight plan options for equipped aircraft.

4.  Since FCs can constantly monitor their trajectories relative to
    adjacent approach traffic, these trajectories can be more tailored
    to FC preferences in terms of maintaining lateral and vertical
    separation as well as specified in-trail SPACING.

Additional Benefits

CE-11 benefits of lesser priority include improved trajectory
predictability, greater airspace access, and ability to scale the ATC
workforce to handle larger traffic volumes. Each is described.

##### Predictability

Participating FCs can diligently monitor MERGING and in-trail SPACING
clearance adherence and provide high predictability of their aircraft
trajectory. Increased trajectory adherence increases the predictability
of intended path conformance, which in turn increases the predictability
of arrival traffic throughput.

##### Access

This refers to the ability of FCs to obtain greater access to airport,
airspace, and ATC services. The concept of MANEUVERING along flexible
approach zones to account for dynamic traffic and weather conditions
allows greater access to all TRACON airspace and more continuous access
to the airport than does a fixed route environment that exists today.
Flexible routing within designated approach zones provides improved
access to off-route airspace within that zone.

##### Scalability

Scalability refers to the capability of the air traffic system to
continue to operate successfully with continually increasing traffic
volumes.

Each additional equipped participating aircraft contributes its own
surveillance infrastructure and provides its own separation assurance.
This system accommodates growth better than a centralized system that
may have limits in capacity to handle traffic growth. Whereas the
current paradigm of centralized human planner / controller does not
scale with large traffic growth, a distributed system of self separating
FCs of participating aircraft that grows with the traffic is readily
scalable. In today's system, controller workload is a strong function of
traffic volume since every aircraft is individually managed. Under
CDTI/FMS-based MERGING and in-trail SPACING control, equipped aircraft
do not need to be continually managed by the ATSP and therefore
controller workload is a lesser function of traffic volume. Thus traffic
volume could be permitted to increase while using the same level of
controller resources.

The CE-11 ideas postulated for approach traffic within TRACON airspace
is scalable back through before top of descent within the terminal
transition airspace. Furthermore, the concept is scalable to departing
aircraft traffic as well.

# 12. Issues and Key Decisions

This section summarizes the primary issues that must be dealt with in
pursuing the CE-11 ideas. Key decision points in the future research are
also listed.

## 12.1 Issues Summary

The CE-11 concept is still in an early stage of development, although
there has been substantial human-in-the-loop simulation work to examine
the feasibility of using the CDTI for in trail SPACING and the FMS for
directing free MANEUVERING. The main issues concerning CE-11 revolve
around validation / refinement of the basic concept and development of
the operational and technical details. Validation of the concept should
be done with airline and ATSP operations staffs at an early stage to
confirm the concept is correctly defined and addressing areas with the
most potential payoff. Functionality and FC-controller relationships can
be refined through discussions with the same staffs. Lessons learned
from previous NASA and Mitre research can be used to directly refine the
basic technical areas of investigation.

Concurrently, development of key technologies for the concept can
progress in preparation for the development of a CE-11 prototype system.
A list of required avionics development efforts includes:

1.  Development of the FMS so that it can compute flexible routes that
    are within the confines of zonal boundaries, adequately spaced from
    convective weather cells, able to meet an RTA at a downstream merge
    point, and are fuel optimal. This includes abilities to change
    boundary and weather cell constraints as defined by up-linked
    information from the ATSP.

2.  Development of the CDTI in trail spacing cues and other information
    to allow the FC to designate the Lead aircraft, to set the desired
    temporal spacing, to guide to the spacing parameter either via
    flight director or automatically via the FMS, to monitor spacing
    adherence, and to monitor lateral and vertical separation from
    adjacent aircraft. This includes ability to compute and use ghost
    projections of Lead aircraft if the Lead is not on the same route as
    the participating aircraft.

3.  Development of data link interface to enable the FC to interact with
    the ATSP when requesting a flexible route, being cleared to use the
    RTA or ghost for merging control, being cleared to maintain a fixed
    temporal spacing with respect to the designated Lead, and returning
    primary separation responsibility back to the controller.

There is a spectrum of ATSP DST requirements ranging from using today's
STARS environment where the controller would verbally clear an aircraft
FC to use its CDTI for in trail spacing control (and no special DST
requirement) to an advanced DST which might build upon emerging Active
FAST capability. The different environments need to be defined, and
associated DST technology needs to be developed consistent with ATSP
needs within CE-11.

For both avionics and ATSP DST requirements, there is the open question
of what are the minimum equipment and training requirements for an
aircraft or flight crew to participate in CE-11. Also, there is the
issue of what terminal areas can actually benefit from the concept in
terms of traffic density, airspace constraints, and amount of training
required to enable the traffic managers to utilize the concept.

In parallel with needed technology developments, there is also a need to
develop the flight crew-traffic manager procedures consistent with the
three operational modes described in Section 3 and the three operational
conditions describe in Section 9. Furthermore, these need to be expanded
to include provision for handling aircraft with different speed
envelopes and different levels of equipment.

## 12.2 Key Decision Points

Key decision points regarding development of CE-11 include the
following:

1.  It must be shown that there is a legitimate economic payoff in terms
    of increased flight efficiency and runway throughput that would be
    enabled by equipping aircraft and training personnel to use the
    CE-11 technology. Increased flight benefits must exceed costs of
    obtaining such benefits.

2.  It must be shown to be operationally viable that in trail spacing
    responsibility can be delegated to the FC. This includes the
    willingness on the part of the FC to accept this responsibility, the
    ability of the ATSP to continue to monitor progress of an approach
    string, and the ability of the ATSP to resume spacing responsibility
    in case of a failure condition preventing the CE-11 procedure to
    continue in nominal fashion.

3.  The current ADS-B OpEval is examining use of ADS-B and CDTI to make
    approaches into Louisville and Memphis more efficient for the Cargo
    Airlines. This supposes no additional ATSP DST developments.
    However, aspects of CE-11 in the 2015 time frame require advanced
    FMS and ATSP DST capability. Decisions need to be made as to which
    sets of environments should be explored for which time frames with
    the associated technology and procedures being developed.

# Appendix A. Operational Requirements Table

## Operational Requirements -- Traffic Management, Synchronization Area

The following operational needs statements are addressed by CE-11, Self
Spacing for Merging and In-Trail Separation. The numbers provide a trace
to the matrix of operational needs statements supporting the AATT
ATM/OPSCON.

+-------+--------------------------------------------------------------+
| **ONS | **ONS Text**                                                 |
| \#**  |                                                              |
+=======+==============================================================+
| 1_375 | Through a data link to the properly equipped cockpit,        |
|       | provide users- routine communications- updated charts,       |
| 4_370 | current weather, SUA status, and other data- basic flight    |
|       | information services, including forecast weather, NOTAMs,    |
|       | and hazardous weather warnings- airport information,         |
|       | including Runway Visual Range (RVR), braking action and      |
|       | surface condition reports, runway availability, and wake     |
|       | turbulence and wind shear advisories                         |
|       |                                                              |
|       | \- clearances and frequency changes in the form of           |
|       | pre-defined messages.                                        |
+-------+--------------------------------------------------------------+
| 4_311 | Properly equipped aircraft are given authority to maneuver   |
|       | as necessary to avoid weather cells, or to follow such       |
|       | aircraft using self-spacing procedures.                      |
+-------+--------------------------------------------------------------+
| 4_315 | When appropriate, clear properly-equipped aircraft to        |
|       | self-separate and maintain sequence ("station-keeping").     |
| 3_225 |                                                              |
+-------+--------------------------------------------------------------+
| 4_316 | Appropriately equipped aircraft are given clearance to merge |
|       | with another arrival stream, and/or maintain in-trail        |
|       | separation relative to a leading aircraft.                   |
+-------+--------------------------------------------------------------+
| 4_355 | Use collision avoidance and escape guidance logic, real-time |
|       | wake turbulence prediction, and flight deck situation        |
|       | awareness to perform simultaneous approaches to closely      |
|       | spaced runways in Instrument Meteorological Conditions       |
|       | (IMC).                                                       |
+-------+--------------------------------------------------------------+
| 4_555 | Arrival operations also benefit from these tools, {tools     |
|       | that provide more efficient airport surface operations,      |
|       | improved real time assessment of traffic activity in         |
|       | departure and en route airspace, and expanded usage of       |
|       | flexible routes based on RNAV, satellite navigation, and     |
|       | FMS.}                                                        |
+-------+--------------------------------------------------------------+
| 4_575 | In the final portion of the arrival phase, decision support  |
|       | systems facilitate the use of time-based metering to         |
|       | maximize airspace and airport capacity.                      |
+-------+--------------------------------------------------------------+
| 4_585 | On final approach, the service provider may give the pilot   |
|       | responsibility for station keeping to maintain the required  |
|       | sequence and spacing to the runway.                          |
+-------+--------------------------------------------------------------+
| 4_646 | To enhance operations during peak capacity periods, arrival  |
|       | operations are enhanced by taking advantage of aircraft FMS  |
|       | to enable Required Time of Arrivals (RTAs) at designated     |
|       | approach points.                                             |
+-------+--------------------------------------------------------------+
| 4_755 | the pilot will be able to select which route he wishes to    |
|       | follow.                                                      |
+-------+--------------------------------------------------------------+
| 4_765 | pilots \... fly to meet required times of arrival            |
+-------+--------------------------------------------------------------+
| 4_770 | Free maneuvering operations in low density areas is being    |
|       | performed.                                                   |
| 5_355 |                                                              |
+-------+--------------------------------------------------------------+
| 5_115 | The use of en route airborne holding has been reduced with   |
|       | the implementation of other procedures that improve traffic  |
|       | flow patterns and make maximum use of available terminal     |
|       | capacity                                                     |
+-------+--------------------------------------------------------------+
| 5_145 | These metering and merging separation procedures could       |
|       | provide the crew the flexibility to more efficiently manage  |
|       | their flight with respect to aircraft performance, crew      |
|       | preferences, and ATC considerations by allowing aircraft to  |
|       | stay on the cleared route in cases were ATC would otherwise  |
|       | have to vector the aircraft to achieve the desired spacing.  |
+-------+--------------------------------------------------------------+
| 5_200 | remain at that altitude until the point is reached from      |
|       | which an optimum descent profile should commence.            |
+-------+--------------------------------------------------------------+
| 5_345 | When appropriate, use a "metering spacing technique" to      |
|       | provide the user the flexibility to efficiently manage a     |
|       | flight.                                                      |
+-------+--------------------------------------------------------------+
| 5_400 | Perform some spacing activities that were previously         |
|       | performed by the service provider. These activities will be  |
|       | performed for metering or merging purposes. (Flight Deck)    |
+-------+--------------------------------------------------------------+
| 5_450 | Reduced or time-based separation standards will be developed |
|       | based on technology and aircraft capability, further         |
|       | increasing system capacity and safety.                       |
+-------+--------------------------------------------------------------+
| 5_575 | Decision support systems will assist in conflict detection   |
|       | and the development of conflict resolutions.                 |
+-------+--------------------------------------------------------------+
| 5_685 | The service provider will also be involved in the            |
|       | coordination of modified flight trajectories for active      |
|       | flights.                                                     |
+-------+--------------------------------------------------------------+
| 5_885 | While vectoring of aircraft is a high workload for both      |
|       | controllers and pilots, only one clearance is given for this |
|       | metering spacing technique                                   |
+-------+--------------------------------------------------------------+
| 6_285 | Perform some separation and merging activities that were     |
|       | previously performed by the service provider.                |
+-------+--------------------------------------------------------------+
| 6_460 | pilots may coordinate with service providers for clearance   |
|       | to conduct specified cockpit self-separation operations.     |
| 6_370 | \... the pilot's view of nearby traffic supplements the      |
|       | service provider's big picture of longer term traffic flow.  |
+-------+--------------------------------------------------------------+
| 6_465 | Pilots may obtain approval for special maneuvers such as     |
|       | reduced separation in-trail climb, in-trail descent, lead    |
| 6_375 | climb, lead descent, limited duration, station keeping as    |
|       | well as lateral passing maneuvers                            |
| 6_240 |                                                              |
+-------+--------------------------------------------------------------+

# Appendix B. Previous CDTI and Related Research and Development

The previous work can be divided into two categories - pre-1990 and
1990-2000. In the following, we summarize the results of several
references for pre-1990 work. Post-1990 work comes from both literature
review and interviews with domain experts who have been involved in this
work.

## B.1 Pre-1990 CDTI Research

The CTDI concept has been suggested and studied since sometime in the
1940's \[3\]. The TELERAN system, developed by RCA and tested in a Link
trainer and a C-47 in the late 1940's, was based upon television
transmission of ground radar information and map overlays to the
aircraft. In 1947, the MIT Radiation Laboratory proposed two restricted
visibility condition traffic display concepts: an airborne radar
capability to derive traffic, terrain, and weather information and
presentation of ground radar detected information on a cockpit plan view
display. In 1948, the Radio Technical Commission for Aeronautics (RTCA)
recommended that future domestic air traffic control (ATC) be based on
both ground and airborne pictorial situation displays.

Airborne station-keeping equipment has been employed successfully by the
military services for many years to maintain safe air-to-air separation
in formation flying. In 1963, the US Air Force flight tested in a system
in which ground radar information was transmitted via a TACAN data link
to an airborne television to display nearby aircraft. Alphanumeric text
was also transmitted and displayed to demonstrate the feasibility of
providing flight clearances, weather advisories, and other pertinent
information to the pilot. In 1965, a televised picture of the FAA's
Boston Terminal Radar Approach CONtrol (TRACON) display was used to test
navigation and conflict detection concepts for general aviation. In
1974, MIT Lincoln Laboratory developed a digitized version of CDTI. The
target data base was supplied by an Air Force 407L radar system via the
SEEK BUS data link; this concept was later reconfigured for the USAF
Airborne Warning and Control System (AWACS) flight demonstrations.

CDTI studies were continued in the 1970's and 1980's by NASA to
investigate potential applications that could increase airport capacity,
reduce controller stress and workload, and enhance safety of flight.
These studies used simulations of the TCAS, Mode S radar, and other data
link systems. Based on these studies, traffic displays were postulated
and tested under simulated traffic conditions. Pilots and controllers
participated in these tests, and much was accomplished in understanding
the relative vehicle dynamics, the human factors of traffic displays,
and the potential of CDTI to provide benefits. The studies also revealed
potential problems such as increased pilot and controller workload and
possibilities of traffic flow instability, secondary conflicts, and
pilot distraction. \[3-16\]

Airborne traffic displays were developed as optional equipment during
the TCAS II program. The functions of these displays are to:

1.  Aid in visual acquisition of adjacent traffic;

2.  Discriminate threat traffic from other traffic;

3.  Provide range and bearing information on adjacent aircraft; and

4.  Instill confidence in the resolution advisories.

Installation of TCAS II began about 1990 for all air carriers. The
requirement to carry and use a TCAS has been extended to cover all
aircraft carrying more than 30 passengers. In all cases, installations
have included some form of TCAS traffic display. Thus, via the TCAS
program, the inclusion of a cockpit display of adjacent traffic became a
reality.

Three elements of the TCAS or other traffic display design are of
importance to the CDTI applications:

1.  Surveillance -- The accuracy, reliability, and volume of spatial
    coverage of the surveillance and the associated accuracy of the
    tracking algorithm govern to what extent TCAS/CDTI can be used for
    merging and spacing applications. The TCAS II surveillance system
    design was primarily the results of work performed at MIT Lincoln
    Laboratory.

2.  Logic design -- The conflict detection and resolution (CD&R) logic
    used to determine which aircraft to display or to indicate that they
    may pose a threat. This must be interfaced with merging and spacing
    cues. The TCAS II threat detection and collision avoidance logic was
    primarily developed by the Mitre Corporation. Seven revisions of
    this software design have been released to the TCAS manufacturers.
    Many aspects of this logic have to be examined in terms of the
    algorithm and software enhancement requirements for merging and
    spacing application.

3.  Pilot interface -- The human factors aspects of the display and
    other interface mechanisms used by the pilot are critical for
    adapting the system to merging and spacing. The TCAS II display
    design format and other aspects of the flight crew interface were
    investigated at both NASA Ames and Langley Research Centers and
    MITLL.

These elements must be considered (a) when devising technical
enhancements to implement the CDTI applications, and (b) when developing
the procedures for using the displays for the applications.

Previous research in CDTI focused to some degree on the in-trail
following application. This research consisted of both analytical work
and experiments conducted using cockpit simulation. In the mid 1970s,
MIT Flight Transportation Laboratory conducted cockpit simulator studies
that explored many terminal area applications of CDTI.

During the 1980-1984 time period, NASA Ames and Langley Research Centers
both sponsored analytical studies and conducted a series of experiments
to determine (a) what were the important elements that allowed pilots to
use the CDTI for in-trail following; (b) how could the CDTI be
mechanized; and (c) what benefits might be realized from CDTI
implementation. Within the NASA research, several questions were posed
associated with the CDTI-based terminal area traffic tactical control
concepts. These included:

1.  What are the basic dynamic phenomena associated with independently
    controlled aircraft in a string? What condition would produce
    instability in the string?

2.  What information does each pilot need (from the CDTI and other
    sources) to merge his aircraft adequately into the string and then
    to maintain appropriate spacing?

3.  What are the effects of measurement and display errors, wind shears,
    aircraft mixes, spacing constraints, and merge trajectories on the
    dynamics and control performance of the system?

At least six different cockpit simulator studies of aircraft strings
were made at NASA Langley and Ames to produce data to analyze in-trail
dynamics to address these questions.

A simplified generic CDTI display used for in-trail following is shown
in Figure B.1. Here, the pilot views the horizontal positions of his
(Own) aircraft and the surrounding aircraft on the cockpit display.
Own's position is indicated on the heading up display by the chevron
symbol one-third the distance up from the bottom and centered laterally.
The route path and other display features move continuously with respect
to the Own symbol. Adjacent aircraft are indicated by triangles. The
immediate leading aircraft that Own is assigned to follow is referred to
hereafter as the "Lead" aircraft. Own and other aircraft symbols are
preceded by vectors proportional in length to the ground speeds. These
vectors may be curved proportional to bank angle, and they produce a
prediction of where each aircraft will be at a future time.

![](media/image9.wmf)

Figure B.1. Simplified generic display format to enable in-trail
following.

In Figures B.2-4, three different longitudinal separation criteria that
have been investigated are depicted by the symbology. The separation
criterion is the mathematical rule used as part of the CDTI display to
indicate to the pilot, as a spacing cue, what the desired separation
should between his and the Lead aircraft. The criterion must establish a
lower separation limit that is safe; yet, it must keep the aircraft
close enough to provide for airspace and landing efficiency. The
resulting implied acceleration commands must be within the normal limits
of the aircraft. Finally, it should be possible to compute the criterion
simply from available information and to display it to the pilot without
ambiguity. The criteria depicted are:

1.  Constant Range (Figure B.2) -- This is shown by the constant range
    ring arc, and the pilot's objective is to steer Own aircraft so that
    the depicted range ring is on top of the Lead aircraft's current
    position. Current TCAS displays have fixed distance range rings.

The Constant Range criterion has the advantages of its familiarity to
pilots flying with TCAS, its simplicity in terms of understanding, and
its immediate applicability for removing large initial distances in
separation of aircraft. However, the Constant Range criterion is not
suited for tight spacing control. If Own is properly spaced with respect
to the Lead, and the Lead aircraft slows down, this requires that Own
instantaneously match this deceleration to maintain a fixed spacing
distance. This causes a slowdown at an earlier range-to-go for Own
aircraft. Successive aircraft will slow down at increasing distances
from the runway that would produce significant fuel penalties and
operational problems. Effective use of the Constant Range criterion
would require that the target spacing range depicted by the rings would,
in fact, be incrementally reduced as the aircraft slows down for final
approach.

![](media/image10.wmf)

Figure B.2. Depiction of Constant Range separation criterion

2.  Constant Time Predictor (Figure B.3) -- This is the predictor vector
    that is in front of Own aircraft symbol; its length is the product
    of the time constant Tp and the measured ground speed Vg. It shows
    where Own will ideally be Tp seconds from now. The objective is to
    steer the tip of Own's predictor vector to be on top of the Lead
    aircraft's current position. A variation of this separation
    criterion is called "Acceleration cue." It modified the Constant
    Time Predictor criterion to include the effect of Own aircraft's
    measured longitudinal acceleration on the prediction vector.

The advantage of using the predictor vector on the CDTI display is that
it is easy for the pilot to visualize his/her future position and to
steer around turns to reduce separation error. The disadvantage is that
this criterion causes early slowdown of Own followed by later excessive
speed. This disadvantage is compensated by adding the acceleration term
at slightly more complexity.

![](media/image11.wmf)

Figure B.3. Depiction of Constant Time Predictor separation criterion.

3\. Constant Time Delay (Figure B.4) -- This criterion consists of
controlling Own to be where the Lead aircraft was Td seconds earlier.
This position is indicated on the CDTI display by an enlarged "history
dot." Own is steered so that the tip of the chevron symbol is on top of
this moving dot.

The advantage of the Constant Time Delay separation criterion is that
the history dots (or equivalently, a moving slot tied to the targeting
spacing history dot) provide an ideal trail and position for Own
aircraft to follow. There is no inherent time delay in using this
criterion. A disadvantage of this criterion is that the dots by
themselves provide no turn indicator or deceleration cue for Own, so the
addition of a predictor vector is desirable. Another complexity is that
to implement this criterion requires storage of the Lead aircraft's
position relative to the ground.

Either the Constant Time Delay or Acceleration Cue criteria produce
acceptable spacing between successive aircraft in a string. The choice
will have to be made based upon which is easier to mechanize using the
traffic sensor or traffic information broadcast. It may be advantageous
to use features from each of the criteria for an overall display to
facilitate tight in-trail spacing. For more details on the research on
use of CDTI for applications of in-trail following, refer to Refs. 3-16.

![](media/image12.wmf)

Figure B.4. Depiction of Constant Time Delay separation criterion.

## B.2 Post-1990 CDTI Developments

After TCAS was mandated for commercial air carriers, and TCAS I and TCAS
II systems with their traffic displays became commonplace, pilots soon
began to use these displays for other "unofficial" purposes than
collision avoidance. The traffic display could help the pilot/FC with
situational awareness of other traffic. Pilots started to use the
display for in-trail following when cleared for unconstrained
transcontinental routes. It became apparent that the TCAS/CDTI would
provide many useful applications and that these applications should be
identified, documented, and standardized so that operational use could
be orderly.

The first "official" use of the TCAS/CDTI for in-trail following control
was for oceanic en route flight \[17\]. In oceanic airspace where radar
information is not available, FCs must provide position reports that
include aircraft identification position, altitude, and estimated
waypoint crossing time for both the current and the next waypoint. This
information is typically provided via high frequency (HF) radio to ATSP
so they can conduct flight following and provide separation. The flight
crew (FC) can request flight plan changes and ATSP can grant such
changes provided that standard non-radar separation exists.

Requests for altitude changes are the most often requested flight plan
changes. FCs may desire an altitude change for more efficient fuel
consumption or to avoid weather. However, controllers often cannot grant
the changes due to "over crowding" on the oceanic tracks. An inability
to climb or descend can cause not only inefficient flight operations but
also an uncomfortable ride for passengers or non-completion of a
scheduled flight segment due to increased fuel usage.

Currently, the United States has authorized the use of the TCAS II
traffic display for in-trail climb (ITC) and in-trail descent (ITD)
procedures on oceanic routes. The constraints of this operation are that
the trailing Own aircraft FC must see the Lead on the traffic display
and there must be enough initial separation so that the ITC or ITD can
be completed without violating acceptable separation as Own passes
through the Lead's altitude. In an enhanced ITC and ITD procedure, the
CDTI would provide flight identification, speed, altitude, and range
information directly to the FC thereby reducing or eliminating
coordination with the Lead aircraft \[18, 19\].

## B.3 On-going CDTI Research and Development

Two on-going technical developments are further enabling the use of
CDTI: (a) the broadcast of automatic dependent surveillance (ADS-B)
where the aircraft broadcasts its precise state and intent based
typically on GPS navigation; and (b) the broadcast of traffic
information service (TIS-B) where the ground radar system determines
states of aircraft and broadcasts these to those adjacent aircraft as a
supplement to ADS-B or TCAS. The minimum aviation system performance
standards (MASPS) for ADS-B describe nearly 80 potential applications of
CDTI based on ADS-B information \[18\]. These technologies have spurred
activity by RTCA to define further acceptable applications, to develop
requirements and operational procedures for these applications, and to
develop and document minimum operating standards (MOPS) for the CDTI
equipment \[19\].

Reference 19 describes in further detail the following CDTI
applications:

1.  Enhanced Visual Acquisition -- Capability that aids FCs in visually
    acquiring other proximate traffic as well as increasing their
    traffic awareness;

2.  Enhanced Visual Approach -- Provides the FC additional electronic
    information to aid in conduct of an enhanced visual approach.
    Information includes target positions, flight identification,
    closure rates, and ground tracks;

3.  Enhanced In-Trail Climb or Descent -- Described previously;

4.  Oceanic Climb or Descent to Co-Altitude (In-trail or lead) --
    Similar to Enhanced ITC or ITD but enables a lead or trailing
    aircraft to climb or descend to the same altitude as that of the
    participating aircraft;

5.  Approach Spacing -- Enables the FC to both accomplish and enhance
    the controller's in-trail spacing objectives to improve runway
    throughput but maintain acceptable safety in terms of meeting wake
    vortex avoidance and runway occupancy constraints. This application
    is directly related to the motivation of CE11;

6.  Airport Surface Situational Awareness -- Enables FCs to observe
    surface traffic positions on a real-time display and, along with any
    available visual cues and radio communications, infer intent with
    respect to surface movements;

7.  Station Keeping in Oceanic, En Route, and Remote Non-Radar Airspace
    -- FCs use the CDTI to safely conduct IFR in-trail constant or
    decreasing to minimum distance longitudinal separation for extended
    periods of time which may last up to the entire duration of a
    flight.

NASA, FAA, Eurocontrol, and other organizations are addressing the
various issues that these potentially near-term CDTI applications pose.

Safe Flight 21 (SF21) is a current FAA sponsored cooperative
government/industry effort to evaluate enhanced capabilities for Free
Flight based on evolving communications, navigation, and surveillance
(CNS) technologies \[20\]. SF21 will demonstrate the cockpit display of
traffic, weather, and terrain information for FCs and will provide
improved information for controllers. The primary objective of the SF21
program is to facilitate implementing nine operational categories
including "improved terminal operations in low visibility." This
category includes enhanced visual approaches with ADS-B and TIS-B,
approach spacing for visual and instrument conditions, and departure
spacing.

Under SF21, a cooperative government-industry team is conducting a
series of operational evaluations (OpEvals) of various ADS-B
applications in conjunction with the Cargo Airline Association plans to
equip their fleets with advanced TCAS/CDTI based upon ADS-B. In 1999,
enhanced visual approaches and see-and-avoid enhancements were the
subject of the OpEval conducted by 24 participating aircraft in the Ohio
River Valley (Airborne Express facility at Wilmington ILN). This was
preceded by extensive cockpit simulation studies conducted to prepare
for the OpEval by Mitre CAASD \[21, 22\]. This year, the OpEval is being
continued to examine approach spacing concepts using Constant Range and
Constant Time Delay criteria (UPS facility at Louisville SDF).

The goal of Capstone, a related project under SF21, is to implement and
test the CNS technologies mentioned previously on general aviation
aircraft flying out of Bethel, Alaska. This project is pertinent in that
it is bringing the benefits of the cockpit display technology to the low
end general aviation (GA) user.

Recent NASA and Mitre research has addressed the related use of ADS-B
and CDTI to facilitate dual approaches to closely spaced parallel
runways in instrument meteorological conditions (IMC) \[24-28\]. The
objective is to maintain throughput and capacity as under visual
conditions (VMC). Here the CDTI is used by trailing Own aircraft to
maintain a constrained longitudinal spacing relative to the Lead
aircraft approaching the parallel runway. The concept is to provide
adequate separation for both wake avoidance and collision avoidance
purposes. Specifically, United Airlines has proposed using this
application to maintain throughput at San Francisco International
Airport, where throughput is cut in half under IMC. NASA has
investigated a concept called Airborne Information for Lateral Spacing
(AILS) to protect against the case where one of the two parallel
aircraft deviates from its nominal path into the airspace of adjacent
parallel traffic.

# Appendix C. Variations to the Baseline System for CE-11

In the course of the discussions that led to this report, some alternate
ideas were discussed that may have merit in terms of future research
that can be conducted. These ideas are briefly mentioned here.

## Moving Slots

An alternative to following the Lead aircraft directly by use of some
kind of cue tied to the dynamics of the Lead aircraft, each FC could
instead be displayed an ideal moving slot to stay within, where that
slot may be tied to the Lead aircraft or it may be within a stream of
slots generated by the DST. The ATSP monitors all aircraft to ensure
adequate separation and compliance with the established procedure. For
cases where the FC fails to maintain adequate spacing, automated systems
or the ATSP will provide a required correction.

The ATSP DST would compute an ideal stream of moving slots and uplink
these slots to the aircraft to be displayed on the CDTI. Each FC sees
its assigned slot and the slot of the immediate Lead. In this case,
there remains the ideal separation between each aircraft in the stream,
but a following aircraft is not as subject to the small variations in
the speed and position of the preceding Lead aircraft, as the Lead also
is working to remain within its assigned slot.

A mechanization requirement would be that the DST computes and up-links
the desired position and velocity of all aircraft in the string in the
form of a train of moving slots. Then, the job of the CDTI would be to
display that slot for the FC to capture and track. This could be largely
de-coupled from the immediate Lead trajectory, and the FC would focus
not on the position relative to the Lead but the position relative to
the assigned slot. In this latter case, the ATSP monitors the position
of each aircraft relative to the ideal slot position. The controller
takes action to vector an aircraft only if it moves to far outside of
the slot, as assigned.

Within this latter environment, the trailing aircraft can either track a
moving position tied to the trajectory of the immediate leading Lead
aircraft, or it can track an ideal moving slot computed and linked up by
the DST. In the latter case, the string of moving slots is anchored to
the trajectory of the String Leader aircraft.

## Merging

There are three alternate ways in which the Own aircraft may be guided
before it merges behind the assigned Lead aircraft:

1\. It may be given a "ghost" image of the Lead aircraft as computed and
projected on Own's structured route before the merge point;

2\. It may be given an RTA at the merge point that is computed based
upon the Lead aircraft crossing that point at the appropriate time
separation in front of the RTA; or

3\. It may be given a moving slot to capture and track that blends into
the track containing the Lead tracking its assigned slot.

# References

1.  Anon., "Concept Definition for Distributed Air/Ground Traffic
    Management (DAG-TM) Version 1.0," NASA AATT Project Internal
    Document, September 1999.

2.  Anon., "DAG-TM Concept Element 11 Research Plan," NASA AATT Project
    Internal Document, March 15, 2000.

3.  Sorensen, J., Hollister, W., Burgess, M., and Davis, D.,
    "TCAS-Cockpit Display of Traffic Information (CDTI) Investigation,"
    DOT/FAA/RD-91, Federal Aviation Administration, Washington, DC,
    April 1991.

4.  Love, W. D., et al, "A Concept for Reducing Oceanic Separation
    Minima Through the Use of a TCAS-Derived CDTI," NASA CR-172258,
    Washington, D.C., January, 1984.

5.  Chappell, S. L., et al, "Pilots' Use of a Traffic Alert and
    Collision Avoidance System (TCAS II) in Simulated Air Carrier
    Operations," NASA TM-100094, NASA Ames Research Center, Moffett
    Field, CA, 1989.

6.  Sorensen, J. A., and Goka, T., "Analysis of In-Trail Following
    Dynamics of CDTI-Equipped Aircraft," *J. of Guidance, Control, and
    Dynamics*, Vol. 6, No. 3, May-June 1983.

7.  Goka, T., "Enhanced TCAS II/CDTI Traffic Sensor Digital Simulation
    Model and Program Description," NASA CR 172445, Langley Research
    Center, Hampton, VA, October 1984.

8.  Connelly, M. E., "Simulation Studies of Airborne Traffic Situation
    Display Applications," MIT ESL-R-751, Cambridge, MA, May 1977.

9.  Williams, D. H., "Time-Based Self-Spacing Techniques Using Cockpit
    Display of Traffic Information During Approach to Landing in a
    Terminal Area Vectoring Environment," NASA TM-84601, Langley
    Research Center, Hampton, VA, 1983.

10. Williams, D. H., and Wells, D. C., "Jet Transport Operations Using
    Cockpit Display of Traffic Information During Instrument
    Meteorological Conditions," NASA TP-2567, Langley Research Center,
    Hampton, VA, May 1986.

11. Hart, S. G., and Wempe, T. E., "Cockpit Displays of Traffic
    Information: Airline Pilots' Opinions about Content, Symbology, and
    Format," NASA TM 78601, Ames Research Center, Moffett Field, CA,
    August 1979.

12. Palmer, E. A., et al, "Perception of Horizontal Aircraft Separation
    on a Cockpit Display of Traffic Information," *Human Factors*, Vol.
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