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                                          Stéphane Mondoloni, Ph.D. *
                                                  CSSI Inc.
                                               Washington, DC

                                            Michael T. Palmer†
                                              David J. Wing ‡
                                        NASA Langley Research Center
                                              Hampton, VA

                    Abstract                                                  Introduction

A prototype PC-based human-in-the-loop (HITL)                NASA has proposed Distributed Air-Ground Traffic
simu lation capability was developed for the purposes        Management (DA G-TM) [1] as a concept for
of investigating autonomous airborne self-separation         increasing aviation system capacity and enhancing
concepts. The simulat ion provides a combined state-         aviation user flexib ility and efficiency. DA G-TM
based and intent-based conflict detection and                also seeks to ensure that both system safety and user
resolution capability (CD&R) to allo w investigations        access to the National Airspace System (NAS) are
of the impact of variations in supplied intent. The          preserved. As part of this concept, developed under
simu lation capability included such glass -cockpit          the Advanced Air Transportation Technologies
elements as an electronic flight instrumentation             project, NASA has developed a collection of 15
system (EFIS), flight management system (FM S) and           “concept elements” (CE). One of the motivations
mu ltifunction control display unit (MCDU), and              behind this investigation, CE-5, is entitled “En Route
cockpit display of traffic informat ion (CDTI) on the        Free-Maneuvering for User-preferred Separation
navigation display (ND). The simu lation provided            Assurance and Local Traffic Flo w Management
automated traffic and area hazard avoidance                  Conformance.” A key component of this concept is
advisories based upon data received through                  called autonomous airborne operations and can be
simu lated Automatic Dependent Surveillance –                described as follows:
Broadcast (ADS-B) and Weather Information Service                     Appropriately equipped aircraft accept the
(WIS) messages.         These advisories could be                     responsibility to maintain separation from
manually executed via the guidance control panel or                   other aircraft, while exercising the authority
automatically downloaded through the MCDU into                        to freely maneuver in en route airspace in
the FMS and executed via the autoflight system. The                   order to establish and maintain user-
simu lation capability incorporated a set of priority-                preferred trajectories that conform to any
based flight rules (“rules of the road”) for intent-                  active local traffic flow management (TFM)
based conflicts, and allo wed for resolution maneuvers                constraints. [1]
to include traffic flow management constraints such
as required times of arrival (RTA).                          In investigating the feasibility of this concept
                                                             element, one of the key research issues to be

 Chief Scientist, AIAA Member
†Research Engineer, AIAA Member
‡Research Engineer

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addressed is the distribution of separation                      strategic, such that the ownship’s intent is altered
responsibility between air traffic service providers             through a modification to the flight plan. These
(ATSP) and the flight crew [2]. Ho wever, p rior to              modifications may take the form of path alterat ions
conducting such an investigation, tools must be                  (e.g., new waypoints) or additional constraints (e.g.,
developed to assist the researcher in addressing the             altitude constraints). Reference [10] describes the
mu ltitude of issues arising fro m such a concept.               strategic conflict resolution scheme that was applied
Specifically, HITL simu lations provide a safe and               to this simulation capability.
controllable means to investigate many pertinent
issues. This paper describes a prototype airborne                This development effort sought to combine both the
conflict detection and resolution HITL simulat ion               state-based and intent-based approaches into one
capability that was developed for the purposes of                simu lation capability. These could be turned on
investigating the need for intent informat ion                   individually or together depending on the objective of
exchange (see research conducted us ing this                     the research. One of the des ired benefits of such a
capability in [3] and [4]) to successfully accomp lish           hybrid scheme is the blunder-protection provided by
airborne separation from hazards while meet ing TFM              the state-based method should traffic aircraft
constraints.    In addition to an active research tool,          inadvertently stop following its broadcast intent.
this prototype system is serving as the foundation of a          However, the mu ltip le resolution methods require
more capable and robust simulation currently under               that clear alerting schemes be developed. Reference
development [5].                                                 [3] describes the alerting scheme used to display the
                                                                 combined state-based and intent-based resolution
Prior effo rts have developed HITL simulat ion                   maneuvers.
capabilit ies seeking to address the feasibility,
workload and safety of airborne separation assurance             One of the fundamental tenets of CE-5 is the
under “free-flight” scenarios (e.g., [6], [7], [8]). This        requirement that trajectories, resulting from airborne
effort focused on providing a simulat ion environment            self-separation, conform to local traffic flow
with several additional capabilities: a hybrid state-            management constraints. The simulation capability
based and intent-based conflict alerting and                     for this research required a conflict resolution scheme
resolution scheme, the ability to meet required t imes           that was capable of resolving conflicts, while meet ing
of arrival, the integration of aircraft performance              externally imposed RTA’s. The resolution scheme
considerations in the resolutions, the inclusion of              proposed in [10] provided such a mechanism.
priority rules for conflict resolution, and the ability to
consider area hazards as part of co mputed resolution            The initial experiment that drove the development of
maneuvers. We describe each of these capabilit ies in            this prototype simulat ion capability required the
more detail below.                                               inclusion of a priority rule base allowing the
                                                                 specification of which aircraft was to maneuver in a
When we refer to state-based conflict detection and              conflict     based    upon      conflict   geomet ric
resolution [9], we are describing algorithms that                characteristics.
require, as input, the state vector (position and
velocity informat ion) of traffic aircraft and the state         In addition to traffic hazards, the inclusion of area
vector of the ownship. Under such a scheme, conflict             hazards was required in the simulat ion capability.
detection is based entirely on a co mparison of the              Not only should the conflict detection consider area
forward projection of the state of both traffic and              hazards, resolution maneuvers should not produce a
ownship.      Resolution maneuvers for state-based               conflict with known area hazards.
conflicts are tactical maneuvers, consisting of
instantaneous modificat ions to the ownship’s state              Many other requirements imposed on the intent-
vector such as a heading or speed change and                     based conflict resolution scheme described in [10]
involving no return to course.           Reference [6]           were not used in this prototype simu lation capability,
describes the state-based conflict detection and                 but are included for future extensions. For examp le,
resolution method that was applied.                              the ability of the intent-based resolution scheme to
                                                                 consider user-preferences in the selection of
In contrast to state-based methods, intent-based                 maneuvers is not activated in the prototype
methods refer to conflict detection and resolution               simu lation.
methods that rely on a forecast trajectory of both
traffic and ownship based upon computed or
broadcast trajectory change points (TCPs).
Resolution maneuvers for intent-based conflicts are

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                   Approach                                  functions. This component sends flight plan and
                                                             trajectory data to other components, and receives
                                                             both Autonomous Operations Planner (pAOP) data
A high-level view of the overall simulat ion                 and the initial flight plan (fro m TM X).
architecture is illustrated in Figure 1. A brief
description of each component follows.                       GCP: The glareshield control panel (GCP)
                                                             component receives simulat ion data fro m and sends
TMX: The Traffic and Events Manager (TMX) was                pilot-selected mode and target state information to
obtained from the NLR (Nat ional Aerospace                   the rest of the ASTOR co mponents. It is through the
Laboratory of the Netherlands) and modified by the           GCP that a pilot would perform such actions as
original developers for this simulat ion capability.         entering altitude clearances, engaging heading-select
This component simu lates all the traffic aircraft and       modes, and engaging lateral navigation (LNA V) and
area hazards, and provides the initial flight plan for       vertical navigation (VNA V) modes.
the ownship (the subject-pilot-controlled simu lated
aircraft).  The ownship is controlled by other                         ACP: The au xiliary control panel (ACP)
components, and TMX receives that ownship data.              component provides many of the controls and
TMX manages the exchange of all ADS-B and WIS                indicators that would normally be located on the
messages to all aircraft.                                    center aisle stand of the MD-11 flight deck, including
                                                             thrust lever, speed brake, gear, and flap controls and
Adapter: The adapter serves as a translator and buffer       indicators. The positions of these controls are
between the TMX and the rest of the simulation.              transmitted to the other ASTOR components.
This was necessary because TMX messaging and
ASTOR (see below) messaging did not follo w the                         TM X                        Adapter
same network transmission protocol.

ASTOR: The Aircraft Simulat ion for Traffic
Operations Research (ASTOR) represents all the                                             WIS/ADSB
simu lated flight deck co mponents for the ownship,
which is modeled after a McDonnell-Douglas MD-11                                      FDS       EAD
transport aircraft. ASTOR was a heavily modified
version of a desktop simulat ion previously employed                                   ND        PFD
for other research efforts (e.g., [11] and [12]). Each
component is at least one executable process, and
TCP/ IP socket messages are used to pass informat ion                       NDCP             ACP
between the various components. These components
are described below.
                                                                            GCP           PCPlane
          PCPlane : This component contains the
ownship aerodynamic equations of motion and                                                    CDU
engine      model,     guidance     algorithms,   and
autopilot/autothrottle system. Reference [13] details
the original development of this component and                                 pAOP           Traffic
describes its capabilit ies. PCPlane receives input                                         Processing
fro m the Flight Management Co mputer (FM C) /
MCDU, Glareshield Control Panel (GCP) and                                                   Processing
Auxiliary Control Panel (ACP) co mponents and
provides simulation data, specifically aircraft state,                                          Area
guidance targets, and autopilot/autothrottle mode
status information, to all other ASTOR co mponents.                                  Intent-     State-
                                                                                     based       based
         FMC/ CDU: The         flight  management                                    CD&R        CD&R
computer/control     display    unit   (FMC/CDU)
component contains the logic for all flight
management functions (e.g., navigation database,
flight planning, and 4-D lateral/vertical trajectory            Figure 1. High-level view of simulat ion layout.
generation) and the on-screen CDU interface to these

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         NDCP: The navigation display control panel
(NDCP) co mponent allows the pilot to control the
display of new traffic information and pAOP
advisories on the navigation display (ND). This
component receives GCP mode information and
sends data to the flight display suite.                            Area
          FDS: The flight display suite (FDS) contains
the three main electronic flight instrumentation
system (EFIS) displays on the MD-11 flight deck: the
primary flight display (PFD), the navigation display                Conflict
(ND), and the engine and alerting display (EAD).                  Prevention
The FDS receives as input NDCP data, simulat ion                 Heading Bands
data, FMC/ CDU data, ADS-B data, WIS data and
pAOP data. The ND has been modified to display a
collection of new informat ion including: relat ive
traffic location and velocity vectors, area hazard
informat ion, conflict alert ing informat ion, and
resolution advisories. Figure 2a illustrates the ND
with some of this information being displayed. The
PFD (see Figure 2b) has also been modified (in a
manner similar to [14]) to display conflict prevention
bands in both speed and climb rates.                           Figure 2a. Illustration of navigation display (ND)
                                                               with added area hazards, traffic informat ion, and
                                                              pAOP-generated conflict prevention heading bands.
          pAOP:       The (prototype) Autonomous
Operations Planner (pAOP) contains both intent-
based and state-based conflict detection and
resolution methods. This component is largely the
focus of this paper. The pAOP component receives
ownship simulat ion data fro m PCPlane and flight
plan data from the FMC/ CDU. Traffic data is
received through ADS-B messages. Area hazard
informat ion is received through WIS messages. This                                          speed bands
component provides conflict detection and resolution
informat ion to both the FDS and FM C/CDU. Both
state and intent-based detection and resolution
informat ion are provided by pAOP.

             pAOP Description

As illustrated in Figure 1, the (prototype)                               Speed
autonomous      operations    planner (pAOP)        is                    Bands
decomposed into several high-level functions:
 Traffic processing is responsible for receiv ing
    ADS-B data pertain ing to traffic, assembling
    these messages into traffic intent data for use in
    intent-based CD&R, and extracting traffic state
    data for use in state-based CD&R.
 Ownship processing takes flight plan and                   Figure 2b. Illustration of primary flight display (PFD)
    simu lation data to provide ownship intent data to          with added pAOP-generated conflict prevention
    intent-based CD&R and ownship state data to                               vertical-speed bands.
    state-based CD&R.

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    Area-hazard processing receives WIS data                   present position) in a flight plan was in the middle of
     messages and packages these into area hazard               a turn.
     informat ion for both state and intent-based
     CD&R.                                                      Receipt of CDU Data
 Intent-based CD&R receives ownship and traffic
     intent data, along with area hazards. This                 Upon receipt of CDU data, the pAOP follows the
     informat ion is used to provide intent-based               following process, similar to the receipt of simulat ion
     conflict detection and conflict resolution                 data:
     advisories. The intent-based CD&R function                  The data are added and validity checks are
     operates in a separate thread from pAOP.
                                                                     performed.      If these succeed, the process
 State-based CD&R receives ownship and traffic                      continues below.
     state, along with area hazard informat ion. These           If the CDU data contains an active flight plan,
     are used to provide state-based conflict detection
                                                                     the flight plan is converted into a format suitable
     and resolution advisories in addition to conflict
                                                                     for intent-based CD&R using the most recent
     prevention information. The state-based CD&R                    available state information as the present
     operates in serial with pAOP.
pAOP is event-driven. Actions are initiated through
                                                                 A change monitor function is invoked to
the receipt of messages from other simulat ion
                                                                     determine if the most recent state and updated
components. The response of pAOP to each message
                                                                     flight plan should be sent to the intent-based
received is described below.
                                                                     CD&R function. For the prototype, updates do
                                                                     not occur during a turn, but updates do occur at a
Receipt of Simulation message                                        user-configurable maximu m rate.

Upon receipt of simu lation data, pAOP follows the              Receipt of ADS-B Data
following process:
 The data are added and validity checks are
                                                                ADS-B type messages received by pAOP contain
     performed on the data. If this succeeds, the
                                                                both state and intent information for traffic.
     process continues below.                                   However, intent informat ion is packaged in multiple
 The simu lation data is converted to a format                 ADS-B type messages. Each one of these messages
     useful to intent-based CD&R and a separate                 received (fro m TMX) may contain multiple aircraft
     format useful for state-based CD&R. pAOP                   ADS-B messages. For each aircraft, the following
     determines if the flight is conforming to its flight       relevant information is included: aircraft identifier,
     plan. If this process is successful, the process           position, velocity, equipment type, and reporting
     continues below.                                           time. In addition to such instantaneous information,
 A change monitor function is invoked to                       each message may include one waypoint in the list of
     determine if the ownship state used for state-             waypoints defining the traffic intent. The number of
     based CD&R should be updated.             For the          waypoints in this list is configurable as a research
     prototype, this function ensures that updates to           parameter in the simulation. Thus, several ADS-B
     ownship state are not too frequent.                        messages have to be received for each traffic aircraft
 A separate change monitor function determines                 in order to determine the intent. This method is used
     if a message should be sent to the intent-based            for software arch itectural purposes only and is not
     CD&R thread containing the updated state and               intended to model data exchange in actual ADS-B
     flight plan. The flight plan message must also be          data links. Other ADS-B parameters are configurable
     sent since the intent-based CD&R considers the             in the simulat ion, such as transmitter range, broadcast
     flight plan to begin at the present position and           rate, failu re status, and message drops.
     alters its copy of the flight plan as such. This
     change monitor function does not update the                Upon receipt of an ADS-B message, the following
     flight plan and state during a turn maneuver, and          process is followed.
     updates at a user-configurable maximu m rate                The data are added and validity checks are
     (nominally every 30 seconds).                                  performed.      If these succeed, the process
Updates to intent-based CD&R during a turn were                     continues below.
deliberately excluded in this prototype since large              ADS-B messages are converted into a format
differences could occur between a forecast trajectory               suitable for detection and resolution. State data
and the flown trajectory when the init ial point (and               is extracted and stored for each flight in the
                                                                    ADS-B type message. Intent information is

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    extracted and pieced together into a temporary             Intent-based CD&R Thread
    traffic trajectory. Once an aircraft’s intent is
    complete, the traffic intended trajectory is placed
                                                               Intent-based conflict detection and resolution is
    into a list of traffic intent. For each flight, pAOP
                                                               accomplished through an independent thread based
    verifies whether the aircraft is in conformance
                                                               upon the CD&R method described in [10]. This
    with its intended trajectory.
                                                               function receives (and removes) area hazards and
   A change monitor function is invoked to                    intended traffic trajectories to be used for conflict
    determine if the list of traffic state should be           detection against the ownship. The ownship state and
    updated for state-based CD&R.                For the       flight plan are used to obtain a forecast ownship
    prototype, this function ensures that updates              trajectory. The trajectory is a four-dimensional
    occur at a maximu m user-specified rate.                   estimation of the future aircraft flight path. This
   A separate change monitor function is invoked to           trajectory is computed from the flight plan by calling
    determine if the list of traffic intent should be          the same functions as used by the FMS (these are
    sent to the intent-based CD&R thread. This                 however, statically lin ked into the pAOP).
    function ensures that messages are sent
    according to a user-specified maximu m rate. A             Upon receiving any new message in this simulation,
    traffic message is then packaged to be sent to the         the intent-based CD&R follows a consistent process:
    intent-based CD&R thread.                                   If a resolution is underway, the maneuver is
                                                                    assumed to be outdated, and resolution
Receipt of WIS Data                                                 calculations are halted. It is for this reason that
                                                                    one does not wish to update the intent-based
In a manner similar to the ADS-B messages, data for                 resolution too frequently, or resolutions would
a particular area hazard (e.g., weather cell, special-              not have time to co mplete.
use airspace) can be contained in multiple WIS                  The appropriate data is updated based on the
messages. Furthermore, WIS messages may be of                       message received.
three types, corresponding to: creating a new                   Conflict detection is performed between the
polygon, adding to an existing polygon, or deleting a               ownship flight trajectory and all known area and
polygon. Upon receipt of a WIS message, the                         traffic hazards. Detection between ownship and
following process is follo wed.                                     traffic hazards uses user-configurable separation
 The data is verified. If successful, the process                  values usually set to a lateral separation of 5
     follows below.                                                 nautical miles and a vertical separation of 999
 The received data is converted into an pAOP                       feet (e.g., reduced vertical separation min ima
     format. Depending on the message type, new                     (RVSM ) for all altitudes, although current values
     polygons are created, old area hazard data is                  can also be configured). Only that portion of the
     deleted, or existing incomp lete area hazard data              trajectory fro m the current time to some
     is filled with data. Upon completion of an                     configurable loo k-ahead time is co mpared to
     incomp lete area hazard, a list is maintained with             hazards for conflict detection.
     complete area hazards. If a message is received            If a conflict is detected, a message is sent to the
     to remove an area hazard , a separate “remove                  pAOP thread containing conflict information.
     area hazard” message is packaged and sent to the               Furthermore, rules of the road determine if the
     intent-based CD&R thread.                                      ownship should maneuver for the specified
 A change monitor function is invoked for the                      conflict. Reference [3] describes the rules of the
     purposes of updating the state-based CD&R                      road implemented for this prototype.
     function. This ensures that updates occur after a          If the ownship is responsible for maneuvering,
     certain number of WIS messages are received.                   then a resolution thread is initiated. This thread
 A separate intent-based change monitor function                   seeks to find a min imu m distance lateral
     is invoked to determine if the stored area hazards             maneuver that removes the detected conflict and
     should be sent to the intent-based CD&R thread.                does not create any new conflicts within the
     If successful, the area hazards are packaged in a              resolution    look-ahead      horizon.       Upon
     message to be sent to the intent-based CD&R                    convergence to a resolution maneuver, a new
     thread.                                                        conflict-free flight plan is sent to the pAOP
                                                               The resolution thread contains a few user-
                                                               configurable parameters that may affect the
                                                               resolution maneuvers. A user-configurable buffer is

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contained so that resolution maneuvers do not
separate by the same lateral separation value as used
for detection. This helps to prevent oscillat ions
resulting from trajectory uncertainties, due to the
discrepancy between FMS-co mputed trajectories and             Turn-dynamics
PCPlane generated flight paths.

Resolution also must use a longer look-ahead horizon
than conflict detection. In a determin istic world,                                                  Freeze-
conflicts will init ially occur at the edge of the look-                                             horizon
ahead horizon. If resolutions used the same look-                                  Latency
ahead time as detection, resolution maneuvers would
simp ly delay the conflict beyond the look-ahead                                                 Own-ship
horizon. This results in maneuvers constantly being
recomputed. Figure 3 illustrated the need for longer                          Figure 4. Freeze horizon.
look-ahead times for resolution.
                                                               Non-message Processing
       Conflict after
                                           Original            Subsequent to the prior processing of mes sages
                                           conflict            received by pAOP, the state-based CD&R function
                                                               will execute using the latest available traffic, area and
      Look-ahead                                               ownship data. This function executes in the same
     horizon - CD                                              thread as the main pAOP and sends messages to the
                                                               flight display suite. Specifically, conflict prevention
   Resolution using                                            informat ion and state-based conflict detection and
                                                               resolution advisories are sent to the FDS for display
    CD look-ahead
                                                               on the ND and PFD. These follow many of the same
                                                               guidelines as used in references [3], [6] and [11]. Re-
                                                               stating this approach for clarity, figure 5 illustrates
                                                               these features on the navigation display:
                          Own-ship                              Conflict prevention bands illustrate state values
                                                                    (e.g., heading) that would cause a state-based
 Figure 3. Resolution must look-ahead further than                  conflict (amber within 5 minutes, red within 3).
                     detection.                                 A Level 1 conflict alert ind icates an impending
                                                                    loss of separation based on state-vector
One additional parameter required for this                          projections; the conflict, however, is predicted to
implementation of conflict resolution is a freeze                   be resolved by the broadcast intent of the traffic
horizon, as illustrated in Figure 4. Any new flight                 aircraft. Alternatively, the Level 1 conflict alert
plan calculated by the conflict resolution function                 indicates an intent conflict with a traffic aircraft
will drop a waypoint along the path at a user-                      that has lower-priority status, thus requiring the
specified distance from the present position. The                   traffic aircraft to generate a resolution. In either
flight plan will not be altered between the present                 case, the ownship pilot is not required to take
position and the freeze horizon point. There are two                action at this time, but is provided a traffic
reasons for the freeze horizon: latency and turn                    advisory to increase situation awareness.
dynamics. Latency results from the time to compute              A Level 2 conflict alert ind icates an impending
the resolution flight plan, and fro m delays in the                 loss of separation based on intent projections.
flight crew approving a suggested maneuver.                         Ownship is responsible for resolving the conflict
Assuming that all waypoints are fly-by waypoints,                   when a Level 2 alert is presented.
any turn at the freeze horizon waypoint will result in          A state-based resolution advisory recommends a
a turn being initiated some distance prior to the                   variation in state to resolve the conflict.
waypoint. If the turn is init iated too close to the           Note that the state-based data will only be displayed
present position, this could lead to sizeable ownship          if the user has enabled the state-based detection and
positional errors in the forecast trajectory (thereby          resolution function. Should the flight crew elect to
leading to errors in detection).                               follow the state-based instructions for resolution, the

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resolution instructions can be flown          manually         The display of a modified flight plan is illustrated in
through the GCP.                                               Figure 6. The new flight plan path is shown as a
                                                               dashed line, and is input into the CDU as a secondary
                                                               flight plan. The flight crew has the option of
                                                               accepting or rejecting the proposed flight plan via
                                                               line select keys on the CDU. While the flight crew
                                                               decides what to do with this maneuver, no further
                                                               resolution maneuvers are sent to the crew. For future
                                          Resolution           designs, the flight crew will be capable of requesting
                                          M aneuver            other resolution choices.

                                                                  Reco mmended

    Figure 5. Illustration of state-based resolution

Intent-based resolutions are initiated in a separate
thread fro m pAOP. Ho wever, conflict detection and
resolution messages are received from the intent-
based CD&R thread. Unlike the state-based method,
which sends complete data at every cycle, the intent-
based approach sends conflict information only upon               Figure 6. Display of an intent-based resolution
change of status. Upon detection of an intent-based                                 maneuver.
conflict, the conflict in formation is sent to the pAOP
thread. This conflict informat ion is only forwarded           Upon acceptance of the resolution maneuver by the
to the FDS if all flights in conflict are in conformance       flight crew, the accepted flight plan becomes the new
with their intent. For the simu lation prototype, the          active flight plan and is displayed as such by the
ownship is deemed to be in conformance with its                navigation display. The intent-based CD&R function
flight plan if the flight is flying with both LNA V and        updates the ownship flight plan with the new flight
VNA V modes engaged.                                           plan such that any resolved conflict(s) is (are)
If the priority rules indicate that the ownship should
not maneuver, the conflict is displayed to the flight          Should the flight crew reject the intent-based
crew as a Level 1 alert, but no resolution maneuver is         resolution maneuver, the active flight plan remains as
computed or displayed to the flight crew. It was               it had been, and the intent-based CD&R will continue
assumed that the flight crew is aware of the priority          to seek a resolution maneuver at its next invocation.
rules and realize that the ownship has right-of-way.           Thus, unless the flight-crew manually alters the flight
When the priority rules indicate ownship                       plan or path, they will continue to receive new
responsibility for resolution of a conflict, the intent-       conflict-free flight plans at some interval.
based resolution algorithm co mputes a resolution
maneuver. For this prototype, lateral maneuvers are                                    Results
selected that minimize the flying distance. A new
flight plan is computed and sent to both the CDU and
                                                               This prototype simulat ion provides a collection of
the FDS for d isplay to and review by the flight crew.
                                                               capabilit ies used for the experiment described in [3]

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and [4].     Specifically, the system provides the
capability to conduct HITL simu lations of airborne
self-separation concepts while meeting externally
specified RTAs. The system incorporates the ability
to receive area and traffic hazard informat ion through
messages simu lating ADS-B and WIS type messages.
The prototype integrated PC-based tools simu lating a
commercial transport glass cockpit (including CDU,
FMS, ND, PFD, and GCP). Cockpit d isplay of traffic
informat ion was accomp lished through the navigation

Figure 7 illustrates the capability of the simulation to
provide area-hazard resolution maneuvers, wh ile
considering neighboring traffic as a constraint on the
resolution maneuver.        The original flight path,
shown as a solid line, would have infringed upon the
area hazard. The traffic aircraft does not conflict
with the flight path, but may conflict with some flight
paths avoiding the area hazard. The maneuver
selected by the intent-based resolution scheme
(shown as a dashed line) provides a path that is
conflict-free fro m both the original area hazard, and
the interfering traffic.                                           Figure 8. Illustration of conflict resolution with
                                                                                     traffic aircraft.

                                                                Figure 9 illustrates the selection of an optimal
                                                                resolution maneuver that min imizes distance
                                                                traveled, but may not immediately appear intuitive.
                                                                A crossing conflict is shown at which t ime both
                                                                aircraft will collide with a closest point of approach
                                                                of zero nautical miles. The ownship has a required
                                                                waypoint beyond the conflict.          The min imu m
                                                                distance maneuver is not a symmetric maneuver in
                                                                this case, but is one of the two maneuvers illustrated
                                                                in the figure. One of these solutions was proposed by
                                                                the resolution algorithm and verified through an
                                                                exhaustive search to be the optimal maneuver in this
                                                                circu mstance.

                                                                The specific co mbination of state-based and intent-
                                                                based resolution maneuvers provides an additional
                                                                level of protection in the event of traffic b lundering
                                                                past their intent. Figure 10 illustrates a situation in
                                                                which a traffic aircraft intends to maneuver behind
                                                                the ownship. If the traffic aircraft misses the
                                                                waypoint, the state-based CD&R function would
   Figure 7. Illustration of area-hazard resolution             notify the pilot of the conflict situation allowing for a
   (dashed line) capability with interfering traffic.           safe conflict avoidance maneuver to occur.

Figure 8 illustrates a maneuver (dashed line)
resolving a conflict with traffic aircraft. Note that the
maneuver is selected to avoid both the conflicting
aircraft in addition to other traffic aircraft and area
hazards. The orig inal flight plan (solid line) would
have conflicted with a crossing flight.

                                  American Institute of Aeronautics and Astronautics

                                                                 A simulat ion capability was presented which
                                                                 demonstrated an ability to conduct PC-based human-
                                                                 in-the-loop simulat ions of airborne self-separation
                                                                 concepts. The simulation provided decision support
                                                                 tools to the flight crew allo wing for the avoidance of
                                                                 both traffic and area hazards simu ltaneously.
                                                                 Furthermore, integration of the resolution capability
                                                                 with a simu lated flight management system permits
                                                                 the investigation of airborne self-separation concepts
                                                                 under the presence of local traffic flow manage ment
    Minimu m distance                                            constraints, particularly t ime-based constraints.
    optimal resolutions
                                                                 The prototype simulat ion provides simulated ADS-B
                                                                 and WIS messages, thereby enabling future HITL
                                                                 studies with variations in message content,
                                 Own-ship                        frequency, and latency.

                                                                 Simp le priority-based resolution rules were
                                                                 incorporated into this simu lation for the original
  Figure 9. Optimal maneuver for crossing conflict.              experiment described in [3] and [4], but can be
                                                                 extended to allow further studies of the impact of
The specific co mbination of state-based and intent-             priority rules under various airborne self-separation
based resolution maneuvers provides an additional                concepts.
level of protection in the event of traffic b lundering
past their intent. Figure 10 illustrates a situation in          The integration of a state-based and intent-based
which a traffic aircraft intends to maneuver behind              conflict avoidance and prevention system into one
the ownship. If the traffic aircraft misses the                  simu lation allows the development of experiments
waypoint, the state-based CD&R function would                    seeking to determine the operational impact and
notify the pilot of the conflict situation allowing for a        benefits of different resolution schemes, including
safe conflict avoidance maneuver to occur.                       mixed state and intent schemes.

                          Blunder path leads to a
                          state-based conflict
                                                                 [1] “Concept Defin ition for Distributed Air/ Ground
                                                                 Traffic Management (DA G-TM )”, Version 1.0,
                                                                 NASA Advanced Air Transportation Technologies
                                                                 Project, Sept. 1999.

                                                                 [2] Ballin, Mark G.; Wing, David J.; Hughes, Monica
                                                                 F.; and Conway, Sheila R.: “Airborne Separation
                            Original Intent
                                                                 Assurance and Traffic Management: Research of
                                                                 Concepts and Technology”, AIAA-99-3989.

                                                                 [3] Wing, David .J.; Adams, Richard J.; Duley,
                   Own-ship                                      Jacqueline A.; Legan, Brian M.; Barmo re, Bryan E.;
                                                                 and Moses, Donald: “Airborne Use of Traffic Intent
  Figure 10. Blunder protection afforded through a
                                                                 Information in a Distributed Air-Ground Traffic
      combined state and intent-based CD&R.                      Management Concept: Experiment Design and
                                                                 Preliminary Results”, NASA / TM -2001-211254,
                                                                 Nov. 2001.

                                  American Institute of Aeronautics and Astronautics
[4] Wing, David J.; Ballin, Mark G.; Barmore, Bryan           and Ergonomics Society 44th Annual Meeting, San
E.; and Krishnamurthy, Karthik: “Use of Traffic               Diego, CA, July 30-August 4, 2000.
Intent Information by Autonomous Aircraft in
Constrained Operations”, AIAA-2002-4555, Aug.                 [12] Gray, W m. M ichael; Chappell, Alan R.;
2002.                                                         Thurman, David A.; Palmer, Michael T.; and
                                                              Mitchell, Christine M; “The VProf Tutor: Teaching
[5] Peters, Mark E.; Ballin, Mark G.; and Sakosky,            MD-11 vertical profile navigation using GT-ITACS”,
John S.:      “A Multi-Operator Simu lation for               Hu man Factors and Ergonomics Society 44th Annual
Investigation of Distributed Air/ Ground Traffic              Meeting, San Diego, CA, Ju ly 30-August 4, 2000.
Management Concepts”, AIAA-2002-4596, Aug.
2002.                                                         [13] Palmer, M ichael T.; Abbott, Terence S.; and
                                                              Williams, Dav id H: “Develop ment of workstation-
[6] Hoekstra, J.M., et al: “Conceptual Design of Free         based flight management simu lation capabilities
Flight with Airborne Separation Assurance”, AIAA              within NASA Langley’s Flight Dynamics and
GN&C Conference, Boston, MA, 1998.                            Controls Div ision”, Ninth International Sy mposium
                                                              on Aviation Psychology, Colu mbus, Ohio, April
[7] “Freer     Flight Simu lation”,       Eurocontrol         1997.
Experimental Center, June 2000.
                                                              [14] “Improving the ASAS Concept without Intent
[8] Bramer, E.: “ASAS: Investigations into Airborne           Knowledge: PREDASA S”, fro m NLR website:
Separation Assurance in a Distributed Simulat ion             http://www.nlr.n l/public/hosted-
Environment – Final Report”, Berlin University of             sites/freeflight/predasas.htm (1999)
Technology, Aug. 1999.

[9] Eby, Martin S.: “A Self-Organizat ional Approach
for Resolving Air Traffic Control Conflicts”, Lincoln         Acknowledgements
Laboratory Journal, 1994, Vo l. 7, No. 2.
                                                              This development effort involved the contribution of
[10] Mondoloni, S.; and Conway, S.: “An Airborne              many individuals dedicating long hours to this task.
Conflict Resolution Approach Using A Genetic                  In no particular order, the authors acknowledge the
Algorith m”, AIAA 2001-4054, AIAA GN&C                        contributions of the following individuals: Ian
Conference, Montreal, Aug. 2001.                              MacLure, Bryan Barmore, Tom Britton, Mark Peters,
                                                              Frank Bussink, Bart Heesbeen, Jacco Hoekstra, Ari
[11] Palmer, Michael T.; Chappell, A lan R.;                  Stassart, Jackie Duley, Katrin Helbing, Brian Legan,
Thurman, David A.; and Mitchell, Christine M.:.               Chace Hall and John Schade.
“GT-COM ET: An architecture to enhance error
tolerance in safety-crit ical systems”, Hu man Factors

                                American Institute of Aeronautics and Astronautics

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