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This paper describes a concept for a coplanar airborne self-separation display, which is designed to aid pilots in their separation task, by visualizing the possibilities for conflict resolution that the airspace provides. This study is a part of an ongoing research toward the design of a constraint-based 3-D separation assistance interface that can present all the relevant properties of the spatiotemporal separation problem. A display concept is proposed that presents speed, heading, and altitude action possibilities in two planar projections of the maneuver action space. The interface also visualizes how these projections interact with each other.
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Design of a Co-Planar Airborne Separation Display
Joost Ellerbroek, Student member, IEEE, Koen C. R. Brantegem, M. M. (Ren´e) van Paassen, Member, IEEE,
Max Mulder
Abstract—This paper describes a concept for a co-planar
airborne self-separation display, that is designed to aid pilots in
their separation task, by visualizing the possibilities for conflict
resolution that the airspace provides. This work is part of an
ongoing research towards the design of a constraint-based 3-D
separation assistance interface that can present all the relevant
properties of the spatio-temporal separation problem. A display
concept is proposed that presents speed, heading and altitude
action possibilities in two planar projections of the maneuver
action-space. The interface also visualizes how these projections
interact with each other.
Index Terms—Ecological Interface Design (EID), Airborne
Separation Assistance System (ASAS), self-separation, situation
IN THE CURRENT Air-Traffic Management (ATM) con-
cepts for unmanaged airspace, aircraft will fly optimized,
four-dimensional trajectories, that have been determined and
coordinated completely before the actual flight [1], [2]. To
resolve traffic (or other) conflicts that result from uncertainties
that arise during flight (e.g., bad weather, departure delays),
automated systems will be used to detect conflicts, and provide
resolution advisories to the pilot.
Although automation provides the resolutions, pilots will
ultimately be responsible for the validity of that resolution.
They should therefore be able to monitor the traffic situation,
and the proper functioning of the automation, and should
be able to intervene in case the automation fails. In other
words, pilots should be able to detect, and act upon very
infrequent situations that were not anticipated in the design
of the automation. It is therefore of paramount importance
for automation to be transparent and understandable to the
operator [3]–[6].
The work presented in this paper is part of an ongoing
study on the design of a 3-D separation assistance interface.
The study employs a constraint-based approach, inspired by
Ecological Interface Design (EID). EID is a proven design
paradigm from the domain of process control [7], [8], that has
in recent years also been applied in several aviation-related
interface concepts [9]–[16]. In this approach, work-domain
analysis tools such as the Abstraction Hierarchy (AH) are used
to identify relevant constraints and relations on multiple levels
Published in IEEE Transactions on Human-Machine Systems, 01/2013;
43(3):277-289, DOI:10.1109/TSMC.2013.2242888. This work has been co-
financed by the European Organisation for the Safety or Air Navigation
(EUROCONTROL), under its Research Grant Scheme launched in 2008, and
by the National Aerospace Laboratory NLR. The content of the work does
not necessarily reflect the official position of EUROCONTROL or the NLR
on the matter.
The authors are with the Control and Simulation section of the Faculty of
Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629
HS Delft, The Netherlands. Email:
of abstraction [17], [18]. An extensive analysis of the work
domain relevant to airborne separation was performed during
the design of previous constraint-based concepts [12], [13],
[16]. This paper will summarize the relevant constraints and
relations that were identified in the analysis, which stands at
the basis of the concept presented in this paper. For a more
exhaustive description of the actual work-domain analysis, the
reader is referred to the previous publications.
The aim of this study is to create an interface that realizes
proper support for airborne separation, by showing the impli-
cations of other traffic for the affordancesof locomotion, and
how they relate to limitations of the own aircraft [12], [13].
The interface presented in this study does not explicitly relate
to specific automation functions. Instead, it visualizes work
domain information, which invariably forms the premise on
which both automation and the human operator should base
their actions. By going beyond visualizations that relate only to
the automation logic, these displays help pilots gain a deeper
understanding of the functions and relations within the work
domain [17], [18], [21], which will be invaluable to pilots
when they need to judge the automation’s functioning [22].
These displays should provide support in routine as well as
unforeseen situations, where pilots may have to rely on their
own problem-solving skills to resolve a conflict.
This study has led to three display concepts [12], [13], [16].
Each of these concepts presents a planar projection of the own
aircraft’s 3-D maneuver space. All three projections represent
simplified, 2-D versions of the maneuver space. They in-
escapably discard information, a problem that is inherent to the
presentation of multi-dimensional data on a 2-D surface [23],
[24]. Our ultimate goal is therefore to find a representation
that mitigates as much as possible the problems of presenting
multi-dimensional data on a 2-D surface.
Several studies have investigated the effects of 3-D vi-
sualization methods for airborne traffic information displays
[25]–[27]. They compare between perspective and (co-)planar
displays that give a basic representation of traffic. For the
current work, however, the focus lies not only on representing
traffic, but also on what such traffic means to pilots in terms
of achieving functional goals, and how it relates to other
functions and constraints in the work domain. This will pose
different demands on the method of presentation.
This paper argues for a co-planar representation, on the
basis of previous concepts and the corresponding work-domain
analysis. It will describe how to mitigate the problems that
James J. Gibson defined affordances as opportunities for action, provided
by an object or by the environment. An affordance is considered always in
relation to the actor, and therefore dependent on the actor’s capabilities and
goals [19], [20]. For instance, with respect to an engine, air affords propulsion,
but with respect to a wing, air affords lift.
arise when information which is intrinsically three dimensional
is distributed across two 2-D displays, and how to show the
interactions that can occur between the two planar presen-
tations. The decision for a co-planar display is a departure
from the aim of the previous concept [16], which was to
create a single, integrated perspective presentation of the
3-D constraints. The following section will therefore discuss
the rationale for choosing a co-planar display. Section III
introduces functional presentations for relevant work-domain
constraints. The fourth section describes the co-planar display
concept. A fifth section illustrates how the visualizations in the
display concept link back to the work-domain analysis. The
paper concludes with a discussion of the benefits, drawbacks,
and remaining challenges of this concept.
For the visualization of a 3-D space on a 2-D screen, two
options can be distinguished: perspective displays and co-pla-
nar displays, each with their own benefits and drawbacks [23],
[24]. The decision to choose either of these two methods will
depend on the task requirements for the resulting display. A
co-planar display has uniform, undistorted axes in its viewing
planes, which benefits precise position and angle judgments.
Perspective displays, on the other hand, have more “pictorial
realism”: they correspond more closely to the 3-D world [26],
[28], [29]. Perspective displays can also employ texture and
shading to increase realism, and improve spatial awareness
[30]. Perspective displays might therefore be preferred when
the task requires complex shape understanding. St. John et
al. also differentiate between tasks that involve only separated
spatial dimensions, and tasks that involve integrated spatial
dimensions, where co-planar displays are better suited for the
former, and perspective displays for the latter [24].
A drawback of co-planar displays in the current context
is that some of the information on the interaction between
locomotion constraints is lost, when these constraints are
presented using separate horizontal and vertical projections.
Also, distributing the information across two displays requires
the pilot to mentally integrate the information from both
displays. Perspective displays, on the other hand, suffer from
perspective distortions, which can induce errors in judging
distances and angles on the display [27], [31], [32]. The
presentation of 3-D structures also suffers from problems of
occlusion: when viewed from a fixed angle, the front facing
side of the structure hides the internal details of the structure.
A. Motivation for a co-planar display concept
In the design of a separation assistance display concept, the
choice between visualizations should depend on the specifics
of the separation task, and how it is performed. From previous
studies and experiments, several arguments can be found for
the use of a co-planar display. First, experiments performed in
this study, as well as other studies, showed that pilots have a
strong preference for single-axis resolution maneuvers [26],
[33]–[36]. Second, the design of previous constraint-based
separation assistance displays illustrated that traffic constraints
can become complex, yet precise judgment of these constraints
is valuable for safe and efficient conflict resolution. They
also illustrate that the planar projections of the constraints
show an intuitive relation with the absolute geometry of
the conflict, which benefits situation awareness. Perspective
distortion makes this relation less visible in a perspective
projection, a problem that also hampered the previous concept
for a perspective 3-D interface [16]. Although that concept
employed constant-velocity cutting planes to reduce the com-
plexity of the constraint visualization, it did not reproduce the
intuitive visual relation with the spatial representation of the
Two of the three current constraint-based separation assis-
tance displays will be used as a basis for the co-planar concept
[12], [13], [16]. The three current display concepts provide
three orthogonal projections of the maneuver space: a top-
down projection [12], a side-view projection [13], and a front-
facing, ego-centric equidistant cylindrical projection [16]. The
first two are presented on the Horizontal Situation Display
(HSD), and Vertical Situation Display (VSD), respectively.
The third concept does not have an equivalent existing display
in the cockpit. Because the first two concepts feature the most
intuitive maneuver space projections, and as they correspond
closest to current re-planning tasks and displays, these will
be used in the co-planar display concept. Each of the two
original display concepts assumes that a traffic conflict lies
exactly in its plane, and the challenge in the design of the
co-planar concept discussed in this paper lies in showing the
interactions that exist between the projection planes of the co-
planar display. This will be discussed in Section III.
B. Comparison with other 3-D displays
Although most current research on airborne separation as-
sistance systems focuses on the development of automated sys-
tems that assist pilots with the separation task [37]–[41], there
are also several display concepts have been developed as aids
in the task of self-separation [39], [42]–[44]. There are two
distinct types of conflict representation that are used in these
displays: a spatial representation, which is similar to traditional
situation displays, and a maneuver-space representation, i.e.,
visualizing how proximate traffic limits ownship maneuver-
ability in terms of airspeed, heading, and vertical speed. Some
displays use only one of these representations, others combine
them. An important benefit of spatial representations is that
they offer an intuitive overview of the situation, familiar to
anyone who has ever used a map. Maneuver-space displays,
on the other hand, are useful because they reveal a direct
link between constraints or commands, and the applicable
maneuver dimensions.
A second distinctive factor between displays is whether
they show explicit (automated) commands, or constraints on
maneuvering. Benefits of explicit command displays are that
they suffer relatively little from display clutter, and that
they reduce workload. Automation also has the potential of
providing the most efficient conflict resolution options. The
most important drawback of displays that show only explicit
resolution commands is that they hide the rationale behind the
automation. These displays do not support human information
seeking, and, in case of automation failure, the pilot is not
supported in recognizing failure, nor in seeking alternatives.
In these situations, performance can even be worse than when
completely unaided [22].
Constraint displays, on the other hand, give a continuous
view on maneuver options and limitations, which allow pilots
to evaluate their own resolution maneuvers. Depending on
how constraints are visualized, these displays can show the
structure of, and relations within the work domain, and can
therefore provide a useful basis for illustrating automation
logic. By showing higher level information and relations, these
displays also allow pilots to investigate the validity of the
data. An important drawback of constraint displays is that
they can result in more display clutter, compared to showing
only explicit commands. According to Tufte’s views on the
use of details (“To clarify, add detail”), however, this is not
necessarily a drawback for a well designed display [45].
The existing concepts described in [39], [42], [44] show
that the concepts presented in the current study are not the
only displays that show constraints on maneuvering instead
of only presenting explicit commands. The concept presented
in [42] provides a spatial representation of constraints in the
form of no-go areas on horizontal and vertical map displays.
A different approach was taken in [39], where constraints
are indicated with colored conflict-bands on the compass, the
speed tape and the vertical speed indicator on the Primary
Flight Display, the HSD, and the VSD. The concept described
in [44] introduces a new display that presents constraints in a
perspective, 3-D maneuver space.
The concepts in the current study aim to improve upon such
constraint-based concepts by visualizing the structure of the
work domain, and by illustrating the relations between lower-
level elements and higher-level information. The remainder of
this paper will describe how properties of the own aircraft and
the surrounding traffic are related to each other, and to higher-
level constraints and functions, in a way that is made visually
apparent on the display.
For airborne trajectory planning and self-separation, several
relevant constraints have been identified [12], [13], [16]. These
constraints fall broadly into two categories: constraints internal
to the own aircraft, and constraints external to the aircraft.
The internal constraints relevant to the problem of separa-
tion relate to the various limitations on aircraft performance.
In addition to these internal limitations, the maneuver space is
further constrained by external factors such as weather, terrain,
other traffic, and airspace boundaries (such as those from
restricted airspace areas). For airborne separation, the focus
obviously lies on the constraints imposed by other traffic.
Functional presentations of these constraints, and the rela-
tions between these constraints, should provide a description
that is compatible to the means that are available to the con-
troller. For trajectory planning, this implies that the description
should relate the goals and affordances of the system, to inputs
that match common flight practice. In cruise flight, pilots
control their aircraft by manipulating velocity, track angle,
and altitude settings, using the autopilot or by modifying the
planned route in the Flight-Management System (FMS). A
successful separation assistance interface should relate these
control variables and their operational limits to the affordances
of the airspace.
A. Velocity action space
A modern glass cockpit supports trajectory planning through
the Horizontal Situation Display and the Vertical Situation
Display, which show horizontal and vertical projections of
task-relevant information such as the planned route, terrain,
weather, and other traffic. While these visualizations clearly
identify the elements of the airspace that constrain the ma-
neuver options of the aircraft, they do not show how these
elements shape the possibilities for pilot action. Because the
operator action space is not shown in a meaningful way, it
remains difficult to relate higher level goals and constraints to
the available actions and inputs.
The design philosophy employed in this study proposes to
achieve this by combining the existing spatial representation
of airspace elements, with a velocity action space, that relates
own aircraft maneuver variables velocity, track angle and
vertical speed, to the identified internal and external constraints
[12], [16]. This action space is defined as the reachable subset
of the 3-D vector space that contains all possible velocity
vectors (i.e., all combinations of velocity, track angle and
vertical speed).
When zero wind is assumed, an aircraft velocity vector in
this vector space can be defined as follows:
cos (χ) cos (γ)
sin (χ) cos (γ)
sin (γ)
where VT AS is the (true) airspeed of the aircraft, χthe track
angle, and γthe flight-path angle, or climb angle of the
aircraft. Vertical maneuvering is more commonly expressed
in vertical speed, which can be derived from VT AS and γ:
V S =VT AS sin (γ). The presence of wind can be of influence
on the velocity action space representation, but only when
ownship and intruder aircraft experience significantly different
wind conditions. It is therefore kept out of the current analysis.
B. Internal constraints
The reachable area that defines the velocity action space is
bounded by constraints that are internal to the own aircraft.
These constraints relate to the various limitations on the perfor-
mance of the aircraft, such as bank limits, maneuver dynamics,
available engine thrust, stall, structural considerations, buffet
characteristics, and requirements on productivity, emissions
and passenger comfort. These limitations result in several
constraints relevant to the task of trajectory planning, such
Track angle differs from aircraft heading in the presence of wind. Heading
indicates in which direction the aircraft nose is pointing, as indicated e.g. on
the magnetic compass. The track angle gives the direction in which the aircraft
is flying. With no wind, these angles are equal. With cross-wind, however,
there will be an offset between heading and track (the drift angle).
as maximum turn rates, maximum and minimum operating
speeds, fastest and steepest steady climb and descent, and the
steepest steady climbing and descending turn. Some of these
constraints also show interactions: For example, in turning
flight, increasing the bank angle will also affect the minimum
velocity and the maximum attainable climb angle [46].
Vmin Vmin
(a) (b)
Fig. 1. The 3-D velocity action space illustrated in two planar projections.
(a): Horizontal maneuvering can be expressed in combinations of airspeed
(VT AS ) and track angle (χ). The minimum (Vmin ) and maximum (Vmax)
obtainable airspeeds are the main constraints for horizontal maneuvering. (b):
Vertical maneuvering can be expressed in combinations of airspeed (VTAS )
and vertical speed (V S). It is also constrained by the minimum (Vmin) and
maximum (Vmax) obtainable airspeeds. Aircraft performance with maximum
(Tmax) and minimum (Tmin ) thrust settings determine maximum steady
climb and descent, respectively.
Fig. 1(a) and Fig. 1(b) illustrate how these constraints can
be visualized in the horizontal and vertical plane, respectively.
The horizontal maneuver space is shaped by the limitations on
airspeed, which reduce it to the ring-shaped area in Fig. 1(a).
Vertical maneuvering is also constrained by airspeed limita-
tions, as well as by the steady climb and descent performance.
Fig. 1(b) shows how these constraints combine in the vertical
plane. The vertical edges of the action space result from the
limits on airspeed. The curved edges at the top and bottom
of the vertical action space visualize the maximum obtainable
steady climb and descent at each velocity, respectively. The
resulting contour is also known as the flight envelope of the
aircraft (refer to [13] for the derivation of this contour).
protected zone
Fig. 2. A conflict situation. Ownship and intruder are in conflict when the
line that extends from the relative velocity vector (Vrel) crosses the intruder
Protected Zone (PZ). The PZ is a flat, 3-D disc around each aircraft, that
should remain clear of other traffic. Common dimensions for this PZ are
RP Z = 5 nmi and hP Z = 2,000 f t.
C. External constraints
In unmanaged airspace, the reachable area that is enclosed
by the internal aircraft constraints is further restricted by
several external factors, such as weather, terrain, and traffic.
For a self-separation interface the focus obviously lies on the
constraints imposed by other traffic. These traffic constraints
are shaped by a minimum horizontal and vertical separation
between any two aircraft, that should be adhered to at all times.
With common values of 5 nautical miles horizontal, and 1,000
feet vertical separation, this results in a 3-D Protected Zone
(PZ): A flat, 3-D disc around each aircraft, that should remain
clear of other traffic (illustrated for the intruder aircraft in
Fig. 2) [33], [47]. Intrusion of this space is referred to as a
loss of separation. A conflict is defined as a predicted future
loss of separation, within a certain time horizon [48]. For this
concept, the time horizon was set between 60 seconds and 10
to 20 minutes [16].
xo,rel (t1)
Fig. 3. Relation between ownship motion relative to the intruder, and the
absolute motion of ownship and intruder. Ownship and intruder are shown
at time t0and t1.xo,rel shows how ownship has moved relative to the
intruder. Line ashows that the change in orientation between ownship and
intruder along the relative path is equal to the change along the absolute paths.
Although ownship and intruder intent can influence the
constraints imposed by the intruder, the current study will only
employ the current states to derive these constraints. Previous
studies did incorporate the effect of intent on maneuver
affordances [49]–[51], however, this is beyond the scope of the
current study. Under the assumption that intruder and ownship
state remain unchanged in the near future, a conflict can be
predicted using the speed of ownship, relative to the intruder:
Vrel,own =Vown Vint (2)
Relative velocity vector Vrel,own indicates how ownship
moves with respect to the intruder aircraft, see Fig. 3. When
the relative track of ownship (the line extended from the
relative velocity vector) crosses the intruder protected zone,
a loss of separation will occur in the near future, see Fig. 2.
For any given traffic geometry, a set of relative velocity
vectors VF A can be defined that would result in a conflict
between the involved aircraft (i.e., all possible relative velocity
vectors where the resulting relative tracks cross the intruder
PZ). Fig. 4 gives an illustration of VF A . It is a construct of
two slanted cones, connected by straight sections. Both cones
have their apex at the ownship location, and their curvature is
Note that the relative motion of intruder symbols on a traffic display is
opposite to this relative velocity: There, the own aircraft symbol is standing
still, and intruder aircraft symbols move relative to the own symbol on the
display. The current concept observes relative motion from the perspective of
ownship (Vrel,own) in order to find constraints on own maneuvering.
protected zone
Fig. 4. The 3-D forbidden area VF A consists of a conical area that is aligned
with the edges of the intruder protected zone, as seen from the ownship, and
has its apex situated at the center of ownship. Cross-section (a) shows how
the thickness of the protected zone changes the shape of the forbidden area
from what would otherwise be a pure slanted cone, to a combination of two
slanted cones, connected by a straight section.
aligned with the upper and lower circles of the intruder PZ.
Cross-section (a) in Fig. 4 illustrates the effect of the thickness
of the intruder protected zone: if the PZ had been flat, VF A
would have been a single slanted cone, and cross-section (a)
would have been an ellipse. The thickness of the PZ introduces
straight sections in the shape of VF A , and in cross-section (a).
The 3-D vector set VF A marks the constraints that other traf-
fic imposes on ownship (relative) motion with respect to that
intruder aircraft, and will be referred to as the 3-D forbidden
area in the remainder of this paper. This representation only
varies as a function of time. This means that for the current
time, the 3-D forbidden area represents the complete set of
relative velocities that would result in a loss of separation.
V Sint
(a) (b)
Fig. 5. Constraints on ownship velocity, expressed in an absolute velocity
field. (a) illustrates horizontal intruder constraints, (b) illustrates vertical
constraints. ownship and intruder are in conflict when the tip of the ownship
velocity vector is inside the 3-D forbidden area. The apex of the shifted
area corresponds with the intruder velocity vector, and can be used on the
display to determine for instance relative track, and intruder vertical speed.
The forbidden area bisector shows relative bearing of the intruder.
The concepts preceding the current study illustrated that
the relation between the forbidden areas and the own velocity
vector can be made visible by translating the forbidden area
and relative velocity vector by the intruder velocity vector, see
Fig. 5 [12], [16]. This would be equivalent to adding Vint on
both sides of the equal sign in Eq. (2). The shifted forbidden
area represents the constraints imposed by an intruder aircraft
in a way that directly relates to the ownship maneuver options.
An added benefit, illustrated in Fig. 5, is that the apex of the
shifted forbidden area corresponds with the intruder velocity
When neither aircraft maneuver, the opening angle of the forbidden area
will expand or contract only as a function of the closing speed of the intruder
aircraft, with respect to ownship. When ownship and intruder are not on a
collision course, the orientation of the forbidden area will also change as both
aircraft pass each other.
vector, and that the direction of the bisector of the forbidden
area is equal to the relative bearing of the corresponding in-
truder, properties that can be useful when assessing a conflict.
Note that this derivation of the forbidden area assumes
instant state changes. It can be shown that this is a safe as-
sumption when a predicted conflict is still in the far future [52],
[53]. However, maneuver dynamics will start to play a larger
role when conflicts become more imminent: in the case of
tactical maneuvers (within 10 minutes of a predicted conflict),
unmodeled dynamics will cause significant errors, particularly
speed maneuvers [52]. The previous horizontal display concept
compensated for this by observing the constraint area at time
tcur +tturn. Here, tturn is the maneuver duration for the
heading solution that corresponds to the respective forbidden
area leg. A new relative position is extrapolated using the
current aircraft velocities, which in turn is used to calculate
the corrected position for the forbidden area leg. This method
can also be applied to the current concept.
F Aver
F Ahor
Fig. 6. Translated 3-D forbidden area and horizontal and vertical projections.
F Ahor is the horizontal projection of the 3-D forbidden area, and presents
constraints for ownship track and horizontal velocity. FAv er is the vertical
projection of the same area, and presents constraints for vertical motion.
D. Planar constraint projections
There are several visualization techniques for 3-D con-
structs such as the forbidden area, each with its specific
benefits and drawbacks [23]. The drawback these methods
have in common, however, is loss of context. This, however,
is unavoidable, as some form of reduction is necessary to
present multi-dimensional data on a 2-D surface [23], [24].
The challenge is to determine crucial parts of the context, and
to find a visualization that maintains the relevant information.
The previous horizontal and vertical constraint displays both
show an orthogonal projection of the 3-D forbidden area
(illustrated in Fig. 6). These projections take the 3-D shape,
and discard the coordinate that is orthogonal to the projection
plane. The resulting shapes are triangular, with the triangle
In reality, a heading or speed change that is taken to resolve a conflict
will take a certain amount of time. In that time, the constraint area will have
grown slightly, and it might occur that what initially seemed a valid solution,
will in fact not resolve the conflict, due to the expansion of the constraint
area during the course of the maneuver.
apex at the (projected) ownship position, and the triangle legs
aligned with the edges of the projected protected zone.
In Fig. 6, F Ahor is the horizontal projection of the 3-D
forbidden area, and presents the constraints on relative track
and horizontal velocity, for all values of vertical speed com-
bined. F Aver is the vertical projection of the 3-D forbidden
area, and presents the constraints on relative vertical motion,
for all values of the ownship track angle combined.
The orthogonal projections, then, provide a global contour
of the forbidden area in their respective dimensions. Visualiza-
tion of these contours has many benefits. Most importantly, the
relation between the triangular contour and the geometry of
the conflict is easily interpretable. The triangles reveal several
key parameters of a conflict, such as spatial proximity, intruder
bearing, intruder heading and velocity, and relative velocity,
and how these parameters relate to each other. These cues also
help to relate the forbidden areas to the traditional intruder
symbols on the display [36].
However, because the projections show constraints for all
values of the orthogonal coordinate, the constraints can be
conservative when the conflict does not lie exactly in that
plane. For instance, a certain combination of speed and track
angle may lie in a horizontally projected forbidden area, but
can still be conflict-free if there is enough vertical separation,
or if vertical separation is obtained before a horizontal loss of
separation occurs. This distinction cannot be made with the
projected forbidden areas alone.
E. Interactions between projection planes
What these projections fail to show, then, is how the
orthogonal planes interact with each other. The horizontal pro-
jection does not reflect vertical separation and maneuvering,
and vice versa. Cutting plane visualization partly reveals this
interaction, by showing a part (a ‘slice’) of the 3-D shape,
for a given constant value of the third dimension [23]. In
combination with the planar projection of the 3-D shape, it
reveals which part of the projection is valid, for a specific
point along the dimension that is orthogonal to the projection
plane. The result of a cutting-plane intersection will therefore
always be a subset of the planar projection of the 3-D shape.
Fig. 7 shows a horizontal cutting plane, that intersects with
the 3-D constraint area for a certain given value of ownship
vertical speed, i.e., the vertical offset of the cutting plane
is equal to ownship vertical speed. The resulting ‘slice’ of
the constraint area represents horizontal velocity constraints,
taking into account the relative vertical motion and orientation.
This reduced forbidden area is a subset of the horizontal
projection of the 3-D constraint zone (also illustrated in Fig. 7).
When ownship and intruder are not vertically separated, and
the relative vertical speed is equal to zero, the horizontal
reduced forbidden area will be equal in shape and size to the
projected horizontal forbidden area.
Fig. 8 shows a vertical cutting plane, aligned with the
ownship track, that intersects with the 3-D constraint area. The
resulting slice of the constraint area presents vertical velocity
constraints, taking into account how a conflict is oriented in the
direction orthogonal to the ownship track (i.e., intruder cross-
track distance and relative intruder track angle). This reduced
cutting plane
V Sown
V Sown
Fig. 7. The horizontal reduced constraint area is given by the intersection
of the 3-D constraint area, and a horizontal cutting plane, offset vertically
by the ownship vertical speed. The horizontal reduced area illustrates exact
constraints on horizontal maneuvering, also for conflicts with non-zero relative
vertical distances and velocities.
forbidden area will always be a subset of the vertical (side-
view) projection of the 3-D constraint zone (see also Fig. 8).
When ownship and intruder are flying with zero cross-track
distance and relative track angle χ= 180or χ= 0
(ownship and intruder are flying head-on or are overtaking,
respectively), the vertical reduced forbidden area will be equal
in shape and size to the projected vertical forbidden area.
cutting plane
Fig. 8. The vertical reduced constraint area is given by the intersection
of the 3-D constraint area, and a vertical cutting plane, aligned with the
ownship track. The vertical reduced area illustrates exact constraints on
vertical maneuvering, also for conflicts with non-zero cross-track distances
and velocities.
Because these reduced forbidden areas depend on the vari-
able that is perpendicular to their plane of projection, they
effectively reveal an important part of the interaction between
the planes of projection. The reduced areas are invariant
with respect to state changes that lie in their own plane
of projection, and therefore provide a consistent set of 2-D
constraints, under the assumption that the third (perpendicular)
variable is kept constant. In this case, the reduced areas only
vary with time, similar to the forbidden area projections.
When the perpendicular variable is varied, the reduced area
will always change in a predictable fashion. If the change in
that variable is away from the tip of the 3-D forbidden area, the
reduced area will also move away from the tip of the triangle
in its respective projection. Similarly, if the change moves the
velocity vector closer to the tip of the 3-D forbidden area,
the reduced area will move towards the tip of the triangle in
its respective projection. Also, because the planar projections
Fig. 9. Concept for a co-planar separation assistance display. This figure shows a Horizontal Situation Display () and a Vertical Situation Display (),
with added separation assistance overlays. and are the horizontal and vertical State-Vector Envelope, respectively. is the projected forbidden area both
on the HSD and the VSD. is the reduced forbidden area on both displays. Represents the ownship state vector, is a TCAS-symbol indicating the
relative location of the intruder aircraft. is a speed tape, showing current IAS, selected IAS, and simplified traffic constraints. is a vertical speed tape,
showing the current and selected vertical speed in feet per minute, and simplified traffic constraints.
effectively result in a 2-D contour of the forbidden area at its
widest point, projections of the reduced areas can never extend
beyond the boundaries of the projected forbidden area.
Aside from interactions that result from shortcomings of
methods for showing 3-D data on multiple, 2-D surfaces,
interactions can also be found between several aircraft loco-
motion variables, and their limits. A very direct interaction
can be found between the aircraft’s bank angle, and its climb
performance and stall speed: Increasing the bank angle will
increase the stall speed, and reduce the maximum climb angle.
On a larger time scale, changes in altitude will have an effect
on minimum and maximum operating speeds, and on climb
and descent performance [46].
These interactions can partly be captured in the visualization
when the visualized constraints are dynamically calculated for
the current values of the flight variables that influence it. There
are, however, situations possible where dynamically calculated
constraints do not suffice. Consider for instance a situation
where a traffic conflict is solved by assuming a vertical speed
that is close to the maximum climb performance. It can happen
that although initially this climbing solution seemed to be
a valid solution, the reduction in climb performance due to
increasing altitude causes this solution option to disappear.
This is, however, beyond the scope of the current study.
Fig. 9 illustrates a design concept for a separation assistance
interface, that presents separation-related affordance cues on a
co-planar display. The combination of a Horizontal Situation
Display and a Vertical Situation Display was chosen for
this co-planar display concept, because these displays are
omnipresent in the modern flight deck. These two displays
also provide the most intuitive maneuver space projections,
and they correspond closest to current re-planning tasks.
In this display concept, the 3-D traffic situation is visualized
in two orthogonal, 2-D views: a top-down view (), and a side
view (). Both views present an ownship-centered moving
map, that shows spatial information such as the FMS route and
intruder aircraft positions. In addition, constraints on ownship
maneuvering are shown on both displays through velocity
action-space overlays (,).
The top-down view presents information in a polar co-
ordinate system: spatial information is expressed in relative
bearing and distance, and the velocity action-space shows
constraints for combinations of track angle and airspeed.
The side view uses a Cartesian coordinate system: spatial
information is expressed in along-track distance and relative
altitude. Here, the velocity action-space shows constraints for
combinations of airspeed and vertical speed.
A. Traditional display elements
The moving-map presentations on the HSD and VSD are not
new: the HSD is present in all modern cockpits, and also the
VSD is becoming more common. To match current practice,
intruder aircraft are represented on both displays using TCAS-
like symbology() [54]: an unfilled diamond indicates a non-
conflicting intruder, a filled diamond indicates a conflicting
intruder, with more than five minutes to a loss of separation.
This is considered a low-priority conflict. A conflict is con-
sidered medium priority when a loss of separation is between
three to five minutes away, indicated with a solid circle as
intruder symbol. A high priority conflict is less than three
minutes away, and is indicated with a solid square.
Separation margins are indicated around each intruder on
both displays, which results in a circle on the HSD, and a flat
rectangle on the VSD, see Fig. 10. On the HSD, the intruder’s
horizontal speed vector is shown with a dotted trend line. The
length of this line is scaled such, that it equals the radius of
Note that this is not necessarily the best intruder visualization. Intruder
symbology design, however, is beyond the scope of the current study. TCAS
symbology was therefore chosen to match current practice.
(a) (b)
Fig. 10. Intruder aircraft symbology. Intruders are visualized using TCAS-
style symbology: unfilled diamonds for non-conflicting aircraft, filled dia-
monds for low-priority conflicts, filled circles for medium-priority conflicts,
and filled squares for high-priority conflicts. (a): Intruder symbology as shown
on the HSD. The TCAS symbol is shown together with the separation margin,
a speed vector, a vertical speed arrow, and a flight-level offset. (b): Intruder
symbology as shown on the VSD. The TCAS symbol is shown together with
the separation margin, a vertical speed arrow, the relative bearing in hours o’
clock, and the intruder distance in nautical miles.
the separation circle if the horizontal speed of the intruder is
equal to the ownship horizontal speed.
A small up/down arrow is shown next to the intruder
symbol, when the vertical speed of that intruder exceeds 500
ft/min. A signed number below the intruder symbol indicates
the vertical offset in flight levels (1 flight level equals 100
feet), see Fig. 10(a). On the VSD, the intruder TCAS symbol is
accompanied by a label that shows the relative bearing in hours
o’ clock, and the distance in nautical miles. An up/down arrow
is shown to the right of the TCAS symbol when the vertical
speed of that intruder exceeds 500 ft/min, see Fig. 10(b).
In addition to the map view, the vertical display also
includes a speed tape, and a vertical speed tape. The speed
tape () shows current Indicated Air Speed (IAS), selected
IAS, and simplified speed constraints in knots. The vertical
speed tape () shows the current and selected vertical speed,
and simplified vertical speed constraints, in feet per minute.
B. Velocity action-space overlays
The horizontal State-Vector Envelope (SVE) () shows
the affordance space for horizontal maneuvering in terms of
track angle and airspeed, see Fig. 11(a). Because a horizontal
situation display in expanded mode (as in Fig. 9) does not
show traffic behind the own aircraft, the horizontal state-vector
envelope also shows only solutions with |χ| ≤ 90. Current
horizontal situation displays also have modes that show the
situation behind the ownship. In such a mode, the horizontal
state-vector envelope would be shown as a whole circle. The
vertical State-Vector Envelope () is illustrated in Fig. 11(b),
and shows the affordance space for vertical maneuvering in
terms of airspeed and vertical speed.
Intruder aircraft that are within detection range will further
reduce the available maneuver space in the horizontal and
vertical State Vector Envelopes. The reduced forbidden areas
(), derived in the previous section, give the most precise
representation of these constraints, see Fig. 11. On the HSD,
a reduced forbidden area gives the constraints imposed by an
intruder on ownship track angle and airspeed, for the current
value of ownship vertical speed. On the VSD, a reduced
forbidden area gives intruder-imposed constraints on ownship
airspeed and vertical speed, for the current ownship heading.
Note that each intruder adds a forbidden area to the available
maneuver space. These forbidden areas, however, work in a
cumulative fashion: selecting a ‘clear’ area solves all detected
conflicts, without creating a new conflict. In the current
concept, the derivation of the constraint areas uses only state
information, and will therefore only be valid when there are no
imminent trajectory changes. Although the influence of intent
information has been considered in previous concepts [49]–
[51], this is out of the scope of the current study.
The reduced forbidden areas result from the intersection
between a flat cutting plane, and the 3-D forbidden area: a
compound of two slanted conical shapes, aligned with the top
and bottom of the intruder protected zone. The shapes that
result from this intersection range from circles, to ovals, to
open-ended hyperbolic curves, see Fig. 11.
Ref. [36] describes how the triangular shapes that result
from planar projection of the forbidden area can be used to
derive several relevant cues about the spatial configuration
of a conflict. These cues make it easier to relate forbidden
areas to intruder symbols on the map view, but also provide
more information on the urgency of a conflict, and how
different resolutions would affect the traffic situation. In other
words, while the reduced forbidden areas provide more precise
constraint visualization, they sacrifice important cues that link
several display elements together. The current concept there-
fore combines the reduced forbidden areas with the outline of
the corresponding projected forbidden areas (). In addition to
the added situational information, these outlines also provide
an outer limit on the shape and size of the reduced forbidden
area, when a perpendicular flight parameter is modified.
The current ownship airspeed, track angle and vertical speed
are presented on the two displays by two velocity vectors ()
that extend from the origin of the SVE to a certain point
in the velocity vector space. On both displays, the length
of these vectors is equal to the ownship airspeed. On the
vertical situation display, the ownship vertical speed results
in a rotation γ= arcsin(V S/VT AS )of the velocity vector.
Because the horizontal situation display is oriented track-up,
the horizontal velocity vector has a fixed, vertical orientation.
A dot, four kts wide, attached at the tip of each velocity vector,
acts as a margin for maneuver selection [36].
In combination with the reduced forbidden areas, the veloc-
ity vectors show the affordance of avoidance: placing either of
the velocity vectors outside all of the forbidden areas results in
a conflict-free trajectory. Intruder flight-path vectors are also
shown as dots at the tip of the corresponding forbidden area tri-
angle. On the horizontal display, the distance from the tip of a
triangle to the center of the SVE is equal to the airspeed of the
corresponding intruder, see Fig. 11(a). On the vertical display,
however, this distance is equal to the in-track component of the
intruder airspeed, Vver t,int =VT AS,int ×cos(χint χown).
Here, VT AS,int is the intruder airspeed, χint is the intruder
track angle, and χow n the ownship track angle. Moving the
ownship velocity vector towards one of these dots to resolve
a conflict will lead to a very inefficient resolution, as it will
cause ownship to fly parallel to the intruder [12].
C. Conflict urgency visualization
In addition to varying intruder symbology, conflict urgency
is also indicated using color coding for all of the display
(a) (b)
V Sint
Fig. 11. The horizontal (a) and vertical (b) State-Vector Envelopes. The forbidden areas correspond to one intruder, who is situated below, to the front and
to the right of ownship, crossing ownship from right to left, and climbing at a shallow climb angle. (a): The circular boundaries that constrain the horizontal
maneuver space represent the aircraft minimum and maximum operating speeds. The intruder track is offset from the ownship track by ψ. The triangle
apex represents the intruder velocity Vint .(b): The vertical maneuver space is bounded by minimum and maximum operating speeds, and by minimum and
maximum steady-state climb. The vertical offset of the triangle apex corresponds to the intruder vertical speed, V Sint .
elements that correspond to one intruder. This means that the
aircraft symbols on both displays, as well as the forbidden
area triangles and reduced forbidden areas on both displays
are colored according to the urgency of the conflict between
ownship and the corresponding intruder. Similar to the TCAS
system, four levels of urgency have been defined for the
current concept [54]. The lowest urgency level corresponds
with intruder aircraft that are currently not in conflict with
ownship. For these intruders, the display elements are col-
ored white. The second level of urgency corresponds with
a conflicting intruder, with a loss of separation further than
five minutes away. This is defined as a low-urgency conflict,
and display elements are colored yellow. A medium-urgency
conflict corresponds with a loss of separation between three
and five minutes, and is colored orange on the display. A high-
urgency conflict indicates a loss of separation within less than
three minutes, and is colored red on the display.
D. Visual momentum
When more than one intruder aircraft needs to be shown
on the display, it becomes more important for the display
to provide ways to link the several visual elements on the
display to an intruder and to each other. Visual cues that
facilitate this link increase visual momentum: they facilitate
the integration of information across multiple displays, and
between elements on each display [55]. This integration is
essential for proper SA, as the elements on both displays
are intrinsically tied together. Manipulation in one plane will
often affect constraints in the other plane. Visualizing which
elements belong together should aid pilots when assessing
these relations.
Woods introduces functional data overlap as a method that
“improves the visual momentum across multiple displays by
‘presenting pictorially the functional relationships that cut
across display frame boundaries’ ”. In other words, visual
momentum can be improved by showing particular informa-
tion on both displays, and by visualizing relationships between
information on both displays. The color coding that is used
to indicate conflict urgency is an obvious way to improve
visual momentum. The shape and orientation of the conflict
zones, however, also provide ways to identify display elements
that belong together. Examples are the distance to an intruder,
which also determines the opening angle of the corresponding
forbidden areas, the predicted intruder flight path determines
the location of the tip of the horizontal triangle, and the vertical
speed, that determines the vertical position of the tip of the
triangle on the vertical display.
E. Comparison with previous concepts
The main difference between the current co-planar display
concept and the previous separate horizontal and vertical
display concepts, is the visualization of the interactions that
can occur between the planes of projection. The horizontal dis-
play shows constraints on horizontal maneuvering, under the
assumption that intruding aircraft are flying level, within min-
imum vertical separation. Similarly, the vertical display shows
constraints on vertical maneuvering, assuming zero cross-track
distance and maneuvering. These projected constraints become
increasingly conservative when conflicts deviate from these
assumptions. The reduced forbidden areas show more precise
constraints by taking the conflict orientation orthogonal to each
projection into account.
Fig. 12 illustrates how the constraints imposed by an intrud-
ing aircraft change when the corresponding conflict geometry
can no longer be defined in a single plane of projection. All
four examples in Fig. 12 show how conflict constraints would
be visualized on the new display. Note that on the original
two displays the visualization would be similar, but that all
triangles would always be completely filled.
The first conflict, shown in Fig. 12(a), corresponds to an
intruder that is both on the same track and the same level
as ownship, and both aircraft are flying level. In this case,
neither the assumptions for the original horizontal display,
nor those for the original vertical display are violated. As a
result, the constraints imposed by the intruder are presented
as completely filled triangles on the new display, completely
identical to what the visualization for this conflict would be
on the original displays.
Fig. 12(b) shows how the constraints change when the
intruding aircraft starts to climb. Because the intruder is still
on the same track as ownship, the assumptions for the original
vertical display still hold, and the presented vertical constraints
are still identical to how they would be presented on the
original vertical display. The horizontal constraints, however,
change as a result of the vertical maneuver of the intruder.
Where the original horizontal display would show a conflict,
Fig. 12. Example scenarios. An overview of each scenario is given on the
left. The black aircraft symbols represent ownship, the gray symbols represent
the intruder. (a) Ownship is behind and overtaking the intruder, both are flying
level, at equal altitude. (b) Intruder is climbing. (c) Both are flying level, but
the intruder is to the left of ownship, crossing to the right. (d) Intruder is to
the right of ownship, crossing to the left, descending from a higher altitude.
the reduced forbidden areas reveal that ownship would have
to accelerate to get into conflict with the intruder.
Fig. 12(c) shows how the vertical constraints change when
the intruder is on a different track than ownship. Similar to
the situation in Fig. 12(b), the original vertical display would
show a conflict, while in reality the intruder passes in front
of ownship before they get too close. Fig. 12(d) shows that
both the horizontal and the vertical presentation of constraints
change when the intruder is both off level and off track. In
this example ownship and intruder are still in conflict. The
presentation in Fig. 12(d) shows that maneuvers exist that
solve the conflict, while still being in both the horizontal and
vertical triangles. Such solutions would be impossible to derive
from the original two displays.
The constraint-based approach that was adopted in this
study used work-domain analysis tools such as the Abstraction
Hierarchy to identify constraints and relationships on multiple
levels of abstraction [17], [18]. Although the work-domain
analysis for this concept has not changed significantly since
the previous concept, looking at how the visualizations in
the current concept are related to that work-domain analysis
can provide a useful review of the concept, as well as
relevant insights for future design iterations and experiments.
This section will therefore briefly describe how the different
elements of the display link back to the functions, constraints
and relationships in the abstraction hierarchy (Fig. 1 in [16]).
Ref. [16] describes the work-domain analysis in more detail.
The velocity action-space overlays proposed in the display
concept form the main additions to the horizontal and vertical
situation displays. These overlays give a consistent view on the
relations between locomotion inputs and the primary functions
of productivity, efficiency and safety, and they show how these
relationships are influenced by several identified constraints.
Together with the velocity vectors, the horizontal and
vertical state-vector envelopes relate to the safety goal, by
showing how internal constraints (available power, structural
limits, ...) limit possible velocity vectors. In combination with
the horizontal FMS track on the map display, the horizontal
SVE relates to the production goal through the destination
approximation constraint (deviations from track that are larger
than χ= 90move the aircraft away from its destination).
The reduced forbidden area relates to the safety goal by
showing the affordance of conflict, (the own velocity vector
inside a reduced forbidden area indicates a conflict). Together
with the internal maneuvering constraints from the SVE, it
shows the affordance of avoidance: any vector within the SVE
that is not inside any reduced forbidden area is a possible
solution to a conflict. The reduced forbidden area also relates
to the efficiency goal, through the ‘shortest way out’ principle
[12]. The smallest vector change out of a reduced area will also
result in the smallest path deviation. Note, however, that path
deviation in the horizontal plane does not directly compare to
path deviation in the vertical plane.
The forbidden area outlines link lower-level elements to
higher level constraints. Together with the intruder symbols
on the map displays, they link conflict and separation on the
abstract function level to obstruction (motion), relative motion,
and traffic location on the generalized and physical function
levels. The tip of the triangle conveys intruder motion (the tip
is offset from the center of the SVE by the intruder velocity
vector), and the triangle bisector communicates intruder rel-
ative bearing, and therefore helps to link forbidden areas to
their respective intruders.
An important argument in the current study is that in order
to support operators in unforeseen situations, displays should
go beyond visualizations that relate only to the automation
logic. The interface should provide a window to the reasoning
and functioning of the automation, by visualization of the
affordances of the work domain, and by making clear how
these affordances relate to the actions and advisories of the
automation. The appropriateness of these displays for real-
world applications, however, also depends on how well the
concepts extend to complex situations, such as multiple in-
truder conflicts, complex trajectories, and of course situations
where the automation is failing. This has also been considered
for preceding concepts, and many of the properties illustrated
in those studies apply to the current concept as well [12], [16].
Van Dam, for instance, illustrated that the forbidden areas
work in a cumulative fashion [12]. Because each forbidden
area reveals absolute maneuvering constraints, imposed by
an intruder aircraft, a combination of forbidden areas from
multiple aircraft, superimposed onto each other, will represent
the set of states that would lead to a conflict with any one
(or more) of these aircraft. As a result, any state outside
of this combined constraint area is a solution to all of the
current conflicts. This property is inherent to the presentation
of constraints in an absolute velocity space.
Ellerbroek illustrated that conflicts can be solved in se-
quence, when the priority of each conflict is visualized using
color coding of each forbidden area [16]. The current concept
uses single colors for each forbidden area, where the color
corresponds to the time to loss of separation, given the current
state of ownship and intruder. A possible improvement could
make use of the fact that every point in a forbidden area
corresponds to a certain state vector, which in turn corresponds
to a certain time to loss of separation. This way, each point
in a forbidden area can be colored individually. In addition
to visualization of the priorities of conflicts given the current
state, the interface can then also reveal the viability of possible
intermediate solutions in complex traffic situations.
One of the issues with co-planar displays is that the operator
has to combine information from two displays to obtain a
complete mental picture of the situation. Although the pro-
jected forbidden areas already provide strong links between
the different elements on the displays, a crowded airspace can
still make it difficult to make these links, especially when
workload is already high. One of the techniques Woods pro-
poses to improve this, is to provide across-display perceptual
landmarks [55]. A common color, for instance, can provide a
perceptual link between items on different displays that belong
together. The priority color coding already partly fulfills this
function, but can be improved with a selection system, where
the different display elements that correspond to one intruder
are highlighted upon selection.
The current concept employs the current states to derive the
constraints imposed by other traffic. This method holds under
the assumption that ownship and intruder state remain constant
in the near future. When this is not the case, the affordance
space will change as a function of space and time due to
Trajectory Change Points (TCP), and other changes of state
or intent. Several studies have illustrated methods to visualize
intent in the forbidden areas [49]–[51]. Each of these methods
makes use of the fact that the dimension along the bisector of
the triangular forbidden area is related to the time at which
the closest point of approach with the respective intruder will
occur, with the triangle origin representing tCP A =. A
change in state at t=tT CP will therefore result in a change in
the forbidden area at the point where tC P A =tT C P . A similar
method can be used to include intent in the current concept,
by extending one of these methods to three dimensions.
Although current ATM concepts for unmanaged airspace
suggest a traffic display to be used as a situation awareness
tool for automated self-separation systems, constraint-based
displays are not limited to this level of support. Because the
displays visualize work-domain constraints and relations, they
support the pilot on multiple levels of control, from fully
automated conflict resolution, to manual pilot decision making.
The work presented in this paper is part of an ongoing study
on the design of a trajectory planning aid. The goal is to obtain
a graphical interface that supports pilots in their new task of
airborne reconfiguration of a pre-planned trajectory, in case
of traffic conflicts in unmanaged airspace. The current work
focuses on ways to visualize 3-D data on a 2-D display. A
co-planar display concept has been proposed, that is based on
the previous top-down and side-view display concepts.
There are several reasons why a combination of a Horizontal
Situation Display (HSD) and a Vertical Situation Display
(VSD) was chosen for the co-planar display concept. First,
these two displays provide the most intuitive maneuver space
projections, and support the most straightforward resolution
strategies, such as single-axis maneuvers, and combinations
of speed and heading or vertical speed. A practical factor is
also that these displays are already available on a modern flight
deck, and they correspond closest to current re-planning tasks.
In previous research, the visualization of constraints on the
display implicitly assumed that conflict geometries were flat:
the constraints shown on the horizontal interface assume zero
vertical separation and no vertical maneuvering, and the side-
view display assumes that there is no cross track separation
with intruder aircraft. When these assumptions are violated
these displays will present overly conservative constraints.
Simply combining these displays, therefore, is not sufficient
to create an effective co-planar solution. Aside from the fact
that good visual momentum demands visual cues that link
both displays together, each individual display also requires
modifications so that the presented constraints remain valid
when a conflict can no longer be defined in just one plane. A
co-planar display should reveal how individual planes interact
with each other, and provide pilots with a consistent and
intuitive view on what can be a complex, 3-D traffic situation.
The current concept, therefore, re-implements as much as
possible the strong points of the previous, single-plane dis-
plays. The triangular shapes that result from planar projection
of the 3-D traffic constraints provide strong and intuitive cues
about the conflict geometry, reveal how different elements
on the display belong together, and can help pilots keep an
overview in complex traffic situations with multiple intruders.
These projections are complemented with precisely derived
constraints, that are also valid in combined cross-track and
off-level conflict situations.
Although full, simultaneous 3-D maneuvering is still not
consistently supported, it can be argued that this is a minor
sacrifice when choosing a co-planar display over a perspective
display: Several studies indicate that pilots prefer single-axis
maneuvers [26], [33]–[36]. Also, the benefits (e.g., in terms
of efficiency) of three-axis maneuvers over two- or one-axis
maneuvers are rarely ever significant.
A possible downside of the constraint-based presentation in
this concept is that in a densely populated airspace, the state-
vector envelope can become crowded with forbidden areas,
making it less suitable (or unsuitable) as a situation awareness
tool. Although this is an undesired situation, a de-cluttering
algorithm will increase automation complexity, and reduce
transparency of actions towards the operator. This will be the
topic of a future study.
Current ATM concepts for the future of the structure of
the airspace suggest a highly optimized, and –in certain parts
of the airspace– decentralized system, with a high degree of
automation. In the decentralized parts of airspace, aircraft will
fly optimized, predetermined trajectories, where automation
will be used to resolve problems that result from uncertainties
during the flight. The concepts suggest that a display of traffic
information should be added to help the aircrew judge these
kinds of situations, and solve problems when they arise. The
current study uses a constraint-based approach to design an
interface that supports traffic situation awareness.
When used in combination with an automated system that
provides explicit resolutions, such a display should improve
operator trust and understanding of an automated resolution,
by helping him understand how constraints shape possible
resolutions. Note that this visualization is independent of the
specific implementation of conflict resolution automation. In-
stead, it visualizes work domain information, which invariably
forms the premise on which both automation and the human
operator should base their actions. This method of visualiza-
tion also provides an opportunity to create a visualization that
is consistent across different levels of automation.
This method of display design, however, also implies that
there are certain demands on the design of the automation. The
interaction between automation and the human actor requires
transparent functioning of the automated system. When a
resolution advisory cannot consistently be explained by the in-
formation on a display, for instance because it takes additional
(hidden) constraints into account, a pilot can hardly be asked
to judge the fidelity of this resolution. Consistency between
interface and automation, therefore, requires a common model
of the work domain, from which the automation derives a
resolution, and which the interface visualizes to the operator.
This consistency will be crucial for human actors to share their
cognition and decision-making with the automation.
A separation assistance display was proposed, that presents
constraints on horizontal and vertical maneuvering, in a ve-
locity action space that is overlaid on both a horizontal and
a vertical situation display. A 2-D co-planar presentation
was chosen because it offers an intuitive, undistorted and
precise view on the constraints and the traffic situation. It also
corresponds more closely to current-day flight-deck interfaces,
as well as to pilot resolution preferences.
A follow-up paper (this issue) will present a set of exper-
iments that evaluate this concept in terms of safety, perfor-
mance, and situation awareness, in manual conflict resolution
tasks. Future design iterations will focus on display clutter, in-
tent, and on the interaction with different automated resolution
[1] Radio Technical Commission for Aeronautics, “Airborne Conflict Man-
agement: Application Description V2.5,” Federal Aviation Authorities,
Tech. Rep. RTCA SC-186, 2002.
[2] SESAR Consortium, “SESAR Definition Phase D3: The ATM Target
Concept,” Eurocontrol, Tech. Rep. DLM-0612-001-02-00, 2007.
[3] D. A. Norman, “The “Problem” of Automation: Inappropriate Feedback
and Interaction, not “Over-Automation”,” Philosophical Transactions of
the Royal Society of London, vol. 327, no. 1241, pp. 585–593, Apr.
[4] N. B. Sarter and D. D. Woods, “Pilot Interaction With Cockpit Automa-
tion: Operational Experiences With the Flight Management System,The
International Journal of Aviation Psychology, vol. 2, no. 4, pp. 303–321,
[5] G. Lintern, T. Waite, and D. A. Talleur, “Functional Interface Design
for the Modern Aircraft Cockpit,The International Journal of Aviation
Psychology, vol. 9, no. 3, pp. 225–240, 1999.
[6] A. Q. V. Dao, S. Brandt, V. Battiste, K. P. Vu, T. Strybel, and W. W.
Johnson, “The Impact of Automation Assisted Aircraft Separation on
Situation Awareness,” in Human Interface and the Management of
Information. Information and Interaction. Springer, 2009, pp. 738–
[7] K. J. Vicente, “Ecological Interface Design: Progress and Challenges,
Human Factors, vol. 44, no. 1, pp. 62–78, 2002.
[8] G. A. Jamieson, “Ecological Interface Design for Petrochemical Process
Control: An Empirical Assessment,” IEEE Transactions on Systems,
Man, and Cybernetics, part A: Systems and Humans, vol. 37, no. 6,
pp. 906–920, 2007.
[9] N. Dinadis and K. J. Vicente, “Designing Functional Visualizations for
Aircraft System Status Displays,The International Journal of Aviation
Psychology, vol. 9, no. 3, pp. 241–269, 1999.
[10] C. Borst, H. C. H. Suijkerbuijk, M. Mulder, and M. M. van Paassen,
“Ecological Interface Design for Terrain Awareness,” International Jour-
nal of Aviation Psychology, vol. 16, no. 4, pp. 375–400, 2006.
[11] C. Borst, F. A. Sjer, M. Mulder, M. M. van Paassen, and J. A. Mulder,
“Ecological Approach to Support Pilot Terrain Awareness After Total
Engine Failure, Journal of Aircraft, vol. 45, no. 1, pp. 159–171, 2008.
[12] S. B. J. van Dam, M. Mulder, and M. M. van Paassen, “Ecological
Interface Design of a Tactical Airborne Separation Assistance Tool,
IEEE Transactions on Systems, Man, and Cybernetics, part A: Systems
and Humans, vol. 38, no. 6, pp. 1221–1233, 2008.
[13] F. M. Heylen, S. B. J. van Dam, M. Mulder, and M. M. van Paassen,
“Design and Evaluation of a Vertical Separation Assistance Display,”
in AIAA Guidance, Navigation, and Control Conference and Exhibit,
Honolulu (HI), 2008.
[14] A. van der Eijk, C. Borst, A. C. In ’t Veld, M. M. van Paassen, and
M. Mulder, “Assisting Air Traffic Control in Planning and Monitoring
Continuous Descent Approach Procedures,Journal of Aircraft, vol. 49,
no. 5, pp. 1376–1390, 2012.
[15] C. Borst, M. Mulder, and M. M. van Paassen, “Design and Simulator
Evaluation of an Ecological Synthetic Vision Display,Journal of
Guidance, Control and Dynamics, vol. 33, no. 5, pp. 1577–1591, 2010.
[16] J. Ellerbroek, M. Visser, S. B. J. van Dam, M. Mulder, and M. M. van
Paassen, “Design of an Airborne Three-Dimensional Separation Assis-
tance Display,” IEEE Transactions on Systems, Man, and Cybernetics,
part A: Systems and Humans, vol. 41, no. 6, pp. 863–875, 2011.
[17] K. J. Vicente and J. Rasmussen, “Ecological Interface Design: Theoreti-
cal Foundations, IEEE Transactions on Systems, Man, and Cybernetics,
vol. 22, no. 4, pp. 589–606, 1992.
[18] C. M. Burns and J. R. Hajdukiewicz, Ecological Interface Design. FL:
Boca Raton: CRC Press LLC, 2004.
[19] J. J. Gibson, “The Theory of Affordances, Perceiving, Acting and
Knowing: Toward an Ecological Psychology, pp. 67–82, 1977.
[20] ——, The Ecological Approach to Visual Perception. Houghton Mifflin,
[21] K. Christoffersen, C. N. Hunter, and K. J. Vicente, “A Longitudinal
Study of the Effects of Ecological Interface Design on Deep Knowl-
edge,” International Journal of Human-Computer Studies, vol. 48, pp.
729–762, 1998.
[22] E. J. Bass and A. R. Pritchett, “Human-Automated Judge Learning: A
Methodology for Examining Human Interaction With Information Anal-
ysis Automation, IEEE Transactions on Systems, Man, and Cybernetics,
part A: Systems and Humans, vol. 38, no. 4, pp. 759–776, 2008.
[23] M. S. T. Carpendale, D. J. Cowperthwaite, and F. D. Fracchia, “Ex-
tending Distortion Viewing from 2D to 3D,Computer Graphics and
Applications, IEEE, vol. 17, no. 4, pp. 42–51, 1997.
[24] M. St John, M. B. Cowen, H. S. Smallman, and H. M. Oonk, “The
Use of 2D and 3D Displays for Shape-Understanding versus Relative-
Position Tasks,Human Factors, vol. 43, no. 1, pp. 79–98, 2001.
[25] S. R. Ellis, M. W. McGreevy, and R. J. Hitchcock, “Perspective Traffic
Display Format and Airline Pilot Traffic Avoidance,Human Factors,
vol. 29, no. 4, pp. 371–382, 1987.
[26] A. L. Alexander, C. D. Wickens, and D. H. Merwin, “Perspective and
Coplanar Cockpit Displays of Traffic Information: Implications for Ma-
neuver Choice, Flight Safety, and Mental Workload,The International
Journal of Aviation Psychology, vol. 15, pp. 1–21, 2005.
[27] L. C. Thomas and C. D. Wickens, “Display Dimensionality, Conflict
Geometry, and Time Pressure Effects on Conflict Detection and Res-
olution Performance Using Cockpit Displays of Traffic Information,”
The International Journal of Aviation Psychology, vol. 16, no. 3, pp.
321–342, 2006.
[28] S. N. Roscoe, “Airborne Displays for Flight and Navigation,Human
Factors, vol. 10, no. 4, pp. 321–332, 1968.
[29] C. D. Wickens, “The When and How of Using 2-D and 3-D Displays
for Operational Tasks,” in Proceedings of the Human Factors and
Ergonomics Society, 2000, pp. 403–406.
[30] M. L. Bolton, E. J. Bass, and J. R. Comstock, “Spatial Awareness in
Synthetic Vision Systems: Using Spatial and Temporal Judgments to
Evaluate Texture and Field of View,” pp. 961–974, 2007.
[31] M. W. McGreevy and S. R. Ellis, “The Effect of Perspective Geom-
etry on Judged Direction in Spatial Information Instruments, Human
Factors, vol. 28, no. 4, pp. 439–456, 1986.
[32] M. L. Bolton and E. J. Bass, “Using Relative Position and Temporal
Judgments to Identify Biases in Spatial Awareness for Synthetic Vision
Systems,” The International Journal of Aviation Psychology, vol. 18,
no. 2, pp. 183–206, 2008.
[33] J. M. Hoekstra, “Designing for Safety: The Free Flight Air Traffic Man-
agement Concept,” Ph.D. dissertation, Delft University of Technology,
The Netherlands, 2001.
[34] C. D. Wickens, J. Helleberg, and X. Xu, “Pilot Maneuver Choice and
Workload in Free Flight, Human Factors and Ergonomics Society
Annual Meeting Proceedings, vol. 44, no. 2, pp. 171–188, 2002.
[35] C. L. A. Steens, S. B. J. van Dam, M. M. van Paassen, and M. Mulder,
“Comparing Situation Awareness for Two Airborne Separation Assis-
tance Interfaces,” in AIAA Guidance, Navigation and Control Conference
and Exhibit, Honolulu (HI), 2008.
[36] J. Ellerbroek, M. M. van Paassen, and M. Mulder, “Evaluation of
a Separation Assistance Display in a Multi-Actor Experiment,” IEEE
Transactions on Human-Machine Systems, submitted, 2011.
[37] J. K. Kuchar and L. C. Yang, “A Review of Conflict Detection
and Resolution Modelling Methods,IEEE Transactions on Intelligent
Transportation Systems, vol. 1, no. 4, pp. 179–189, 2000.
[38] R. Ghosh and C. J. Tomlin, “Maneuver Design for Multiple Aircraft
Conflict Resolution, in Proceedings of the American Control Confer-
ence, Chicago (IL), 2000, pp. 672–676.
[39] J. M. Hoekstra, R. N. H. W. van Gent, and R. C. J. Ruigrok, “Designing
for Safety: the Free Flight Air Traffic Management Concept,” Reliability
Engineering and System Safety, vol. 75, pp. 215–232, 2002.
[40] L. Pallottino, E. M. Feron, and A. Bicchi, “Conflict Resolution Problems
for Air Traffic Management Systems Solved With Mixed Integer Pro-
gramming,” IEEE Transactions on Intelligent Transportation Systems,
vol. 3, no. 1, pp. 3–11, Mar. 2002.
[41] D. J. Wing, R. A. Vivona, and D. A. Roscoe, “Airborne Tactical Intent-
Based Conflict Resolution Capability, in 9th AIAA Aviation, Technology,
Integration, and Operations, 2009.
[42] C. Meckiff and P. Gibbs, “PHARE Highly Interactive Problem Solver,”
Eurocontrol, Tech. Rep. 273/94, Nov. 1994.
[43] V. Battiste, W. W. Johnson, N. H. Johnson, S. Granada, and A. Q. V.
Dao, “Flight Crew Perspective on the Display of 4D Information for En
Route and Arrival Merging and Spacing,Human-Computer Interaction:
Interaction Platforms and Techniques, pp. 541–550, 2007.
[44] W. R. Knecht, “Testing a Multidimensional Nonveridical Aircraft Col-
lision Avoidance System,” Human Factors, vol. 50, no. 4, pp. 565–575,
[45] E. R. Tufte, Envisioning Information. Cheshire, CT: Graphics Press,
[46] S. K. Ojha, Flight Performance of Aircraft. AIAA Education Series,
[47] Radio Technical Commission for Aeronautics, “Final Report of the
RTCA Board of Directors’ Select Committee on Free Flight,” RTCA,
Tech. Rep., 1995.
[48] Federal Aviation Administration and Eurocontrol, “Principles of Oper-
ation for the Use of Airborne Separation Assurance Systems,” Federal
Aviation Authorities - Eurocontrol, Tech. Rep. PO-ASAS-V7.1, 2001.
[49] S. B. J. van Dam, M. Mulder, and M. M. van Paassen, “The Use of
Intent Information in an Airborne Self-Separation Assistance Display
Design,” in AIAA Guidance, Navigation, and Control Conference and
Exhibit, 2009.
[50] J. D’Engelbronner, M. Mulder, M. M. van Paassen, S. de Stigter, and
H. Huisman, “The Use of the Dynamic Solution Space to Assess
Air Traffic Controller Workload,” in AIAA Guidance, Navigation, and
Control Conference and Exhibit, 2010.
[51] G. A. Mercado-Velasco, M. Mulder, and M. M. van Paassen, “Analysis
of Air Traffic Controller Workload Reduction Based on the Solution
Space for the Merging Task,” in AIAA Guidance, Navigation, and
Control Conference and Exhibit, 2010.
[52] R. A. Paielli, “Modeling Maneuver Dynamics in Air Traffic Conflict
Resolution,Journal of Guidance Control and Dynamics, vol. 26, no. 3,
pp. 407–415, May 2003.
[53] S. B. J. van Dam, M. Mulder, and M. M. van Paassen, “Airborne
Self-Separation Display with Turn Dynamics and Intruder Intent-
Information,” in IEEE International Conference on Systems, Man and
Cybernetics. Montreal, Canada: IEEE, 2007.
[54] U.S. Department of Transportation and Federal Aviation Administration,
“Introduction to TCAS II Version 7.1,” 2011.
[55] D. D. Woods, “Visual Momentum: a Concept to Improve the Cognitive
Coupling of Person and Computer,International Journal of Human-
Computer Studies, vol. 21, pp. 229–244, 1984.
Joost Ellerbroek received the M.Sc. degree in
aerospace engineering from the Delft University of
Technology, The Netherlands, in 2007, where he
is currently working toward the Ph.D. degree. His
Ph.D. work concentrates on the design and validation
of an interface that supports interaction with airborne
separation automation. The research presented in this
paper is part of his thesis.
Koen C. R. Brantegem received the M.Sc. degree
(cum laude) from the Delft University of Technol-
ogy, The Netherlands, in 2011. He graduated within
the control and simulation section on his thesis enti-
tled “Ecological 2-D Coplanar Airborne Separation
Assurance System”. The results of his work are
incorporated in this paper. He is currently working
towards obtaining a commercial pilot license.
M. M. (Ren´
e) van Paassen received the M.Sc.
degree (1988, cum laude) from the Delft University
of Technology, The Netherlands, and a Ph.D. (1994),
on the neuromuscular system of the pilot’s arm.
He thereafter was a Brite/EuRam Research Fellow
with the University of Kassel, and a post-doc at
the Technical University of Denmark. Currently, he
is associate professor at the faculty of Aerospace
Engineering, Delft University of Technology. His
work ranges from studies of perceptual processes
and manual control to complex cognitive systems.
Max Mulder received the M.Sc. (1992) and Ph.D.
degrees (1999, cum laude) from the Delft University
of Technology, The Netherlands, for his work on
the cybernetics of tunnel-in-the-sky displays. He is
currently Full Professor and Head of the Control and
Simulation Section, Faculty of Aerospace Engineer-
ing, Delft University of Technology. His research
interests include cybernetics and its use in modeling
human perception and performance, and cognitive
systems engineering and its application in the design
of “ecological” human-machine interfaces.
... In the past decades we developed several ecological interfaces for the flight deck. Examples are a Total Energy management display for basic aircraft symmetrical flight control, that enables pilots to understand and act on exchanging their aircraft potential and kinetic energy [20], Separation Assistance displays that allow pilots to better understand and act on other traffic [21][22][23][24], an ecological Synthetic Vision display [25][26][27], and a display to manipulate fourdimensional (position and time) aircraft trajectories [28,29] We also explored various EID designs for air traffic controllers in current and future air traffic management environments [16,17,[30][31][32][33], and controllers of multiple unmanned aerial vehicles [34]. We will discuss two examples of our work in the next sections, focusing on the aircraft separation task performed by pilots in the cockpit, or by air traffic controllers on the ground. ...
... This AH has had numerous iterations, as can be seen in our publications over the years [21][22][23]. Indeed, we have been struggling with it for quite some time as, other than in process control where the abstract and generalized functions can be quickly connected to the physics of the plant being controlled [9], in this separation problem the "physics" were not clear from the beginning. ...
... Figure 4(a) shows the ecological ASAS display, in its most elementary form: a two-dimensional semicircular presentation used as an overlay on the current Navigation Display, Fig. 3 (righ-hand display). Later we also developed vertical [48], co-planar [23,24] and 3-D orthogonal [22] presentations. Figure 4(b) shows the display elements. ...
The purpose of human-machine systems design is to develop interfaces and automation tools which support human operators in performing effective, efficient and safe work. An important prerequisite for the latter is that operators understand the process under control, are aware of what is happening, and have sufficient means to act on the process appropriately. In this chapter, which is an extension of our CHIRA’2017 paper [1], we discuss the ecological approach we adopted to design human-machine systems in aviation. We focus in particular on what, in our opinion, operator situation awareness actually means, and how to improve it. The aircraft separation task will be discussed, using two examples which show how novel visualizations and tools can support pilots and air traffic controllers in their decision making.
... Previous studies in ATC [34,35,36] have evaluated the capabilities of the SSD concept for estimating metrics for airspace complexity and controller workload. Several SSD-based interfaces, in two and three dimensions, have emerged for ATC [37,38] and some have been successfully evaluated as a decision-support tool for pilots, in self-separation tasks [39,40,41]. Recently, the basic, state-based, SSD has been evaluated in studies focusing on basic ATC conflict detection and resolution (CD&R). ...
... Refs. [32,40,48] for the theoretical underpinnings of our displays. ...
Full-text available
Air traffic controller workload is considered to be a limiting factor for further air traffic growth. To reduce workload, increased automation levels and novel decision-support tools are being investigated. This Paper describes the adaptation and evaluation of a previously developed interface, called the Solution Space Diagram, in a route merging task. It portrays both constrained and unconstrained speed and heading combinations and enables the controller, by means of direct manipulation, to safely vector aircraft. The authors hypothesized that this interface enables controllers to use it in their own preferred way, supporting their skills and strategies, reducing their workload. A preliminary experiment was conducted in which 12 participants, grouped according to expertise level, controlled a sector and were faced with different levels of traffic in a route merging task. Results show that the interface aids in finding merging solutions faster; a significant reduction in the number of commands and in perceived workload was observed. The participants changed their strategy to perform less vectoring and issue route interceptions at an earlier stage, without affecting aircraft separation. These changes were also observed with the professional controllers, although they showed to be more conservative to the use of the diagram. This Paper justifies experimentation with a larger number of participants and in a setup of higher operational realism.
... The Ecological Interface Design framework (EID) might provide frugal solutions for designing visual Augmentations for helicopter pilots. EID has proved to be a means of improving levels of performance and safety in a large variety of activities (such as robot teleoperation 13 ; missile and naval missile control 14,15 ; military mission planning 16,17 ), as well as enhancing aircraft pilots' awareness of terrain 18,19 , their control of flight path and speed 20,21 , their monitoring of aircraft separation and conflict resolution 22 , helicopters' ship deck landing 23 , helicopters' obstacle avoidance 24 , and other tasks (for a review, see 25 ). ...
Full-text available
When attempting to land on a ship deck tossed by the sea, helicopter pilots must make sure that the helicopter can develop sufficient lift to be able to safely touchdown. This reminder of affordance theory led us to model and study the affordance of deck‑landing‑ability, which defines whether it is possible to land safely on a ship deck depending on the helicopter’s available lift and the ship’s deck heave movements. Two groups of participants with no piloting experience using a laptop helicopter simulator attempted to land either a low‑lifter or a heavy‑lifter helicopter on a virtual ship deck by either triggering a pre‑programmed lift serving as the descent law if it was deemed possible to land, or aborting the deck‑landing maneuver. The deck‑landing‑ability was manipulated by varying the helicopter’s initial altitude and the ship’s heave phase between trials. We designed a visual augmentation making visible the deck‑landing‑ability, and thus enabling participants to maximize the safety of their deck‑landing attempts and reduce the number of unsafe deck‑landing. The visual augmentation presented here was perceived by participants as a means of facilitating this decision‑making process. The benefits were found to have originated from the clear‑cut distinction it helped them to make between safe and unsafe deck‑landing windows and the display of the optimal time for initiating the landing
... The WDA technique has been used both in the context of developing EID-based interfaces and also for resolving interface issues. In the context of EID, WDA has been performed to improve energy efficiency monitoring Jamieson, 2007, 2014), railway driving performance (Read et al., 2021), road (Baber et al., 2019) and maritime (Van Dam et al., 2006;Morineau et al., 2009;Fay et al., 2018) traffic management, medical engineering (Kwok and Burns, 2005;McEwen et al., 2012;Li et al., 2014), aviation (Amelink et al., 2005;Borst et al., 2007;Van Dam et al., 2008;Borst et al., 2008Borst et al., , 2010Ellerbroek et al., 2011Ellerbroek et al., , 2013 and in ATC (Lodder et al., 2011;Klomp et al., 2014;Mercado Velasco et al., 2015;Beernink et al., 2015;Borst et al., 2017;Ellejmi et al., 2018) In the context of resolving interface issues, Mumaw et al. (2000a,b); Xu (2007) performed WDA and identified gaps and actions needed to resolve pilots' flight deck automation issues. ...
... The solution space diagram (SSD) is used as state data input in [25,34]. SSD was first proposed as an alternative metric to predict workload for ATCos [51], and further extended as a visual aid for possible conflict detection and resolution tasks on navigational display [52]. ...
Full-text available
Deep reinforcement learning (DRL) has been widely adopted recently for its ability to solve decision-making problems that were previously out of reach due to a combination of nonlinear and high dimensionality. In the last few years, it has spread in the field of air traffic control (ATC), particularly in conflict resolution. In this work, we conduct a detailed review of existing DRL applications for conflict resolution problems. This survey offered a comprehensive review based on segments as (1) fundamentals of conflict resolution, (2) development of DRL, and (3) various applications of DRL in conflict resolution classified according to environment, model, algorithm, and evaluating indicator. Finally, an open discussion is provided that potentially raises a range of future research directions in conflict resolution using DRL. The objective of this review is to present a guidance point for future research in a more meaningful direction.
... Hence, we follow the concept on the avoidance planes mentioned in Refs. [17,28]. Avoidance planes help transform the 3D problems into a series of 2D setups. ...
Full-text available
This work provides a generalization of the three-dimensional velocity obstacle (VO) collision avoidance strategy for nonlinear second-order underactuated systems in three-dimensional dynamic uncertain environments. A hierarchical architecture is exploited to deal with conflicting multiple subtasks, which are defined as several rotations and are parameterized by quaternions. An improved VO method considering the kinodynamic constraints of a class of fixed-wing unmanned aerial vehicles (UAV) is proposed to implement the motion planning. The position error and velocity error can be mapped onto one desired axis so that, only relying on an engine, UAVs can achieve the goal of point tracking without collision. Additionally, the performance of the closed-loop system is demonstrated through a series of simulations performed in a three-dimensional manner.
... Solution space diagram (SSD) method was first proposed as an alternative metric to predict workload for an air traffic controller [38], and further extended as a visual aid for possible airborne separation task on navigational display in the cockpit [39]. ...
Full-text available
A novel method for aircraft conflict resolution in air traffic management (ATM) using physics informed deep reinforcement learning (RL) is proposed. The motivation is to integrate prior physics understanding and model in the learning algorithm to facilitate the optimal policy searching and to present human-explainable results for display and decision-making. First, the information of intruders' quantity, speeds, heading angles, and positions are integrated into an image using the solution space diagram (SSD), which is used in the ATM for conflict detection and mitigation. The SSD serves as the prior physics knowledge from the ATM domain which is the input features for learning. A convolution neural network is used with the SSD images for the deep reinforcement learning. Next, an actor-critic network is constructed to learn conflict resolution policy. Several numerical examples are used to illustrate the proposed methodology. Both discrete and continuous RL are explored using the proposed concept of physics informed learning. A detailed comparison and discussion of the proposed algorithm and classical RL-based conflict resolution is given. The proposed approach is able to handle arbitrary number of intruders and also shows faster convergence behavior due to the encoded prior physics understanding. In addition, the learned optimal policy is also beneficial for proper display to support decision-making. Several major conclusions and future work are presented based on the current investigation.
... This research utilizes the Solution Space Diagram (SSD) as input to the neural network. The SSD was originally designed as a decision-support tool that integrates various critical parameters of the Conflict Detection and Resolution problem [33][34][35]. This was later extended to complexity and workload analysis [11,[36][37][38], automated conflict resolution [15,16,39] and even ATC training [40]. ...
Full-text available
Lack of trust and lack of acceptance caused by strategic mismatches in problem-solving have been identified as obstacles in the introduction of workload-alleviating automation in air traffic control. One possible way to overcome these obstacles is by creating automation capable of providing personalized advisories conformal to the individual controller. This paper focuses on performing an exploratory investigation into the tools and methodology required for creating conformal automation. Central in the creation of individualized prediction models is the combination of a visual feature, capturing traffic situations, and a tailored convolutional neural network model trained on individual controller data recorded from a human-in-the-loop simulation. The main advantage of using a visual feature is that it could facilitate “transparency” of the machine learning model. Results show that the trained models can reasonably predict command type, direction, and magnitude. Furthermore, a correlation is found between controller consistency and achieved prediction performance. A comparison between individual-sensitive and general models showed a benefit of individually trained models, confirming the strategy heterogeneity of the population, which is a critical assumption for personalized automation. Future research should be done in refining the model architecture, finding richer visual features that capture the breadth of human decision-making behavior and feedback model outputs back to individuals for measuring controller agreement.
... The goal of the VO method is to ensure a minimum separation between objects, for this a so-called protected zone (PZ) is defined around the ownship. 17 This method is promising as an airborne conflict resolution tool 18,19 and proved itself in autonomous mobile robot navigation 20 capable of avoiding both static and dynamic obstacles. The VO method can be applied in complex dynamic environments 21 and can take the operators intent and the vehicles dynamic restrictions in consideration. ...
Operational demands in safety‐critical systems impose a risk of failure to the operators especially during urgent situations. Operators of safety‐critical systems learn to make decisions effectively throughout extensive training programs and many years of experience. In the domain of air traffic control, expensive training with high dropout rates calls for research to enhance novices' ability to detect and resolve conflicts in the airspace. While previous researchers have mostly focused on redesigning training instructions and programs, the current paper explores possible benefits of novel visual representations to improve novices' understanding of the situations as well as their decision‐making process. We conduct an experimental evaluation study testing two ecological visual analytics interfaces, developed in a previous study, as support systems to facilitate novice decision‐making. The main contribution of this paper is threefold. First, we describe the application of an ecological interface design approach to the development of two visual analytics interfaces. Second, we perform a human‐in‐the‐loop experiment with forty‐five novices within a simplified air traffic control simulation environment. Third, by performing an expert‐novice comparison we investigate the extent to which effects of the proposed interfaces can be attributed to the subjects' expertise. The results show that the proposed ecological visual analytics interfaces improved novices' understanding of the information about conflicts as well as their problem‐solving performance. Further, the results show that the beneficial effects of the proposed interfaces were more attributable to the visual representations than the users' expertise.
Full-text available
An interface was developed to assist controllers in metering, sequencing, and merging aircraft flying a continuous-descent approach. This time space diagram displays predicted aircraft approach trajectories in a way that allows controllers a better insight into the separation between aircraft flying continuous descents. The paper discusses how a baseline time space diagram display can he enhanced with "what-if?" predictions, providing support for routine controller-spacing techniques such as speed instructions and the use of direct-to commands. An experiment with eight professional air traffic controllers compared the time space diagram to a conventional plan-view display in low- and high-traffic-rate scenarios. Use of the time space diagram display yielded a significant reduction in controller workload. It also reduced the number of instructions to the pilots, suggesting a reduced workload for the flight deck as well. Controllers' opinions on the usefulness of the what-if predictions were mixed, and these additions did not lead to any significant performance improvements.
Full-text available
Ecological Interface Design (EID) has been shown to be an effective design approach for building single-operator interfaces for complex systems. However, in such systems, operators typically are also a part of a team. An experiment was conducted to examine the effect of EID on collaboration. Teams of two operated a feed water simulator, DURESSj, through a series of scenarios given either the EID interface or the Non-EID interface. It was found that there was no penalty to using EID, as scenario completion times with the EID interface were equivalent to the Non-EID interface. Indeed, EID operators demonstrated evidence of more robust control and reported significantly lower mental workload especially when dealing with system faults.
Three different cannonical viewpoints into a 3D domain are defined to create a taxonomy of 3D displays. We then show how the information processing demands of each display viewpoint, provides benefits and or imposes costs on four categories of tasks, involving travel, image matching or situation awareness, visual search, and precise judgments. These task-display interactions are illustrated from experiments in aviation display design, battlefield judgments, and data visualization. Conclusions are offered regarding two possible ways of addressing the task-display interactions in design.
This book contains several hundred displays of complex data. Suggestions and examples are given for methods of enhancing dimensionality and density for portrayal of information. The first chapter outlines methods of moving away from single dimensional layout. Through the use of multi-dimensional images, greater clarity can be achieved and amount of information displayed increased. The following chapters cover the use of micro-macro design to illustrate detail, the layering of information and colour. The final chapter covers the display of space and time data. Examples used to illustrate the techniques covered include maps, the manuscripts of Galileo, timetables, dance movement notations, aerial photographs, electrocardiograms and computer visualisations. -R.C.Medler
Conference Paper
In advanced noise abatement procedures, the approach of an aircraft is optimized to reduce the noise on the ground. A drawback of many noise abatement procedures is that, due to the nature of the approach profile and uncertainties in the arrival time, air traffic controllers are forced to increase spacing, leading to a significant reduction of runway capacity. A display, named the Time-Space Diagram, has been developed to assist controllers in metering, sequencing and merging aircraft flying noise abatement procedures such as the Continuous Descent Approach. Initial tests showed that the display supported controllers in their task, however it was recommended that the presentation of information could be enhanced by providing support for common controller spacing techniques. The improved display was tested in an experiment, in both low and high traffic rate scenarios. Eight professional air traffic controllers used the Time-Space Diagram in addition to a conventional plan view display. Results indicate that the use of the Time-Space Diagram display results in a significant reduction of controller workload. It also led to a reduction of the number of instructions to the pilots, suggesting a reduced workload for the flight deck as well. Although controllers appreciated the changes in the display these did not lead to significant performance improvements.
3-D perspective displays are an increasingly popular choice for the depiction of real-world operational settings. Instead of depicting an environment from directly above, as conventional 2-D displays do, 3-D displays show the scene from closer to 45 degrees to the ground plane. By doing so, all three dimensions of the object or scene are visible, and integrated into a single display. Advances in computing are making 3-D displays affordable and fast enough for real-time use for operational tasks. Their naturalistic look and easy feel has led them to be very popular with users in domains such as air traffic control, flight control, and immersive training. But there are warning signs that indicate that 3-D displays do not offer a panacea. First, user preference and user performance do not always correlate well. Second, due to inherent ambiguities in imaging geometry, 3-D displays give up faithful representation of two dimensions for ambiguity (and distortion) in all three. How much of a concern these issues are remains unclear. Critical questions need answering before we should start advocating the widespread adoption of 3-D displays for operational use. When should one choose a 2-D display over a 3-D display and vise versa? How might the cognitive and perceptual demands of a task influence which display type is more useful? Should one orient to a 3-D display but then operate only on a 2-D display? How critical are the exact parameters governing the geometry of 3-D perspective views? Do 3-D displays lead to more rapid situational awareness than 2-D displays? Just how good are 3-D displays for depicting objects (their identity, attributes, and inter-relationships)? If 3-D displays are populated with realistic symbols, is it the realism or the rendering of 3-D space that enhances performance, or both? Are 3-D displays better with egocentric views but worse for exocentric views? How effective is augmentation with other depth cues at reducing line of sight ambiguity in 3-D displays? Are there trade-offs and performance costs associated with these augmentations? This symposium brings together leading experts in the field to answer these telling questions.
Conference Paper
Air traffic controller workload is considered to be an important limiting factor to the growth of air traffic. The difficulty of an air traffic control task can be analyzed through examining the problem's solution space, that is, all possible vector commands that satisfy the constraints of safety, productivity and efficiency. But apart from deriving metrics for workload, a visualization based on the solution space, resulting in the Solution Space Diagram, could help the controller in managing the air traffic. An experiment was conducted in which two different levels of traffic density were tested in order to evaluate the effects of presenting the Solution Space Diagram on controller workload. The experiment entailed the task of merging aircraft into a single route and subjects provided subjective ratings of workload at fixed intervals of time. Depending on traffic level and subject experience, significant effects of the Solution Space Diagram were found on the reduction of controller workload.
Conference Paper
Air traffic capacity is mainly bound by air traffic controller workload. In order to effectively find solutions for this problem, off-line pre-experimental workload assessment methods are desirable. In order to better understand the workload associated with air traffic control, previous research introduced the static Solution Space as a possible workload metric. The Solution Space Diagram is a mapping of intruding aircraft trajectories to the velocity/heading plane in the form of Conflict Zones and safe areas. Choosing a velocity vector in either one will provide an unsafe or a safe solution, respectively. In this paper an improved, dynamic Solution Space will be tested for correlations with air traffic controller workload, measured experimentally. A two dimensional experiment has been conducted, where subjects were required to line up all aircraft in a sector towards a certain waypoint, while continuously providing subjective workload ratings. High correlations were found between several Solution Space parameters and the subjective workload. Even though a conventional workload metric shows also to be highly correlated to the measured workload, the Solution Space could be the scenario independent workload metric that is currently missing in air traffic controller workload determination.