Formalatc 287

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Ubiquitous Computing and Communication Journal
Volume 3 Number 6 Page www.ubicc.org
DESIGN OF FORMAL AIR TRAFFIC
CONTROL SYSTEM THROUGH UML
Shafeeq Ahmad
Azad Institute of Engineering & Technology, INDIA
ahmad_shafeeq@rediffmail.com
Vipin Saxena
Dr. B. R. Ambedkar University, INDIA
vsax1@rediffmail.com
ABSTRACT
In recent years, UML has become most popular among modeling languages and is
commonly used to drive the design and implementation of system and software
architectures. UML models help to achieve functional and non-functional requirements
of system. Furthermore, UML tools have enabled the creation of source code from
UML diagrams in order to initiate the programming phase of building software.
However, due to lack of clearly defined semantics, it has been challenging to create
source code from UML models. The main objective of the paper is to model Air Traffic
Control system by the use of UML. An activity of Air Traffic Control i.e. departure
process which only covers part of the Air Traffic Control functionality has been
considered in this paper. The UML models created using formal naming semantics help
them to convert into source code and also help to achieve functional and non-functional
requirements. The complexity of Air Traffic Control System is also measured which
makes the design simple and visibly understandable.
Keywords: UML model, formal semantics, source code, Air Traffic Control.
1 INTRODUCTION
Nowadays Object Oriented software
development process is widely used in the Software
Industry. The emergence of Object-Oriented
programming has heavily contributed toward a
standardized method of modeling known as the
Unified Modeling Language (UML). In recent years,
UML has become synonym for software modeling
and is commonly used to model the software
architecture problems. Source code can be easily be
generated with the help of different UML diagrams
for building the software. To generate the correct
source code, the main problem is lack of clearly
defined semantics and code generation can only be
done if the UML specification is standard, complete,
precise, and unambiguous. The present work is based
upon the ATC system which is explained below:
1.1 ATC System
ATC system is a service that gives guidance to
aircraft, prevents collisions and manages safe &
orderly traffic flow. It is a vast network of people &
equipment that ensures safe operation of aircrafts.
The first air traffic control (ATC) system was
originally built in the 1960s; since then, air traffic
has increased immensely, and has become
increasingly more difficult to maintain safety in the
sky. As air travel has become an essential part of
modern life, the ATC system has become strained
and overworked. The ATC system has been in a
process of continuous improvement / change. In the
earliest days of aviation, few aircraft were in the
skies that there was little need for automated control
of aircraft. As the volume of air traffic increased and
the control was still fully manual; the system was
considered unsafe as human error has been cited as a
major factor in the majority of aviation accidents and
incidents.
In today’s Air Traffic Control system, air traffic
controllers are primarily responsible for maintaining
aircraft separation. Every aircraft follows several
activities during a flight. These activities are shown
below in Fig.1:
(i) Preflight -This portion of the flight starts on the
ground and includes flight checks, push-back
from the gate and taxi to the runway.

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(ii) Takeoff - The pilot powers up the aircraft and
speeds down the runway.
(iii) Departure - The plane lifts off the ground and
climbs to a cruising altitude.
(iv) En route - The aircraft travels through one or
more center airspaces and nears the destination
airport.
(v) Descent - The pilot descends and maneuvers the
aircraft to the destination airport.
(vi) Approach - The pilot aligns the aircraft with the
designated landing runway.
(vii)Landing - The aircraft lands on the designated
runway, taxis to the destination gate and parks at
the terminal.
En Route
(iii) Outdated design/technology – Obsolete software
design and programming language [11] are
major barriers to upgrades and efficient software
maintenance of the currently used ATC systems
because of which improved capacity and
efficiency can’t be achieved with the current
system. The current computer software limits the
number of aircraft that can be tracked at any
given time, and the dated architecture makes
enhancements, troubleshooting and maintenance
more difficult. Computer outages, planned or
unplanned, are covered by a backup system that
cannot handle the same level of air traffic as the
main system. The result is significantly limited
capacity during backup mode.
(iv) Mixed communication – The communication
Preflight
Departure
Takeoff
Descent
Approach
Landing
between the controllers & pilot currently is a
combination of voice & datalink. The results of
test conducted [18] show that the mixed
communication leads to slow speed which can
be overcome only when the whole
communication takes place in a well defined
manner .
2 RELATED WORK
Figure 1: Activities of Aircraft [6]
1.2 Drawbacks of Current ATC System
ATC systems are highly complex pieces of
machinery, they employ standard verification and
modeling technique to coordinate, distribute and
track aircraft as well as weather information. The
currently used systems need to employ procedures
for improved safety and efficiency which include
flexibility, potential cost savings & reduction in
staffing. This means that there is a lack of advanced
technology and desire to support the controller. Thus
there is a need to build ATC system based on a
method which can handle increased air traffic
capacity/congestion to provide a safety critical
interactive system. The following are the major
drawbacks of current ATC system:
(i) Lack of well-defined human/software interface –
The idea of full automation or minimum human
intervention of the ATC system still remains
unfulfilled. The existing systems do require
human interaction as the system only guides but
actual decision is taken by the controllers in-
charge (ground, local).
(ii) Need for high maintenance – Maintenance of the
system is also an issue which can cause problem
as discussed by Matthew L. Wald [23] about an
incidence in which voice communication
between the pilot & controllers broke down &
the reason behind this was found to be a lack of
maintenance.
The ATC system consists of controllers &
technology. The various controllers involved have
different tasks assigned to them which have been
very well described in [15], [16], [19], [20]. The
ATC real-time system [19] is characterized as
complex, time-driven and potentially distributed. It is
composed from multiple sub-systems, which must
cooperate in order to complete its real time targets.
ATC employs the approach of Parallel Applications
in which the simultaneous execution of some
combination of multiple instances of programmed
instructions and data on multiple processors are
performed in order to obtain results faster. Meilander
[4] has proposed a solution for problems arising due
to parallel processing used in ATC but its
implementation needs reliable software. The reason
for system failure discussed in [8] was found to be
lack of maintenance. For the efficient software
maintenance, [11] emphasizes changes particularly
on the technology front which means modernizing
the current air traffic control system by replacing the
software used in the current ATC systems. The study
done by Verma[18] by implementing changes in the
procedures, roles and responsibilities of the
controllers for redistribution of workload and
communications among them, has also stressed on
the introduction of new automated technologies for
the controllers.
For a critical system like ATC an extremely low
probability of failure is needed as if the system fails
any related type of hazard can occur. The reports on
incidences at non-controlled airports [5] show that

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pilots can’t entirely rely on vision to avoid collision
& also it is necessary to get all the air-traffic related
information correctly. So it is crucial that in such
large and extremely complex systems the software is
designed with a sound architecture A good
architecture not only simplifies construction of the
initial system, but even more importantly, readily
accommodates changes forced by a steady stream of
new requirements. This architectural construct can be
derived from modeling concepts by using the
powerful extensibility mechanisms of UML [1]. The
UML model-based approach helps to manage critical
systems, since the model support the necessary
analysis activities in several ways:
• The formalized structural and behavioral system
description gives the necessary basis for
criticality analysis.
• Providing behavior is expressed in simple state
charts or “English-like” pseudo code, the
behavior should at least be explainable to end-
users.
• The model gives an excellent basis for fault-
tolerance analysis, since the model includes the
dependency structure.
The size and complexity of the ATC system
demand a considerable initial development effort,
typically involving large development teams, that is
followed by an extended period of evolutionary
growth. During this time new requirements are
identified and the system is modified incrementally
to meet them. Under such circumstances an
overriding concern is the architecture of the software.
This refers to the essential structural and behavioral
framework [1] on which all other aspects of the
system depend. Any change to this foundation
necessitates complex and costly changes to
substantial parts of the system. Therefore, a well-
designed architecture is not only one that simplifies
construction of the initial system, but more
importantly, one that easily accommodates changes
forced by new system requirements.
To facilitate the design of good architecture the
domain-specific usage can be implemented using the
UML. Object-oriented requirements analysis using
modular and decomposable use cases provide to be
very powerful method for the analysis of large-scale
ATC systems [6]. It is extremely useful to define
both the static semantics (i.e., the rules for well-
formedness) and the dynamic (run-time) semantics
of the ATC system. These semantic rules fully
defined and consistent, and in conjunction with an
action specification language that is also complete
and consistent can be used to specify the details of
state-transition (Activity diagram of UML) actions,
Interaction (Sequence diagram of UML) and object
methods (class diagram of UML) and the resulting
models will be executable. Furthermore, if all the
necessary detail is included in a model, such model
can be used to automatically generate complete
implementations.
The role of software architecture is similar in
nature to the role architecture plays in building
construction. Building architects look at the building
from various viewpoints useful for civil engineers,
electricians, plumbers, carpenters and so on. This
allows the architects to see a complete picture before
construction begins. Similarly, architecture of a
software system is described as different viewpoints
of the system being built. These viewpoints are
captured in different model views. UML provides a
number of diagram types for creating models. UML
does not specify what diagrams should be created or
what they should contain, only what they can contain
and the rules for connecting the elements. Some
UML diagrams can have different uses and
interpretations. The result of this has been that
behavioral and/or run-time semantics are not well
defined for standard UML modeling elements. The
semantics problems have made it difficult to achieve
model-based programming using standard UML.
Hence semantically correct UML models [12], [17]
are needed to achieve code. Also the UML models
enhance communication as they provide a better way
to understand and communicate among analysts,
designers, developers, testers and with domain
experts for designing a system. Creating and
debugging software code has been and continues to
be a very labor-intensive and expensive process but
having an accurate UML model can ease the work as
if any problem comes and modification is required
then instead of modifying the code the models can be
modified, even the extensibility can be done by
adding new constructs to address new development
issue.
To meet the non-functional requirements [13] such
as modifiability, testability and reliability, the design
and analysis of a system at the architecture design
level must be done. Non-functional requirements
have a critical role in the development of a software
system as they can be used as selection criteria to
help designers with rational decision-making among
competing designs, which in turn affects a system’s
implementation .Thus the ATC system needs to
provide a sufficient amount of dependability and
should support a survivability architecture [10].
In this paper we examine the most important
modeling constructs used for representing the ATC
system handling departure and also describe how
they are captured and rendered using UML. The
models are based on the work presented by Saxena &
Ansari [21].The operations, phases and controllers
involved during departure of an aircraft have been
described in [2], [3], [7], [9], [10], [14], [15], [19] &

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[20] which have been used as base for designing
various UML models.
3 PROPOSED APPROACHED
The existing systems are quite complex and
inefficient. To adapt to the changing demands of
speed and efficiency a reliable software system for
ATC is required to be developed. Software
architecture based on UML models will help in
handling complexities and drawbacks of existing
ATC systems and also help to better understand the
domain. UML is the de-facto standard visual
modeling language which is a general purpose,
broadly-applicable tool supported, industry-
standardized modeling language which offers an
extensive set of diagrams for modeling. The
complexity of the problem domain requires extensive
efforts for the clarification of the initial problem
statement. Moreover, due to the extremely long
lifespan of ATC systems, stable and robust analysis
models enabling the integration of new operational
scenarios are needed which can be efficiently
obtained using UML models. In the design of a
departure activity of ATC system UML will help to
meet safety, reliability, availability, and robustness
demands in an environment of steadily increasing air
traffic density. The code obtained using the UML
models is highly optimized which is one of the main
requirements for the design of a cooperative ATC
The informal activity diagram for departure of an
aircraft is given below in Fig. 2. It simply describes
the main activities performed during departure by the
controller and the aircraft object. In this diagram
some semantic problems are present like if any
clearance has not been granted then what will be
done is not clear i.e. use of decision box hasn’t been
done. Further all the activities have been shown in a
single step and no refinements have been done to
simplify the activities. These loose semantics of the
diagram make this model unable to be executable,
therefore the formal activity diagram for departure of
an aircraft is defined and shown in Fig. 3
Aircraft Controller
Request pushback
clearance
Grant pushback
clearance
Pushback
from gate
Leave ramp
area
Request taxi
clearance
Grant taxi
clearance
system.
Taxiing
4 DESIGN OF ARCHITECTURE
System architecture is a set of design decisions.
These decisions are technical and commercial in
nature. To meet the functional and nonfunctional
requirements of the above said ATC system it is
necessary to model the complete ATC system by the
use of UML. Different types of diagrams are
Request departure
clearance
Depart
Grant departure
clearance
designed and described below in brief:
4.1 UML Activity Diagram
Activity diagram describes the workflow behavior
of a system. It illustrates the dynamic nature of a
system by modeling the flow of control from activity
to activity. An activity represents an operation of a
class, resulting in system state change.
Figure 2: Informal UML activity diagram of
departure activity of a flight
Formal activity diagram [fig.3] contains five
objects aircraft, Gate_controller, Local_ controller,
Ramp_controller & Ground_controller. It describes
various activities which are performed by all the
objects. While these activities/operations are being
done, it also shows in which state the system is like
ready for departure, taxiing etc. There is a request by
the pilot of the aircraft to the gate controller to assign

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Figure 3: Formal UML activity diagram of departure activity of a flight

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it a gate which is the initial state & the final state is
named as depart. When the next activity is performed
the system is taken to the next state that means a
transition causes the change of state. Against the
transitions various conditions are listed which
actually cause the transition. Also the use of decision
box is applied which is required to check whether the
various clearances like pushback, taxiing & final
it lacks concrete, well defined semantics which make
it impossible to help it in bringing to a computational
consistency, therefore formal use case diagram is
drawn for departure activity of a flight and shown
below in Fig. 5.
departure clearance has been granted or not. If
clearance has been granted then only the aircraft
starts pushing back or departing else if because of
some reason clearance could not be granted then it
will again go to the request clearance state where it
Ramp_controller Govern ramp area
clearance to leave ramp
<<extend>> Taxiing
gate freeing
grant pushback fromgate
assign gate
Gate_controller
will be seen & tracked until it is safe for pushback or
departure. During the whole departure activity the
possibilities of any emergency condition is also taken
sequencing at ramp
<<extend>>
enter and leave ramp
<<include>>
<<include>>
<<include>>
clearance for taxiing
Ground_controller
into consideration and shown in the diagram. Hence
the solid and well organized semantics like
pushback
<<extend>>
Taxi-plan
Tracking
<<include>>
Detect actual runway exit
Handle surface movement
conditions for transitions, use of swimlanes, fork,
<<include>>
Departure queue sequencing
join, method calls from objects i.e. able to call
methods from other objects, decision condition
specification etc. have been used in this diagram
which make this model executable.
Aircraft
clearance for takeoff
<<extend>>
<<include>>
select for departure
Local_controller
4.2 UML Use Case Diagram
The use case diagram captures system departure
DepaDrteuprareture
<<include>>
<<extend>> Maintaining safe distance
between planes
<<include>>
Information of weather, speed
and direction
Sectorization
Sequencing at holding points
functionality as seen by users. It shows an outside-in
view of the procedures available in the use of the
system i.e. all the available functionality of the
system is represented at a high level. The UML
<<include>> Monitor runway incursions
models can package the most relevant activities in
use cases and identify important actors. The UML
use case diagram for departure activity of ATC
system is shown in Fig.4.
Monitoring
Taxiing
Grant Clearances
Pushback
Figure 5: Formal UML use case diagram of
In the above model there are all five actors namely
Aircraft, Ground_controller, Ramp_controller,
Gate_controller & Local_ controller which interact
with various use cases. The use cases clearly
describe various main functions during departure and
whether they are dependent on each other directly or
indirectly denoted by extended or included keywords.Controller
Tracking & Maintaining
safe distance
Handle aircraft
movement
Provide information
Departure
Aircraft
It explains actually which functions are governed by
which actors. This diagram provides the functional
basis for creating the remainder of the diagrams
needed to arrive at an executable diagram as it has
well defined semantics like identifying every actor
involved during departure of a flight, specifying all
the functions performed by each actor and use ofAircraft sequencing
Figure 4: Informal UML use case diagram of
This diagram consists of two actors namely
controller and aircraft. The use-cases consists
represent the functionality of both actors. But this
use case diagram provides an informal viewpoint as
extend or include keyword to identify how one
function of an actor is related to another. This formal
use case diagram also drives the design of the class
diagram, and sequence diagram.
4.3 UML Class Diagram
Class diagram identifies & describes the static
structure of the system i.e. the system architecture.

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The following Fig.6 shows the informal class
diagram of the departure activity of a flight.
Controller
Aircraft
controllername
location
area
aircraftname
airlinename
departuretime
monitor()
grantclearance()
tracking()
movementhandling()
pushback()
taxi()
depart()
Figure 6: Informal UML class diagram of
This diagram gives a general view of the classes
involved during departure namely the controller and
the aircraft class. In this diagram only the name of
the attributes and the operations of the classes have
been specified. Even the attribute type or the return
type of the methods hasn't been described. These
loose semantics make this model impossible to
compile and execute. Now let us examine a formal
class diagram of departure activity of a flight shown
below in Fig.7
some distinct attributes (properties) and some
operations which are listed in the 2nd and 3rd section
respectively of the class diagram. The type of
attributes and return type of operations have been
clearly specified in the diagram. This class diagram
has covered almost all the explicitly defined
semantics like class relationships, multiplicities,
associations, properly labeled relationships, strict
naming of classes, attributes, and methods, explicit
parameters/return values which are necessary in
order to allow the model to be computationally
executable.
4.4 Sequence Diagram
A sequence diagram is an interaction diagram that
details how operations are carried out, what
messages are sent and when. Sequence diagrams
have a temporal focus so they are organized
according to time which means what happens when.
The informal sequence diagram of departure
activity of a flight is shown below in Fig 8. It simply
gives a sequence of messages between the controller
Gate_controller
gatename : Variant
gateassignment()
makegateavailable()
pushbackclearance() : Boolean
+gate clearance granting 1
Ground_controller
location : Variant
area : Integer
inactiverunwayname : String
monitoringdevice : String
holdingareas()
controlgroundtraffic()
+taxi clearance granting
controls
+assigned aircraft 0...**
Aircraft
airlinename : String
aircraftnumber : Variant
airplanetype : String
position : Variant
altitude : Integer
departuretime : Date
departureairport : String
speed : Integer
distance : Integer
route : Variant
callsign : String
+assigned aircraft
Local_controller
sector : String location
: Variant
activerunwayname : String
radarcoverage : Long
protectcriticalareas()
departurequeuesequencing()
1 controls trajeventlist : Variant
0..* latitude : Integer
0...** controls givinginformation() : Variant
1 clearance()
handleemergencies()
taxiclearance() : Boolean
+assigned aircraft
longitude : Integer
depart()
taxiing(taxi-out-plan, assigned-runway)
+departure clearance granting
selectfromqueue()
handleemergencies()
sectorization()
runwayassignment()
+assigned aircraft pushback()
0..* getdeparturetime()
assignflightcreww()
maneuvering()
delayflight(number of minutes)
monitorrunwayincursions()
holdingpointsequencing()
consists of
controls
1
+ramp clearance granting
Ramp_controller
ramparea : Long
controlrampoperations()
sequencingatramp()
aircraftservicing()
aircraftloading()
rampclearance() : Boolean
setcallsign(string callsign value)
getcallsign(string callsignvalue)
addtrajevent()
1
Clearance_delivery_controller
aircraft : String
clearancelimit : Long
departurefrequency : Integer
routeassigned : Variant
altitudeassigned : Double
routechecking()
finaldepartureclearance() : Boolean
Figure 7: Formal UML class diagram of
departure activity of a flight Figure 8: Informal UML sequence
diagram of departure activity of a flight
This class diagram of the departure activity of a
flight described in Fig. 7 focuses on the main objects
and associations (relation) between them. During
departure main objects are Gate_controller,
Ramp_controller, Ground_controller,
Local_controller and Aircraft itself. The
Local_controller in turn consists of the
Clearance_delivery_controller which gives the final
departure clearance. Also a controller called
Ramp_controller is present which is just a type of
ground controller which is responsible for operations
at ramp (parking) area. All the objects in turn have
and the aircraft objects. But this model doesn’t
include the semantics necessary for making it
executable as no parameter values are specified, no
guards have been used for any decision point, even
the class definition is not necessarily present in the
called class. Hence this model can not be compiled
and executed, therefore a formal sequence diagram is
defined & shown in Fig.9.

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Figure 9: Formal UML sequence diagram of departure activity of a flight

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In Fig.9 complete departure activity of a flight can
be seen at a glance. The messages are communicated
between the five main objects; aircraft,
Gate_controller, Ramp_controller, Local_controller
and Ground_controller. It clearly describes the
interaction between the objects and gives the order or
sequence in which the actions take place i.e. in what
order messages have been send. The solid lines
represent the call messages while the dotted lines
represent the messages returned between the objects.
The tags are added to clearly specify about the
operation. This model consists of all the clear
semantics like use of tags, explicit parameter values,
explicit return values, combined
fragments(alternatives and parallel), guards needed
for decision points and all the class definitions exist
in the called class. Thus this formal sequence
diagram with well defined and solid semantics makes
it able to be compiled and computationally
executable.
5 CONCLUDING REMARKS
From the above work it is concluded that the
described software architecture design process fully
covers the departure activity of an aircraft. UML
modeling has been used for making the various
models (class, use-case, activity & sequence) using
informal and formal semantics. These designed
models are essential part of architecture of ATC
software system and that can help to achieve
executable code and other functional and non-
functional requirements of software system.
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[21] V.Saxena, and G.A.Ansari, ”UML Models of
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