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This is the author preprint version, not including the changes arising from the review process, of the following final paper (that
should be used for reference and citation):
A Tactical Separation System for Small Air Transport Vehicles, V. Di Vito, G. Torrano, J. Beran
7th EASN 2017 International Conference on Innovation in European Aeronautics Research, 26-29 September 2017, Warsaw,
A TACTICAL SEPARATION SYSTEM FOR SMALL AIR
VITTORIO DI VITO, GIULIA TORRANO
CIRA, Italian Aerospace Research Center
Via Maiorise snc, 81043 Capua (CE), Italy
Honeywell International s.r.o.
V Parku 2325/16
Praha 148 00, Czech Republic
Small Air Transport (SAT) is emerging as the most suitable transportation means in order to allow efficient travel, in particular for
commuters, on a regional range based on the use of small airports. In this framework, the project COAST (Cost-Optimized Avionic
System), funded by the Clean Sky JU and started in the year 2016, aims to deliver key technology enablers for the affordable cockpit
and avionics, including dedicated technology for Tactical Separation decision support to the pilot. This paper provides the description
of the COAST proposed Tactical Separation System (TSS), which is an ADS-B-based advanced self-separation system aimed to extend
the traffic situational awareness and to provide the pilot with suggested manoeuvers aimed to maintain the required separation minima.
TSS will constitute an enabling technology for the implementation of the separation responsibility delegation to the flight segment
(self-separation) in the future SESAR environment. Furthermore, the proposed TSS implementation will constitute a step-forward in
the framework of the development of Airborne Separation Assurance Systems (ASAS) for Small Aircraft. The TSS is expected to
receive consolidated traffic picture (position and velocity of all tracks) from the ADS-B receiver and its own position and velocity from
the GNSS receiver, all consolidated by the Traffic Awareness System (TAS) under development in the COAST platform. Based on this
overall information, the TSS is expected to perform its main assigned functions, i.e. Conflict Detection and Conflict Resolution. In the
paper, a description is first reported of the overall TSS architecture and of its concept of operations. Based on that, an overview of each
functionality implemented in the TSS is reported in the paper, namely with reference to: Coarse Filtering, Conflict Detection, Severity
Assignment, Conflict Resolution and overall TSS Logic.
Keywords Small Air Transport (SAT); Tactical Separation System (TSS); Self-Separation; Traffic Avoidance (TrA); Situational
Awareness; Conflict Detection; Conflict Resolution; Airborne Separation Assurance Systems (ASAS).
In the last decades the concept of Small Air Transport (SAT) gained an ever increasing importance across
Europe as well as in the United States. The concept refers to the use of fixed wing aircraft with 5 to 19 seats (or
similar cargo vehicles), belonging to the EASA CS-23 category, in order to enable the transportation of people
(or goods) over a regional range based on the use of small airports.
Several benefits, in comparison with the use of larger commercial vehicles on the same routes, are associated
to the introduction of the SAT category in the Air Transport System (ATS), such as reduced fuel consumption,
reduced turnaround times, and increased economic viability (EPAST, 2007a). Considerations as the ones above
outlined have motivated the push for the research activities in the SAT domain, starting from the assessment of
the general concept and feasibility, in line with the needs expressed by the ACARE Flightpath 2050 (ACARE,
2011), targeting a goal of 4 hours door-to-door journey for the 90% of travelers in Europe.
The interest for the SAT concept has been clearly expressed in the European Union supported research
activities in the project EPATS (European Personal Air Transport System). Similarly, the SAT concept has been
addressed in the US supported research activities on the PATS (Personal Air Transport System) topic,
implemented by some NASA funded projects such as AGATE (Advanced General Aviation Transport
Experiments) (Scarpellini Metz and Bowen, 2004) and SATS (Small Aircraft Transportation System)
(Dollyhigh, 2002; Trani et al., 2003).
In particular, in the EU the EPATS project emphasized the need for developing proper innovations, based on
research and development activities, in order to fill the gap towards the implementation of the SAT concept of
operations by using specifically designed or upgraded aircraft, especially to enable the SAT aircraft single pilot
operations. In particular, in the EPATS project, among the other things, it has been emphasized that (EPATS
2007a, 2007b; Piwek, 2007), considering the expected SESAR ATM environment, for the single pilot operations
it is needed the availability of advanced self-separation and advanced collision avoidance on-board equipment,
making use of ADS-B and GNSS data. These considerations also emerged, then, from the findings of subsequent
EU projects addressing the domain of Personal Air Transport, such as the EU funded project PPlane (The
Personal Plane Project) (Di Vito et al., 2012).
In the recent years, the SAT topic has been included in the Clean Sky 2 Joint Undertaking in the European
Union’s Horizon 2020 Research and Innovation Programme and the project COAST (Cost Optimized Avionics
SysTem) has been funded under grant agreement No CS2-SYS-ITD-GAM-2004-2015-01. The project, started in
the year 2016, aims to tackle the SAT challenge and to deliver key technology enablers for the affordable cockpit
and avionics, while also enabling the single pilot operations for small aircraft (COAST Consortium, 2015). In
this project, the research and developments activities are carried out addressing some enabling technologies for
the implementation of the SAT vehicles single pilot operations (Di Vito et al., 2017) and, among these
technologies, the Tactical Separation System (TSS) is considered.
The TSS is an ADS-B-based advanced self-separation system aimed to extend traffic situational awareness
and to support the pilot decision making by automatically calculating and suggesting proper manoeuver aimed to
maintain the required separation minima, should a loss of separation be foreseen over the considered time
The development of automatic separation systems to be implemented on-board the flying vehicles is indeed a
research topic that is addressed in literature since more than a decade, even if without specific reference to SAT
vehicles but with reference to mainly RPAS (Remotely Piloted Air Systems) and manned commercial and
general aviation vehicles. The main driver for these studies is the foreseen huge increase of air traffic levels in
the next decades: according to EUROCONTROL data (EUROCONTROL, 2013), it is estimated that the traffic
levels in Europe in 2035 will be, in the most likely scenario, about 14.4 million IFR (Instrument Flight Rules)
movements in the ESRA (EUROCONTROL Statistical Reference Area), which means 1.5 times the traffic in
2012. The relevant growth of air traffic density will in turn increase the risk of occurrence of hazardous
situations among aircraft. In this framework, in the absence of relevant technological improvements in the
aircraft’s avionics equipment, the pilot workload will also consequently be incremented, so inherently leading to
further risks. These considerations especially apply to the small aircraft and general aviation domain, being
usually these aircraft less equipped than the commercial ones (which, for instance, are supported by the TCAS).
Different methods have been proposed in literature for conflict detection and resolution (Kuchar and Yang,
2000). For the most general case of conflicts involving several aircraft the resulting conflict resolution problem
becomes highly combinatorial and can be approached either through a centralized or through a decentralized
resolution strategy (Erdmann and Perez, 1987; Clark et al., 2003). Main advantage associated to the centralized
approach is that the resulting solution is both complete (all conflicts are solved) and optimized at global level
(i.e. taking into account all the involved aircraft); nevertheless, such approach usually requires that the resolution
strategy relies on non-deterministic approach, so resulting computationally intensive and not suitable for real-
time applications. On the other hand, main advantage associated to the decentralized approach is that each
involved aircraft computes in an independent way its resolution trajectory, only taking into account its specific
conflict condition. The resolution strategy in this case usually requires that the resolution problem is, first,
simplified by assigning a priority order to the possible multiple conflicts the considered vehicle is facing and,
then, the problem is solved with respect to the conflict (or conflicts) with highest associated priority. Such a
strategy, therefore, leads to resulting resolution trajectories that are not aimed to constitute a global optimum for
the whole conflict resolution problem, but are designed only with reference to own vehicle. This is of course the
main disadvantage associated with the application of the decentralized approach and an additional drawback can
be that usually such approach is incomplete, because it may fail to find a solution even though it exists. On the
other hand, the main advantage of the decentralized approach relies on the relevant simplification of the problem
to be solved, which in turn leads to the reduction of the associated computational burden and to the possibility of
More in details, geometric conflict detection and resolution strategies are quite effective and popular. An
interesting example of geometric approach is reported in the work by Albaker and Rahim (2005), where the
authors propose a geometrical intersection method for estimation of collision risk, based on the application of a
cylindrical protected zone on all vehicles (considered as point-of-mass) and on the check of protected zones
overlapping. The resolution maneuver is selected among some sets of only horizontal or only vertical
manoeuvers, in such a way to minimize the deviation from the original path. The method has been numerically
assessed and a linear increase of the computation time with the number of involved vehicles has been
Another example of geometric approach can be found in the work by Park et al. (2009), where the authors
consider a conflict situation involving two unmanned vehicles (considered as point-of-mass), during the en-route
phase, assuming that they are able to share navigation information through ADS-B. The conflict detection
criterion is based on the application of a cylindrical protected zone that is checked for violation by calculating
the minimum expected reciprocal distance between the vehicles. For the resolution, the authors propose the
application of the “vector sharing resolution” strategy, based on the increase of the miss distance by modifying
the speed vector reference of each involved aircraft.
In the framework of the research activities addressing conflict detection and resolution strategies, the Italian
Aerospace Research Center (CIRA) devoted significant effort in the last decade, leading to the development of
both Collision Avoidance (Luongo et al., 2010, 2011; Orefice et al., 2014, 2015a) and Self-Separation (Luongo
et al., 2012; Di Vito et al., 2013) systems as well as to the development of an integrated system implementing
and harmonizing both the functionalities (Filippone et al., 2015). These systems reached the Technology
Readiness Level (TRL) 6 by means of proper successful flight test campaigns that have been carried out in
different Italian and international projects led by CIRA (Fasano et al., 2016; Filippone et al., 2015).
Based on this relevant background, gained through applications mainly on RPAS vehicles, CIRA will
develop in the COAST project a specific Tactical Separation System for the implementation onboard of the
Small Air Transport vehicles. This system will be able to support the pilot in real-time in order to ease his/her
tasks in presence of congested traffic scenarios. The proposed system will be able to act in real-time during the
flight in order to: detect possible conflict situations involving own vehicle and surrounding traffic, compute a
safe maneuver to maintain separation with other traffic and, finally, propose the manoeuver to the pilot. This will
allow reducing the pilot’s decision-making process related workload in performing the self-separation task,
while at the same time enhancing the flight safety level.
In this paper, an outline will be provided of the TSS, starting from the overall description of the project
COAST and of the related approach to the SAT cockpit technologies development (section 2). In section 3, then,
the TSS architecture will be outlined and the concept of operations of the system will be described. In section 0,
finally, an overview will be provided of the main functionalities that will be implemented in the TSS.
This paper represents an introduction to the TSS design activities that are carried out by CIRA in the
framework of the COAST project, started since only one year, so it paves the way for future papers where the
description of the evolution of the design activities and of the achieved technical results will be provided.
2. COAST project description
The COAST consortium includes four institutions, Honeywell International (as leader), Italian Aerospace
Research Center (CIRA), Institute of Aviation (ILOT), and Rzeszów University of Technology, each one
bringing in the partnership its proper background of knowledge.
In the previous section 1, it has been already emphasized that, based on the findings of the EPATS project
(EPATS 2007a, 2007b; Piwek, 2007), some enabling technologies for the SAT concept implementation can be
identified. Indeed, a quite big set of technologies, procedures and capabilities can be useful to support the SAT
paradigm implementation, including: on-board systems for self-separation, on-board systems for collision
avoidance, on-board systems for traffic awareness, VTOL, STOL, CDA, GNSS, Performance Based Navigation
(PBN), ADS-B surveillance, automatic emergency landing system, and others.
Many of these technologies are already studied and applied in the aeronautical field, even if not necessarily
in the SAT framework, and the related competences are well represented in the COAST consortium. A cross-
fertilization from the commercial vehicles and from the RPAS (Remotely Piloted Air Systems) domains can be
exploited for the SAT technologies development, as these domains are very well supported by the consortium
members. For instance, Continuous Descent Approach trajectories are under study for commercial vehicles
applications (Errico et al., 2016; Errico and Di Vito, 2017a, 2017b; Errico et al., 2017), automatic landing
systems are addressed by of R&D activities in the RPAS framework since long time (Di Vito et al., 2007; De
Lellis et al., 2011, 2012), as well as Self-Separation (Filippone et al., 2015; Orefice et al., 2015b; Di Vito et al.,
2013; Luongo et al., 2012) and Collision Avoidance Systems (Luongo et al., 2010, 2011; Fasano et al., 2016),
ADS-B applications (Orefice et al., 2014, 2015a) are a constantly growing research filed and also automatic
trajectory generation for Unmanned Aerial Vehicles (UAVs) navigation (Platts et al., 2007; De Lellis et al.,
2013; Morani et al., 2013, Di Vito et al., 2009) is a common topic that can be useful in the SAT domain.
Nevertheless, a selection of the most relevant technologies to be addressed in the COAST project has been
performed, in order to make possible the technological development according to the allocated timeframe and
budget. In particular, the COAST project addresses the development of the following technologies:
Traffic Awareness System (TAS), which integrates transponder and ADS-B transceivers, considers
incorporation of the future ACAS-X system, and reduces the maintenance cost by performance monitoring.
Tactical Separation System (TSS), which is an ADS-B-based advanced self-separation system to extend
traffic situational awareness and to provide the pilot with suggested manoeuvers aimed to maintain the
required separation minima.
Flight Reconfiguration System (FRS), which is an emergency flight path management system in case of
Advanced Weather Awareness System (AWAS), which provides complete awareness of weather situation
with (both observed and forecasted) information assisting the pilot in avoiding entry into atmospherically
Scalable GNSS Receiver (GNSS), which provides improved accuracy, integrity, and reliability by
introducing the Dual-Frequency and Multiple Constellation (DFMC) GNSS receiver and by using new HW
and SW paradigms.
Compact Computing Platform (CCP), which is a scalable, reusable, and reliable platform featuring compact
HW design, innovated SW architecture and enabling simple customization for different aircraft platforms.
High-Integrity Electronics (HIE), which aims enabling smart actuators and sensors, health monitoring and
prognostics resulting in the reduced operational costs and reducing the aircraft integration complexity.
A more comprehensive description of the COAST project technologies is provided in the reference paper by
Di Vito et al. (2017).
In the following of this paper, more details are provided about the TSS technology under study and
development in the COAST project by CIRA, which is specifically dedicated to support the situational
awareness and the decision making of the pilot in managing the self-separation task.
This system is included in the overall COAST proposed concept of the avionics and cockpit architecture can
be summarized as depicted in the following Fig. 1. It acknowledges three layers of technology elements/systems:
the cockpit interaction layer, which represents the displays with the user interface and control panels and
will consider cockpit multimodality and integration of Electronic Flight Bags (EFBs);
the cockpit functions layer, which represents the applications (algorithms and logic) required for
airworthiness of the aircraft;
the avionics platform, which represents the avionics HW and SW infrastructure required for the cockpit
Fig. 1. Overall concept of the COAST SAT avionics and cockpit (COAST Consortium, 2015)
The overall architecture integrated in the cockpit framework is represented in the following Fig. 2.
Fig. 2. COAST system architecture of the SAT avionics and cockpit (COAST Consortium, 2015)
As indicated in the figures above and as better described in the following section 3, the TSS is included in
the cockpit functions layer and in the cockpit interaction layer. In the cockpit functions layer it is implemented,
hosted by the Compact Computing Platform, the TSS core software devoted to the processing of the input data in
order to elaborate the situational awareness and the suggested separation maneuver to support the pilot decision-
making. The consolidated traffic picture and the suggested separation manoeuver are provided, then, to the pilot
through a proper page on the portable electronic device (tablet) hosted in the cockpit, so the TSS includes also a
small software piece in the cockpit interaction layer aimed to elaborate information provided by the core TSS
software in order to properly display them to the pilot. The TSS, finally, receives the data provided by mainly the
GNSS and the ADS-B IN, which are parts of the avionics platform.
3. TSS architecture and concept of operations
The Tactical Separation System is aimed to extend the TAS function by aiding the pilot with tactical separation
management. It constitutes an enabling technology for implementation of the separation responsibility delegation
to the flight segment (Self-Separation) in the future SESAR environment (SESAR Consortium, 2007) and its
implementation represents a step-forward in the framework of the development of Airborne Separation
Assurance Systems (ASAS) (FAA/EUROCONTROL, 2001; EUROCONTROL, 2005) for Small Aircraft.
The major TSS functions are the ones of Conflict Detection, Conflict Prioritization, and Conflict Resolution,
properly supported by dedicated overall TSS Logic. The conceptual scheme of the TSS is depicted in the
following Fig. 3.
Fig. 3. COAST Tactical Separation System (TSS) high-level architecture (COAST Consortium, 2015)
As represented in the previous figure, the TSS receives consolidated traffic picture (position and velocity of
all tracks) from the ADS-B receiver and its own position and velocity from the GNSS receiver, all consolidated
by the TAS platform. The Conflict Detection module checks each track’s projected trajectory with respect to its
own projected trajectory, in order to assess if potential violations of separation specified volumes exist. Based on
this check, all the detected tracks are classified and the most dangerous identified by the Severity Assignment
module. The information concerning the most dangerous track representing a loss of separation risk is then sent
to the Conflict Resolution algorithm, which is activated by the TSS Logic module in order to elaborate suitable
manoeuver to be executed to restore the safe separation minima.
This classification of the detected tracks and the suggested separation manoeuver is sent to the multi-function
display (MFD), or to a proper portable device (tablet), to support the pilot’s decision-making and, concurrently,
an alert is provided using aural feedback to the pilot. In the Fig. 3 it is also represented a future improvement of
the TSS, improvement that indeed is out of the scope of the COAST project, involving the communication of the
TSS status in broadcast to all surrounding vehicles, by adding proper information to the ADS-B Out message.
The suggested manoeuver to restore the separation is subject to a predefined strategy: the TSS provides
purely horizontal or purely vertical or purely speed magnitude manoeuver or their combination up to a full 4D
manoeuver. The system is designed to consider further requirements, such as compliance with the Rules of the
Air, optimization of fuel consumption, etc. In particular, the TSS design process properly takes into account the
Coordination: the TSS is expected to act at tactical separation level, so addressing a time horizon that is
longer than the emergency time horizon, considered by the collision detection and resolution systems
implemented on-board (TSAA and ACAS). Hence, the TSS is expected to prevent the activation of such
Compliance: the TSS is expected to be compatible with the TCAS/ACAS devices on-board surrounding
traffic vehicles. In normal operations, the TSS is expected to prevent the TCAS/ACAS activation in the
surrounding traffic, due to its action at tactical level.
Self-compatibility: the TSS is expected to assure that TSS equipment on-board different vehicles provide
For what concerns the TSS concept of operations, this system assists the pilot, particularly by formulating an
adapted solution to resolve the predicted conflict situation. The system is expected to propose correction
trajectory to the pilot to aid him/her to remain away from self-separation minima, while also complying with the
Rules Of the Air. The separation assurance functionality, therefore, is here implemented as the tactical capability
of keeping the aircraft away from other airborne aircraft by at least predefined separation minima. As better
described in the following section 4.2, the separation minima can be defined starting from their definition
provided by EUROCONTROL with reference to RPAS applications: “Where an RPA pilot is responsible for
separation, he should, except for aerodrome operations, maintain a minimum distance of 0.5 NM horizontally or
500 ft vertically between his RPA and other airspace users, regardless of how the conflicting traffic was detected
and irrespective of whether or not he was prompted by a Sense & Avoid system.” (EURCONTROL, 2007).
Starting from this specification, the separation assurance function is expected to be capable of assuring
separation provision with respect to the surrounding vehicles when the minimum distance between the aircraft is
predicted to be smaller than the assigned separation minima over the considered time horizon. As a result, the
TSS is expected to provide the pilot with traffic information within a sufficient timeframe, i.e. at tactical level, so
also allowing to prevent the activation of an emergency collision avoidance manoeuvre (ACAS resolution
More in details, the TSS is expected to provide the pilot with a clear traffic picture including all the relevant
information about the surrounding traffic aircraft, such as positions, altitudes, speeds, headings, for his
situational awareness and separation duty. Furthermore, the TSS will support the pilot with information on the
“severity level” related to the surrounding traffic (i.e. the level at which each surrounding track is considered
dangerous for the ownship, according to proper criteria implemented in the TSS) and, if needed, the TSS will
suggest to the pilot a suitable separation maneuver.
For what concerns the provision of advanced situational awareness about the surrounding traffic to the pilot,
graphical representation means will be used in the TSS HMI in order to provide the information about the alert
state related to each surrounding aircraft, by means of traffic symbol modifications in terms of shape and color.
For instance, different colors can be associated to the severity level of the surrounding aircraft while proper
symbol shape modification can be associated to a vehicle for which a separation assurance manoeuver has been
The separation manoeuver, then, is suggested to the pilot in an immediate and intuitive way on the TSS
interface. For instance, if a velocity change is needed, an appropriate indication of this change will be added to
the ownship symbol and a numerical indication of the final velocity to be reached will be provided; similar
considerations apply for suggested track and altitude change or for a combination of them.
As a future development, it could be considered the implementation of the possibility for the pilot of
customizing the TSS display on the portable device (tablet) hosted in the cockpit. For instance, it would be
useful to implement the possibility for the pilot of modifying the display range/map scale and/or the possibility
of zooming. Furthermore, it would be useful the possibility of selecting the surrounding airspace volume to be
considered, in terms of a selectable altitude band and of a selectable range with respect to the ownship position.
Such functionalities have been implemented in previous work carried out by CIRA in the project RAID
shown in the following Fig. 4 where the RAID HMI developed by CIRA is reported. This HMI developed by
CIRA for the RAID project was designed to support the integrated system for collision avoidance and separation
assurance, designed by CIRA and implemented on the CIRA dedicated RPAS experimental vehicle, that has
been validated in the RAID project through real-time simulations (Filippone et al., 2015) and real flight test
Fig. 4. The HMI developed by CIRA in the project RAID (Filippone et al., 2015)
Nevertheless, in the COAST project the TSS implements, as already well known, only tactical separation and
does not consider collision avoidance; furthermore, the system is designed to be applied on a manned CS-23
SAT vehicle instead of an RPAS. Therefore, in the COAST prototypal implementation of the TSS HMI, the
above-indicated possibilities of display customization will not be included, so they are out of the scope of the
COAST project, and the TSS HMI that CIRA is going to develop will be a strongly simplified version with
respect to the one developed for the RAID project.
4. TSS functionalities overview
The analysis of the overall TSS architecture represented in the previous Fig. 3 emphasizes that the TSS receives,
based on the information provided by the on-board ADS-B In surveillance sensor, traffic position and velocity
data about surrounding ADS-B Out equipped aircraft. The TSS is designed to operate in the context of a DO-
317B Surveillance Processing application (RTCA, 2014), aimed to perform the processing of the raw data
provided by the ADS-B In surveillance system, in order to allow their use by the TSS. The TSS receives also its
own-ship position and velocity data from the Global Navigation Satellite System (GNSS) receiver. These data
are elaborated to derive a consolidated traffic picture, to detect possible loss of separation risks over the future
considered time horizon and to elaborate, in this case, a proper separation maneuver that is suggested to the
pilot, as described in the previous section 3.
In order to perform the above outlined tasks, as represented in the conceptual architecture reported in Fig. 3
as well as in the Simulink TSS SW model that CIRA is designing in the COAST project, represented in the
following Fig. 5, some main functionalities can be identified as constituting the overall Tactical Separation
Fig. 5. TSS SW architecture implemented by CIRA in Simulink environment
In particular, the main TSS functionalities are the ones listed in the following:
Traffic Prioritization (indicated in Fig. 3 as “Severity Assignment”),
A description of each functionality is reported in the following subsections, in order to show the approach
that is applied by CIRA in the design of the Tactical Separation System.
4.1. Coarse Filtering
In order to provide a pre-selection of the traffic, due to the ADS-B In equipment capability of detecting traffic
located very far that does not need immediate consideration by the TSS, a suitable coarse filtering function is
needed, able to select the surrounding aircraft to be then processed by conflict detection functionality. This
allows reducing the computational burden.
The basic principle that may be preliminary proposed for the coarse filtering consists in excluding from the
conflict detection the traffic vehicles whose distance from the ownship is greater than a specified threshold,
suitably tuned during the project activities in order to take into account for the tactical time horizon of the TSS
4.2. Conflict Detection
Once received traffic information about surrounding aircraft resulting from the coarse filtering processing, the
conflict detection function will check potential conflicts between ownship and each surrounding aircraft,
determining if, over a specified time horizon, their future positions could experience a separation loss.
For the separation assurance functionality a suitable safety volume, also referred as “protected zone”, needs
to be defined. It represents a non-intrusion zone, around the considered traffic aircraft, that the TSS has to assure
not to be breached over the considered time horizon. According to the separation minima defined by
EUROCONTROL for military RPAS integration into the civil airspace (as indicated in section 3), the baseline
TSS avoidance zone shall be a cylindrical volume of airspace, centred on the aircraft, with a horizontal radius of
0.5 NM and a vertical height of 500 ft, as represented in the following Fig. 6.
Fig. 6. TSS baseline separation volume (EUROCONTROL, 2007)
The separation volume baseline dimensions above indicated, then, need to be increased in order to consider
that the above-indicated values are the ones set by EUROCONTROL for the inclusion of the RPAS in the civil
airspace but in the COAST project it is not considered an RPAS vehicle but a manned vehicle. In particular, in
the COAST project the TSS addresses a future application scenario that is the SESAR one in presence of the
delegation of the self-separation responsibility to the flight segment. The related requirements, therefore, will be
developed and suitably updated, if needed, during the project execution, based on the evolution of the
international standards and on the outcomes of projects supporting the standardization of self-separation (for
instance, in Europe the EDA funded project MIDCAS).
This implies that the nominal separation volume considered by the TSS in the COAST project will be
suitably greater than the baseline one indicated in Fig. 6 and will be properly set in order to take into account the
application on manned vehicle flying into a managed airspace.
Furthermore, an additional extra-size contribution need to be properly considered in order to take into
account the uncertainties affecting the sensors measurements and the aircraft manoeuvring dynamic. These
aspects will be considered by implementing an incremental sizing factor that increases separation volume
dimensions proportionally to the closure rate between the conflicting aircraft.
The resulting TSS protected airspace zone, therefore, obtained as sum of the nominal size and of the closure
rate dependent calculated extra-size, will be variable over the considered TSS time horizon as a function of the
closure rate variations.
Based on the considered protected airspace zone, the conflict detection is performed, consisting in checking
if, over the future assigned time horizon, the separation volume surrounding the traffic aircraft is predicted to be
penetrated by the ownship propagated trajectory.
The baseline trajectory propagation method is the linear extrapolation, over a specified time horizon, of the
trajectory starting from the actual inertial velocity vector of the aircraft, for both the ownship and the considered
surrounding aircraft (Hoekstra et al., 2002; Bilimoria, 2000; Dowek et al., 2001; Di Vito et al., 2013; Luongo et
al., 2010, 2011, 2012; Orefice et al., 2014). The possibility of storing past state vectors to detect traffic turns
without any intent information may be considered (Butler and Lewis, 2013) in order to improve state-based
The conflict detection check is performed by conducting pairwise comparisons between the ownship
propagated trajectory and each traffic aircraft propagated trajectory, in order to determine whether any of them
pose a possible separation loss with the ownship. The approach considers a cylindrical body in 3-D space
surrounding each aircraft, such that a conflict is equivalent to an intrusion of another airplane into its protected
zone: the 3-D conflict detection problem is then defined as the ownship trajectory intersection with an aircraft
protected zone (Dowek et al., 2001; Di Vito et al., 2013; Luongo et al., 2010, 2011, 2012; Orefice et al., 2014,
2015a, 2015b; Fasano et al., 2016).
The conflict detection TSS functionality is designed by CIRA in COAST based on its background
technological solutions, starting from the algorithms that CIRA developed in previous projects (Di Vito et al.,
2013; Filippone et al., 2015; Luongo et al., 2010, 2011, 2012; Orefice et al., 2014, 2015a; Fasano et al., 2016),
of course under proper modifications due to the specific COAST project application.
Once the conflict search has been performed, each traffic aircraft is evaluated about the predicted violation of
the assigned separation volume: if no violation has been predicted, the aircraft is not considered as a dangerous
one, so it is tagged as traffic aircraft. If a violation of the separation volume has been predicted for the
considered aircraft, it is tagged as conflicting aircraft.
4.3. Traffic Prioritization
The traffic prioritization functionality, also indicated as “severity assignment”, receives the data processed by the
conflict detection module in order to classify all the tracks. The classification can be provided in terms of
different severity levels, indicating:
the conflict status, i.e. if the considered track is predicted to pose a loss of separation condition with respect
to ownship (such tracks are classified as “conflicts”);
the normal traffic status, i.e. the considered track is predicted to not pose risks to ownship (such tracks will
be classified as “traffic”);
the information about ACAS (TCAS) behavior, provided through the TAS platform.
Furthermore, due to the circumstance that, based on the above-described checks performed with reference to
each vehicle resulting from coarse filtering, multiple conflicts and multiple threats may be detected. Therefore,
the Traffic Prioritization module will implement a proper prioritization criterion in order to individuate the most
The adopted criterion for that sorting can be based on the consideration of the time-to-go (ttg) measure
(Orefice et al., 2014), as considered in current state-of-the-art approaches (Munoz et al., 2013) and as can be
derived by simplification of usual ACAS tau parameter evaluation (ICAO, 2006).
The severity assignment functionality represents the basis for the visualization on the TSS dedicated HMI of
all the relevant information related to the surrounding traffic, collected and elaborated by the Conflict Detection
functionality and classified according to the associated severity level of risk, in order to individuate and
differentiate all the surrounding aircraft.
4.4. TSS logic
The overall situational awareness information elaborated by the Traffic Prioritization module is processed by the
TSS Logic module, in order to establish which the action to be taken by the system is. More in details, the logic
module is aimed to process the information about the vehicles in order to verify if the activation of the Conflict
Resolution module is needed and to identify which is the vehicle to be addressed for the elaboration of the self-
It is fundamental to emphasize here that the Tactical Separation System is primarily intended to provide the
elaboration of the resolution manoeuver with respect to only the most dangerous conflict (i.e. the most dangerous
vehicle posing loss of separation risk with respect to ownship), so preventing the insurgence of emergency
collision conditions in the traffic picture. Indeed, in order to become a threat (i.e. a vehicle posing collision risk),
a vehicle needs prior to be a conflict (i.e. a vehicle for which potential loss of separation is detected). Therefore,
by supporting the pilot in restoring the safe separation minima with respect to the conflicts, the TSS prevents that
they become threats.
In the cases where for various possible reasons (pilot not able to implement the suggested separation
manoeuver, suggested manoeuver not suitable, incorrect input data to the TSS, …) a threat emerges, the
responsible for collision avoidance will be always the pilot, supported by dedicated ACAS such as the TCAS if
4.5. Conflict Resolution
Based on the received activation by the TSS logic and indication of the vehicle (if any) with respect to which a
self-separation maneuver is needed, the Conflict Resolution function processes the corresponding conflicting
vehicle data in order to elaborate a suitable maneuver aimed to restore the separation minima. The maneuver is
elaborated in terms of suggested changes in track and/or altitude and/or speed of the ownship able to allow
avoiding the infringement of the suitable safe separation volume set around the considered conflicting vehicle.
It is worth to emphasize here that the proposed Tactical Separation System provides the elaboration of a self-
separation manoeuver with respect to the most dangerous conflicting vehicle, selected by the TSS logic based on
the data provided by the Traffic Prioritization function. Nevertheless, in the project evolution it will be also
considered the possibility to develop and implement suitable multi-conflict resolution algorithm able to elaborate
safe separation restoration manoeuver with respect to all the conflicting vehicles and not only to the most
The resolution manoeuvre is aimed to:
resolve the conflict condition by proper modification of the own-ship trajectory in order to restore the
suitable separation margin with respect to the considered conflicting vehicle;
comply with the Rules of the Air;
consider the compatibility with ACAS;
consider the compatibility with other TSS-equipped aircraft;
ensure, if possible, the optimization of the fuel consumption and the minimum deviation from the original
avoid the insurgence of new conflicts with the surrounding traffic aircraft.
For what concerns the compatibility with ACAS, this is obtained based on information about ACAS
behaviour provided through the TAS platform. For what concerns the compatibility with other TSS-equipped
aircraft, then, this is obtained without the need of manoeuver coordination with the conflicting aircraft, because
the compliance with the Rules of the Air in the calculation of the separation manoeuver implicitly guarantees the
self-compatibility among TSS equipped vehicles. The coordination of the separation manoeuver with the
conflicting aircraft is not needed here, so the TSS implements not cooperative method for separation manoeuvre
elaboration, due to the circumstances indicated in the following:
the compliance of the calculated manouver with the Rules of the Air;
the real-time computation of the manoeuver, meaning that it is continuously updated based on the current
conflict geometry and, therefore, the system continuously checks if the suggested manoeuvre is still
applicable or its update is needed (in such a case, the update is automatically performed).
Once the conflict resolution manoeuvre elaboration has been performed, the new trajectory is suggested to
the pilot in terms of one or more of the following indications:
vertical speed or altitude change.
These indications can lead to a purely horizontal or purely vertical or purely speed magnitude manoeuver or
their combination up to a full 4D manoeuvre (Bilimoria, 2000; Dowek et al., 2001; Di Vito et al., 2013;
Filippone et al., 2015; Luongo et al., 2012)
After its application by the pilot, the separation assurance manoeuvre is considered completed when the
condition of clear of conflict is satisfied, i.e. when:
no infringement of the considered separation volume is still predicted,
AND ownship distance with respect to the considered conflicting vehicle is increasing.
The conflict resolution TSS functionality is designed by CIRA in COAST based on its background
technological solutions, starting from the algorithms that CIRA developed in previous projects (Di Vito et al.,
2013; Filippone et al., 2015; Luongo et al., 2012), of course under proper modifications due to the specific
COAST project application.
In the paper, the CIRA approach to the design of the Tactical Separation System (TSS) in the COAST project
has been presented. This system is aimed to implement a self-separation enabling technology to support single
pilot operations for the SAT vehicles. As indicated in the paper, some technologies have been considered in
COAST as outstanding for the implementation of single pilot operations in SAT vehicles. The TSS, in particular,
is aimed to provide the pilot with increased situational awareness and with proper decision-making support, also
in compliance with the expected pilot responsibilities in the future SESAR environment.
From the TSS conceptual description reported in the paper, it is possible to derive that relevant features
characterize this technology. In particular, the innovation introduced by the Tactical Separation System is first
represented by the extension of situational awareness and collision avoidance by tactical separation management
including resolution manoeuvers. Furthermore, TSS delivers functionality assuring compatibility with ACAS,
which is planned by SESAR Phase 2 or Phase 3. Further relevant features are self-compatibility (TSS-TSS) and
real-time update of the suggested manoeuver in case of sudden change in conflict geometry. Manoeuvers are
compliant with the Rules of the Air and are optimized to minimize deviations from the original trajectory, while
at the same time reducing, if possible, the fuel consumption. Future improvement of the TSS, which is out of the
scope of the COAST project, involves the communication of the TSS status in broadcast to all surrounding
vehicles, by adding proper information to the ADS-B Out message.
Furthermore, the introduction of the tactical separation support to the pilot by implementing the TSS
represents a relevant advance for commercial products, considering that currently no commercial technology is
available providing full tactical separation management functionality (only collision avoidance systems are
commercially available). The advent of SESAR rules and the potential delegation of traffic separation
responsibility to pilots will lead to an increase of their stress and workload; TSS represents a solution to this
workload increase. In summary, the TSS will represent an affordable enabler for single-pilot operations and will
be aligned with the SESAR introduced concept of delegation of the separation responsibility to the flight
Future papers will describe the evolution of the design of this technology and the achieved results.
This work has been supported by the project COAST (Cost Optimized Avionics SysTem), funded by the Clean
Sky 2 Joint Undertaking in the European Union’s Horizon 2020 research and innovation programme, under grant
agreement No CS2-SYS-ITD-GAM-2004-2015-01.
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