Aerospace and Electronic Systems Magazine September 2016 - 18

Feature Article:

DOI. No. 10.1109/MAES.2016.150129

Onboard Visual Sense and Avoid System for Small
Aircraft
Tamás Zsedrovits, Pazmany Peter Catholic University, Budapest, Hungary
Péter Bauer, Borbála Jani Mátyásné Pencz, Antal Hiba, István Go˝zse, Máté Kisantal,
Máté Németh, Zoltán Nagy, Bálint Vanek, Ákos Zarándy, József Bokor, Hungarian
Academy of Sciences, Budapest, Hungary

INTRODUCTION
The sense and avoid (SAA) capability is a key issue in the integration of Unmanned Aircraft Systems (UAS) into the common
airspace [1]. Besides the lack of a regulatory framework in most
countries, some technological challenges are still not resolved
which holds back routine UAS applications. Most UAS or Remotely Piloted Aircraft Systems (RPAS) have no SAA capability.
The separation for these aircrafts must be done solely by the Air
Traffic Control (ATC) which is not cost effective. Furthermore,
in the case of a lost link, there should be some kind of collision
avoidance capability onboard; otherwise, if we want to maintain
the current safety level of the air traffic system, a single unmanned
airplane would block a significant sector of the airspace.
Besides the safety and cost issues in the RPAS case, general
aviation could benefit from a solid SAA system as well. An SAA
instrument could support pilots in the case of conflicting instructions coming from the different layers of collision avoidance (for
example ATC and onboard devices) and in some cases it could extend the range for the non-cooperative collision avoidance done by
the pilots [2].
An ideal SAA system would be cost effective, lightweight, and
low-power in order to minimize the negative effect on cost, make
application onboard small unmanned aerial vehicles (UAVs) possible, and maximize flight time of the aircraft.

Authors' current addresses: T. Zsedrovits, Pazmany Peter
Catholic University, Faculty of Information Technology and Bionics, Prater utca 50/a, Budapest, 1083 Hungary; P. Bauer, B.J.
M. Pencz, A. Hiba, I. Go˝zse, M. Kisantal, B. Vanek, A. Zarándy,
J. Bokor, Hungarian Academy of Sciences, Systems and Control Laboratory, Institute for Computer Science and Control,
1111 Budapest, Kende u. 13-17., 1083 Hungary; M. Németh, Z.
Nagy, Hungarian Academy of Sciences, Computational Optical Sensing and Processing Laboratory, Institute for Computer
Science and Control, 1111 Budapest, Lágymányosi u. 11, 1083
Hungary, E-Mail: (zsedrovits.tamas@itk.ppke.hu).
Manuscript received July 15, 2015, revised December 4, 2015,
and January 14, 2016, and ready for publication May 3, 2016.
Review handled by D. Maroney.
0885/8985/16/$26.00 © 2016 IEEE
18

There are numerous SAA system developments and achievements all over the world. Therefore, if one would like to develop
such systems every decision about the development directions
should be done carefully. First, the targeted system capabilities
should be decided considering the existing SAA capabilities and
solutions in the national airspace.
Applying a layered approach, the SAA systems can be wellplaced inside the structure of air traffic management. In air traffic
management, the rules of safe flight operations are given. In order
to reduce the risk of mid-air collisions (MAC) and prevent accidents caused by wake turbulence, aircraft must retain a separation
distance from any other aircraft [3]. This separation is well defined
in the regulations and maintained by the air traffic controllers. The
given rules take into account different types of aircraft, different
types of safety equipment, as well as different scenarios.
Besides the traffic management rules, there are airborne collision avoidance systems (ACAS). The objective of ACAS is to
provide a backup collision avoidance system for the existing ATC
system without the need of any ground services and with minimum
false alarm rates; this holds for encounters where the collision risk
does not warrant escape maneuvers [4]. These methods are considered to be cooperative collision avoidance because the ACASs of
the two aircraft, which are participating in the scenario, are communicating with each other.
However, in general only big and expensive aircraft are
equipped with ACAS. On smaller and less expensive aircraft, the
pilot is primarily in charge of handling collision avoidance.
Most of the time, safe operation is possible in this way as well,
because the operating altitude and the maximum speed of these
smaller airplanes is much lower and slower, respectively, than for
the larger aircrafts. If two aircraft are not communicating with each
other, each of them has to run non-cooperative collision avoidance
considering the right-hand rule. However, in case of unmanned aircraft, human vision and pilot decision making can rarely be used to
implement SAA because usually no pilot is in charge; the mission
goals for the UAS are set by a system operator. For this reason,
in the case of a human pilot, the concept is called see and avoid.
However, in case of a UAS, the concept is called SAA. The UAS
case requires an automated onboard detection and decision making
system tightly coupled with the autopilot of the aerial vehicle to

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