Aerospace and Electronic Systems Magazine September 2016 - 53

Feature Article:

DOI. No. 10.1109/MAES.2016.150163

Proof-of-Concept Airborne Sense and Avoid System
with ACAS-XU Flight Test
Tatsuya Kotegawa, General Atomics Aeronautical System, Inc., Poway, CA, USA

INTRODUCTION
Today, unmanned aircraft system (UAS) operations are typically
conducted in segregated airspace and in limited combat theaters. In
order to integrate UAS into the National Airspace System (NAS),
UAS must operate safely, efficiently, and be compatible with existing manned aircraft traffic so that the overall safety and efficiency
of the airspace is not degraded [1]. There are many challenges that
lie ahead to allow UAS unrestricted access to NAS, but perhaps the
greatest technical challenge involves fulfilling the "See and Avoid"
requirements (14CFR 91.111 and 91.113).
Sense and avoid (SAA) is a technical capability for a UAS
to fulfill the "See and Avoid" requirements employing onboard
sensors, hardware and software elements to detect and track
nearby air traffic, convey awareness of potential conflicts to
the pilot, and provide guidance to resolve such conflicts [2].
SAA functionality comprises two distinct layers of protection:
self-separation (SS) and collision avoidance (CA). SS is the
tactical process whereby the UAS remains "well clear" of other airspace users, thus fulfilling the separation provision layer
within the ICAO Global ATM Operational Concept [3]. CA, on
the other hand, is the final layer of protection against midair
collision risk when progression of a potential collision is not
avoided by SS.
In 2013, a partnership was formed between General Atomics Aeronautical Systems, Inc. (GA-ASI), the Federal Aviation
Administration (FAA) Traffic alert and Collision Avoidance
System (TCAS) Program Office, National Aeronautics and
Space Administration (NASA), Honeywell Inc., and BAE Systems to develop and flight-test a proof-of-concept SAA system.
The system is comprised of Pilot-in-the-Loop SS functionality
implemented in the ground control station (GCS) and airborne
CA functionality using the FAA's Airborne Collision Avoidance
system variant for UAS (ACAS XU). Both systems rely on surveillance provided by a suite of airborne sensors integrated onto
a Predator B UAS. Flight tests were conducted in November-DeAuthor's address: General Atomics Aeronautical Systems,
Inc., 4200 Kirkham Way, Poway, CA 92064. E-mail: (tatsuya.
kotegawa@ga-asi.com).
Manuscript received July 30, 2015; revised February 10, 2016,
February 25, 2016; ready for publication April 8, 2016.
Review handled by G. Fasano.
0885-8985/16/$26.00 © 2016 IEEE
SEPTEMBER 2016

cember 2014 involving preplanned encounters between the SAA
test article, NASA's Ikhana Predator B UAS, as well as manned
and unmanned intruders. This paper describes the SAA systems
that were flown, how they performed, and lessons learned for
future development.

FLIGHT TEST OBJECTIVES AND OVERVIEW
The objective of the 2014 flight test was to demonstrate and assess
CA and SS performance of the SAA system integrated onto a UAS
in a live flight environment. The test was conducted in two phases: demonstration and assessment of the CA and SS component
of SAA. The SAA system here also serves as a proof-of-concept
system for the RTCA Special Committee 228 (SC-228) to generate
Minimum Operational Performance Standards (MOPS) for a certifiable SAA system on UAS. Specific flight test objectives, combining the research interests of the partners involved in this effort,
are summarized below.
[CA/SS] Flight test a suite of surveillance sensors for SS and
CA including ADS-B and air-to-air radar (ATAR).
[CA/SS] Flight test the surveillance and tracking module
(STM) using surveillance information fused from multiple
sensor sources.
[CA] Flight test the threat resolution module (TRM) logic for
vertical and horizontal maneuvers.
[CA] Flight test the interoperability of ACAS XU with legacy
TCAS II systems (responsive coordination) and with other
ACAS X systems (active coordination emulation using ADS-B).
[CA] Flight test automatic maneuvering in response to a
resolution advisory (RA).
[SS] Examine effectiveness of several prototype SAA displays
for pilot situation awareness and guidance algorithms for
recommendations to maintain SS with potential conflicts.

COLLISION AVOIDANCE: ACAS X U
The FAA TCAS Program Office is developing the Next Generation Airborne Collision Avoidance System, ACAS X. ACAS X is
a modular family of variants supporting a variety of aircraft platforms and operating environments with plug-and-play capabilities

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