Aerospace and Electronic Systems Magazine September 2016 - 54

to leverage additional surveillance sources. ACAS XU is a variant
tailored for the performance characteristics of unmanned aircraft.
[4]. The prototype ACAS XU system comprises two high-level
components: the STM and TRM. The STM tracks and computes
the estimated states of a threat aircraft and passes the data to the
TRM. Variations of the STM were used in this test to investigate
the utility of surveillance sources including ADS-B, a prototype
ATAR, TCAS-II, and the fusion of data. The TRM selects a CA
maneuver according to an optimized logic table. Two variations of
TRM were used to generate advisories to resolve a collision threat:
vertical TRM (TRM V) to generate vertical advisories (climb or
descend), as currently implemented in TCAS II against cooperative intruders; or horizontal TRM (TRM H) to generate horizontal
advisories (turn left or right to a heading) against noncooperative
intruders.
The flight test employed preplanned encounters consisting of
specific ground tracks, ground speeds, and altitude profiles for
ownship and the intruder to arrive at a planned closest point of approach (CPA). To ensure safety, the encounter scenarios were designed to trigger an alert without artificial offsets while preserving
both altitude and horizontal separation between the intruder and

ownship, not relying on the system under test (ACAS XU) to function correctly to preserve separation and prevent near-midair collision (NMAC). The scenarios were constructed using representative
horizontal geometries and operationally relevant vertical profiles
including 500-1000 ft altitude separation and vertical blunders.
Figure 1 illustrates the horizontal geometries and two vertical profile examples, including the special ATAR vertical profile.
The ACAS X U prototype implemented on Ikhana for this
flight test was integrated into the flight control software to
provide selectable maneuvering modes for response to ACAS
X U RAs. In advisory mode, RA messages are displayed to the
UAS pilot in the ground control station (GCS) on the headsup display (HUD) for pilot action to execute the RA. In auto
mode, the flight computer automatic maneuvered the unmanned aircraft in response to an RA 3 s after receiving and
displaying an RA to the pilot. The pilot maintains the ability
to regain positive control of the aircraft anytime during an
automatic CA maneuver. Intruder aircraft did not respond to
RAs in this flight test, so the automatic maneuvering function
was disabled for the GA-ASI Predator B used as the unmanned
intruder.

Figure 1.

ACAS Xu flight test encounter geometries.

54

IEEE A&E SYSTEMS MAGAZINE

SEPTEMBER 2016



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