Aerospace and Electronic Systems Magazine September 2016 - 58


Proof-of-Concept Airborne Sense and Avoid System

Figure 6.

Encounter summary and recap.

sentative of HTS performance during all flight tests. The HTS is
initiated at t=0 as the ADS-B and TCAS Mode-S measurements
become available. Between t=0 and t=200, the estimated track
correctly fused the ADS-B and TCAS measurements together.
HTS also correctly differentiated radar IDs 72/73 from 74 (belonging to ICAO ID 11264663) and initiated separate tracks for
them. When fusing measurements from multiple sensors originating from the same intruder aircraft, the HTS places more statistical significance on the most accurate measurement. ADS-B
is overall a more accurate sensor than TCAS Mode-S, and HTS
algorithms reflect this by distributing weights on sensor measurements accordingly. HTS continued to track the intruder aircraft
until the ADS-B and TCAS Mode S measurements became unavailable at t=200, but reinitialized the fused track with the same
fusion ID as they returned at t=300. As the intruder entered the
radar field of view, HTS also fused its measurements with the
ICAO ID 11264663 track starting at t=480. The vertical radar
measurements caused the estimated track to vary slightly away
from the ADS-B measurements as the HTS began to incorporate
the statistical accuracy of the radar. The radar dropped the track
at t=560 and the HTS continued to track the intruder using the
ADS-B and TCAS Mode S measurements. The HTS maintained
radar ID 74 with the track in case of measurement drop out for
an additional 9 s and then deleted its association from the track
completely. The HTS deleted the track completely 4 s after the
ADS-B and TCAS Mode S measurements were stopped in the
test scenario.
58

COLLISION AVOIDANCE FLIGHT TEST RESULTS
CA flight tests took place over six flight days in November and
December 2014, including four days of encounters between Ikhana
and manned intruders, and two days of encounters between Ikhana
and another Predator B (N308HK) UAS. The six total days of testing yielded 113 scripted encounters, 22 nonscripted encounters,
and 13 aborted encounters. Nonscripted encounters imply accidental triggering of ACAS-XU alerts during maneuvers to reposition aircraft for the next scripted encounter. While unplanned,
these nonscripted encounters provided valuable data for ACAS-XU
V&V. Of the 113 scripted encounters, 103 were against "cooperative" intruders (series 10-30) and 10 were against "noncooperative" intruders (series 40 special ATAR). The totals for the test are
summarized in Figure 6. Of the 113 scripted encounters, the ACAS
XU logic generated the scripted/expected alert for 74 encounters
(65%). For 39 encounters (35%), the ACAS XU logic generated
an alert that was not as scripted/expected. However, most of these
encounters can be classified as acceptable and the alerts were appropriate given the particulars of the encounter as flown. Specifically, the nomenclature "not as scripted/expected" does not imply
that the alerts were unsafe. Many of these alerts were in encounters
where no alert was desired, but the alert only served to increase
safety. Deviations in actual ownship and intruder trajectory from
the scripted flight path (such as altitude fluctuations, winds, acceleration timings) also contributed to a different RA than expected,
but was nevertheless safe given the profile as flown.

IEEE A&E SYSTEMS MAGAZINE

SEPTEMBER 2016



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