Pr(AC1 alert) 1.70 · 10.1 2.50 · 10.1 2.50 · 10.1 4.77 · 10.1
Pr(AC1 strengthening) 5.60 · 10.3 5.90 · 10.3 7.90 · 10.3 6.78 · 10.3
Pr(AC1 reversal) 1.94 · 10.5 5.04 · 10.4 2.51 · 10.5 3.91 · 10.3
E[AC1 RA changes] 1.80 · 10.1 2.57 · 10.1 2.56 · 10.1 6.48 · 10.1
E[AC1 RA duration] 2.30 · 100 4.59 · 100 4.60 · 100 1.08 · 101
The Joint-Indep strategy outperforms TCAS on all metrics: Joint-Indep is almost twice as safe and alerts almost three times less frequently. Because there is no sensor noise, Uncoord-Indep is just as safe as Joint-Indep but at the expense of more alerts. The Uncoord-Compat strategy is over twice as safe as Uncoord-Indep with the same alert rate. This is because Uncoord-Compat, unlike Uncoord-Indep, ensures compatibility. Uncoord-Compat is over four times safer than TCAS while alerting almost half the time.
4000 3000 2000 1000 0 .1000 .2000
Altitude (ft)
North (ft)
1
0.5
0
.0.5
.1
.1.5 East (ft) ×104
(b) Horizontal pro.le.
The
following
four
coordination
strategies
were
evaluated
with
sensor
noise
(Section
7.1)
on
the same encounters selected from the correlated encounter model:
.
joint policy / independent action selection (Joint-Indep),
.
uncoordinated policy / independent action selection (Uncoord-Indep),
.
joint policy / centralized action selection (Joint-Cent), and
.
uncoordinated policy / compatibility action selection (Uncoord-Compat).
The
results
are
summarized
in
Table
12.
TABLE 12
Performance evaluation of coordination strategies with the TCAS sensor
Joint-Indep Uncoord-Indep Joint-Cent Uncoord-Compat TCAS
Pr(AC1 alert) 2.97 · 10.1 4.07 · 10.1 3.05 · 10.1 4.07 · 10.1 5.12 · 10.1
Pr(AC1 strengthening) 1.74 · 10.2 1.92 · 10.2 1.84 · 10.2 1.87 · 10.2 6.54 · 10.3
Pr(AC1 reversal) 8.99 · 10.4 5.00 · 10.3 1.53 · 10.4 1.40 · 10.3 3.60 · 10.3
E[AC1 RA changes] 3.27 · 10.1 4.31 · 10.1 3.36 · 10.1 4.26 · 10.1 6.94 · 10.1
E[AC1 RA duration] 4.73 · 100 8.44 · 100 4.84 · 100 8.48 · 100 1.13 · 101
The Joint-Indep strategy performs markedly worse than TCAS. Because the aircraft can disagree over the belief state, they can issue incompatible advisories that if followed lead to collision. The Uncoor-Indep strategy, though it also has the potential of issuing incompatible advisories, is safer than Joint-Indep because it alerts earlier, perhaps allowing additional time to reverse the advisory if necessary. Joint-Cent, which solves this problem in part by using only the belief state of the aircraft with the lower address, has an NMAC probability ten times lower than Joint-Indep with comparable alert probability. The Joint-Cent strategy still has a somewhat greater probability of NMAC compared to TCAS but alerts signi.cantly less. The Uncoord-Compat strategy is over twice as safe as TCAS and alerts less often. One potential reason why Uncoord-Compat is safer than Joint-Cent is because Uncoord-Compat, using the uncoordinated logic which assumes that intruders do not maneuver to avoid collision, alerts more aggressively.
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