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Mode AiC replies per second is approximately 20 per cent of
the reply rate for a transponder operating without ACAS in a
busy area of multipIe Mode AiC ground sensor coverage.
3.2.3.6.1 As an example, when interrogation limiting is
not invoked, the overall Mode A/C and Mode S interrogation
rates of a directional ACAS unit would typically be as follows:
the Mode A/C interrogation rate kt is typically constant at 83
whisper-shout interrogations per second. Assume that the sum
of the normalized whisper-shout powers, i.e. the Mode AIC
contribution to the left-hand side of inequality (I), is
approximately 3. The Mode S interrogation rate depends on
the number of Mode S aircraft in the vicinity. In en-route
airspace it is typically an average of about 0.08 interrogations
pcr second for each Mode S aircrafl wiUlin 56 h11 (30 NM).
In a uniform aircraft density of 0.006 aircraft per square km
(0.02 aircraft per square NM), the number of aircraft within
56 km (30 NM) is 57. If 20 per cent of these are ACAS
equipped, n, = 12 and the variable term on the right-hand side
of inequality (1) is 21.5. If the number of ACAS aircraft in the
area does not exceed 26, the fixed term continues to govern
and no limiting occurs until there are approximately 100
Mode S aircraft within 56 km (30 NM).
3.2.3.6.2 Similar considerations hold for inequalities (2)
and (3). In inequality (2) the r~lutual supp~~ssioinl erval
associated with each top antenna interrogation is 70
microseconds. The bottom antenna mutual suppression interval
is 90 microseconds. Thus the Mode AiC contribution to the
left-hand side of inequality (2) is 0.0059 and the Mode S
interrogation rate can be as high as 59 top antenna
interrogations per second before violating the limit. With a
typical whisper-shout sequence, the left-hand side of
inequality (3) is approximately 3. The number of ACAS
aircraft within 56 km (30 NM) can be as high as 26 without
violating inequality (3).
3.2.3.6.3 When the interrogation rate or density increases
to the point at which one of the limits is violated, either the
Mode AIC or Mode S normalized interrogation rate or both
must be reduced to satisfy the inequality. If the density were
to reach 0.029 aircraft per km2 (0.1 aircraft per NM~)
uniformly out to 56 krn (30 NM), there would be 283 aircraft
within a 56 krn (30 NM) radius. If 10 per cent of these were
equipped with ACAS, n, = 28. The right-hand limits in
inequalities (1) and (3) would then be 9.66 and 2.76
respectively. To satisfy these lower limits, the Mode A/C and
Mode S contributions to the left-hand side of inequality (1)
would both have to be reduced. As a result, the surveillance
range of both Mode NC and Mode S targets would be less.
3.2.3.6.4 Inequality (1) contains an exponent a which
serves to match the inequalities to the specific local ACAS
aircraft density such that a "victim" transponder operating in
the vicinity of ACAS that are distributed within the limits of
uniform-in-area to uniform-in-range will never detect more
than 280 ACAS interrogations in a one-second period.
The value of a defines the local ACAS distribution
characteristic within the vicinity of own ACAS.
It is based on the relative numbers of ACAS within 56 km
(30 NM), within 11.2 km (6 NM) and within 5.6 km (3 NM)
as derived from ACAS broadcast interrogations and from
ACAS surveillance. The value of a is the minimum of:
a) the logarithm of the ratio of the number of ACAS
aircraft, n, , within 56 km (30 NM) to the number of
ACAS aircraft, nb, within 11.2 km (6 NM) divided by
the logarithm of 25; and
b) one fourth of the ratio of the number of ACAS aircraft,
nb, within 11.2 km (6 NM) to the number of ACAS
aircraft, n,, within 5.6 krn (3 NM).
A uniform-in-area distribution of ACAS aircraft within 56 krn
results in an u value of 1.0 and a uniform-in-range distribution
results in a value of 0.5. Since decreasing values of a result in
greater power reduction and therefore shorter surveillance
ranges, the minimum value of cx is constrained to 0.5 in order
to preserve adequate surveillance range for collision avoidance
in the highest density terminal areas. Additional constraints are
imposed on the value of a, to account for special situations in
which the measured local ACAS distribution is:
1) based on numbers so small as to be inconclusive
(nb = l), in which case ul is constrained to 1;
2) is inconsistent with a relatively high overall ACAS
count (nb < 4, n, < 2, n, > 25), in which case Ul is
constrained to 1; or
3) is inconsistent with a relatively low overall ACAS
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