Home' RTCA Documents for Review : ACAS X MOPS DRAFT Vol.1 Contents 59
ACAS X MOPS
Interference Limiting Procedures
At the beginning of each surveillance update interval, and NTA shall (1024) be
determined as indicated in §126.96.36.199.1. The value shall (1025) then be used to:
a. modify each normalized Mode C and Mode S interrogation value in the left hand
limits of inequality 1 prior to its summation over the surveillance update interval, and
b. modify the fixed term in the right-hand limits of inequality 1.
NTA shall (1026) then be used to evaluate the current right-hand limits in inequalities 1
and 3. Smoothed values (8 second averages) of the Mode S contributions to P(i) and
M(i) shall (1027) also be used to calculate the left hand limits of inequalities 1 and 2.
Air-to-air coordination interrogations shall (1028) be transmitted at full power. These
interrogations need not be included in the summations of Mode S interrogations in the
left-hand terms of these inequalities. If the smoothed value of the left-hand side of either
inequality 1 or 2 equals or exceeds the current limit and ownship is below 18,000 ft
barometric altitude, both the Mode S and the Mode C surveillance parameters shall
(1029) be modified to satisfy the inequalities. If the left-hand side of inequality 3
exceeds the current limit and own ACAS X is below 18,000 ft barometric altitude,
Mode C surveillance shall (1030) be modified to satisfy the inequality. In both cases,
hysteresis of ±500 ft relative to the 18,000 ft altitude boundary shall (1031) be applied to
prevent fluctuations in interference limiting due to small altitude deviations.
Mode C surveillance sensitivity shall (1032) be modified by reducing the interrogation
power and by increasing the receiver MTL during the Mode C listening period. If a high
resolution whisper-shout interrogation sequence is used (§188.8.131.52.4.1.2), Mode C
surveillance shall (1033) be modified by sequentially eliminating steps from the whisper -
shout sequence. If the minimum basic whisper-shout interrogation sequence or a single
interrogation is used (§184.108.40.206.4.1.1 or §220.127.116.11.4.1.5 respectively), Mode C surveillance
shall (1034) be modified by reducing the power level of each step within the whisper -
shout sequence or each single interrogation sequentially for each beam. Each whisper -
shout or power reduction step is uniquely associated with a receiver MTL setting. Thus,
the receiver sensitivity in Mode C surveillance periods will be automatically tailored to
match these power reductions.
The overall surveillance sensitivity for Mode S targets shall (1035) be reduced by
reducing the interrogation power and by increasing the receiver MTL during all Mode S
squitter listening periods. Once such a change has been made the only change allowed
during the ensuing 8 seconds is a reduction in the number of whisper -shout steps if such
is needed to satisfy Inequality 3. This 8-second freeze allows the effect of the Mode S
changes to become apparent since the 8-second averages used in Inequalities 1 and 2 then
will be determined by the behavior of the system since the change.
Note: This will indirectly reduce the Mode S interrogation rate by reducing the target
count. Many Mode S interrogations are acquisition interrogations transmitted to
targets of unknown range. It is thus not effective to directly control the Mode S
interrogation rate simply by dropping long-range targets from the track file.
The Mode C and Mode S surveillance sensitivity reductions shall (1036) be
accomplished such that the ACAS X equipment is not prematurely limited and has the
capability of using at least 75% of the allowance specified in the three limiting equations
for all mixes of target types and for all densities up to the maximum densi ty capability of
the system. When the value of any of the smoothed limits is exceeded, the appropriate
action shall (1037) be taken to limit interference within one surveillance update interval.
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