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ACAS X MOPS
1. the (Cl) flag is not set AND no more than one ATCRBS aircraft exists within the
reliable surveillance range, OR
2. the (Cl) flag is not set AND the (Tc) flag is not set AND () indicates a switch to
the next lower level is acceptable.
Exceptions to the above rules for whisper-shout selection occur under the following
a. When ACAS X is powered on, the high resolution whisper-shout sequence shall
(1280) be used for all top directional beams and the bottom omni for the first 60
seconds to allow track acquisition and stabilization.
b. When a TA is issued on a Mode C intruder, the high resolution whisper-shout
sequence shall (1281) be used in the beam providing surveillance on the threat for
the duration of the advisory.
c. When a RA is issued on a Mode C intruder, the high resolution whisper-shout
sequence shall (1282) be used in all beams for the duration of the advisory.
22.214.171.124.4.1.5 Surveillance in Areas of No ATCRBS Intruder Aircraft
When there are no ATCRBS aircraft within the ACAS X surveillance region of a
directional or omnidirectional beam, it is not necessary to t ransmit a whisper-shout
interrogation sequence in that beam for degarbling purposes. However, it is still
necessary to transmit a single interrogation in a beam in order to detect the presence of an
ATCRBS-equipped aircraft and to initiate whisper-shout interrogations for Mode C
acquisition and tracking.
ACAS X shall (1283) transmit a single Mode C interrogation from a top-mounted
directional beam or from the bottom omnidirectional beam only if an established or
candidate Mode C track does not exist within the full ACAS X surveillance range of that
beam. The power level of the single interrogation in a specific beam and the
corresponding MTL shall (1284) be equivalent to the highest whisper-shout level
transmitted in that beam if using the high resolution sequence defined in Figure 2-10.
When interrogation power is reduced to limit interference (§126.96.36.199.2), each single
interrogation and its related receiver MTL shall (1285) be reduced by 1 dB in the order
shown for the beams in the column labeled Interference Limiting Priority in Figure 2-8
with the lowest number reduced first. The timing of individual pulses in each
interrogation is as defined in Figure 2-1 except that a whisper-shout suppression pulse is
ACAS X shall (1286) switch from a single interrogation within a beam to the Minimum
Basic whisper-shout sequence in that beam whenever:
a. two successive Mode C replies correlate in range such that the second Mode C reply
occurs within a 5000 ft range correlation window centered either at the measured
range of the previous Mode C reply or at a range offset by ±13 μsec from the
measured range of the previous reply, or
b. an established Mode C track or a Mode C track in the process of being acquired
traverses into that beam from another beam.
Mode C track initiation (§188.8.131.52.4.1.2) shall (1287) only be accomplished through use of
a whisper-shout interrogation sequence. ACAS X shall (1288) switch back to the single
interrogation after 10 surveillance update intervals in which no established Mode C track
or Mode C track in the process of being acquired was present within the beam and no two
successive correlating replies, indicating the potential for initiation of a Mode C track,
were received within the beam.
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