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whisper-shout sequence for top antenna
omnidirectionally. If a directional-receive antenna is used, the
.MTL values must be adjusted to account for the antenna gain.
For example, for a net antenna gain of 3 dB, all MTL values
in the table would be raised by 3 dB; and the MTL for step
number 1 would be -71 dBm rather than -74 dBm.
3.2.2.5 The power is defined as the effective radiated
power for the interrogation. All power levels are to be within
*2 &of nominal. The tolerance of thc step increments is to
be *I12 dB and the increments are to be monotonic throughout
the entire power range of the sequence.
3.2.2.6 Most of the interrogations.are transmitted from the
top antenna because it is less susceptible to multipath
interference from the ground.
3.2.2.7 Selection of the appropriate whisper-shout
subsequence for a particular antenna beam is performed each
interrogation cycle based on the current or anticipated level of
Mode A/C synchronous garble in that beam as determined by
ACAS surveillance. The high density whisper-shout
subsequence is selccted for an antenna bcam whcncver
synchronous garble is present in that beam as evident from the
existence of at least one low confidence altitude code bit in
two consecutive Mode C rcplies. The 6-levcl whisper-shout
sequence is selecled for an antenna beam if either:
a) a single Mode A/C aircraft exists within the surveillance
range nf that beam and synchronous garble is not
present; or
b) synchronous garble is not present, Mode AIC targets
are not within garble range of each other, and the
Mode AIC aircraft density within the reliable
surveillance range is equal to or less than 0.23
aircraftkm (0.43 a i r c r a m ) . Whenever a TA is
generated on a threat within a particular antenna beam,
the high lcvcl sequence is used for that beam for the
duration of the advisory. Whenever an RA is generated,
the high level sequence is used for all antenna beams for
the duration of the advisory.
3.2.2.8 If no established Mode A/C surveillance track nor
any candidate track, consisting of three correlating Mode C
acquisition replies, exists within the surveillance range of an
antenna bcam, degarbling is unnecessary and ACAS transmits
a single Mode C interrogation in that beam. The power level
of the single interrogation and its associated MTL in each
be.am is equivalent to the highest allowable powel- levcl of the
corresponding low level whisper-shout subsequence as determined
by interference limiting. Single Modc C interrogations
are susceptible to uplink mode conversion due lo multipath
and may result in a mixture of Modc A and Modc C replies
Atlachment Annex 10 - Aeronautical Telecornmunicatwns
from an intruder that are separated by 13 microseconds.
ACAS, therefore, selects the low level whisper-shout
subsequence for a beam for reliable surveillance acquisition
and tracking whenever:
a) a single interrogation in that beam results in a Mode NC
reply that occurs within a 1 525 m (5 000 ft) range
window centred either at the measured range of a Mode
A/C reply received in the previous surveillance update
interval or at a range offset from the previous reply
range by +I 3 microseconds; 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. ACAS switches back to the single
interrogation after ten surveillance update intervals in
which two correlating acquisition replies were not
received.
3.2.3 INTERFERENCE LIMITING
3.2.3.1 ACAS equipment conforms to a set of three
specific inequalities (Chapter 4, 4.3.2.2.2.2) for controlling
interference effects. The three inequalities, applicable to
ACAS operating below a pressure altitude of 5 490 rn (18 000
ft), are associated with the following physical mechanisms: (1)
reduction in "on" time of other transponders caused by ACAS
interrogations, (2) reduction in "on" time of own transponder
caused by mutual suppression during transmission of
interrogations, and (3) Mode A/C fruit caused by ACAS Mode
AIC interrogations. Setting n, to 1 in inequalities (1) and (3)
for ACAS operating above pressure altitude of 5 490 m
(18 000 ft), prevents a single ACAS from transmitting
unlimited power by providing an upper limit on the ACAS
one-second interrogation powerlrate product.
3.2.3.2 Inequality (I) ensures that a "victim" transponder
will never detect more than 280 ACAS interrogations in a
one-second period from all the ACAS interrogators within
56 km (30 NM) for any ACAS distribution, surrounding the
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