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ACAS X MOPS
184.108.40.206.4.1.3 Determination of Whisper-Shout Based on Synchronous Garble
Each scan, ACAS X shall (1270) determine the appropriate whisper-shout sequence (i.e.,
the high resolution sequence or the minimum basic sequence) to be used with each top
antenna directional beam according to criteria based on the expected level of synchronous
garble within the reliable ACAS X surveillance range of that beam.
The reliable ACAS X surveillance range in the absence of interference limiting is
considered to be nominally 14 NM in the forward beam, 8.8 NM in the left and right
beams and 5.0 NM in the aft beam (§220.127.116.11.1). When interference limiting is imposed,
the reliable surveillance range used in each beam to determine the relative range of
Mode C aircraft and Mode C aircraft density (see below) is reduced by a factor
equivalent to the square root of the interference limiting power reduction for that beam.
For example, if the largest interrogation in the forward beam is reduced 6 dB because of
interference limiting, the reliable surveillance range is reduced from 14 NM to 7 NM.
The reliable surveillance range used to determine the level of low confidence bits in each
received Mode C reply is always 14 NM in the forward beam, 8.8 NM in the side beams
and 5.0 NM in the aft beam regardless of the level of interference limiting.
The following information on Mode C aircraft within each antenna beam and within the
reliable ACAS X surveillance range defined above, shall (1271) be determined each scan
in order to select the proper whisper-shout sequence for that beam:
a. The level of low confidence bits in each received Mode C reply.
b. The relative range of Mode C aircraft.
c. The Mode C aircraft density.
The whisper-shout sequence for a specific top directional beam is determined and
selected solely on the garble situation in that beam and is independent of the situation
or use in the other beams. In other words, beams can simultaneously have different
levels of degarbling capability. When interference limiting calls for the elimination
of a whisper-shout step or a 1-dB reduction in the interrogation power level, the order
of priority in a mixed-beam-interrogation-sequence follows the order specified in
Figure 2-8 and Figure 2-10 for the specific interrogation sequence and beam. For
example, regardless of sequence used, the forward beam priority always follows 1, 5,
9, 13, 17, 21 etc., the side beam priorities always follow 2&3, 6&7, 10&11, 14&15
etc., the rear beam priority always follows 4, 8, 12, 16 etc., and the bottom always
follows 80, 81, 82, and 83.
d. Low Confidence Bit Measurement (Cl)
Each scan, ACAS X shall (1272) examine successive Mode C replies from an
intruder for the existence of low-confidence altitude code bits in order to provide an
early detection of synchronous garble, particularly during Mode C intruder
acquisition, and to indicate whether a switch to an appropriate whisper-shout
interrogation sequence is necessary. The existence of low-confidence code bits in an
intruder reply must persist for two or more successive scans to qualify for
synchronous garble and to eliminate fruit interference.
If two successive replies from an intruder contain at least one low-confidence altitude
code bit, regardless of its position in each reply, a flag shall (1273) be set for that
beam to indicate synchronous garble. This information is then used in §18.104.22.168.4.1.4
to select the appropriate interrogation sequence for that beam.
Note: During reply decoding, the confidence flag for a reply pulse position is set
LOW whenever there exists another received reply (either real or phantom)
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