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for correlation. Reports that correlate are used to extend the
associated tracks. Rcports that do not correlate with existing
tracks may be compared to previously uncorrelated reports to
start new tracks. Before a new track is started, the replies that
lead to its initiation may be tested to ensure that they agree in
all of the most significant altitude code bits. A geometric
calculation may be performed to identify and suppress
specular false targets caused by multipath reflections from the
terrain.
3.1.2.5 Tracks being initiated may be tested against track
validity criteria prior to being passed to the collision avoidance
Attachment Annex 10 - Aeronautical Telecommunications
algorithms. The purpose of these tests is to reject spurious
tracks caused by garble and multipath. Spurious tracks are
generally characterized by short track life.
3.1.2.6 Aircraft not reporting altitude in Mode C replies
are detected using the Mode C reply framing pulses. These
aircraft are tracked using range as the correlation criterion. The
additional use of bearing for correlation will help to reduce the
number of false non-Mode C tmcks.
3.1.2.7 Reply merging. Multiple replies may be generated
by a Mode AIC target that responds to more than one whispershout
interrogation during each whisper-shout sequence or by
a target that responds to interrogations from both the top and
bottom antennas. The equipment is expected to generate no
more than one position report for any target even though that
target may respond to more than one interrogation during each
surveillance update interval.
3.1.2.8 Mode A/C surveillance initiation. The equipment
will pass the initial position reports to the collision avoidance
algorithms only if the-conditions in a) and b) below are
satisfied:
a) initially, a Mode C reply is received from the target in
each of three consecutivc surveillance update periods,
and:
1) the replies do not correlate with surveillance replies
associated with other tracks;
2) the range rate indicated by the two most recent
replies is less than 620 m/s (1 200 kt);
3) the oldest reply is consistent with the above range
rate in the sense that its range lies within 95.3 m
(312.5 ft) of a straight line passing through the two
most recent replies;
4) the replies correlate with each other in their altitude
code bits:
b) a fourth correlating reply is received within five
surveillance update intervals following the third reply of
the three consecutive replies in a) above arid is within
+60 rn (+200 ft) of the predicted altitude code estimate
determined in a) 4) above.
3.1.2.8.1 The following is an example of an acceptable set
of rules for assessing correlation of reply code bits and
determining the initial altitude track code estimate for a target.
Three replies correlate only if:
a) all eight of their D, A and B code pulses agree; or
b) seven of their D, A, and B code pulses agree and at least
one of their C code pulses agree.
3.1.2.8.2 The test for code agreement among the three
replies is made individually for each of the reply pulse
positions. This test i s based on the presence of code pulses
alone; agreement occurs for a given reply pulse position if all
three replies are detected with ONE in the position or all three
replies are detected with a ZERO in that position. The
confidence associated with those pulse detections does not
affect agreement.
3.1.2.8.3 The confidence flag for a reply pulse position is
set "low" whenever there exists another received reply (either
real or phantom) that could have had a pulse within
k0.121 microsecond of the same position. Otherwise, the
confidence flag is set "high".
3.1.2.8.4 When agreement among the three replies does
not occur for a given reply pulse position, the initial track
pulse code estimate for that position is based on the values of
the individual pulse codes and the confidence flags associated
with those pulse codes in three replies.
3.1.2.8.5 When agreement faiis for a given pulse position,
the rules for estimating the initial track code for that position
are based on the principle that "low" confidence ONES are
suspect. The rules are as follows:
a) If in the most recent (third) reply the detected code for
a given pulse position is "high" confidence or a ZERO,
the initial track pulse code estimate for that position is
the same as the code detected in that position in the most
recent reply.
b) If in the most recent reply the detected code for a given
pulse position is a "low" confidence ONE, the initial
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