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ACAS X MOPS
shall (1376) be set to the maximum signal strength of the DF=11 and DF=17 squitters
received since the last estimated signal strength was determined.
The purpose of these requirements is to ensure a timely update rate of an
intruder’s estimated signal strength. Using the maximum received signal
strength provides a more conservative estimate of the received signal strength.
Filtering of signal strength to avoid frequent changes between Hybrid
Surveillance and Extended Hybrid Surveillance provides no benefit since the
requirements of §18.104.22.168.22.214.171.124.7 preclude unnecessary re-validation
interrogations due to Extended Hybrid to Hybrid Surveillance transitions.
If DF=17 messages are used to estimate the signal strength, then it is acceptable
to just use position messages.
126.96.36.199.188.8.131.52 Acquisition Using Interrogations
The equipment shall (1347) transmit an acquisition interrogation (UF=0, AQ=1) to
determine the range of each valid active acquisition target as defined in §184.108.40.206.4.2
Squitter Processing, or from which inadequate altitude information has been received.
Error correction decoding (§220.127.116.11.2.2d) shall (1348) be applied to the received replies.
If two or more valid replies are received in response to a single Mode S acquisition
interrogation, only the reply that is closest in range shall (1349) be retained.
If an acquisition interrogation fails to elicit a valid reply, additional interrogations shall
(1350) be transmitted. The total number of acquisition interrogations addressed to a
single target shall (1351) not exceed three within a single surveillance update cycle and a
total of nine within the first six surveillance update cycles. The first acquisition
interrogation shall (1352) be transmitted using the top antenna or the antenna most likely
to elicit a reply. If two acquisition interrogations to a target fail to elicit valid replies, the
next two acquisition interrogations to that target shall (1353) be transmitted using the
opposite antenna than used for the first acqusition interrogations . If acquisition
interrogations fail to elicit replies within 6 surveillance update intervals, the acquisition
process shall (1354) cease until enough additional squitters/fruit are received to indicate
that successful acquisition is likely enough to warrant a subsequent acquisition attempt.
One acceptable means of accomplishing this is described in Appendix A, Section A.8.
If additional attempts are made to acquire the target, they shall (1355) conform to the
pattern described in the requirement above for the first attempt except that:
a. On the second and third attempt, only one interrogation is made during each single
surveillance update interval; and in the absence of valid replies, six interrogations are
transmitted during the first six surveillance update intervals.
b. Any further attempts consist of a single interrogation during the entire six
surveillance update intervals.
When a valid acquisition reply is received, the VS field in the reply shall (1356) be used
to determine whether the aircraft is on the ground as described in §18.104.22.168.2.2. Mode S
aircraft that are determined to be on the ground shall (1357) be monitored passively with
replies (DF=0 or 4) or squitters (CA field of DF=11 or 17). In the absence of passive
monitoring, an active interrogation once every five surveillance update intervals shall
(1358) be used to monitor the air/ground status as long as the aircraft remains on the
ground and continues to transmit squitters.
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