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ACAS X MOPS
The CA field in squitters from DO-181A or later transponders shall (1337) be used to
determine whether the aircraft is on the ground as described in §188.8.131.52.2.2. The
equipment shall (1338) not interrogate an aircraft for acquisition in the following three
cases: (1) the aircraft’s CA field indicates that the aircraft is currently on the ground and
the aircraft does not have an operational Active CAS, (2) the aircraft’s CA field indicates
that the aircraft is currently on the ground, the aircraft has an operational Active CAS, but
own ACAS X is above 2000 feet AGL, or (3) the aircraft qualifies for acquisition using
passive position reports as specified in §184.108.40.206.220.127.116.11.
For Mode S targets with DO-181 transponders from which a squitter has been received or
Mode S targets with DO-181A or later transponders from which a squitter has been
received that indicates the target to be airborne, altitude shall (1339) be determined by
monitoring transmissions received with any, some, or all of DF=0, DF=4, or DF=17
messages; or, in the absence of such reply transmissions, by actively interrogating to
elicit a short special surveillance reply. Validity of extended squitter transmissions and
of altitude data is specified in §18.104.22.168.3.2.8 and §22.214.171.124.126.96.36.199 respectively.
The equipment shall (1340) monitor squitters and altitude replies to other interrogations
whenever it is not transmitting or receiving replies to its own interrogations. Monitoring
shall (1341) be accomplished using an antenna capable of simultaneous reception over
360 degrees of azimuth.
To reduce the number of ineffectual interrogations, no interrogations shall (1342) be
transmitted to a target if so few squitters and altitude replies are received that the RF link
seems unreliable. Also, the equipment shall (1343) not interrogate for acquisition a
target whose altitude information indicates that it is not within ±10,000 ft of own altitude
when ownship is airborne, or not within ±3,000 ft of own altitude when ownship is on the
Targets that are within ±10,000 ft of own altitude when ACAS X is airborne, or within
±3,000 ft of own altitude when ACAS X is on the ground, and that indicate a reliable RF
link are called valid targets. In order to provide timely acquisition of targets that
transition the altitude surveillance boundary, the altitude of targets that are indicated to be
outside of ±10,000 ft of own altitude when ACAS X is airborne, or outside of ±3,000 ft
of own altitude when ACAS X is on the ground, shall (1344) continue to be monitored
using any, some, or all of DF=0, DF=4 or DF=17 transmissions or, in the absence of such
transmissions, by interrogating once every 10 seconds.
Ownship shall (1345) interrogate targets from which it does not receive altitude
information but does continue to receive error-free squitters.
The ACAS X squitter processing function shall (1346) have memory capacity for at least
150 ICAO 24-bit Aircraft Addresses.
Note: Although the required surveillance range of ACAS X is 14 NM, Mode S squitters
will typically be received from targets out to the nominal detection range of 30
NM. In a density of 0.3 Mode S aircraft per sq. NM, there will be about 141
aircraft within 30 NM of ACAS X.
Appendix A, Section A.8 describes one acceptable method of processing squitters and
altitude replies to reduce unnecessary interrogations.
A valid Mode S target shall (1368) be acquired, through the exclusive use of extended
squitters, provided that the following criteria are met:
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