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ACAS X MOPS
The only other circumstance in which RA intent is reversed arises when the master
detects that its current choice of intent must be reversed in order to resolve the encounter.
The master then reverses intent, cancels the previous VRC and sends a new one forcing
the slave to reverse its intent.
When both aircraft are ACAS X or TCAS II, both aircraft have have discrete 1030/1090
MHz interrogation/reply capability and the ICAO aircraft address es are used for
master/slave determination. The aircraft with the lower ICAO address is the master and
the aircraft with the higher ICAO address is the slave.
Future Coordination Schemes
ACAS X and TCAS are both intended for comercial transport aircraft with good
performance charaterristics (e.g., an ability to climb at 2500 fpm) and a discrete
1030/1090 MHz interrogation/reply capability. Such aircraft can be considered to be
peers of each other. In future, less capable aircraft or aircraft not linked to a discrete
1030/1090 MHz interrogation/reply capability might equip with CAS. This document
includes provisions to enable ACAS X to coordinate RAs with such new CAS, fitted to
aircraft that would not be considered peers of ACAS X.
When the two aircraft are not peers, master/slave determination is accomplished
according to §188.8.131.52.3.2. Capability information is exchanged that announces the
coordination schemes available to each CAS. The options include using ADS-B instead
of 1030/1090 MHz interrogation and reply for coordination messages, and also an
announcement that an aircraft should not be considered a peer of other ACAS X or TCAS
Three coodination schemes are envisaged: Active, Passive, and Responsive Coordination.
Active Coordination is that described above for ACAS X and TCAS and involves active
1030/1090 MHz interrogation and reply. Passive Coordination would be between two
aircraft both using ADS-B to coordinate and considered to be mutual peers (and using
ICAO 24-bit Aircraft Address to determine master/slave status). Responsive
Coordination would be used when one aircraft uses 1030/1090 MHz interrogation and
reply and the other uses ADS-B to send coordination messages, and also when, for any
reason, it is determined that the two aircraft are not peers and and one should be the
master regardless of the ICAO 24-bit Aircraft Addresses.
Responsive Coordination is the scheme used by the slave when the two aircraft are not
peers. The slave must ensure that it selects RAs that are compatible with an intent
received from the master. The master, which in this document would be ACAS X, uses
Modified Active Coordination or Modified Passive Coordination, the modification being
that it is the master regardless of ICAO 24-bit Aircraft Address.
Multiple Threats, Sense and Intent, and Reversals
In an encounter with a single threat the concepts of “RA sense” and “RA intent” are
equivalent: the sense is up and the intent is to pass above the threat if the red arc on an
IVSI display prohibits or limits descent; and the sense is down and the intent is to pass
below the threat if the red arc on an IVSI display prohibits or limits a climb. The word
“intent” is particularly useful when both aircraft are ACAS equipped and the RAs are
Once an initial RA has been announced, the logic is designed so that the frequenc y of
reversals of RA sense (intent) is minimised, subject to other constraints. In the first
operational version of TCAS II, the first action of TCAS II when a threat was identified
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