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specific state information for ACTIONARBITRATION. st_int.st_cost_on stores context-specific state
information for online cost estimation functions in ONLINEUNCOORDINATEDACTIONCOSTESTI-
MATION and ONLINECOORDINATEDACTIONCOSTESTIMATION. For TRMOWNSTATE, st_multi-
threat_cost_balancing and st_arbitrate maintain state information that is associated with the global
state. st_own.st_multithreat_cost_balancing stores context-specific state information for MULTI-
THREATC OSTBALANCING. st_own.st_arbitrate stores context-specific state information for AC-
The state variables associated with processing are set at initialization, in INTRUDERPREP, or as part
of output processing as described in Section 3.7. The state variables associated with coordination
are set at initialization, in INTRUDERPREP, in UPDATEINTRUDERINPUTS, or as part of output
processing as described in Section 3.7.
The parameters contained in params, of type PARAMSFILE_TYPE are referenced by name in the
The parameters in that JSON file are described in Appendix F.
3.1.3 Intruder Designation and Protection Modes
Like its predecessor, TCAS, ACAS X is designed to prevent collision with multiple simultaneous
intruders. ACAS X supports Operational Modes that pertain across intruders: Traffic Advisory/Res-
olution Advisory (TA/RA), Traffic Advisory Only (TA-Only), and Standby. While in TA/RA mode,
ACAS X allows different intruders to be treated differently and allows the same intruder to be
treated differently at different times. An intruder can be designated for special processing or it can
be designated for standard processing with different parameters and lookup tables, used in STATE-
ANDCOSTESTIMATION. When different parameters and lookup tables are used they are referred to
collectively as a protection mode. The parameters that depend on protection mode can be found in
Appendix F.2, under the “modes” section in the JSON parameter specification.
Two general classes of ACAS X TA/RA processing are described in this document: ‘Xa’ is the class
of a general intruder, representing the primary focus of development of ACAS X. ‘Xo’ is the class
tailored for intruders in a special operational mode, such as Closely Spaced Parallel Operations
For Xo, an intruder can be designated for special processing, Designated No Alerts (‘DNA’), or for
standard processing with different parameters and lookup tables, ‘CSPO 3000’. Xo operational use
and requirements are described in the ACAS Xo System Requirements Document (SRD) .
ACAS X traffic designation support is provided via the Aircraft Surveillance Applications (ASA)
System described in DO-317B . The Aircraft Surveillance and Separation Assurance Processing
(ASSAP) function in the ASA System provides the interface to the ACAS-X system.
In the TRM, Xo ‘DNA’ operation is independent of protection mode and is indicated by the dna
flag in TRMINTRUDERINPUT. DNA suppresses Resolution Advisories and Traffic Advisories for
the designated intruder only. TA-Only Operational Mode, Standby Operational Mode, and DNA
processing are handled by the TRM as special cases that call for adjustments to the outgoing coor-
dination messages and/or the pilot display. All three are discussed in GENERATE TRM OUTPUT.
Xo ‘CSPO 3000’ is the protection mode tailored for an intruder aircraft paired with own aircraft in
Closely Spaced Parallel Operations (CSPO). The currently defined operational standard is CSPO
The codes for identifying the two currently supported protection modes are given in CONSTANTS
ACAS Xa/Xo MOPS, Vol. II
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