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ACAS X MOPS
surveillance methodology (such as ADS-B) may allow future development of horizontal
RAs. This is not being considered for this ACAS Xa/Xo MOPS document, but will be
part of the ACAS Xu standards.
If the threat aircraft is itself equipped with Active CAS equipment that generates
resolution advisories, a coordination procedure via the air -to-air Mode S data link is
performed. This procedure assures that the aircraft resolution advisories are compatible.
Coordination is discussed further in §1.3.3.
The resolution advisories displayed to the pilot can be divided into two categories:
Corrective advisories, which instruct the pilot to deviate from the current flight path (e.g.,
don’t climb when the aircraft is climbing); and preventive advisories which do not
require a modification to the current flight path (e.g., don’t climb when the aircraft is in
The traffic advisories displayed to the pilot describe the positions of air craft that are, or
could become, collision threats. Traffic advisories indicate the range, altitude, and
bearing of the threat up to 15 seconds before the time a resolution advisory is displayed.
The display of traffic advisories may assist the flight cr ew of ownship in the following
Alert the crew to the presence of potentially threatening traffic and provide intruder
position data that can aid visual acquisition.
Provide the crew a graphic depiction of the conflict situation prior to the time the
resolution advisory is displayed, thereby facilitating a reduction in the time taken by
a pilot to respond to the resolution advisory.
Coordinating RAs Against ACAS-Equipped Treats
Coordination of RAs is pairwise: ACAS X transmits to each threat its intent with respect
to that threat (i.e., whether to pass above it or below it), and receives from each threat the
intention of that threat with respect to own aircraft. Even in multiple encounters, when
ACAS X generates an RA against several threats simultane ously, the negotiation of
complementary RAs involves only two aircraft at a time; it results in a contract between
only two parties, and coordination takes place with each ACAS equipped threat
When the threat is TCAS or ACAS X-equipped, ACAS X sends a “TCAS Resolution
Message” to the threat. The information is in the form of a negative command, e.g., ‘do
not pass above,’ and is encoded as a Vertical Resolution Advisory Complement (VRC)
subfield. ACAS X will send a ‘do not pass above’ VRC to an equipped threat if it has
selected a climb sense advisory against that threat. Likewise, if it has selected a descend
sense advisory against an equipped threat, it will send a ‘do not pass below’ VRC to that
Before selecting an RA, ACAS X must first check to see if it has received a VRC from
the threat. If no VRC has been received, ACAS X will select the optimum RA based on
the encounter geometry. If a VRC has been received, in most cases (not always), ACAS
X will initially select an RA based on an intent complementary to that of the threat.
In the vast majority of cases the two aircraft see each other as threats at slightly different
moments in time. In contrast, “simultaneous” coordination occurs when both aircraft
select RAs before a VRC is received from the other aircraft. In the case of simultaneous
coordination, the RA intents can be be incompatible and and one aircraft must give way
to the other by reversing its intent. The aircraft that is obliged to give way is referred to
as the slave and the aircraft that prevails is referred to as the master.
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