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ACAS X MOPS
X equipment periodically transmits interrogation signals. These interrogations are
received by Air Traffic Control Radar Beacon System (ATCRBS) or Mode S altitude
reporting transponders. ACAS X will detect the presence of Mode S-equipped aircraft by
listening to Mode S transmissions received with DF=11 (squitter transmissions) and
DF=17 (Extended Squitter, as described in §126.96.36.199.4.2.1 of this document. In reply to
the interrogations, the transponder transmits a signal which reports its altitude. The
ACAS X equipment computes the range of the intruding aircraft by using the round-trip
time between the transmission of the interrogation and the receipt of the reply. Altitude
information is provided by decoding the information in the received reply.
All ACAS X systems will include the use of hybrid surveillance (making use of Mode S
Extended Squitter information) to extend display range and limit interference. ACAS X
will also use surveillance information provided by ADS-B and ADS-R. ACAS X will not
require any of the requirements for ADS-B reception included in DO 317 as a minimum
capability, but will require some capability to receive DF=17 Messages per DO 260B.
The quality of ADS-B surveillance is expected to enhance the performance of Collision
Avoidance due to the quality of the surveillance data . ACAS Xa/Xo will process ADS-R
messages which are re-broadcast by ADS-B ground stations when this service is
available, in a manner consistent with the processing of other ADS-B Messages, provided
the ADS-R service is certifiable for use in critical applications.
ADS-B Message data from other aircraft is received and processed to provide the
location and velocity of those aircraft as well. Data from transponder replies and
potentially ADS-B on those intruders are tracked, and the system provides a probabilitisic
estimate of the location of all intruders. These data are used to determine whether some
action must be taken to provide awareness or a maneuver should be conducted to avoid
the intruding aircraft. Each threat aircraft is processed individually to permit selection of
the minimum safe resolution advisory based on track data and coordination with other
Active CAS-equipped aircraft with a resolution advisory generating capability.
Bearing estimates are used by the collision avoidance algorithm which determines if the
predicted horizontal separation at closest point of approach is large enough not to declare
the intruder a threat. Bearing estimates are also used to display traffic advisories in the
cockpit of ownship.
Note that ADS-B data is used in ACAS X for both hybrid surveillance and threat
resolution. For TCAS II, ADS-B was only used for hybrid surveillance, but not threat
resolution. For ACAS X, TAs and RAs can be issued against targets providing ADS-B
data as long as it is validated with active surveillance. If ADS-B fails validation, the
system reverts back to using active surveillance for threat resolution. Validated ADS-B
is required for generation of RAs. ACAS X also has the optional capability to generate
TAs against ADS-B only or ADS-R only targets. A display that indicates to the pilot that
a TA against an ADS-B only or ADS-R only target can never progress to an RA is
required for this optional capability. If the optional capability is not implemented, then
ADS-B and ADS-R only targets will be displayed as non-threats and proximate traffic as
appropriate, but TAs will not be generated against them.
As in TCAS II, RAs indicate vertical maneuvers that are predicted to either increase or
maintain the existing vertical separation from threatening aircraft. TAs indicate the
positions of intruding aircraft that may later cause resolution advisories to be displayed.
The resolution advisories employed in ACAS Xa/Xo equipment do not provide
horizontal escape maneuvers and therefore are not intended to increase horizontal
separation. Increased horizontal position accuracy available through the use of new
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