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ACAS X MOPS
ACAS Xo functionality
ACAS Xo will require all functionality from ACAS Xa systems. ACAS Xo provides the
capability of applying different, more operationally appropriate conflict detection and
resolution logic to a particular “designated” intruder.
The designation function is specified in the ASA System MOPS requirements.
Therefore, aircraft that equip with ACAS Xo will also require integration with an ASA
System or equivalent for Xo mode designation and display requirements. ASA System
integration provides the flight crew interface necessary for ACAS Xo traffic designation,
as well as other CDTI display capabilities such as a unified traffic picture and directional
This document includes requirements for two initial ACAS Xo modes, DNA and CSPO-
3000, and limits the use of ACAS Xo to one mode and one aircraft at a time. Designated
No Alert, or DNA allows suppression of alerts for the designated aircraft while CSPO-
3000 includes logic for closely spaces parallel operations down to 3,000 ft. While use of
the ASA System or equivalent is required to provide the controls and displays necessary
for ACAS Xo, the initial ACAS Xo modes are designed for stand-alone use (i.e., no
corresponding ASA System applications).
Future versions of the ACAS Xa/Xo MOPS are expected to provide additional ACAS Xo
modes designed to specifically support future ASA System applications. These
applications may automatically designate the ACAS Xo mode. The ACAS Xo system
may be required to support designation of multiple aircraft concurrently, and also
operation of multiple ACAS Xo modes concurrently.
Figure 1-4 shows a possible implementation of ACAS Xa/Xo equipment that could be
installed on aircraft. This depiction is intended as a conceptual aid and not intended to
serve as a prescription for the architecture of the system as implemented by
The functional groupings, shown by the dotted black lines in the figure, will be described
in the paragraphs that follow. The smaller blocks show the first level functions (ex.,
“Surveil Aircraft”, “Estimate Score”, etc.) which are defined in further detail in the
ACAS X Functional Architecture Document (FAD) (Ref. AA). The numbers in the
figure correspond to the function identification number used in the ACAS X FAD.
The Signal Processing Module (SPM) / Front End Surveillance is responsible for (a)
converting analog (RF) signals into digital, demodulating, and encoding for transfer to
the software for processing, (b) decoding, modulating and converting digital signals to
analog (RF) signals for transmission to the ground and other aircraft, and (c) determining
which targets need to be interrogated. Requirements for transmission powers, rates,
directions / ports would fall within the functions in this module. Additionally,
measurement of intruder range and bearing is done here.
The Surveillance and Tracking Module (STM) is responsible for tracking the relative
3D position and velocity of ownship and targets, and passing relevant track file
information to threat logic. The STM represents the uncertainty in its estimates of track
parameters (such as position, velocity, etc.) using samples that embody the uncertainty in
the estimates. The STM uses active surveillance to validate ADS-B data. The STM also
determines, based on validation and ADS-B quality metrics, which data source’s tracks
(ADS-B or active) should be transmitted to the TRM for threat detection and threat
resolution purposes. For ACAS Xo applications, the STM will receive target
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