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ACAS X MOPS
Avionics in the Traffic Alert and Collision Avoidance System II (TCAS II) use
secondary surveillance radar to interrogate Mode A/C and Mode S transponder -equipped
aircraft and obtain information needed to track their position / speed (ID, barometric
altitude, range, etc.). The equipment uses this information to predict the time to closest
point of approach (CPA), and determine the altitude difference at that point. Depending
on time to CPA, and the particular threshold being violated, TCAS II will issue an
advisory. TCAS II avionics employ rule/heuristics-based logic (based on decades of
research / simulation). The measured bearing information provided by TCAS II antenna
systems is inadequate to support horizontal advisories. It is used to provide the bearing
for display of traffic, and some horizontal filtering.
TCAS II Version 6.0 was first introduced in the United States in 1990 with collision
avoidance logic Version 6.0 and later 6.04 and 6.04a. TCAS II Version 7.0 was
introduced in the United States and Europe in 2000 and was subsequently mandated by
the International Civil Aviation Organization (ICAO) for all commercial turbine -powered
transport aircraft worldwide having more than 19 passenger seats or having a maximum
take-off weight above 5700 kg. TCAS II Version 7.1 addressed several shortfalls in
Version 7.0 logic. TCAS II Version 7.1 was required by Europe by December 1st
(Ref. EU Commission Regulation No. 1332/2011, subsequently amended by Regulation
2018/583). TCAS II is referred to as the Airborne Collision Avoidance System II (ACAS
II) in ICAO terminology. TCAS II Version 7.1 is still in operation and was standardized
in RTCA DO-185B/EUROCAE ED-143. Moreover, ICAO Annex 10 (volume IV) states
that all ACAS installations after 1 January 2017 shall be compliant with version 7.1. At
the time of this publication, there are no active efforts to modify TCAS II beyond its
current minimum operational standards. All references to TCAS in this document, unless
otherwise noted, refer to TCAS II.
The evolving global airspace presented an opportunity for a new system, ACAS X, to
improve upon TCAS II. The ACAS X concept ensures interoperability with current and
future ATM systems (including TCAS II), meaning that advisories provided on different
aircraft in the same encounter are complimentary, maneuvers performed by pilots
following advisories are compatible, and that CA operations are minimally disruptive to
other ATM operations in that airspace.
There are four major improvements of ACAS X over TCAS II. In addition to the
spectrum congestion mitigation changes introduced as part of the TCAS II Hybrid
Surveillance MOPS update, ACAS X will incorporate the following:
Improved risk ratio, reduction of unnecessary alerts, and a reduced total number of
resolution advisories (RAs) issued.
Easier tuning and optimization to match the system’s surveillance and tracking
capability or to match the aircraft performance capabilities.
The capability to resolve conflicts using a Decision Theoretic approach, specifically
Markov Decision Process (MDP) rather than a rule/heuristics-based approach. The
new logic is referred to as Optimized Resolution Logic and is reflected in the Threat
Resolution Module (TRM).
The capability to accept and process new surveillance sources like GNSS and ADS-
B. The new sources, in addition to Secondary Surveillance Radar (SSR), may be
used for tracking cooperative aircraft more accurately.
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