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This document provides the Algorithm Design Description (ADD) for the Surveillance and Tracking
Module (STM) and the Threat Resolution Module (TRM) of the next generation Airborne Collision
Avoidance System (ACAS X). The algorithms are described at a sufficiently high level to allow
for implementation in a variety of software languages and hardware platforms, thereby providing
maximum freedom to manufacturers while ensuring the intended output from the system. This
document provides descriptions of the algorithm’s operations. Every effort has been made to ensure
the accuracy of this text, however, in case of a conflict between the description and the function of
the algorithm, the algorithm function is correct.
This document assumes the reader is familiar with ACAS X, including planned variants of the
system software and its modular breakdown. If not, the reader is encouraged to review the ACAS
X Concept of Operations (ACAS X CONOPS) .
The ACAS X algorithm design consists of two separate modules: (1) the Surveillance and Tracking
Module (STM), which detects aircraft in the vicinity and tracks their position, and (2) the Threat
Resolution Module (TRM), which identifies threats and provides resolution guidance. The algo-
rithms in this ADD are limited to ACAS Xa and ACAS Xo, the variants of ACAS X intended for
installation on aircraft that currently require TCAS II equipage. This document contains modifica-
tions informed by operational suitability and performance evaluations conducted via modeling and
simulation as well as analysis of flight tests in 2013 and 2015 as well as the Operational Evaluation
conducted in 2017.
Although this document provides justification for certain aspects of the TRM algorithms, it is not
intended to overview the analyses that contributed to the various design decisions. This document
does not discuss the process of optimizing the lookup tables. For such details, see .
The Traffic alert and Collision Avoidance System (TCAS), currently mandated worldwide on all
large transport and cargo aircraft, has been shown to significantly reduce the risk of mid-air colli-
sion. TCAS uses an on-board surveillance system to monitor nearby air traffic. The surveillance
information is then fed into the threat resolution logic to determine whether to alert the pilots of a
potential collision. If deemed necessary to prevent collision, TCAS will issue a resolution advisory
to the pilots to climb or descend at a particular rate.
Developing robust collision avoidance logic that reliably prevents collision without excessive alert-
ing is challenging due to sensor error and uncertainty in pilot response and, consequently, the future
paths of the aircraft. The current TCAS logic was the result of many years of development and
involved the careful engineering of many heuristic rules. Unfortunately, due to the complexity of
the logic, it is difficult to revise it to accommodate the evolution of the airspace and the introduction
of new surveillance technologies and procedures.
ACAS Xa/Xo MOPS, Vol. II
© 2018 RTCA, Inc.
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