Purpose of this Document

This document describes Airborne Collision Avoidance System X (ACAS X) functionality and provides the necessary interface definitions and protocols to accommodate the requirements of RTCA DO-385: Minimum Operational Performance Standards for Airborne Collision Avoidance System X (ACAS X) ACAS Xa and ACAS Xo) (latest version applies) and the requirements of RTCA DO-386: Minimum Operational Performance Standards for Airborne Collision Avoidance System X (ACAS X) ACAS Xu (latest version applies).

Additionally, this document describes interfaces and protocols necessary to accommodate Cockpit Display of Traffic Information (CDTI) based on reception of Automatic Dependent Surveillance – Broadcast (ADS-B) data and Traffic Information Services – Broadcast (TIS-B) data. The equipment becomes ACAS X with ADS-B IN applications added, as defined by RTCA DO-317C: Minimum Operational Performance Standards for (MOPS) for Aircraft Surveillance Applications (ASA) Systems (latest version applies).

ACAS X hybrid surveillance is an integral part of RTCA DO-385 and DO-386 MOPS.

ARINC 735C compliance requires conformity to the relevant RTCA MOPS for the prescribed application.

COMMENTARY

The control and Human Machine Interface (HMI) for CDTI and ADS-B IN have not been standardized. Therefore, this Characteristic attempts to provide interfaces which could potentially support many different options for control and interfacing. It is expected that actual aircraft installations will use a subset of these.

Older Traffic Alert and Collision Avoidance System (TCAS) equipment (ARINC Characteristic 735 and ARINC Characteristic 735A compliant) were designed to be backward compatible with Mode S transponders defined by ARINC Characteristic 718, which are only compatible with RTCA DO-185. ARINC 735A equipment is compatible with Mode S transponders defined by ARINC Characteristics 718 and 718A, which are compatible with both RTCA DO-185 and RTCA DO-185A.

ARINC 735B TCAS equipment was intended to be compatible with ARINC 718A transponders that provide extended squitter (i.e., ADS-B OUT, RTCA DO-260/A/B) capability, which are compatible with RTCA DO-185B.

An ARINC 735C ACAS X traffic computer is intended to be compatible with transponders that support the Transponder to TCAS Protocol and TCAS to Transponder Protocol (XGD/TGD) protocol; it is not expected that an ARINC 735C ACAS X traffic computer would support the older ARINC 735 protocol. An ARINC 735C ACAS X traffic computer is also intended to be compatible with RTCA DO-260B/C compliant transponders which will transmit squitters with Resolution Advisory (RA) data.