Purpose of this Document

This Characteristic describes a Traffic Alert and Collision Avoidance System (TCAS) that provides the necessary interface definitions and protocols to accommodate the requirements of RTCA/DO-185A.

RTCA/DO-185A, "Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment", requires an increase in the amount of data to be transferred from TCAS to the transponder than was required by RTCA/DO-185. Specifically, the Resolution Advisories Report and Data Link Capability Report are expanded. In addition, the protocol is expanded to allow TCAS to request data contained in the ground initiated COMM-B buffers of the on-board MODE S transponder and provides definition of the protocol used by the transponder for delivery of such data. The TCAS/Transponder Interface and Protocol definitions provided in Attachments 6A through 6Z are not used to satisfy the requirements of RTCA/DO-185A. They are retained to enable the upgraded TCAS to operate in accordance with the requirements of RTCA/DO-185 when paired with an older transponder that cannot support the requirements of RTCA DO-185A.

COMMENTARY

The TCAS system described by this document is designed to be backward compatible with Mode S transponders defined by ARINC Characteristic 718 which are only compatible with RTCA/DO-185. The TCAS system is also compatible with Mode S transponders defined by ARINC Characteristic 718A, which are compatible with both RTCA/DO-185 and RTCA/DO-185A