Purpose: This document describes the "Mark 2" version of the airborne components of the Aircraft Communications Addressing and Reporting System (ACARS). This system is intended for use in conjunction with existing airborne radio equipment to enhance the effectiveness of air-ground operational control communications. This enhancement accrues from the abilities of the system to provide air-toground and ground-to-air data communications. Airlines and other ACARS airborne sub-system users should note that part of any commitment to the system is the definition of their planned use of its data communications capability. Such definitions are needed by both the Data Link Service Provider (DSP) on the ground and airborne equipment manufacturers to permit them to generate appropriate software.

The intent of this document is to provide general and specific design guidance for the development and installation of the "Mark 2" airborne equipment. As such, this guidance will cover the desired operational capability of the system and the standards necessary to achieve interchangeability of the hardware. Necessary background to this is a summary description of the overall system. This may be found in ARINC Specification 620, wherein are described both the link-control-related functions, (i.e., those functions essential to maintaining the air-ground communications link) and the service-related functions of the system. While the airborne sub-system must accommodate all the link control functions in order to be considered part of the system, it need accommodate only those service-related functions requested (per paragraph one above) by the user.

Equipment manufacturers should note that this document aims to encourage them to produce maintenance-free, high performance equipment. They are at liberty to accomplish this by the use of design techniques they consider to be the most appropriate. Their airline customers are more interested in the end result than in the means to achieve it.