Purpose: This document contains the physical form and fit dimensions, the electrical interface definition and a description of the functional operation of the airborne components of the High Frequency Data Link (HFDL) communications system. An overview of the associated ground system is also provided. The protocols used in the HFDL system are defined in ARINC Specification 635 "HF Data Link Protocols".

The intent of this document is to provide general and specific design guidance for the development and installation of the airborne equipment. As such, this guidance covers the desired operational capability of the HFDL system and the standards necessary to achieve interchangeability of the airborne hardware. The HF Data Link system has various modes of operation which are described more fully in Chapters 3, 4 and 5.

COMMENTARY: 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.