This interface control document (ICD) defines the subsystem communications interfaces and message content for a class of digital intermediate frequency (IF), or “all digital,” strategic or large fixed satellite communications (SATCOM) terminals. The intent of this document is to act as an Open Standard Digital-IF Interface (OSDI) allowing lower cost subsystem development by a variety of vendors, taking full advantage of the migration to digital IF SATCOM architectures, with ensured interoperations via compliance to a set of defined open interfaces. This ICD begins with a top-level overview of the Future Advanced SATCOM Technologies (FAST) architecture, identifying the processing subsystems and interface objects/classes. The remainder of the document is organized into a series of brief overviews of the subsystems, highlighting their top-level functional requirements and interfaces, and then a detailed decomposition of the interface objects and the associated classes (components of the interface objects within a specific subsystem), the messaging types and communications roles/concept, and ultimately the message content governing how the subsystems interoperate.