This document is a functional description of the intended configuration and operation of the Apollo Unified S-band Telecommunications and Tracking System. In particular, this description applies to the link between the command-service module and the ground tracking station.The lunar excursion module-to-ground link is similar, and an equivalent description applies. The document begins with a short resume of the system's developmental history. Then the basic spacecraft and ground system concept are simply explained. Following are detailed descriptions of the communications signals, modulation techniques, and subsystem configurations. The document concludes with explanations of system operational techniques.