A study of the telecommunication performance of a space communications relay satellite with simultaneous multiple-vehicle access capability



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The problem of providing adequate communication coverage for Earth orbiting space vehicles using only ground-based terminals is investigated. It is shown that for low orbital altitudes, coverage is provided for a very small percentage of the orbital period. Use of three synchronous satellites to relay data between spacecraft and ground is shown to provide continuous communication coverage. The geometric and dynamic relationships between the relay satellite and orbiting spacecraft which affect communication system performance are determined. Relationships between output performance requirements and intermediate frequency signal-to-noise ratios at the spacecraft and ground terminals are derived. These equations and specified performance requirements for Apollo communication functions, are used to determine the required terminal receiver signal-to-noise ratios. A relay satellite system capable of supporting three orbiting spacecraft, each with three ground-to-spacecraft and three spacecraft-to-ground carriers is defined. A frequency translation repeater is chosen for the relay device, with a retrodirective phased array for the satellite-spacecraft antenna. A link analysis of the total system is accomplished. The system is analyzed as four separate links, ground-to-satellite, satellite-to-spacecraft, spacecraft-to-satellite, and satellite-to-ground, using standard link calculations. An equation is derived to relate the individual link performance to total system performance. It is shown that all required communication functions can be accomplished when using the spacecraft directional antenna. Only a spacecraft-to-satellite-to-ground voice link (down link) is possible if the spacecraft omnidirectional antenna is used. The other two required functions using this antenna, a down-link telemetry channel and a ground-to-satellite-to-spacecraft voice link, cannot be established. A change in modulation techniques is required to provide these capabilities.