Applications technology satellites F and G communications subsystem

The communications subsystem comprises the 30-ft parabolic reflector, the composite feed system, and the multiple frequency transponder. For high reliability, redundancy is used as required in these assemblies. The high effective radiated power (ERP) provided by this subsystem allows it to function as part of the experiments, in addition to performing as the interface between the spacecraft and the ground stations. Therefore, in addition to meeting the communications design requirements, the subsystem has been designed to meet the requirements for performance of several technical objectives, namely accurate pointing of a radiated beam to either an earth station or another satellite, and high ERP. Successful performance of these objectives is a requirement in the Position Location and Aircraft Communications Experiment, Tracking and Data Relay Experiment, and Instructional Television experiments. The composite feed subassembly, which provides radiating and receiving elements for the S, L, C, and UHF frequency bands, is used in conjunction with the high-gain RF reflector to provide optimum antenna performance. The transponder is the interface between the onboard experiments and ground stations and comprises the receiver, IF amplifier, and transmitter assemblies which perform frequency generation, translation, and power amplification functions. The receivers, operating at C, S, and L band, perform preamplification, and down-convert the RF signal to the 150-MHz IF. The IF amplifier assembly uses input switching to allow connection of a down-converter output to any of the three IF amplifiers; and output switching is used to connect an IF output to the desired up-converter. This achieves high reliability with maximum flexibility. The transmitters up-convert the IF signals which are then amplified by a traveling-wave tube amplifier (TWTA) or solid-state amplifier and fed to the appropriate composite feed elements. A synthesizer provides multiple mixing frequencies for up and down conversion in coherent and noncoherent modes of operation.