Communications Satellite

Overview

A communications satellite is a relay platform parked in geostationary orbit, where its 24-hour period matches Earth's rotation so it appears fixed over one longitude and ground antennas never have to track it. The spacecraft splits cleanly into a service bus and a payload. The bus keeps the satellite alive and pointed; the payload does the paid work of receiving weak uplink signals, shifting them in frequency, amplifying them by ten orders of magnitude and beaming them back down.

The structure is built around a Spacecraft Structure central thrust tube that carries launch loads into a ring of honeycomb sandwich panels, which double as equipment radiators. Bolted to those panels is the Communications Payload: a bent-pipe repeater whose receive reflectors, low-noise amplifiers, frequency converters and high-power amplifiers form the signal chain. Power comes from the Electrical Power Subsystem subsystem, with two sun-tracking solar wings and a Li-ion battery to ride through eclipse. The Attitude Control Subsystem holds the antennas pointed at their coverage region, and the Propulsion Subsystem subsystem raises the orbit after launch and trims the station for fifteen years.

How it works

After separation the satellite is in a transfer orbit and must climb to GEO. The liquid apogee engine in the propulsion subsystem fires at successive apogees to circularize the orbit, after which xenon ion thrusters take over the small, relentless north-south corrections that fight luni-solar perturbation. Once on station, the solar wings deploy and the solar array drive mechanisms rotate them to face the Sun continuously, feeding the power conditioning unit that regulates the bus and charges the battery.

The repeater runs continuously. Each transmit reflector is illuminated by a feed horn, and the receive chain hands signals through low-noise amplifiers and converters before the Command & Data Handling computer-managed amplifiers and channel filters drive them out again. Across all of it, the Thermal Control Subsystem control subsystem dumps amplifier waste heat through radiators and heat pipes while survival heaters protect cold components, and the TT&C Subsystem link keeps operators in contact with the spacecraft.