Every satellite is composed of a bus—that is, the set of systems that keep it going—and a payload that accomplishes its mission. The bus plays a supporting role; its functions include power generation and distribution, attitude control, and propulsion. On a communication satellite, the payload is the communication subsystem, which carries out the communications mission (receiving and transmitting information). The payload of a communication satellite has one or more antennas, receivers, and transmitters, as well as hardware and software that perform some information processing. Redundant (spare) units are included for all equipment except the antennas, with each spare corresponding to one or more operating units.
Antennas serve as interfaces between uplinks (signals transmitted from Earth) and downlinks (signals transmitted to Earth) and the electronics inside the satellite. Earth-coverage antennas receive signals from all points on Earth with approximately equal sensitivity and/or transmit signals to all points on Earth with approximately equal power. Spot-beam antennas concentrate their receiving sensitivity or transmitting power on a limited area on Earth; the location of this area can be fixed or steerable.
Receivers are designed to amplify the very weak (less than one-millionth watt) received signals while minimizing noise. They also filter the received signal to reject out-of-band noise and interference from unwanted signals. Processing can be as little as a frequency translation from the uplink frequency to the downlink frequency; if this were not done, the transmitter's power would prevent all reception on the same frequency. However, processing often includes additional filtering and routing different groups of received signals to different transmitters, and it can also include demodulating and decoding the uplink information, then remodulating and recoding it for the downlink.
Modern communication subsystems divide received signals into many separate groups for efficient transmission and for routing to multiple antenna beams. Each group of signals and the associated transmitter is called a repeater (or transponder). The core of each transmitter is a high-power (typically 10- to 100-watt output) amplifier. A transmitter can also include post-amplifier filters, as well as switches that route the signals to various antennas.