Communications satellites are similar to other unmanned spacecraft, but are refined for the communications mission in GEO. Within the satellite are two major sections:
- The spacecraft bus (likewise, the bus).
- The communications payload (or simply, the payload).
The bus contains the support vehicle and control subsystems that allow the payload to perform its mission as a microwave repeater in space. Within the bus we find:
- Tracking, telemetry, command and ranging (TTC&R) to provide a remote control and housekeeping link to the satellite operator.
- Solar panels to provide all of the prime power.
- Batteries to operate the satellite when it is hidden from the sun by the earth or moon (called eclipse, happening for up to 70 minutes a day during equinox season).
- Reaction control system (propulsion) to be used to correct the orbit.
- Attitude and spacecraft control processing to assure that the antennas are pointed at the ground.
- Thermal control to maintain the electronics and other components within a safe temperature range over the life of the satellite.
- Structure to hold everything together and protect the subsystems during launch and after deployment on orbit.
All of these subsystems are important because they must work properly throughout the satellite's lifetime. Problems in these areas can reduce satellite performance and even threaten its ability to operate.
On the left is a fully-deployed satellite as it appears on orbit.
On the right is its stowed configuration for launch by an expendable booster rocket.
The payload is the business-end of the satellite, consisting of:
- Repeater (microwave receivers, RF multiplexers, power amplifiers, channel processing and switching). Contained within the repeater are the transponders,
- Antennas (reflectors, feeds, feed networks, support structure and pointing mechanisms). The antennas create "footprint" coverage but require the repeater to receive and transmit the actual signals from and to the ground.