Satellite Subsystems
Irrespective of the intended application, is it a communications satellite or a weather satellite or even an Earth observation satellite, different subsystems comprising a typical satellite include the following:
- Mechanical structure
- Propulsion
- Thermal control
- Power supply
- Tracking, telemetry and command
- Attitude and orbit control
- Payload
- Antennas
The structural subsystem provides the framework for mounting other subsystems of the satellite and also an interface between the satellite and the launch vehicle.
The propulsion subsystem is used to provide the thrusts required to impart the necessary velocity changes to execute all the maneuvers during the lifetime of the satellite. This would include major maneuvers required to move the satellite from its transfer orbit to the geostationary orbit in the case of geostationary satellites and also the smaller maneuvers needed throughout the lifespan of the satellite, such as those required for station keeping.
The thermal control subsystem is essential to maintain the satellite platform within its operating temperature limits for the type of equipment on board the satellite. It also ensures a reasonable temperature distribution throughout the satellite structure, which is essential to retain dimensional stability and maintain the alignment of certain critical equipments.
The primary function of the power supply subsystem is to collect the solar energy, transform it to electrical power with the help of arrays of solar cells and distribute electrical power to other components and subsystems of the satellite. In addition, the satellite also has batteries, which provide standby electrical power during eclipse periods, during other emergency situations and also during the launch phase of the satellite when the solar arrays are not yet functional.
The telemetry, tracking and command (IT &C) subsystem monitors and controls the satellite right from the lift-off stage to the end of its operational life in space. The tracking part of the subsystem determines the position of the spacecraft and follows its travel using angle, range and velocity information. The telemetry part gathers information on the health of various subsystems of the satellite encodes this information and then transmits it. The command element receives and executes remote control commands to effect changes to the platform functions, configuration, position and velocity.
The attitude and orbit control subsystem performs two primary functions. It controls the orbital path, which is required to ensure that the satellite is in the correct location in space to provide the intended services. It also provides attitude control, which is essential to prevent the satellite from tumbling in space and also to ensure that the antennae remain pointed at a fixed point on the Earth’s surface.
The payload subsystem is that part of the satellite that carries the desired instrumentation required for performing its intended function and is therefore the most important subsystem of any satellite. The nature of the payload on any satellite depends upon its mission. The basic payload in the case of a communication satellite is the transponder, which acts as a receiver, amplifier and transmitter. In the case of a weather forecasting satellite, a radiometer is the most important payload. High resolution cameras, multispectral scanners and thematic mappers are the main payloads on board a remote sensing satellite. Scientific satellites have a variety of payloads depending upon the mission. These include telescopes, spectrographs, plasma detectors, magnetometers, spectrometers and so on.
Antennas are used for both receiving signals from ground stations as well as for transmitting signals towards them. There are a variety of antennas available for use on board a satellite. The final choice depends mainly upon the frequency of operation and required gain. Typical antenna types used on satellites include hom antennas, centre-fed and offset-fed parabolic reflectors and lens antennas.
