Satellites are physical objects that are developed by humans, and are placed in the Earth's orbit. They are used for various scientific and technological purposes. They have a number of instruments, and subsystems to accomplish the many tasks assigned to them. Basically, a subsystem is a set of devices that are responsible for carrying out the operations of the satellite, without any interruptions. The most important of these subsystems are described below:
The communication subsystem is the most vital part of a satellite. The key components of a communication subsystem are - antennas, receivers, and transmitters. Their main function is to receive and send signals all across the globe. Some satellites, and ground stations have specially designed radio dishes that are used to exchange data. The curved section of these dishes helps in the reflection of both incoming as well as outgoing signals, from its central horn. The antennas are needed for the purpose of transmitting images at different frequency levels, and with varying coverage. They are also capable of receiving instructions from their command centers on Earth. The beacons and transponders fitted in some satellites, help in ground tracking.
Command and Data Handling
This subsystem comprises computers that collect data, and processes them on board. These computers are capable of delivering results according to the instructions received from the command centers. They are used for the physical measurement of distant objects. The inputs for this purpose are received from various sensory devices, and processed to convert them into encoded data, and the same is transmitted to the ground station for further analysis. This type of data transmission is one of the most important functions of the satellite.
The work of satellites is completely disrupted without the power supply subsystem. The main task of the power supply subsystem is to generate, store, and supply adequate amounts of electrical power for the satellite. The power requirements depend on the total area covered, kinds of instruments used, and the communication load.
Chemical batteries, solar cells, or radioisotope devices are used to power them. Solar cells are most widely used in the power supply subsystem. Solar cells convert the solar energy into electrical energy. They are therefore the most reliable source of power supply. Chemical batteries oxidize the fuel for the production of power. They are used to supply electricity, when the satellite enters the shadow of the Earth. The disadvantage of these batteries is their bulky size. For this reason, they are used mostly for short-term missions.
Some satellites are also provided with turbo-generators, or thermionic generators. These generators use heat to produce electricity. The heat required is either obtained from the sun, or from small nuclear reactors fitted with the subsystem. However, currently these nuclear subsystems have only been used on an experimental basis.
A satellite can work only when all these subsystems perform their functions correctly. Malfunction in any of these subsystems can lead to total breakdown of this machine.