Satellite communication is the term used to describe any form of communication that occurs between two earth stations using a satellite. Electromagnetic waves are used as carrier signals in this communication. In this article, we are going to discuss the various satellite subsystems. In satellite communication system, various operations take place, among which the main operations are orbit controlling, altitude of satellite, monitoring and controlling of other subsystems. When we are talking that the satellite is in a designated orbit, it means it must remain there and performs all necessary operations. So, during that particular like requirement various things we need to monitor like whether the satellite is in the correct orbit, what is the height of the satellite, and monitoring and controlling other subsystems. Satellite communication mainly consists of two segments:
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Space segment: Space segment means the satellite is in space so the components or the subsystems of this segment are the communication satellite itself, satellite payloads, launch vehicles, etc. Major space segment subsystems are:
- AOC Subsystem(altitude and orbit control subsystem)
- TT&C Subsystem(telemetry tracking and command subsystem)
- Power and Antenna Subsystem
- Transponders
- Earth segment: When we are talking about the earth segment which is also known as a ground segment, it means transmitting equipment, receiving equipment, and antenna system. The main components of the earth segment are called earth stations. It presents in the ground segment which has the ability to access the satellite repeater in order to provide communication between users.
Through communication links, the earth segment and space segment subsystems can communicate with one another.
Satellite Subsystems
We can see the diagram which shows the satellite subsystems. Satellite subsystems can be broadly divided into four categories you can see four columns have been shown in diagram.
- First is the power supply subsystem
- Second is the attitude and orbit control
- Third is the telemetry tracking in command and
- Fourth is the communication subsystems
So we are going to discuss about these four satellite subsystems in detail.
Power Supply Subsystems
First is the power supply subsystem when we are talking about the power supply subsystem we can see there are many elements in this subsystem which are solar array, battery, power supply control circuitry. When we are talking about the satellite like the communication satellite, electrical power supply is required because it is used to run the spacecraft so when the satellite is in the stationary orbit, the power supply is required.
- Solar Array: Now we will discuss about solar cell which is an important component of the satellite system. Solar cells are photovoltaic cells and it converts the solar radiation into electricity and then it was going to be converted into the required voltage level in a power supply and the conversion efficiency of a typical solar cell it is about 12 percent to 15 percent. Typical satellite solar cells generate approximately 1 to 2 kilo watt of power. Thousands of minute solar cells are placed on the surface of the satellite. If the surface area is not adequate, so that required number of solar cells can be installed, then a separate projection can be used over the satellite and this kind of separate projection is known as a solar panel, which convert sun energy directly into the electrical energy.
- Battery: When there is no availability of the sun during the unfavourable weather conditions like during snowfall, during night out, etc, there is a requirement of the battery. Also when the like solar power is not available, the battery pack up is used.
- Power Supply Control Circuitry: Power supply control circuitry to control and monitor the various units over there. So this is all about the power supply subsystem.
Attitude And Orbit Control
Second category is the Attitude & Orbit Control. This is very important subsystem of a satellite attitude and orbit control means it is concerned about controlling the orbit of the satellites besides maintaining stabilization and its position. There are various forces which are acting on the satellite see satellite is at a height of approximately 42,000 kilometer and there are number of forces which are working on the satellites and which may change the attitude and orbit of a satellite. We know that earth is not a true sphere there are certain bulging flattening so there are certain forces that may impact the satellite gravitational forces due to sun, moon and other planets pressure due to solar radiation, magnetic field of the earth. So there are various orbital perturbations which may impact the satellite orbit. So there is a requirement of attitude and orbit control unit which is like parts are propulsion system, sensors, attitude control, antenna control, and orbit control.
In case of the attitude control two types of sensors are used one is the earth sensor and second is the sun sensor. :
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Propulsion System: Propulsion system is a reaction control system which is carried by the satellite in the geostationary orbit and usually there are three kinds of units for the propulsion system the units are below.
- low thrust actuators
- high thrust motors and
- with space shuttle.
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Attitude Control: There is a requirement to identify whether the satellite is stable or unstable or whether the correct attitude is being maintained so there are mainly four operations are required. The operations are:
- Detection of the satellite attitude: first you have to identify at which particular height the satellite is and there are variety of sensors which are placed on the body of the satellite subsystem. So those sensors are used to sync the data, sends the data to the ground station. So first step is detection of satellite attitude.
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Comparison with the reference success: obviously whatever the data we are getting we need to compare with the reference one in case there is any difference then there is a requirement to identify or evaluate the correct torque and once the torque is being calculated, then there is a requirement of the correction. We must remember that there are two types of control:
- The active attitude control- the satellite is unstable or sufficiently stable within the desired attitude configurations.
- The passive attitude control- In case of the passive attitude control what is the requirement the attitude corresponds to a position of a satellite.
Telemetry Tracking And Command
We can see there are three terms in the name telemetry, tracking, and command. So the function of the telemetry is to receive the data. As we already know that there are a number of sensors which are placed on the body of the satellite so telemetry system collects the data from many sensors and sends this particular data to the controlling earth station. The data may be in the form of the pressure and fuel tank, the temperature of the subsystems, voltage current in the power conditioning unit and many more. so this is the function of the telemetry.
Second is the tracking. Tracking determines the slant range for accurate determination of satellite orbit as if we are saying we are tracking some one it means we are continuously monitoring and identifying what it is doing. The tracking system is located on earth station and it provides the information about range elevation, azimuth angle so that if there is any change in the orbital elements, then corrections can be made accordingly. This is the function of the tracking.
Third is the command. Command means if there is a requirement to convey certain commands or certain information according to which some action needs to be taken out then this particular system works. So this is the function of the command subsystem.
Communication Subsystem
Communication subsystem is one of the major component of the communication satellite because this subsystem consists of more than one antennas and antennas may be transmitting antennas, receiving antennas, which are going to transmit over a wide bandwidth, and set of transmitter and receiver as we can see in the diagram above. So this transmitters and receivers are used to amplify, re-transmit the incoming signals and this particular set in general combination of this is known as a transponder.
In the case of the satellite communication two types of transponders are used.
- bent pipe
- regenerative type
So in the diagram we can see there is a frequency converter. There is a uplink frequency and downlink frequency. At uplink frequency, signal is being transmitted towards the satellite while for downlink frequency, signal is transmitted from the satellite towards the earth station. The uplink frequencies and downlink frequencies get different so the frequency conversion is required from at which particular point uplink frequency is being converted into the downlink and then the corresponding signal or the information is being retransmitted back to the earth station.
There are various satellite antennas which are commonly used reflector antenna, horn antenna, wire antenna, Cassegrain antenna. So they can be considered as a separate unit but they have a very important function in the case of the communication satellite because we can say that the antenna is an integral part of the satellite subsystem.