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Electronics & Communication Engineering Department
Asansol Engineering College
Kanyapur, Sen Raleigh Road, Asansol 713304, Burdwan, India
The first satellite that was used for communication purpose in INDIA was ARYABHATTA and it was launched in 19th April.1975. It was made and assembled by an organization called Indian Space Research Organization (ISRO). In the year 1981, a satellite named APPLE was launched in space which was the first Indian Experimental communication satellite. The unique feature of it was that it was a three axis stabilization geosynchronous satellite and weighed around 645 kg. The term APPLE is an abbreviation for Ariane Passenger Payload Experiment. It consisted of a (6/4 Ghz) processing equipment called Transponder. Various experiments were carried out with APPLE, [SITE, STEP (Other satellite telecommunication experiment projects)] and the results obtained from these experiments provided an impetus for Govt. of India to have its own multipurpose Geosynchronous Earth Orbit satellite under INSAT (Indian National Satellite) program. The first satellite INSAT-1A was launched in the year 1982 which was under this INSAT program, but this effort went in vain as the power house of this satellite consisting of solar cells did not operate properly( failed to open) and this satellite was unused latter on. The average electrical power required by INSAT-1 was approximately 1000W and was provided by the power house subsystem of the satellite. The payload was one C-band transponder and two S-band transponders. Later succession of INSAT-1 series was launched like INSAT-1B, INSAT-C and INSAT-D. After this due to the success of the first generation satellites, INSAT-2 series was launched viz. INSAT-2A, INSAT-2B, INSAT-2C, INSAT-3D and INSAT-2E which provided variety of services.
WHAT IS SATELLITE?
A Satellite is a solid object which revolves around some heavenly body due to the effect of gravitational forces which are mutual in nature. We can categorize satellites in two types, namely Passive Satellites and Active satellites. Passive satellites are not like active satellites. Even a moon can be a passive satellite. Thus passive satellites are relay stations in space. A passive satellite can be further subdivided into two types, namely Natural satellites and artificial satellites. A moon is a natural satellite of earth. But spherical balloon with metal coated plastic serve as artificial satellites.
Active satellites are complicated structures having a processing equipment called Transponder which is very vital for functioning of the satellite. These transponders serve dual purpose i.e. provides amplification of the incoming signal and performs the frequency translation of the incoming signal to avoid interference between the two signals.
All ABOUT SATELLITE COMMUNICATION
The term Satellite communication is very frequently used, but what is satellite communication? It is simply the communication of the satellite in space with large number of earth stations on the ground. Users are the ones who generate baseband signals, which is processed at the earth station and then transmitted to the satellite through dish antennas. Now the user is connected to the earth station via some telephone switch or some dedicated link. The satellite receives the uplink frequency and the transponder present inside the satellite does the processing function and frequency down conversion in order to transmit the downlink signal at different frequency. The earth station then receives the signal from the satellite through parabolic dish antenna and processes it to get back the baseband signal. This baseband signal is then transmitted to the respective user via dedicated link or other terrestrial system. Previously satellite communication system used large sized parabolic antennas with diameters around 30 meters because of the very faint and weak signals received. But nowadays satellites have become much stronger, bigger and powerful due to which antennas used have become automatically smaller in size. Thus the earth station antennas are now not large in size as the antennas used in olden days. A satellite communication system operates and works in the millimeter and microwave wave frequency bands from 1 Ghz to 50 Ghz. There are various frequency bands utilized by satellites but the most recognized of them is the uplink frequency of 6 Ghz and the downlink frequency of 4 Ghz. Actually the uplink frequency band is 5.725 to 7.075 Ghz and the actual downlink frequency band is from 3.4 to 4.8 Ghz. The major components of a Satellite Communication system is spacecraft and one or more earth earths.
THE EXCITING COMPONENTS OF SATELLITE i.e ITS SUBSYSTEMS
Attitude & orbit control system:
This subsystem comprises of rocket motors that keeps the correct orientation of the satellite in space by moving it back to the correct orbit. Various external forces cause to change the parking position of the satellite. The primary factors are gravitational forces of sun, moon earth and also other planets of solar system. Other factors include solar pressure on the antennas and solar sails, which is present on the body of the satellite. All these factors are hugely responsible for misbalancing of the satellite and also responsible for changing the parking position of the satellite. Apart from this the earth’s magnetic field is also playing a major role in changing the parking position of satellite. The earth’s magnetic field generates eddy currents in the metallic structure of the satellite as the satellite moves through the magnetic field. Thus the body of the satellite gets rotated called as wobble of the satellite.
Remedy for Misbalancing of the satellite: station keeping: It is a method of periodically accelerating the satellite in the opposite direction against the forces acting on the body of the satellite like gravitational forces, eddy currents etc. in order to maintain the correct orientation of satellite in space and maintaining its orbit. The two most common methods employed to keep the satellite stable in orbit are: spin stabilization and three axes body stabilization.
TTC and M SUBSYSTEMS:
These subsystems are found partly on the satellite and partly on the earth stations. Data obtained from the sensors present on the spacecraft are sent by the Telemetry systems through telemetry link to the controlling earth stations. The telemetry system monitors the condition of the spacecraft. Furthermore the Tracking system is present on the earth station which is all concerned about range, azimuth angles and elevation angles of the spacecraft by providing necessary information on it. There are various techniques used for tracking of satellite:
Velocity and acceleration sensors on the satellite can be used to establish the change in orbit.
Doppler shift of the telemetry carrier from the earth station or beacon transmitter may be measured to determine the rate at which the range is changing.
Ranging tones may be used for range measurement.
This is required to run satellite’s housekeeping and communication system. The block diagram of the power subsystem is shown as:
Solar panels generate direct current which is used to operate different subsystems. The batteries like Nickel-Cadmium batteries are charged by the DC power by employing the battery chargers. The stabilized low voltage is supplied to power various subsystems which are generated by the voltage regulator circuits. A dc to dc converter circuit generates high voltage dc which is used for operating the traveling wave tube amplifiers. Generation of ac from dc is done by dc to ac inverter circuits for running ac devices.
This subsystem can also be called as a reaction control subsystem. It is carried by the satellite in the GEO orbit. The dominant functions of it are:
· It helps the spacecraft to move to its assigned position in orbit and also helps to maintain it in that position.
· It is also used to maintain the direction of spin axis attitude control against the perturbation forces.
The main components of propulsion subsystem are: Low thrust actuators, High thrust motors eg: apogee kick motor, Apogee boost motor and finally Perigee kick motor. Low thrust actuators are further classified as Chemical thrusters and Electrical thrusters. These thrusters are used for attitude and orbit corrections. Moreover the Electric thrusters are mainly of two types 1.> Plasma thrusters 2.> Ion thrusters.
SPACECRAFT ANTENNA (subsystem)
Antenna subsystem is also an essential component of satellite system. Basically four main type of antennas are used: these are Monopoles and dipoles (wire antennas) which are mainly used in VERY HIGH FREQUENCY AND ULTRA HIGH FREQUENCY to provide communication for TTC and M subsystem. 2.> Horn antennas are mainly used at microwave frequencies. Horns are actually used as feeds for reflector. 3.> Array antennas are actually phased array antennas which are used on satellites to form multiple beams from single aperture. 4> Reflector antennas are commonly used for earth station antennas and the most widely employed shape of it is the paraboloid with a feed placed at its focus. The patterns for different satellite antennas are shown as:
THE KEY ELECTRONIC EQUIPMENT IN A SATELLITE OR TRANSPONDERS:
It is the key electronic component in a satellite. The transmitter receiver combination in a satellite is known as a Transponder. It performs two major functions 1.> It provides amplification of the signal thus providing a gain of around 110dB. 2> It also does the frequency down conversion or frequency translation of the uplink signal in order to avoid interference between the received and the transmitted signal.
Types of Transponders: 1. Bend pipe type Transponder 2. Regenerative type Transponder.
Bend pipe type transponders are also called conventional type transponders.
Diplexer (acting as a two-way microwave gate) is the device which is responsible or used by the satellite for both receiving the uplink signal and transmitting the downlink signal. The frequency down conversion is done in the carrier processor. Amplification of the weak received signal is done in the front end. The downlink frequency is brought to a sufficient power level by amplification by the power amplifier such as Traveling Wave tube. The carrier processing equipment determines whether the transponder is of conventional or regenerative type
Regenerative Transponders: The regenerative transponder is one where there is provision for detection and demodulation process. The main advantages for these kind of transponders are:
· The signal to noise ratio is improved.
· These are simpler and more flexible to implement.
· At low baseband frequency the amplification is easier to obtain in case of regenerative type.
Types of multi channel transponder systems:
Dual channelized system.
The various frequency translation schemes in use:
FOR CONVENTIONAL TRANSPONDERS
RF-RF Translation: This is a single mixer system. The diagrams of it is shown below:
RF-IF-RF translation schemes: This is a double conversion scheme using a single stable oscillator. This kind of translation scheme provides two advantages over RF-RF conversion scheme: 1. The process of carrier filtering is done at the IF band. 2. Before the return transmitted signal the uplink carriers can be easily removed. The diagram of it is shown below:
FOR REGENERATIVE TRANSPONDERS
The two common schemes are:
IF Remodulation scheme: In this technique the uplink RF spectrum is first translated down to low IF band , which is then modulated on to return RF.
Demodulation- Remodulation scheme: The remodulation removes the uplink noise and interference from return modulation.
SATELLITE LAUNCH VEHICLES:
Satellites are launched into its orbit by the satellite launch vehicles. These satellite launch vehicles are basically multistage rockets. It is classified into two types:
Expendable launch vehicle (ELV) eg: Ariane, Delta etc. These vehicles get destroyed in space and it also carries more than one satellite with it.
Reusable launch vehicle (RLV). Also known as space transportation system (STV) eg: Space shuttle. In case of these satellites the vehicle will return back to the earth after leaving the satellite in space. Thus they can be reused again and again.
Components of Launch vehicle:
Auto piloting system
Interactive steering subsystem
DIFFERENCE OF COMMUNICATION SATELLITE FROM COMMUNICATION RELAY:
For communication satellites the range is much higher than that of communication relay. Communication Satellite can cover up to several thousand kilometers.
For communication satellite the uplink and the downlink frequency is the same. But for communication satellites the uplink and the downlink frequencies are different in order to avoid interference.
Different frequency bands used in satellite communication:
Ultra high frequency band (UHF).