Electronics & Communication Engineering Department
Asansol Engineering College
Kanyapur, Sen Raleigh Road, Asansol 713304, Burdwan, India
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
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
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
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.
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
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.
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:
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
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
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
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
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:
launch vehicle (ELV)
eg: Ariane, Delta etc. These vehicles get destroyed in space and it also
carries more than one satellite with it.
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:
DIFFERENCE OF COMMUNICATION SATELLITE FROM
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
Different frequency bands used in