(TT & C)
Architecture of Satellite Communication System
Satellite communication –
Satellite communication systems can be broadly
divided into two segments :
Space segment : It includes the satellites, ground
facilities needed to keep satellites operational,
referred to as Tracking, Telemetry & Command
Earth segment : It consists of transmit & received
Equipment carried aboard the Satellite can be classified as :
Payload : refers to the equipment used to provide the service
for which satellite has been launched.
Bus : it not only refers to the vehicle which carries the payload
but also, to the various sub - systems which provide :
Command & telemetry functions
Payload & Bus Elements
Primary electrical power for operating the electronic
equipment is obtained from solar cells.
Individual cells can generate only small amounts of power,
therefore arrays of cells in series – parallel connection are
Higher powers can be achieved with solar panels arranged in
the form rectangular “Solar sails”.
They must be folded during launched phase & extended when
During Eclipse, power is provided by two nickel-cadmium
long life batteries, which will deliver 830 Watts of power.
Nickel-Hydrogen batteries are being used in Hughes HS 601
& in Intelsat IV.
Attitude of satellite refers to its orientation in space.
Much of equipment carried aboard a satellite is for purpose of
controlling the attitude.
It is necessary to ensure that directional antennas point in
In case of earth environmental satellites, earth sensing
instruments must cover the required regions of the earth:
requires attitude control.
Number of forces, disturbance torques can alter the attitude
e.g. being solar radiations, meteorite impacts etc.
Attitude control contd……..
Infrared sensors refer to as Horizon detectors are used to
detect the rim of the earth against the background of space.
With the use of four such sensors, one for each quadrant, the
center of the earth can be readily established as a reference
Any shift in orientation is detected by one or other of the
sensors & a corresponding control signal is generated which
activates a restoring torque.
An attitude maneuver is executed where a shift in attitude is
The control signals needed to achieve this maneuver may be
transmitted from earth stations.
Types of attitude control
Passive attitude control : it refers to the use of
mechanism which stabilize the satellite without
putting a drain on the satellite energy supply. E.g.
spin stabilization & gravity gradient stabilization.
Active attitude control : in this case there is no
overall stabilizing torque present to resist the
Instead, corrective torques are applied as required in
response to disturbance torque. Methods used to
generate active control talks include em coils, gas jets
How satellite attitude is defined ?
It is defined by three axes : roll, To Roll axis
pitch & yaw center Yaw
All three axes pass through the Roll rotation
center of gravity of satellite.
Movement of satellite about the
roll axis moves the antenna
footprint north & south.
Movement about the pitch axis Pitch rotation
moves the footprint east & west.
Movement of satellite about the
yaw axis rotates the antenna Pitch axis (normal to orbital
Spin Stabilization used with
Satellite constructed so that it is Satellite
mechanically balanced about one spin
particular axis & is then set N
spinning around the axis.
For GEO spin axis is adjusted
parallel to the N-S axis of the
In absence of disturbing torques,
the spinning satellite would S
maintain its correct attitude
relative to the earth.
In presence of disturbing torques, Geostationary Orbit
spin rate decrease & direction of
angular spin axis will change.
Jets can be used to increase the
spin rate & to shift axis back to its
correct N-S orientation.
Three Axis Stabilization
There are stabilizing elements for each of the three axes,
roll, pitch & yaw.
Because the body of the satellite remain fixed relative to
earth, this stabilization is also known as body
Active Attitude control is used with three axis
This may take the form of control jets fired to correct
the attitude of satellite.
Reaction wheels can also be used.
Orbital control (Station keeping)
Station keeping : used for maintaining a satellite in its correct
Equatorial ellipticity of the earth causes GEO to drift slowly
along the orbit, to one of two stable points, at 75 deg east
&105 deg west.
To counter this drift, an oppositely directed velocity
component is imparted to the satellite by means of jets.
This results in satellite drifting back through its nominal
station position, coming to a stop & recommencing the drift
along the orbit until the jets are pulsed once again these are
called east-west station keeping maneuvers.
Orbital Control Contd….
Satellites in the 6/4-GHz band must be kept within +-
1deg of designated longitude, & in the 14/12 GHz band,
Satellite which is in GEO will also drift in latitude, the
main perturbing forces being the gravitational pull of the
sun & the moon.
These forces causes the inclination to change at a rate
of .85deg / year.
Counteracting jets must be pulsed to correct the
These maneuvers are termed as north-south station
Satellites are subject to large thermal gradients, receiving the
sun’s radiation on one side while the other side faces into
Thermal radiation from the earth & the earth’s albedo, which
is the fraction of radiation falling on earth which is reflected :
significant for low altitude earth orbiting satellites.
Negligible for GEO.
Equipment in the satellite also generates heat which has to be
Thermal blankets & shields which may be provided for
Radiation mirrors are often used to remove heat from payload.
Telemetry, tracking, & command subsystem:
Telemetry : means measurement at a distance. It refers to
generating an electrical signal proportional to quantity being
measured, & encoding & transmitting this to a distant station,
which for the satellite is one of the earth station.
Data which are transmitted as telemetry signals include
attitude information, environment information etc.
This subsystem transmits information about the satellite to the
earth station while command subsystem receives command
signals from earth station, in response to telemetred
TT&C Subsystem contd…….
The command subsystem demodulate & decodes the command
signals & routes these to appropriate equipment needed to
execute the necessary action.
Command signals are often encrypted.
Tracking : it is accomplished by having the satellite transmit
signals which are received at TT&C earth stations.
It is important during the transfer & drift orbital phases of the
A GEO satellite will tend to shift as a result of the various
Therefore it is necessary to track the satellites movements and
send correction signals as required.
Satellite range is also required for time to time.
This can be determined by measurement of propagation delay of
signals specially transmitted for ranging purposes.
Satellite Control System
Telemetry Tracking &
Ranging commands &
Other Voice & Data Links
Stations Data Communication
Analysis center Computer
A transponder is the series of interconnected units
which forms a single communication channel
between the receive and transmit antennas in a
Some of the units utilized by a transponder in a given
channel may be common to a number of
Transponder consist of wideband receivers, input de-
multiplexer, power amplifier components.
Frequency arrangement of C band
Transmission Band: C Band
4 GHz (Down Link), 6 GHz (Uplink).
Bandwidth allocated for C-band service: 00 MHz.
This is divided into sub bands, one for each
A typical Transponder BW is 36 MHz allowing for a
4 MHz guard band between transponders.
12 such transponders can be accommodated in
500 MHz BW.
Frequency arrangement of C band
Polarization isolation can be used to double the no. of
Polarization isolation refers to the fact that carriers,
which may be on the same frequency but with opposite
senses of polarization, can be isolated from one another
by receiving antennas matched to the incoming
With linear polarization, vertically & Horizontally
polarized carriers can be separated in this way.
Because carriers with opposite senses of polarization
may over lap in frequency , this technique is referred to
as Frequency Reuse.
Horizontal 6085 6125
Satellite Transformer Channels
O/P MUX Power gain
I/P MUX blocks
Wide Band 3.920
BPF Receiver 3.960
From 3.7 To
as GHz 4.080 3.7-
Satellite Wideband Receiver
A duplicate receiver is provided so that , if one
fails, the other is automatically switched in.
The combination is referred to as redundant
receiver, meaning although two are provide one
is active at a time.
The first stage is LNA.
Then is the mixer stage followed by amplifiers
(may utilize BJTs or FETs).
Satellite Wideband Receiver
I/P To I/P
Filter Pre amp DEMUX
The I/P DEMUX separates the broad band
input, covering the frequency range 3.7-
4.2 GHz, into the transponder frequency
The channels are arranged in even
numbered & odd numbered groups.
This provides greater frequency separation
b/w adjacent channels in a group, which
The channels are usually arranged in even numbered &
odd numbered groups.
This provides greater frequency separation b/w adjacent
channels in a group, reducing ADI.
The O/P from Rx is fed to a power splitter which in turn
feeds to separates circulators.
Broadband signal is transmitted along each chain,
channelizing is achieved by means of channel filters
connected to each circulator.
Each filter has BW of 36 MHz & is tuned to appropriate
Power Amplifier for each
Power amplifier provides the output power
Each power amplifier is preceded by an attenuator.
Attenuator has a fixed or a variable section.
Fixed attenuator is needed to balance out variations in
the input attenuation so that each transponder channel
has the same nominal attenuation.
The variable attenuator is needed to set the level as
required for different type of services (for e.g.
requirement for input power back – off ). The variable
attenuator adjustment is an operational requirement, it
must be under the control of the ground TT&C station.
Power Amplifier contd…
Traveling wave tube amplifier (TWTAs) are widely used in
transponders to provide the final output required to the transponder
Block Diagram showing relative
levels in a Transponder
1. Input filter 6. Amplifier
2. Wide band Rx 7. Mux 60 dB
3. 3 dB coupler
4. DeMux 102.5 dB
The antenna carried abroad a satellite provide the dual functions of
receiving the uplink and transmitting the downlink signals.
Ranges from dipole type antenna where omni – directional
characteristics are required to the highly directional antennas
required for telecommunications, TV relays and
broadcast.directional beams are produced by means of reflector
type antennas, the paraboloidal reflector being the most common.
The gain of the reflector is given as
G= n(i) [3.14 *D]^2 / L^2
n(i) = aperture efficiency = 0.55 (typically)
D = reflector diameter
L = wavelength of the signal
Antenna Subsystem contd….
The beam width is given by
th (3 dB) = 70 L/D degree
As the gain increases the beam width becomes narrower, by
increasing the reflector size or decreasing the wavelength.
Wide beams for global communication are produced by simple horn
antennas at 6/4 GHz as these horns beam the signal directly to the
earth without the use of the reflectors.
A simple biconical dipole antenna is used for the tracking and
The same feed horn may be used to transmit and receive carriers
with the same polarization.Transmit & received signals are
separated by a device called diplexer.
Communications Satellite Components
Satellite system link model
Satellite system consists of
three basic sections :
Modulator IF Mixer BPF HPA
LNA MIXER BPF LPA
MW shift oscillator
To Earth station 4 or
From Earth station 6 To other 12 GHz
or 14 GHz transponders
Down Link Model
BPF LNA MIXER BPF Demodulator
MW Generator Baseband Out
4 or 12 GHz