The document discusses various subsystems of communication satellites, including: - Spacecraft structure, power, attitude control, thermal control, and propulsion systems - Communication payloads including transponders and supporting subsystems - Telemetry, tracking and command systems that allow communication between the satellite and ground stations for control and monitoring - Attitude control is needed to point antennas and instruments, and is achieved through methods like spinning, momentum wheels, and thrusters - Thermal control and solar arrays are used to regulate temperatures and provide power to satellite subsystems

1. EC8095 Satellite Communication
II Unit
By
Sk. Hedayath Basha, Asst. prof
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2. II Unit - Space Segment
Syllabus:
 Spacecraft Technology - Structure, Primary
Power, Attitude and Orbit Control
 Thermal Control and Propulsion
 Communication Payload and Supporting
Subsystems
 Telemetry, Tracking and Command
 Transponders – The Antenna Subsystem
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3. Today’s Topics – 22.01.2022
 Spacecraft Technology - Structure,
 Primary Power
 Attitude and Orbit Control
 Thermal Control and Propulsion
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Note: The On-Board Computer (OBC) is the brain of the satellite.

4. Satellite
Communication
Systems - Segments
Ground Segment Space Segment
Satellite Communications System
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5. Satellite Structure
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attitude & orbit control system (AOCS)

6. Space Segment
• The space segment will obviously include the
satellites, but it also includes the ground
facilities needed to keep the satellites
operational, these being referred to as the
tracking, telemetry, and command (TT&C)
facilities.
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7. Transponder
• In a communications satellite, the equipment
which provides the connecting link between
the satellite’s transmit and receive antennas is
referred to as the transponder.
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8. The Power Supply
• The primary electrical power for operating the
electronic equipment is obtained from solar
cells.
• Individual cells can generate only small
amounts of power, and therefore, arrays of
cells in series-parallel connection are required.
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9. Types of Solar Cells
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11. Solar Array
• Solar arrays that convert energy to electricity
on the International Space Station are made
of thousands of solar cells, made
from purified chunks of the element silicon.
These cells directly convert light to electricity
using a process called photovoltaics.
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12. Vanguard 1 satellite
• The first spacecraft to use solar panels was
the Vanguard 1 satellite, launched by the US
in 1958.
• This was largely because of the influence of Dr.
Hans Ziegler, who can be regarded as the
father of spacecraft solar power.
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13. • In geostationary orbit the telescoped panel is
fully extended so that both are exposed to
sun- light.
• At the beginning of life, the panels produce
940 W dc power, which may drop to 760 W at
the end of 10 years.
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14. Attitude Control & Orbit Control
• The attitude of a satellite refers to its
orientation in space.
• Much of the equipment carried aboard a
satellite is there for the purpose of controlling
its attitude.
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16. • Attitude control is necessary, for example, to
ensure that directional antennas point in the
proper directions.
• In the case of earth environmental satellites,
the earth-sensing instruments must cover the
required regions of the earth, which also
requires attitude control.
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17. Disturbance torques
• . A number of forces, referred to as
disturbance torques, can alter the attitude,
some examples being the gravitational fields
of the earth and the moon, solar radiation,
and meteorite impacts.
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• Passive attitude control refers to the use of
mechanisms which stabilize the satellite
without putting a drain on the satellite’s
energy supplies; at most, infrequent use is
made of these supplies, for example, when
thruster jets are impulsed to provide
corrective torque.
• Examples of passive attitude control are spin
stabilization and gravity gradient stabilization.

20. Active Control Torques
• Methods used to generate active control
torques include momentum wheels,
electromagnetic coils, and mass expulsion
devices, such as gas jets and ion thrusters.
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(a) Roll, pitch, and yaw axes. The yaw axis is directed toward the earth’s center, the pitch
axis is normal to the orbital plane, and the roll axis is perpendicular to the other two.
(a) RPY axes for the geostationary orbit. Here, the roll axis is tangential to the orbit and lies
along the satellite velocity vector.

22. Spinning satellite stabilization
• Spin stabilization may be achieved with
cylindrical satellites. The satellite is
constructed so that it is mechanically balanced
about one particular axis and is then set
spinning around this axis.
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23. • For geostationary satellites, the spin axis is
adjusted to be parallel to the N-S axis of the
earth, as illustrated in Fig. 7.5. Spin rate is
typically in the range of 50 to 100 rev/min.
Spin is initiated during the launch phase by
means of small gas jets.
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24. • In the absence of disturbance torques, the
spinning satellite would maintain its correct
attitude relative to the earth
• Disturbance torques are generated in a
number of ways, both external and internal to
the satellite.
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25. • Solar radiation, gravitational gradients, and
meteorite impacts are all examples of external
forces which can give rise to disturbance
torques.
• Motor- bearing friction and the movement of
satellite elements such as the antennas also
can give rise to disturbance torques.
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Spin stabilization in the geostationary orbit.
The spin axis lies along the pitch axis, parallel to the earth’s N-S axis.

27. Thermal Control
• The need for a Thermal Control System (TCS)
is dictated by the technological/functional
limitations and reliability requirements of all
equipment used onboard a spacecraft and, in
the case of manned missions, by the need to
provide the crew with a suitable
living/working environment.
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28. • Almost all sophisticated equipment has
specified temperature ranges in which it will
function correctly. The role of the TCS is
therefore to maintain the temperature and
temperature stability of every item on-board
the spacecraft within those pre-defined limits
during all mission phases and thereby using a
minimum of spacecraft resources.
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29. Thermal Control
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30. • Interaction with the environment
The external surfaces of a spacecraft may either
need protection from the local environment or
improved interaction with it, involving:
• the reduction or increase of absorbed
environmental fluxes
• the reduction or increase of heat losses to the
environment
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31. • Heat provision and storage
In some cases, to reach or maintain the desired
temperature level, heat has to be provided and/or
a suitable heat-storage capability has to be
foreseen.
• Heat collection
In many cases, dissipated heat has to be removed
from the equipment in which it is generated to
avoid an undesirable increase in the unit's, and/or
the spacecraft's temperature.
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32. • Heat transport
Generally speaking, it is not possible to reject
the heat directly where it is generated, and
appropriate means have to be used to transport
it from the collection device to the radiating
device.
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33. • Heat rejection
The heat collected and transported has to be
rejected at an appropriate temperature to a heat
sink, which is usually the surrounding space
environment.
The rejection temperature depends on the amount
of heat involved, the temperature to be controlled
and the temperature of the environment into which
the device radiates the heat.
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34. The major parameters driving the TCS
design are
• the environment in which the spacecraft has to
operate
• the total amount of heat dissipated on board the
spacecraft
• the distribution of the thermal dissipation inside
the spacecraft
• the temperature requirements of the various
equipment items
• the configuration of the spacecraft, and its
reliability/verification requirements.
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36. Propulsion
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37. Today’s Topics – 24.01.2022
 Communication Payload and Supporting
Subsystems
 Telemetry, Tracking and Command
 Transponders – The Antenna Subsystem
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38. Communication Payload
• In a nutshell, the payload is
the communications antennas, receivers, and
transmitters.
• The rest of the satellite, the bus, supports
the payload by providing a structure, power,
commanding, and telemetry, an appropriate
thermal environment, radiation shielding, and
attitude control
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39. Supporting Subsystems
• The communications
subsystem is responsible for
ensuring telecommunication
between the satellite and
another system, which may be
either another satellite or a
ground station.
• The communications
subsystem receives and
demodulates uplink signals and
modulates and transmits
downlink signals.
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BSS: Broadcasting Satellite Service
FSS: Fixed Satellite Service

40. Communication Payload
The second major module is the communication
payload, which is made up of transponders.
In a communications satellite, the equipment
which provides the connecting link between the
satellite’s transmit and receive antennas is
referred to as the transponder.
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41. Transponder
A transponder is capable of :
• Receiving uplinked radio signals from earth
satellite transmission stations (antennas).
• Amplifying received radio signals
• Sorting the input signals and directing the output
signals through input/output signal multiplexers
to the proper downlink antennas for
retransmission to earth satellite receiving stations
(antennas).
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42. Satellite Communication Services
There are two categories in which the satellite
communication services can be classified:
• One-way satellite communication
• Two- way satellite communication
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43. One-way satellite communication
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44. Two-way Satellite Communication
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45. Telemetry, Tracking and Command
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S – Band: 2 to 4 GHz
C – Band: 4 to 8 GHz

46. Telemetry, Tracking and Command
• Telemetry, Tracking and Command (TTC) are
vital functions of a spacecraft. They allow data
to be communicated between the ground and
the spacecraft for spacecraft control and
command.
• The communication is through a
telecommunication link established between
the control station on the ground and
the satellite
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47. • Telemetry is the process of recording and
transmitting the readings of an instrument.
• A tracking system, also known as a
locating system, is used for the observing of
persons or objects on the move and supplying
a timely ordered sequence of location data for
further processing.
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48. Telemetry
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50. Commanding subsystem
• Commanding subsystem is necessary in order to
launch the satellite in an orbit and its working in
that orbit.
• This subsystem adjusts the altitude and orbit of
satellite, whenever there is a deviation in those
values. It also controls the communication
subsystem.
• This commanding subsystem is responsible for
turning ON / OFF of other subsystems present in
the satellite based on the data getting from
telemetry and tracking subsystems
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51. T T &C Subsystem
• The telemetry, or telemetering, function could
be interpreted as measurement at a distance.
• Data which are transmitted as telemetry
signals include attitude information such as
that obtained from sun and earth sensors.
• Environmental information such as the
magnetic field intensity and direction, the
frequency of meteorite impact, and so on.
• Spacecraft information such as temperatures,
power supply voltages, and stored-fuel
pressure.
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52. • Telemetry and command may be thought of as
complementary functions.
• The telemetry subsystem transmits
information about the satellite to the earth
station,
• while the command subsystem receives
command signals from the earth station, often
in response to telemetered information.
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53. • The command subsystem demodulates, if
necessary, decodes the command signals and
routes these to the appropriate equipment
needed to execute the necessary action.
• Thus attitude changes may be made,
communication transponders switched in and
out of circuits, antennas redirected, and
station-keeping maneuvers carried out on
command.
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54. • It is important to prevent unauthorized
commands from being received and decoded,
and for this reason, the command signals are
often encrypted.
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55. Tracking
• In this the satellite movement is tracked and
correction signals are sending to satellite
because various disturbing forces are acting
on the satellite.
• This system is located at the earth station
providing information on elevation and
azimuth angles of the satellite.
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56. • Tracking is obviously important during the
transfer and drift orbital phases of the satellite
launch.
• Once it is on station, the position of a
geostationary satellite will tend to be shifted
as a result of the various disturbing forces.
• Therefore, it is necessary to be able to track
the satellite’s movement and send correction
signals as required.
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57. • Tracking beacons may be transmitted in the
telemetry channel or by pilot carriers at
frequencies in one of the main
communications channels, or by special
tracking antennas.
• Satellite range from the ground station is also
required from time to time.
• This can be determined by measurement of
the propagation delay of signals especially
transmitted for ranging purposes.
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58. Command Sub-System
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59. Examples of common commands are:
1) Transponder switching
2) Switch matrix reconfiguration.
3) Antenna pointing control.
4) Controlling direction and speed of solar drive
array.
5) Battery reconditioning
6) Beacon switching
7) Thrusters firing
8) Switching heaters of various subsystem
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60. Command Sub System
• The receiving antennas used are
omnidirectional antennas to maintain contact
for all orientation of the satellite, so that the
satellite can receive the signals during launch,
orbit transfer and other periods prior to
attitude stabilization.
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61. • The receiver then converts RF signal to base
band signal and command decoder decodes
the command.
• Then verification is done in which involves
transmitting decoded commands to satellite
control centre via telemetry carrier and the
command is stored in the satellite till
verification is done
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62. • The command receiver also provides
baseband ranging tone which is modulated on
beacon telemetry and sent to satellite control
centre.
• The antennas used for telemetry and
command signals are parabolic reflectors.
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63. Transponder Subsystem
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