Space segment

Details about Space Segment

1. EC8095 Satellite Communication II Unit By Sk. Hedayath Basha, Asst. prof IV - ECE - 2020 - 2021 1

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 IV - ECE - 2020 - 2021 2

3. Today’s Topics – 22.01.2022 Spacecraft Technology - Structure, Primary Power Attitude and Orbit Control Thermal Control and Propulsion IV - ECE - 2020 - 2021 3 Note: The  On-Board Computer (OBC) is the brain of the satellite.

4. S atellite Communications System IV - ECE - 2020 - 2021 4

5. Satellite Structure IV - ECE - 2020 - 2021 5 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. IV - ECE - 2020 - 2021 6

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 . IV - ECE - 2020 - 2021 7

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. IV - ECE - 2020 - 2021 8

9. Types of Solar Cells IV - ECE - 2020 - 2021 9

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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 . IV - ECE - 2020 - 2021 11

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. IV - ECE - 2020 - 2021 12

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. IV - ECE - 2020 - 2021 13

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. IV - ECE - 2020 - 2021 14

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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. IV - ECE - 2020 - 2021 16

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. IV - ECE - 2020 - 2021 17

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19. IV - ECE - 2020 - 2021 19 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. IV - ECE - 2020 - 2021 20

21. IV - ECE - 2020 - 2021 21 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. 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. IV - ECE - 2020 - 2021 22

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. IV - ECE - 2020 - 2021 23

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. IV - ECE - 2020 - 2021 24

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. IV - ECE - 2020 - 2021 25

26. IV - ECE - 2020 - 2021 26 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. IV - ECE - 2020 - 2021 27

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. IV - ECE - 2020 - 2021 28

29. Thermal Control IV - ECE - 2020 - 2021 29

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 IV - ECE - 2020 - 2021 30

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. IV - ECE - 2020 - 2021 31

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 . IV - ECE - 2020 - 2021 32

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. IV - ECE - 2020 - 2021 33

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. IV - ECE - 2020 - 2021 34

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36. Propulsion IV - ECE - 2020 - 2021 36

37. Today’s Topics – 24.01.2022 Communication Payload and Supporting Subsystems Telemetry, Tracking and Command Transponders – The Antenna Subsystem IV - ECE - 2020 - 2021 37

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 IV - ECE - 2020 - 2021 38

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. IV - ECE - 2020 - 2021 39 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 . IV - ECE - 2020 - 2021 40

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). IV - ECE - 2020 - 2021 41

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 IV - ECE - 2020 - 2021 42

43. One-way satellite communication IV - ECE - 2020 - 2021 43

44. Two-way Satellite Communication IV - ECE - 2020 - 2021 44

45. Telemetry, Tracking and Command IV - ECE - 2020 - 2021 45 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 IV - ECE - 2020 - 2021 46

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 . IV - ECE - 2020 - 2021 47

48. Telemetry IV - ECE - 2020 - 2021 48

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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 IV - ECE - 2020 - 2021 50

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. IV - ECE - 2020 - 2021 51

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. IV - ECE - 2020 - 2021 52

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. IV - ECE - 2020 - 2021 53

54. It is important to prevent unauthorized commands from being received and decoded, and for this reason, the command signals are often encrypted. IV - ECE - 2020 - 2021 54

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. IV - ECE - 2020 - 2021 55

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. IV - ECE - 2020 - 2021 56

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. IV - ECE - 2020 - 2021 57

58. Command Sub-System IV - ECE - 2020 - 2021 58

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 IV - ECE - 2020 - 2021 59

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. IV - ECE - 2020 - 2021 60

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 IV - ECE - 2020 - 2021 61

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. IV - ECE - 2020 - 2021 62

63. Transponder Subsystem IV - ECE - 2020 - 2021 63