1. Lect 03© 2012 Raymond P. Jefferis III1 Satellite Communications Satellite subsystems Overview Position control system Attitude control system Power control system Environmental control system Telemetry Transponders Antennas Beam shaping Reliability design Global Positioning System (GPS), NASA

2. Lect 03© 2012 Raymond P. Jefferis III2 Satellite Subsystems Overview The communications mission of the satellite is supported by subsystems to maintain its position, orientation, electric power, and internal environment. Fulfilling the mission may require producing a shaped communications beam to communicate.

3. Lect 03© 2012 Raymond P. Jefferis III3 Orbital Position Control A geosynchronous satellite must remain located within a 3-dimensional box despite the effects of gravitational anomalies and solar wind. (This is necessary for accurate ground station location of the satellite.) Station-keeping is effected by thruster “burns” (For example: xenon ion engine) Fuel/energy is expended to do this, which limits the effective life of the satellite.

4. Lect 03© 2012 Raymond P. Jefferis III4 Schematic of Orbital Position Control Orbital position measured by ground stations Thruster burns calculated Burn times sent to satellite Orbital corrections made by thruster “burns”

5. Simple Orbital Dynamics Lect 03© 2012 Raymond P. Jefferis III5 whereμ = 3.986004418E5 r = orbital radius v = orbital velocity Note: As orbital radius decreases, velocity increases.

6. Orbital Maneuvers In: Orbital and Celestial Mechanics Website http://www.cdeagle.com http://www.cdeagle.com/html/ommatlab.html Recommended download: Orbital Mechanics with MATLAB, Orbital Maneuvers http://www.cdeagle.com/ommatlab/maneuvers.pdf Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 6

7. Lect 03© 2012 Raymond P. Jefferis III7 Attitude Control Earth station coverage requires that satellite remain in a fixed orientation with respect to the earth On-board sensors measure orientation with respect to the earth, sun, and stars. Attitude corrections are made by control thrusters in three axes. Little energy required for attitude corrections.

8. Lect 03© 2012 Raymond P. Jefferis III8 Attitude Measurements Orientation measurements Sun orientation Star(s) orientation(s) Earth orientation A-A´ angle and orientation B-B´ angle and orientation

9. Attitude Control Lect 03© 2012 Raymond P. Jefferis III9 The governing differential equation is: Where J is the moment of inertia of the satellite around the axis of interest, θ is the attitude angle with respect to a fixed reference direction, and T is the applied torque, supplied by thrusters.

10. Simple Attitude Algorithm Estimate the required correction angles Burn thrusters to provide torques Let angles drift to calculated value Cancel angular velocity with opposing the thrusters Repeat until the alignment is correct Lect 03© 2012 Raymond P. Jefferis III10

11. Lect 03© 2012 Raymond P. Jefferis III11 Power Control Orients solar panels normal to solar radiation, for maximum output Regulates system voltages and distributes current loads Maintains battery conditioning by maintaining charge and discharge cycles for expected outages of 70 minutes (orbital darkness cycle) Limits discharge to 70%, for battery protection

12. Solar Power Silicon solar cells produce electric power from “incident radiation” –Direct sunlight –Sunlight reflected from earth (albedo) Power is proportional to incident energy Temperature affects conversion efficiency As solar cells age, power output is reduced Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 12

13. Solar Panel Characterization Short circuit currrent, Isc Open circuit voltage, Voc Maximum power point voltage, Vmpp Maximum power point current, Impp Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 13

14. Simple Circuit Model Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 14

15. Simple Model Explanation Iph -> current delivered by photocell D -> Diode characteristic of photocell (Some loss current, I D flows in diode) Rs -> Equivalent internal series resistence of photocell Ip -> effective current delivered (Iph – I D ) Vp-> effective photocell output voltage Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 15

16. Mathematical Model Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 16

17. Mathematica ® Photocell Model Function Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 17

18. Photocell Output Voltage Calculation Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 18

19. Calculated Photocell Output Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 19

20. Current vs Voltage Output of Solar Cell Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 20

21. Maximum Power Point of Solar Cell Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 21

22. Class Work Plot graphs for photocell panels with the following power outputs: –500 W/m 2 –250 W/m 2 See notes Lect 03© 2012 Raymond P. Jefferis III Lect 00 - 22

23. Lect 03© 2012 Raymond P. Jefferis III23 Temperature Control Temperature regulation (0 - 75 degC) Temperature and its cycling stresses all components, shortening operational life Satellites have multiple heat sources –Solar cell power dissipated on board –Direct absorption of solar radiation Waste heat can be dumped into space by: –Radiation –Evaporation (if fluid available)

24. Heat Balance Lect 03© 2012 Raymond P. Jefferis III24

25. Lect 03© 2012 Raymond P. Jefferis III25 Telemetry Transmitted data about conditions in a satellite –Operational status information Subsystems data Utility data –Environmental data Temperatures Pressures (propellant tanks)

26. Lect 03© 2012 Raymond P. Jefferis III26 Telemetry Block Diagram

27. Lect 03© 2012 Raymond P. Jefferis III27 Ground Station Telemetry Telemetry data received by earth station(s) Satellite tracking data is generated by the earth station Orbital control processing is calculated on the earth External data & commands are sent by earth station(s) to the satellite using telemetry channels

28. Layered Telemetry Model Lect 03© 2012 Raymond P. Jefferis III28 Waqas Afzal and Adnan Mahmood, Proceedings of the International MultiConference of Engineers and Computer Scientists 2008,Vol II IMECS 2008, 19-21 March, 2008, Hong Kong

29. Lect 03© 2012 Raymond P. Jefferis III29 Transponders Receive weak communication signals on one frequency Amplifly these weak signals Simultaneously retransmit the communication on another frequency at much higher power. Electromagnetic isolation is required to prevent transmitted signals from interfering with the reception of weak signals from a ground station.

30. Lect 03© 2012 Raymond P. Jefferis III30 Channel Isolation Methods Frequency separation between Reception and Transmission channel frequencies Electronic filtering (bandpass amplifiers) Transmitting antennas oppositely polarized (electromagnetically decoupled) from the receiving antennas in each channel Circulators with high degree of isolation

31. Lect 03© 2012 Raymond P. Jefferis III31 Double Conversion Transponder H and V indicate Horizontal and Vertical polarization, respectively.

32. Data Processing Transponder Lect 03© 2012 Raymond P. Jefferis III32 H and V indicate Horizontal and Vertical polarization, respectively.

33. Lect 03© 2012 Raymond P. Jefferis III33 In-Band Frequency Allocations Transmission and reception frequencies are channelized into discrete bands Band allocation (1 of n) 1 - 36 MHz Channel (useful capacity) 2 - 2 MHz guard bands Bands on 40 MHz centers

34. Lect 03© 2012 Raymond P. Jefferis III34 Transponder Frequency Plan The Intelsat GALAXY-11 communications satellite uses the plan that follows. Each channel has 36 MHz bandwidth, with a 2 MHz guard band on each end Transmit (XMT) and Receive (RCV) pairs of frequencies are about 2500 MHz apart, to provide enough isolation that simultaneous reception and transmission can take place.

35. Lect 03© 2012 Raymond P. Jefferis III35 G11 C-Band Transponders Downlink (XMT) Uplink (RCV) Note 2225 MHz RCV/XMT separation on each channel.

36. Lect 03© 2012 Raymond P. Jefferis III36 G11 Ku-Band Transponders Uplink (RCV) Downlink (XMT) Note 2300 MHz RCV/XMT separation on each channel.

37. Lect 03© 2012 Raymond P. Jefferis III37 Antennas Receive weak signals and couple them to a low noise amplifier Transmit power signals and shape the beam for planned reception patterns on the ground Note: Satellite is stabilized in all axes Can be horizontally or vertically polarized. Polarized signals are received best by similarly polarized receiving antennas. Have “gain” due to focusing of energy

38. Lect 03© 2012 Raymond P. Jefferis III38 Antenna Types Wire (monopoles and dipoles) Low gain (4 - 8 dB) not focused Horn (tapered waveguide) Intermediate gain (23 dB), 10˚ beam focus Often used to feed dish antenna Reflecting (dish, many wavelengths in diameter) High gain (45 dB), 3˚ beam focus Array (multiple phased antennas in pattern) Adjustable gain and beam shape possible

39. Lect 03© 2012 Raymond P. Jefferis III39 Dipole Antennas Broad radiation pattern (78 degrees) Low gain (2.15 dB over isotropic for half- wave antenna) Longer versions have more gain (radiation pattern is altered) Low gain limits missions to only those that can be accomplished with low orbit

40. Reflector Antennas Narrow radiation pattern (for wavelength λ and diameter D, and factor k), (k = 60 for parabolic antenna) High gain (for diameter D, wavelength λ, and area, A), Lect 03© 2012 Raymond P. Jefferis III40

41. Lect 03© 2012 Raymond P. Jefferis III41 Parabolic Dish Antenna Symmetric The feed faces the reflector at its focal point Wikipedia

42. Lect 03© 2012 Raymond P. Jefferis III42 Center Fed Parabolic Dish Antenna

43. Lect 03© 2012 Raymond P. Jefferis III43 Offset Parabolic Dish Antenna Asymmetric Feed is offset; faces the reflector Reflector is shaped above feed horn line to compensate for offset Wikipedia

44. Lect 03© 2012 Raymond P. Jefferis III44 Offset-Fed Parabolic Dish

45. Lect 03© 2012 Raymond P. Jefferis III45 Cassegranian Antenna Symmetric Feed horn extends through center of reflector Hyperboloid secondary reflector positioned at focus of primary reflector Wikipedia

46. Cassegranian Antenna Lect 03© 2012 Raymond P. Jefferis III46 Cassegrain radar antenna at Sondrestrom, Greenland ( Diameter: 32 m Normal operating frequency: 1290 MHz ) Photo by L. Chang (wikipedia)

47. Lect 03© 2012 Raymond P. Jefferis III47 Double Reflector Antennas Cassegranian –Feed horn through center of reflector –Hyperboloid secondary reflector at the focus of the primary reflector Gregorian –Feed horn through center of reflector –Ellipsoid secondary reflector at focus of the primary reflector Offset Gregorian –Gregorian with feed horn at edge of the primary reflector

48. Gregorian Antenna Feed Lect 03© 2012 Raymond P. Jefferis III48 Flickr Photo: http://flickr.com/photos/ekirsche/87736375/ http://flickr.com/photos/ekirsche/87736375/

49. Lect 03© 2012 Raymond P. Jefferis III49 Antenna Gain The angular dependence of radiation from an antenna is its radiation pattern. It is measured as radiated power per solid angle. The ratio of radiated power per solid angle to that of an isotropic dipole is the gain of the antenna.

50. Antenna Power Flux Density Isotropic Radiated power per unit spherical area Equivalent to the square of the RMS E-field voltage divided by the impedance of free space, 377 Ohms. Ψ = P/4πr 2 = E 2 /Z FS [Watts/m 2 ] Lect 03© 2012 Raymond P. Jefferis III50

51. Antenna Aperture The received power, P r, is equal to the raditated flux density, Ψ, multiplied by the effective aperture, A eff, of the receiving antenna P r = A eff ψ = ηAψ [Watts] A eff is a fraction, η, of the actual antenna aperture area because of edge effects and other losses. Lect 03© 2012 Raymond P. Jefferis III51

52. Lect 03© 2012 Raymond P. Jefferis III52 Gain of Aperture Antenna G = aperture antenna gain   = aperture efficiency A = aperture area [m 2 ]  = operating wavelength [m] G = aperture antenna gain   = aperture efficiency D = aperture diameter [m]  = operating wavelength [m] For circular aperture,

53. Lect 03© 2012 Raymond P. Jefferis III53 Example Calculation A circular antenna has D/  = 25 [wavelengths]  A = 63% Gain = 0.63*(  *25) 2 = 3886 = 36 dB

54. Lect 03© 2012 Raymond P. Jefferis III54 Beamwidth of Aperture Antenna θ = 3dB beamwidth in degrees λ = operating wavelength D = aperture diameter [m] Ref: J. D. Kraus and R. J. Marhefka, Antennas for All Applications, Third Edition, McGraw-Hill, 2002.

55. Lect 03© 2012 Raymond P. Jefferis III55 Example Calculation If a circular antenna has D/  = 25 [wavelengths],  3dB = 75/25 = 3 [degrees] Note that the same reflector diameter will yield a gain of 6 dB at half the wavelength

56. Lect 03© 2012 Raymond P. Jefferis III56 Antenna Beamwidth where, θ = 3dB beamwidth [degrees] λ = operating wavelength [m] D = aperture diameter [m] Note: For  = 3˚, D/  = 25

57. Approximate Gain vs Beamwidth Run Mathematica(R) program: mAntGain LECT 04© 2012 Raymond P. Jefferis III Lect 00 - 57

58. Lect 03© 2012 Raymond P. Jefferis III58 GALAXY-11 Calculation Intelsat GALAXY-11 at 91W (NORAD 26038) 39.1 dBW on C-Band (20W, 24 ch, Bw: 36 MHz) 47.8 dBW on Ku-Band (75/140W, 40 ch, Bw: 36 MHz)

59. Lect 03© 2012 Raymond P. Jefferis III59 Beamwidth of Ku-Band Antenna Antenna diameter:1.8 [m] Frequency:12 [GHz] Wavelength:0.025 [m] Beamwidth ≈ 75/(1.8/0.025) ≈ 1.05 ˚

60. Lect 03© 2012 Raymond P. Jefferis III60 Example - Ku-Band antenna gain 3dB beamwidth = 3˚ D/  = 25  = 0.63 G = 3886 G db = 36

61. Lect 03© 2012 Raymond P. Jefferis III61 Sample Calculation of Antenna Gain eff = 0.63; beamw = 3; f = 12*10^9; c = 2.99792458*10^8; lam = c/f; app = 75.0/beamw diam = app*lam G = eff*p^2*app^2 lG = 10*Log[10, G]

62. Lect 03© 2012 Raymond P. Jefferis III62 Phased Array Antennas For N antenna sources phased ϕ degrees apart in an array of aperture radius, a Physical spacing, typically  /4 Resulting beam intensity and angle,  (from Wikipedia) are:

63. Lect 03© 2012 Raymond P. Jefferis III63 Transmitter Antenna Gain For a circular antenna (parabolic dish), where, A e = Effective aperture [m 2 ]   = aperture efficiency d = aperture diameter [m] G = aperture antenna gain  = operating wavelength [m]

64. Lect 03© 2012 Raymond P. Jefferis III64 Reliability Satellites cannot easily be maintained Reliability methods: –Component qualification –Burn-in (100 - 1000 hours) –Redundancy –Component switching

65. Lect 03© 2012 Raymond P. Jefferis III65 Component Qualification Conditions Components manufactured with 100% tested materials Raw material tracked to component lots Component failure rates characterized –Specified operating conditions (-85 to +125 ˚C) –Many components tested (some destructively) –Failure rates calculated Lot numbers qualified for further use

66. Lect 03© 2012 Raymond P. Jefferis III66 Burn-in Most component failures occur early on Running under power (burn-in) causes weak components to fail early Used to catch systematic problems - bad lots Does not reduce life of most components Burn-in times of 100 - 1000 hours is considered optimal

67. Lect 03© 2012 Raymond P. Jefferis III67 Measures Used To Provide Redundancy Multiple redundant pathways (Repair by ground command) Median voting (Self-repair) Switched alternative circuits (Repair by ground command)

68. Lect 03© 2012 Raymond P. Jefferis III68 Multiple Redundant Pathways All components operate simultaneously Results can be rescaled for correct values There is a common point of failure at output Assumes advantageous failure modes!

69. Lect 03© 2012 Raymond P. Jefferis III69 Parallel Amplifier Example Output of C will either be double or half of its correct value, assuming A or B fails in OFF mode (advantageous failure mode) Note: Permits repair by ground station command (Gain change)

70. Lect 03© 2012 Raymond P. Jefferis III70 Median Voting Circuit Its rules: –Reject largest value –Reject smallest value –Take median value as true Rejects up-scale and down-scale failures Expensive! A voter circuit is a common point of failure

71. Lect 03© 2012 Raymond P. Jefferis III71 Median Voting Schematic Note: Self-repairing

72. Simple Probability Calculations Given that a single channel has a failure probability (p = 10 -6 ), per unit time, the failure probability is For three equal channel failure probabilities, p, the probability of two simultaneous failures for (p = 10 -6 ) is, Lect 03© 2012 Raymond P. Jefferis III72

73. Conclusion The triple redundancy system failure probability with voting is nearly the square of the single-element system failure The voter circuit is a common point of failure to be considered Up-scale or down-scale failure (advantageous failure mode) is assumed Lect 03© 2012 Raymond P. Jefferis III73

74. Lect 03© 2012 Raymond P. Jefferis III74 Switched Alternative Circuits Two-way redundant paths built into signal path Switching between paths are provided to select preferred component Both outputs analyzed on the ground Switching is effected to select the chosen component Cheaper –Uses fewer components –Saves power

75. Lect 03© 2012 Raymond P. Jefferis III75 Switching Circuit Example Note: permits repair by ground station command, when either Amplifier A or Amplifier B fails.

76. End Lect 03© 2012 Raymond P. Jefferis III76