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Introduction General concepts Needs, advantages, and disadvantages Satellite characteristics

Satellite Communications General concept. Introduction General concepts Needs, advantages, and disadvantages Satellite characteristics Orbits Earth coverage System components and design Power sources Communication characteristics Spectrum and Bandwidth Channel capacity

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Introduction General concepts Needs, advantages, and disadvantages Satellite characteristics

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  1. Satellite CommunicationsGeneral concept • Introduction • General concepts • Needs, advantages, and disadvantages • Satellite characteristics • Orbits • Earth coverage • System components and design • Power sources • Communication characteristics • Spectrum and Bandwidth • Channel capacity • Frequency and Wavelength • Path losses • Antennas and beam shaping Textbook: Satellite Technology: Principles & Applications, Third Edition, Anil. K. Maini. V. Agrawal, John Wilen & Sons, 2014.

  2. Other Useful References Ippolito, Louis J., Jr., Satellite Communications Systems Engineering, John Wiley, 2008. Kraus, J. D., Electromagnetics, McGraw-Hill, 1953.  Kraus, J. D., and Marhefka, R. J., Antennas for All Applications, Third Edition, McGraw-Hill, 2002.  Morgan, W. L. , and Gordon, G. D., Communications Satellite Handbook, John Wiley & Sons, 1989. Proakis, J. G., and Salehi, M., Communication Systems Engineering, Second Edition, Prentice-Hall, 2002. Roddy, D, Satellite Communications, Fourth Edition, Mc Graw-Hill, 1989. Stark, H., Tuteur, F. B., and Anderson, J. B., Modern Electrical Communications, Second Edition, Prentice-Hall, 1988. Tomasi, W., Advanced Electronic Communications Systems, Fifth Edition, Prentice-Hall, 2001.

  3. General Concepts of Satellites: • They orbit around the earth • Have various orbital paths (to be discussed) • They carry their own source of power • They can communicate with: • Ground stations fixed on earth surface • Moving platforms (Non-orbital) • Other orbiting satellites

  4. Needs, Advantages & Disadvantages • Communications needs • Advantages of using satellites • Disadvantages of using satellites

  5. Satellite Communications Needs • Space vehicle to be used as communications platform (Earth-Space-Earth, Space-Earth, Space-Space) • Space vehicle to be used as sensor platform with communications • Ground station(s) (Tx/Rx) • Ground receivers (Rx only)

  6. Advantages of Using Satellites • High channel capacity (>100 Mb/s) • Low error rates (Pe ~ 10-6) • Stable cost environment (no long-distance cables or national boundaries) • Wide area coverage (whole North America, for instance) • Coverage can be shaped by antenna patterns

  7. Disadvantages of Using Satellites • Expensive to launch • Expensive ground stations required • Cannot be maintained • Limited frequency spectrum • Limited orbital space (geosynchronous) • Constant ground monitoring required for positioning and operational control

  8. Satellite Characteristics • Orbiting platforms for data gathering and communications – position holding/tracking • VHF, UHF, and microwave radiation used for communications with Ground Station(s) • Signal path losses - power limitations • Systems difficult to repair and maintain • Sensitive political environment, with competing interests and relatively limited preferred space

  9. Mission Dependent Characteristics • Orbital parameters • Height (velocity & period related to this) • Orientation (determined by application) • Location (especially for geostationary orbits) • Power sources • Solar (principal), nuclear, chemical power • Stored gas/ion sources for position adjustment

  10. Satellite Application Examples • Telecommunications • Military communications • Navigation systems • Remote sensing and surveillance • Radio / Television Broadcasting • Astronomical research • Weather observation

  11. Orbits • Have particular advantages and disadvantages (See text Chapter 1) • Are determined by satellite mission • Keppler’s Laws of planetary motion describe certain orbital properties (Covered in Lecture 2)

  12. Orbital Properties • Altitude (radius to center of the earth) • Inclination with respect to the earth axis • Period of rotation about the earth • Ground coverage by the satellite • Communications path length(s)

  13. Types of Orbit Dr. Leila Z. Ribeiro, George Mason University

  14. Missions Associated with Orbit Types • GEO • Primarily commercial communications • MEO • Military and research uses • LEO • Remote sensing • Global Positioning Systems

  15. LEO and MEO Features • Earth coverage requires multiple passes • Typical pass requires about 90 minutes • Signal paths relatively short (lower losses) • Small area, high resolution ground image • Earth station tracking required • Multiple satellites for continuous coverage (Decreases with increasing altitude - “Telstar”)

  16. The Geostationary (Clarke) Orbit • Arthur C. Clarke, Wireless World, February, 1945, p58.

  17. Geo-Synchronous Satellite (GEO) Features • Appears fixed over point on earth equator • Each satellite can cover 120 degrees latitude • Orbital Radius = 42,164.17 km • Earth Radius = 6,378.137 km (avg) • Period (Sidereal Day) = 23.9344696 hr (86164.090530833 seconds) • Long signal path - large path losses

  18. GEO Features (continued) • Ground image area (instantaneous) • Ground track coverage (multiple orbits) • Stationarity (geostationary orbit) • Space coverage (satellite-satellite)

  19. Orbital Altitudes and Problems • Low Earth Orbit (LEO) • 80 - 500 km altitude • Atmospheric drag below 300 km • Medium Earth Orbit (MEO) • 2000 - 35000 km altitude • Van Allen radiation between 200 - 1000 km • Geostationary Orbit (GEO) • 35,786 km altitude (42,164.57 km radius) • Difficult orbital insertion and maintenance

  20. Orbital Inclinations • Equatorial • Prograde – inclined toward the east • Retrograde – inclined toward the west • Inclined • Various inclination angles with respect to the spin axis of the earth, including polar • Geostationary (on equator; no inclination) • Sun synchronous

  21. Earth Coverage Calculation By the Law of Sines: and, The elevation angle is approximately,

  22. Earth Coverage Calculation (continued) • The total coverage area on the surface of the earth, using the previously calculated value of δ) is given by the equation,

  23. Alternate Earth Coverage Calculation • Coverage variation as a function of satellite altitude (rsat) rsat is the radius to the satellite from the center of the earth

  24. Calculation: CoverageArea.nb re = 6378.137; (* km *)rs = re + hs; alpha = ArcSin[re/rs]ad = alpha/Degreedelta = ArcSin[(rs/re)*Sin[alpha]] - alphadd = delta/DegreeA = 2 p re^2 (1.0 - Cos[delta])Plot[A, {hs, 1000, 2000}, AxesLabel -> "Coverage [km^2]", Frame -> True, FrameLabel -> {"Altitude [km]", "Coverage [km^2]"}]

  25. Advanced Earth Coverage Calculations In: Orbital Mechanics with MATLAB http://www.cdeagle.com/html/ommatlab.html Recommended download: Coverage Characteristics of Earth Satellites http://www.cdeagle.com/ommatlab/coverage.pdf

  26. “Satellite System” Components • Satellite(s) • Earth station(s) • Computer systems • Information network(Example: Internet)

  27. Satellite System Design Satellite network with earth stations.

  28. Satellite Components • Receiver (receives on an uplink) • Receiving antenna • Signal processing (decode, security, encode, other) • Transmitter (transmits on a downlink) • Transmitting antenna (beam shaping) • Power and environmental control systems • Attitude control • (De)multiplexing (used in rotating satellites) • Position holding (mission dependent option)

  29. Satellite Power Sources • Solar power panels (near-earth satellites) • Power degrades over time - relatively long • Radioactive isotopes (deep space probes) • Lower power over very long life, rarely used. • Fuel cells (space stations with resupply) • High power but need maintenance and chemical resupply, rarely used. • Example: International Space Station

  30. Solar Power • Power available in orbit: ~1400 watts of sunlight per square meter • Conversion efficiency: ~25% • Useful power: ~350 Watts/square meter • Panel steering required for maximum power • Typical power levels: 2 - 75 kW • Photocell output degrades over time

  31. Typical Solar Power Panel Example Type: GaAs/Ge Voltage: 53.1 Volts Power: 1940 Watts ( Effective Load + Source Resistance: 1.45341 Ω ) Geostationary Operational Environmental Satellites (GOES) - Ground testing of solar panels, NASA

  32. Satellite Communication Characteristics • Via electromagnetic waves (“radio”) • Typically at microwave frequencies • High losses due to path length • Many interference sources • Attenuation due to atmosphere and weather • High-gain antennas needed (“dish”) to make up for path loss and noise • Spectrum and Bandwidth • Channel capacity • Frequency and Wavelength • Path losses

  33. Spectrum and Bandwidth • Electromagnetic spectrum allocations (“DC to light” – see next slide) • Bandwidth: the size or “width” (in Hertz) of a spectrum frequency band • Frequency band: a range of frequencies in the available spectrum. • Channel capacity increases with the bandwidth (see Slide 42)

  34. Electromagnetic Spectrum Wikipedia

  35. Channel Capacity • The number of error free bits of information transmitted and received per second • Shannon (BSTJ, Vol. 27,1938) The capacity C [bits/s] of a channel with bandwidth W, and signal/noise power ratio S/N is

  36. Frequency and Wavelength Formula • Microwave energy, at a given frequency, f [Hz] • Moves at a velocity, v [m/s] • And has a wavelength (distance between peak intensities),λ[m] • Formula: λ= v / f (v = c for space) Note: The speed of light, c, in a vacuum (space) is fixed at, c = 299 792 458 [m/s]

  37. Frequencies of Interest for Satellites • Generally between 300 MHz and 300 GHz. • The microwave spectrum • Allows efficient generation of signal power • Energy radiated into space • Energy may be focused (beam shaping) • Efficient reception over a specified area. • Properties vary according to the frequency used: • Propagation effects (diffraction, noise, fading) • Antenna Sizes

  38. Microwaves • Include frequencies from 0.3 GHz to 300 GHz. - Line of sight propagation (space and atmosphere). - Blockage by dense media (hills, buildings, rain) - Wide bandwidths compared to lower frequency bands. - Compact antennas, directionality possible. • Reduced efficiency of generation • 1 GHz to 170 GHZ spectrum divided into bands with letter designations (see next slide)

  39. Designated Microwave Bands Standard designations For microwave bands Common bands for satellite communication are the L, C and Ku bands. Wikipedia

  40. Common Microwave Frequency Allocations • L band • 0.950 - 1.450 GHz • Note: GPS at 1.57542 GHz • C band • 3.7 - 4.2 GHz (Downlink) • 5.925 - 6.425 GHz (Uplink) • Ku band • 11.7 - 12.2 GHz (Downlink) • 14 - 14.5 GHz (Uplink)

  41. Common Microwave Frequency Allocations • Ka band • 18.3 - 18.8, 19.7 - 20.2 GHz (Downlink) • 30 GHz (Uplink) • V band • 40 - 75 GHz • 60 GHz allocated for unlicensed (WiFi) use • 70, 80, and 90 GHz for other wireless

  42. L-Band • Frequencies: 0.950 – 1.450 GHz (λ ~30cm) • Uses: • Amateur radio communications • GPS devices • Features: • Patch antenna used for GPS receivers • Low rain fade - Low atmospheric atten. (long paths) • Low power • Small receiver configurations

  43. C-Band • Frequencies: 3.7 - 6.425 GHz (λ~5cm) • Uses: • TV reception (motels) • IEEE-802.11 WiFi • VSAT • Features: • Large dish antenna needed (3m diameter) • Low rain fade - Low atmospheric atten. (long paths) • Low power - terrestrial microwave interferences

  44. Ku-Band • Frequencies: 12 - 18 GHz (λ ~ 2cm) • Uses: • Remote TV broadcasting • Satellite communications • VSAT • Features: • Rain, snow, ice (on dish) susceptibility • Small antenna size - high antenna gain • High power allowed

  45. Ka-Band • Frequencies: 18 - 40 GHz (λ ~ 1cm) • Uses: • High-resolution radar • Communications systems • Deep space communications • Features: • Obstacles interfere (buildings, vegetation, etc.) • Atmospheric absorption

  46. V-Band • Frequencies: 40 to 75GHz. (λ~ 5 mm) • Uses: • Millimeter wave radar research (very expensive!) • High capacity millimeter wave communications • Point-to-point fixed wireless systems (WiFi) • Features: • Rain fade • Obstacles block path • Atmospheric absorption • Expensive equipment

  47. Millimeter Waves • Planck space exploration satellite • Planck is a flagship mission of the European Space Agency (Esa). It was launched in May 2009 and moved to an observing position more than a million km from Earth on its "night side".It carries two instruments that observe the sky across nine frequency bands. The High Frequency Instrument (HFI) operates between 100 and 857 GHz (wavelengths of 3mm to 0.35mm), and the Low Frequency Instrument (LFI) operates between 30 and 70 GHz (wavelengths of 10mm to 4mm). • Johnson noise problems addressed • Some of its detectors operate at minus 273.05C

  48. Path Losses • The loss of a radiated signal with distance • Losses increase with frequency • Satellites typically require long path lengths ( Path lengths can be over 42,000 km )

  49. Causes of Path Loss • Dispersion with distance • Atmospheric absorption (Calculated in Lecture 11) • Rain, snow, ice, & cloud attenuation (Calculated in Lecture 12) • Atmospheric noise effects resulting in increased Bit Error Rate (BER) (Calculated in Lecture 6)

  50. Simple Path Loss Model • Free-space power loss = (4πd / λ)2In dB this becomes, where: d is the path distance in km f is the frequency in MHz

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