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Satellite Communication

Satellite Communication

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Satellite Communication

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  1. Satellite Communication

  2. Important Milestones (1950-1970) Early Satellite Communication • 1957: First satellite launched by former USSR (Sputnik, LEO) • 1958: First US satellite launched (Explorer-1, LEO) • 1960: First passive communication satellite launched (EchoI&II) • 1962: First active comm. satellite launched (Telstar, MEO) • 1963: First satellite launched into geostationary orbit (Syncom1) • 1964: Intelsat created • 1965: First satellite launched into geostationary orbit for commercial use Early Bird (re-named Intelsat 1)

  3. Important Milestones (1970-1990) GEO Applications Development • 1972: First DOMSAT operational (Canada), Inter-Sputnik founded • 1977: 1979: Inmarsat established • 1981: First reusable launch vehicle flight • 1982: International maritime communications made operational • 1984: First DBS operational (Japan) • 1987: Successful trials of land mobile comm. (Inmarsat) • 1989-90: Global mobile communication service extended to land mobile and aeronautical use (Inmarsat)

  4. Important Milestones (1990 - Current) NGSO development and GEO expansion • 1990-95: NGSO systems proposed for mobile communications. Continuing growth of VSATs and DBS • 1997: Launch of first batch of LEO for hand-held terminals (Iridium). Voice service telephone-and paging service- pocket size mobile terminals launched (Inmarsat) • 1998: Iridium initiates services • 1999: Globalstar Initiates Service • 2000: ICO initiates Service.Iridium fails; system sold to Boeing, Thuraya I launched into GEO for MSS • 2003: Thuraya II launched into GEO

  5. Basic Satellite System Elements

  6. Satellite Footprints

  7. Communication Satellite Services • Fixed Satellite Services (FSS) • Broadcast Satellite Services (BSS) • Mobile Satellite Services (MSS) - Aeronautical • Maritime • Land

  8. Development of Satellite Communication Systems

  9. Microwave Frequencies Frequency Band Name 30 – 300 MHz VHF 300 MHz – 1 GHz UHF 1 GHz – 2 GHz L band 2 – 4 GHz S band 4 – 8 GHz C band 8 – 12 GHz X band 12 – 18 GHz Ku band 18 – 27 GHz K band 27 – 40 GHz Ka band 40 – 50 GHz Q band 50 – 60 GHz U band 60 – 80 GHz V band

  10. International Regulations The radio spectrum is a limited natural resource, which should be shared by all types of radio services. International Telecommunication Union (ITU) allocates the frequencies and also specifies the power allowed for each type of services on a global and regional basis. To facilitate the planning the ITU has divide the world into three regions: 1. Europe, Africa, Former SU, Mongolia, Middle-East 2. North and South America and Greenland 3. Remainder of Asia, Australia and the South-West Pacific

  11. Satellite Orbits • Can be circular or elliptical • Can orbit around the equator (equatorial orbit) or pass over the poles (polar orbit) or can be at any angle between these • Lower height limit of about 300 km due to atmospheric drag

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  13. Satellite Orbits • Categorized as • Low Earth Orbit (LEO) • Medium Earth Orbit (MEO) • Geostationary Earth Orbit (GEO) • Non-Geostationary satellites are sometimes called orbital satellites (though all satellites are in orbit)

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  15. Van Allen Belts • Regions of high radiation that can damage satellites • Extend from about 1500 to 5000 km and 10, 000 to 25 000 km above the earth • LEO satellites are below the first belt • MEO satellites are between the two belts • GEO satellites are above the belts

  16. LEO satellites • Altitude 300 to 1500 km • Many satellites needed for continuous coverage • Short propagation distance leads to strong signals and short propagation times

  17. MEO Satellites • Altitude from 8000 to 10 000 km • Fewer required for complete coverage, but signals are weaker and propagation delays longer than for LEO

  18. GEO Satellites • Altitude approx. 36 000 km • Must have circular equatorial orbit and must orbit in same direction as earth’s rotation. • Appear to remain stationary above a point on earth • One satellite can cover almost an entire hemisphere • Very large propagation distance leads to weak signals and long propagation time

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  20. GEO Orbit • Three satellites could cover entire globe EXCEPT polar regions • Angle of elevation to satellite decreases near poles causing spreading of beam and reduced signal strength

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  22. Propagation Time • For a GEO satellite, the time for a signal to propagate to a satellite and return is approx. 0.25 second • This causes an inconvenient though not unacceptable delay in a telephone conversation • Also can cause problems in data comm.

  23. The Spacecraft Communication considerations: • Type of service • Communication capacity (Bandwidth, EIRP) • Coverage area

  24. Transponders • Transponder is the component of the communication subsystem of the satellite which receives signals, shifts it frequency for retransmission. • Two types of transponders: • Bent Pipe • Regenerative • Beam Switching

  25. Bent Pipe Transponder .

  26. . Regenerative Transponder

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  28. Cross Links • Satellites that are in view of one another can communicate directly without going through an earth station

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  30. Earth Station Antennas • Since many GEO sats use the same frequency bands, earth station antennas must have narrow beamwidth of 2 degrees or less • Smaller beamwidth is associated with higher gain and larger diameter. Some Intelsat earth stations use 0.1 degree.

  31. GEO Applications • Television/radio broadcasting • Fixed Telephony • Data • Shipboard and mobile communication

  32. Broadcasting • Used for communication within TV networks, to CATV headends etc • Now also used by individuals (DBSS) • Uses transponders in C and Ku bands • C band: 6 GHz up, 4 GHz down • Ku band: 14 GHz up, 12 GHz down • DBS uses high power satellites which uses of 200 W per transponder

  33. Anik E-2 Footprint

  34. Satellite Telephony • Not as high quality as fiber optic systems largely due to time delay • Three main types: • Frequency division multiplexing - frequency modulation (FDM-FM) • Single channel per carrier (SCPC) • Time-division multiple access (TDMA)

  35. FDM-FM • Whole transponder is used, with one main carrier, FM modulation • A number of telephone signals are multiplexed using conventional SSB-FDM techniques, then the whole signal (with frequency content up to about 8 MHz) is used to FM modulate the main carrier

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  37. SCPC • Transponder bandwidth is divided up and portions are given to service providers, which may be at different locations • Each carrier is modulated by one voice signal using FM • Allows small users but is less efficient of bandwidth • Requires linear power amps on satellite

  38. TDMA • Each earth station uses the whole transponder satellite for a portion of the time • Efficient but complex: timing is critical

  39. Satellite Data • Conventional bent-pipe transponder can support many types of data • One popular use is called Very Small Aperture Terminal system (VSAT) • VSAT refers to the size of the dish at remote terminals (1.2 to 2.4 m, which is small for a non-DBS GEO satellite)

  40. VSAT • VSAT networks are usually in the form of a star. • Central hub has large dish (5 to 7 m) and transmits at 256 to 512 kb/s • Remote terminals have lower data rate of about 12 to 19.2 kb/s • Most systems are two-way, one-way systems are also possible

  41. VSAT Applications • Two-way • Branch offices connect to head office • Portable credit/bank card setups • One-way • Stock market information • Wire services to radio stations

  42. Mobile GEO Systems • Example • Inmarsat • Thuraya

  43. Inmarsat • International Marine Satellite Organization • Has 9 GEO sats, 4 used, 5 spares or leased to others • Each sat. has one hemisphere beam and 5 spot beams • Spot beams have about 48 dBW EIRP • Operates in L Band (1.5/1.6 GHz)

  44. Inmarsat Services • Services intended mainly for ships: • A service: analog telephony • B service: digital telephony • Portable service: • Uses spot beams mainly on land • Notebook-size terminal with flat antenna

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  46. LEO Systems • These satellites have many uses. We concentrate on two: telephony and related uses (“Big LEO’s”) and paging and other related low-bandwidth applications (“Little LEO’S”)

  47. Big LEO’s • Three contenders • Iridium (went bankrupt but operating again) • Globalstar (operating but still losing money) • Teledesic (currently on hold)

  48. Iridium • Uses 66 satellites with an elaborate system of crosslinks • Complete worldwide coverage • Voice, data at up to 10 kb/s using compression, using TDMA • Flawed Marketing Strategy

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  50. Iridium Phone