Satellite_Comm

• Satellite communication


  Long distance communication using artificial satellites stationed in space for telecommunications
• Satellite communication


  • Avoids need for large number of repeaters required in terrestrial communication links
  • Can cover large areas that are difficult to reach with terrestrial links
• Communications satellite


  An artificial satellite stationed in space for the purpose of telecommunications
• Arthur C. Clarke published article "Extra-terrestrial Relays" describing geostationary satellites for communications


  1945
• Geostationary orbit


  Satellite revolves around the earth at a constant speed once per day over the equator at a height of 35,786 km

  
• Conditions for geostationary orbit

  1. Period of revolution same as earth's rotation (23 hours 56 minutes 4 seconds)

  2. Altitude of 35,786 km

  3. Orbit in equatorial plane

  4. Movement in easterly direction same as earth's rotation

  
• Geostationary satellite


  • Large antenna at earth station easy to track
  • 24-hour communication possible with single satellite
  • Satellite appears stationary, radiates highly effective power
  • Wide visibility, global communication possible with 3 satellites
• Geostationary satellite drawback


  Delay caused in long distance transmission
• Advantages of satellite communications

  • Large coverage area
  • High quality and reliability
  • High capacity
  • Flexibility
  • Speed of installation
  • Suitability for mobile, short-term and emergency communications
  • Suitability for point-to-multipoint and broadcasting applications
• Applications of satellite communications

  • Telephony
  • Satellite television
  • Fixed service satellite
  • Direct broadcast satellite
  • Satellite radio
  • Satellite internet
  • Navigation (GPS)

  
• Basic satellite communication components

  • Uplink (transmission from ground to satellite)
  • Downlink (retransmission from satellite to ground)
  • Receiver and receive antenna
  • Transmitter and transmit antenna
  • Means to connect uplink to downlink
  • Prime electrical power

  
• Satellite payload


  Equipment used to provide the service, including transponders and antennas

  
• Satellite support subsystems


  Altitude and orbit control, power, telemetry and telecommand
• Frequency bands used for satellite communication

  • C-band (5.925-6.425 GHz uplink, 3.7-4.2 GHz downlink)
  • Extended C-band (6.725-7.025 GHz uplink, 4.5-4.8 GHz downlink)
  • Ku-band (14.0-14.5 GHz uplink, 10.95-11.2 GHz and 11.45-11.7 GHz downlink)

  
• Transmission path of satellite communication link is about 72,000 km (36,000 km uplink and 36,000 km downlink)

  
• Electromagnetic waves take about 240 ms to travel the satellite communication link
• Satellite subsystems


  • Power subsystem
  • Telemetry and tele-command (TTC) subsystem

  
• Frequency bands used for satellite communication

  • C-Band
  • Extended C-Band
  • Ku-Band

  
• 1. Band


  U/L: 5.925 - 6.425 GHz, D/L: 3.7 - 4.2 GHz, Total 500 MHz BW

  
• Extended C-Band


  U/L: 6.725 - 7.025 GHz, D/L: 4.5 - 4.8 GHz, Additional 300 MHz BW

  
• Ku-Band


  U/L: 14.0 - 14.5 GHz, D/L: 10.95 - 11.2 and 11.45 - 11.7 GHz, Total 500 MHz BW

  
• Transmission path of a communication link through a satellite is about 72000 km long (36000 km uplink path and 36000 km downlink path)

  
• Electromagnetic waves travelling at 3 x 10^5 km/sec take about 240 msec. from one end to the other
• Such large propagation delay is at the limit of psychologically tolerable values in telephony
• The propagation delay results in "echo" in a telephony channel
• A special equipment called echo suppressors is incorporated in the earth station to counteract the echo
• Propagation delay also restricts the number of satellite hops for building up a telephony circuit to one as the resulting propagation delay will be much beyond the acceptable value for more than one hop
• Satellite orbits


  • LEO (Low Earth Orbit): 150 to 1,000 km, 1.5 to 1.8 hours
  • MEO (Medium Earth Orbit): 5,000 to 10,000 km, 3.5 to 6 hours
  • Geosynchronous: 36,000 km mean altitude, 24 hours
  • GEO (Geostationary): 36,000 km, precisely in the plane of the equator, 24 hours

  
• Geostationary Satellite


  Precisely 35,786 km in the plane of the equator

  
• Satellite footprint


  The ground area that its transponders offer coverage
• Primary Coverage Area


  3 dB contour, EIRP 32 dBw
• Secondary Coverage Area


  6 dB contour, EIRP 29 dBw
• Tertiary Coverage Area


  9 dB contour, EIRP 26 dBw
• VSAT


  Very Small Aperture Terminal, a device used to send and receive wireless transmissions by satellite

  
• VSAT


  • Typically about 2 to 5 feet (0.55-1.8 meters) in diameter for Ku-band systems
  • Capable of both receiving and sending satellite signals
  • Can be designed to serve both broadcast and interactive applications whether data, voice or video

  
• VSAT Equipment


  • ODU (Outdoor Unit)
  • IDU (Indoor Unit)
• ODU (Outdoor Unit)


  Consists of transmit/receive dish (antenna), block up-converter (BUC), low noise block-down-converter (LNB), and feed assembly
• BUC


  Converts incoming I.F. (from IDU) to R.F. transmitting frequency, amplifies it and passes it to feed
• LNB


  Amplifies incoming R.F. from feed using low noise amplifier, converts it to I.F. and passes it to IDU
• IDU (Indoor Unit)


  Converts I.F. from ODU to baseband signals (data, video or voice) on receiving side, and converts baseband signals to I.F. and passes them to ODU on transmitting side