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The Future of Satellite Communications

The Future of Satellite Communications. Joel Klooster ENGR 302 May, 2002. History. Sputnik - Russian (Oct 4, 1957) Only lasted 92 days in orbit Echo - 1st TV satellite (1960) Telstar - 1st active TV satellite (1962) Syncon - 1st earth synchronous (1964). Types of Orbits. Geosynchronous

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The Future of Satellite Communications

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  1. The Future of Satellite Communications Joel Klooster ENGR 302 May, 2002

  2. History • Sputnik - Russian (Oct 4, 1957) • Only lasted 92 days in orbit • Echo - 1st TV satellite (1960) • Telstar - 1st active TV satellite (1962) • Syncon - 1st earth synchronous (1964)

  3. Types of Orbits • Geosynchronous • Same rotational velocity as earth • Maintains position relative to earth • Altitude - 35 786 km • Velocity - 11 300 km/hr • Asynchronous • Much lower altitude = much higher velocity • Position over earth constantly changing

  4. Operation of Satellites • Transmission • Reception • Low Noise Converter • Polarization • Tuner • Antennas

  5. Radio Signal Propagation • Free Space Propagation Model • Line-of-sight with no obstructions • Friis free space equation

  6. Path Loss (PL) • Fading of electromagnetic signal • Positive difference between received power and transmitted power (in dB)

  7. The Solution?? Lasers!!

  8. Advantages of Laser Communications • Frequencies 7 - 8x higher • Higher bandwidth • Smaller beam divergence • Smaller antennas • Higher data rates

  9. What type of Laser is used? • Neodymium: yttrium, aluminum, garnet (Nd:YAG) most common type • Rod of crystalline YAG lightly doped with Nd is used as the amplifier • Optical energy excites Nd atoms to higher energy state • Return to normal energy state emits energy at wavelength of 1.664 um

  10. Modulation • What actually drives the communication • Birefringence modulator most useful • Uses electric-field induced birefringence of the crystal to rotate polarized light

  11. Optical Detectors • Receiver must somehow convert the laser energy to an electric current • Photon-wave theory of light • Photons have some momentum that exerts a force on the receiver • Frees electrons from atoms of cathode • Photoelectrons attracted to anode, creating a current

  12. Optical Modulation Formats • Based on short energy pulses with a high peak power and low duty cycle • Three Types • Pulse Gated Binary Modulation (PGBM) • Pulse Polarization Binary Modulation (PPBM) • Pulse Interval Modulation (PIM)

  13. Pulse Interval Modulation • Most efficient type of modulation • N separate time slots in pulse interval • Transmit log2(N) bits per pulse • Pulse sent during one of these time slots • Time slot is value of the word • Translated in binary data

  14. Example of PIM

  15. Conclusion • Satellites very important for modern communications • Radio Frequency communication reaching the end of its usefulness • Laser Communications will eventually be the method of choice for satellites

  16. Questions

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