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

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Presentation on theme: "The Future of Satellite Communications Joel Klooster ENGR 302 May, 2002."— Presentation transcript:

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 km Velocity 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 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 log 2 (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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