The Future of Satellite CommunicationsJoel KloosterENGR 302May, 2002

HistorySputnik - Russian (Oct 4, 1957)Only lasted 92 days in orbitEcho - 1st TV satellite (1960)Telstar - 1st active TV satellite (1962)Syncon - 1st earth synchronous (1964)

Types of OrbitsGeosynchronousSame rotational velocity as earthMaintains position relative to earthAltitude - 35 786 kmVelocity - 11 300 km/hrAsynchronousMuch lower altitude = much higher velocityPosition over earth constantly changing

Operation of SatellitesTransmissionReceptionLow Noise ConverterPolarizationTunerAntennas

Radio Signal PropagationFree Space Propagation ModelLine-of-sight with no obstructionsFriis free space equation

Path Loss (PL)Fading of electromagnetic signalPositive difference between received power and transmitted power (in dB)

The Solution??Lasers!!

Advantages of Laser CommunicationsFrequencies 7 - 8x higherHigher bandwidthSmaller beam divergenceSmaller antennasHigher data rates

What type of Laser is used?Neodymium: yttrium, aluminum, garnet (Nd:YAG) most common typeRod of crystalline YAG lightly doped with Nd is used as the amplifierOptical energy excites Nd atoms to higher energy stateReturn to normal energy state emits energy at wavelength of 1.664 um

ModulationWhat actually drives the communicationBirefringence modulator most usefulUses electric-field induced birefringence of the crystal to rotate polarized light

Optical DetectorsReceiver must somehow convert the laser energy to an electric currentPhoton-wave theory of lightPhotons have some momentum that exerts a force on the receiverFrees electrons from atoms of cathodePhotoelectrons attracted to anode, creating a current

Optical Modulation FormatsBased on short energy pulses with a high peak power and low duty cycleThree TypesPulse Gated Binary Modulation (PGBM)Pulse Polarization Binary Modulation (PPBM)Pulse Interval Modulation (PIM)

Pulse Interval ModulationMost efficient type of modulationN separate time slots in pulse intervalTransmit log2(N) bits per pulsePulse sent during one of these time slotsTime slot is value of the wordTranslated in binary data

Example of PIM

ConclusionSatellites very important for modern communicationsRadio Frequency communication reaching the end of its usefulnessLaser Communications will eventually be the method of choice for satellites

Questions

Transmission: most common method is frequency modulation (FM). Uplink at 14 GHz, Downlink at 10.7 - 12.7 GHz (KU-Band)Use transponder to receive, convert freq, and transmitReception:Receiving antenna on earth or another antenna picks up signalFootprint diagram shows area of coverage for each satelliteLow Noise Convertor: on antennaconvert signal to lower frequency (950 - 2150 MHz) with minimal noisePolarization: concentrates beam and gives it directionleft- or right-hand circular, horizontal or vertical lineardepolarizor selects one or the otherTuner:lowers frequency even more (to 47 - 850 MHz) for TV tunerAntenna:Prime Feed Focus Dish - concave, low efficiency, large diameterOffset Dish - LNC off to side, better efficiency, smallerDual Offset Dish - larger dish get signal from sat., directs them to smaller dish, 80% efficiencyFlat Antenna - very compact, limited use

Satellites communicate to each other and earth via electromagnetic radio waves that propagate through free spaceFriis Free Space Equation: power of received signal decays as the square of the distance between the transmiter and receiverBirefringence modulation: modulates the laser output by rotating the polarized incident lightif no voltage is applied, linearly polarized light exits the modulator in same plane it enteredwhen apply a voltage, the induced birefringence slows the rotation of one of the circularly polarized vectors, bbut not the other. This is the retardation of the light vector. Emerging light is rotated by some angle - the retardation angle