30-01-2013, 04:35 PM
Satellite Communications
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Introduction
Types of satellite services
1. Fixed satellite service (FSS)
• Links for existing telephone networks
• Transmitting TV signals to cable companies.
2. Broadcasting Satellite Service (BSS)
• Direct to home (DTH) =Direct broadcasting satellites (DBS)
3. Mobile satellite service (MSS)
• Land mobile , maritime mobile and aeronautical mobile
4. Navigation satellite service (GPS)
• Global positioning system (S&R)
5. Meteorgolical satellite service (Weather Forecast)
6. Deep Space Satellites
Advantages of satellite communications
1. Mobile/Wireless Communication, independent of location
2. Wide area coverage:country,continent, or globe
3. Wide bandwidth available throughout
4. Independence from terrestrial infrastructure
5. Rapid installation of ground network
6. Low cost per added site
7. Uniform service characteristics
8. Total service from a single provider
9. Small Fading margin (3dB)
Disadvantages of satellite communications
1. High cost for satellite
2. Short life time maximum of 15 years
3. Redundancy in component!
Frequency Spectrum concepts:
• Frequency: Rate at which an electromagnetic wave reverts its
polarity (oscillates) in cycles per second or Hertz (Hz).
• Wavelength: distance between wavefronts in space. Given in
meters as: λ= c/f
Where: c = speed of light (3x108 m/s in vacuum)
f = frequency in Hertz
• Frequency band: range of frequencies.
• Bandwidth: Size or “width” (in Hertz) or a frequency band.
• Electromagnetic Spectrum: full extent of all frequencies from
zero to infinity.
Radio Frequencies (RF)
• RF Frequencies: Part of the electromagnetic spectrum
ranging between 300 MHz and 300 GHz. Interesting
properties:
– Efficient generation of signal power
– Radiates into free space
– Efficient reception at a different point.
Differences depending on the RF frequency used:
- Signal Bandwidth
- Propagation effects (diffraction, noise, fading)
A t Si
- Antenna Sizes
Types of Satellite Stabilization
• Spin Stabilization
– Satellite is spun about the axis on which the
moment of inertia is maximum (ex., HS 376,
most purchased commercial communications
satellite; first satellite placed in orbit by the
Space Shuttle.)
• Three-Axis Stabilization
– Bias momentum type (ex., INTELSAT V)
– Zero momentum type (ex., Yuri)