Prepared By: Ismail Mehrez- Mohamed Khaled. General concepts Satellite characteristics System...
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Transcript of Prepared By: Ismail Mehrez- Mohamed Khaled. General concepts Satellite characteristics System...
• General concepts• Satellite
characteristics• System components• Orbits• Power sources• Communications
Frequencies Path losses
GPS Satellite - NASA
Satellite is in the orbit of the eartho Special orbits have particularly useful propertieso Carries its own source of power
Communications possible with:o Ground station fixed on earth surfaceo Moving platform (Non-orbital)o Another orbiting satellite
Orbital parameterso Height o Orientationo Location
Power sourceso Principally solar powero Stored gas/ion sources for position adjustment
VHF, UHF, and microwave radiation used for communications with Ground Station(s)
Signal path losses - power limitations
Low Earth Orbit (LEO)o 80 - 500 km altitudeo Atmospheric drag below 300 km
Medium Earth Orbit (MEO)o 8000 - 18000 km altitudeo Van Allen radiation
Geostationary Orbit (GEO)o 35,786 km altitude o Difficult orbital insertion and maintenance
By the Law of Sines:
and,
rssin()
d
sin( )
90
The elevation angle is approximately,cos() rs sin( ) / d
Inclination Angle
Elevation Angle
Receiving antenna Receiver Processing (decode, security, encode, other) Transmitter Transmitting antenna Power and environmental control systems Possible position control (geosynchronous)
Solar panels (near-earth satellites)o Power degrades over time - relatively long
Radioactive isotopes (deep space probes)o Lower power over very long life
Fuel cells (space stations with resupply)o High power but need maintenance and chemical
resupply
Via electromagnetic waves (“radio”) Typically at microwave frequencies High losses due to path length Many interference sources Attenuation due to atmosphere and weather High-gain antennas needed
The capacity C [bits/s] of a channel with bandwidth W, and signal/noise power ratio S/N is
Wavelength = Velocity/Frequency
where,velocity ≈ velocity of light in vacuum
( about 3 x 108 meters/sec)
C W log2 1S
N
• Generally between 300 MHz and 300 GHz.i.e. The microwave spectrum
- Line of sight propagation (space and atmosphere).- Blockage by dense media (hills, buildings)-Wide bandwidths compared to lower frequency bands.
• Properties vary according to the frequency used: Propagation effects (diffraction, noise, fading) Antenna Sizes
Wikipedia
Standard designationsFor microwave bands
Common bands for satellite communication are the L, C and Ku bands.
Dish-Antenna Power Gain:
G =
o A is the area of the antenna apertureo D is the diameter of the parabolic reflectoro lambda is the wavelength of the radio waves.o eA: is a dimensionless parameter between 0 and 1 called
the aperture efficiency.
Example:Calculate the Power gain of a Ku-Band antenna With
average aperture efficiency of 0.6 at a wavelength of 0.02m. The diameter of the reflector is known to be 80cm.
Solution: Power Gain = 0.6*(3.14*40)2 = 9465
GdB = 10 log10[Power Gain ] = 40 dB
Example 2:Repeat example 1 with D = 9m
Solution:GaindB = 10 log10 (d/)2 = 60 dB
Conclusion?.....Bigger antennas have higher gain.
Losses increase with frequency Long path lengths (dispersion with distance)
( Path lengths can be over 42,000 km ) Atmospheric absorption Rain, snow, ice, & cloud attenuation
Pt = transmitted powerPr - received powerAt = transmit antenna apertureAr = receive antenna apertureLp = path lossLa = atmospheric attenuation lossLd = diffraction losses
Free-space power loss = (4d / )2
In dB this becomes,
where:d is the path distance in mf is the frequency in Hz
Example:Calculate the power loss of a Ku band
geosynchronous satellite with the given parameters:f = 15,000 MHzd = 42,000 km
Solution:LossdB =
20 log10(40,000) + 20 log10(15,000) – 147.55 = 208 dB
High gain antennas High transmitter power Low-noise receivers Error correcting codes Frequency selection
Telecommunications Military communications Navigation systems Remote sensing and surveillance Radio / Television Broadcasting Astronomical research Weather observation
High channel capacity (>100 Mb/s) Low error rates (Pe ~ 10-6) Stable cost environment (no long-distance
cables or national boundaries) Wide area coverage (whole North America, for
instance) Coverage can be shaped by antenna patterns
Expensive to launch Expensive ground stations required Very hard to be maintained Limited frequency spectrum Limited orbital space (geosynchronous) Constant ground monitoring required for
positioning and operational control Sensitive political environment, with competing
interests and relatively limited preferred space
Space vehicle used as communications platform(Earth-Space-Earth, Space-Earth, Space-Space)
Ground station(s) (Tx/Rx)
Texto Satellite Communications, Second Edition, T. Pratt, C.
Bostian, and J. Allnut, John Wilen & Sons, 2003.
Ippolito, Louis J., Jr., Satellite Communications Systems Engineering, John Wiley, 2008.
Kraus, J. D., Electromagnetics, McGraw-Hill, 1953. Kraus, J. D., and Marhefka, R. J., Antennas for All Applications,
Third Edition, McGraw-Hill, 2002. Morgan, W. L. , and Gordon, G. D., Communications Satellite
Handbook, John Wiley & Sons, 1989.Proakis, J. G., and Salehi, M., Communication Systems
Engineering, Second Edition, Prentice-Hall, 2002.Roddy, D, Satellite Communications, Fourth Edition, Mc Graw-Hill,
1989.Stark, H., Tuteur, F. B., and Anderson, J. B., Modern Electrical
Communications, Second Edition, Prentice-Hall, 1988.Tomasi, W., Advanced Electronic Communications Systems, Fifth
Edition, Prentice-Hall, 2001.