Lecture 9
Prepared By:
Ismail Mehrez Mohamed Khaled
GPS Satellite NASA
• General concepts
• Satellite characteristics
• System components
• Orbits
• Power sources
• Communications
Frequencies
Path losses
Satellite is in the orbit of the earth o
Special orbits have particularly useful properties o
Carries its own source of power
Communications possible with: o o o
Ground station fixed on earth surface
Moving platform (Non-orbital)
Another orbiting satellite
Orbital parameters o
Height o o
Orientation
Location
Power sources o o
Principally solar power
Stored gas/ion sources for position adjustment
VHF, UHF, and microwave radiation used for communications with Ground Station(s)
Signal path losses - power limitations
Dr. Leila Z. Ribeiro, George Mason University
Low Earth Orbit (LEO)
o o
80 - 500 km altitude
Atmospheric drag below 300 km
Medium Earth Orbit (MEO)
o o
8000 - 18000 km altitude
Van Allen radiation
Geostationary Orbit (GEO)
o o
35,786 km altitude
Difficult orbital insertion and maintenance
Elevation Angle
By the Law of Sines: r s sin( )
d sin( ) and,
90
The elevation angle is approximately, cos( ) r s sin( ) / d
Inclination Angle
Satellite(s)
Ground station(s)
Computer systems
Information network
Satellite network with earth stations.
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
C W log
2
1
S
N
Wavelength = Velocity/Frequency where, velocity ≈ velocity of light in vacuum
( about 3 x 10 8 meters/sec)
•
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
Wikipedia
Standard designations
For microwave bands
Common bands for satellite communication are the L , C and Ku bands.
Dish-Antenna Power Gain:
G = o o o o
A is the area of the antenna aperture
D is the diameter of the parabolic reflector lambda is the wavelength of the radio waves.
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
G dB
= 10 log
10
[Power Gain ] = 40 dB
Example 2:
Repeat example 1 with D = 9m
Solution:
Gain dB
= 10 log
10
EA ( p d/ l ) 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
L p
L a
L d
P t
P r
A t
A r
= transmitted power
- received power
= transmit antenna aperture
= receive antenna aperture
= path loss
= atmospheric attenuation loss
= diffraction losses
Free-space power loss = (4 p d / l ) 2
In dB this becomes,
where:
d
is the path distance in m
f
is the frequency in Hz
Example:
Calculate the power loss of a Ku band geosynchronous satellite with the given parameters: f = 15,000 MHz d = 42,000 km
Solution:
Loss dB
=
20 log
10
(40,000) + 20 log
10
(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 (P e
~ 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)
Text o
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.
