Transcript Elements of Simulation

Satellite Communications
Introduction
•
•
•
•
•
•
General concepts
Satellite characteristics
System components
Orbits
Power sources
Communications
 Frequencies
 Path losses
GPS Satellite - NASA
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 1
Text
• Text
– Satellite Communications, Second Edition, T.
Pratt, C. Bostian, and J. Allnut, John Wilen &
Sons, 2003.
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 2
Other Useful References
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.
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 3
General Concepts
• Satellite is in (earth) orbit
Special orbits have particularly useful properties
Carries its own source of power
• Communications possible with:
– Ground station fixed on earth surface
– Moving platform (Non-orbital)
– Another orbiting satellite
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 4
Satellite Communications
• Advantages
• Disadvantages
• What is involved
• Why use space
• Frequency spectrum
• Satellite components and systems
• System design considerations
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 5
Advantages of Satellites
• 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
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 6
Disadvantages of Satellites
•
•
•
•
•
•
Expensive to launch
Expensive ground stations required
Cannot be maintained
Limited frequency spectrum
Limited orbital space (geosynchronous)
Constant ground monitoring required for
positioning and operational control
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 7
Satellite Communications Needs
• Space vehicle used as communications
platform
(Earth-Space-Earth, Space-Earth, Space-Space)
• Space vehicle used as sensor platform with
communications
• Ground station(s) (Tx/Rx)
• Ground receivers (Rx only)
•
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 8
Satellite Characteristics
• Orbital parameters
– Height (velocity & period related to this)
– Orientation (determined by application)
– Location (especially for geostationary orbits)
• Power sources
– Principally solar power
– Stored gas/ion sources for position adjustment
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 9
Satellite Characteristics
• Orbiting platforms for data gathering and
communications – position holding/tracking
• VHF, UHF, and microwave radiation used for
communications with Ground Station(s)
• Signal path losses - power limitations
• Systems difficult to repair and maintain
• Sensitive political environment, with competing
interests and relatively limited preferred space
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 10
Application Examples
•
•
•
•
•
•
•
1/13/09
Telecommunications
Military communications
Navigation systems
Remote sensing and surveillance
Radio / Television Broadcasting
Astronomical research
Weather observation
© 2010 Raymond P. Jefferis III
Lect 01 - 11
Orbits
• Have particular advantages and
disadvantages
• Are determined by satellite mission
• Obey Keppler’s Laws
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 12
Types of Orbit
Dr. Leila Z. Ribeiro, George Mason University
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 13
Orbital Altitudes and Problems
• Low Earth Orbit (LEO)
– 80 - 500 km altitude
– Atmospheric drag below 300 km
• Medium Earth Orbit (MEO)
– 2000 - 35000 km altitude
– Van Allen radiation between 200 - 1000 km
• Geostationary Orbit (GEO)
– 35,786 km altitude (42,164.57 km radius)
– Difficult orbital insertion and maintenance
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 14
LEO and MEO Features
•
•
•
•
•
•
Earth coverage requires multiple passes
Typical pass requires about 90 minutes
Signal paths relatively short (lower losses)
Small area, high resolution ground image
Earth station tracking required
Multiple satellites for continuous coverage
(Decreases with increasing altitude - “Telstar”)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 15
The Clarke Orbit
• Arthur C. Clarke, Wireless World, February,
1945, p58.
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 16
Geostationary Orbit (GEO)
•
•
•
•
•
Appears fixed over point on earth equator
Each satellite can cover 120 degrees latitude
Orbital Radius = 42,164.17 km
Earth Radius = 6,378.137 km (avg)
Period (Sidereal Day) = 23.9344696 hr
(86164.090530833 seconds)
• Long signal path - large path losses
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 17
Orbital Features
•
•
•
•
Ground image area (instantaneous)
Ground track coverage (multiple orbits)
Stationarity (geostationary orbit)
Space coverage (satellite-satellite)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 18
Orbital Inclination Angles
• Equatorial
– Prograde - toward the east
– Retrograde - toward the west
• Inclined
– Various inclination angles, including polar
• Geostationary
• Sun synchronous
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 19
Earth Coverage
By the Law of Sines:
rs
d

sin(  ) sin( )
and,
  90  
The elevation angle is approximately,
cos( )  rs sin( ) / d
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 20
Earth Coverage (continued)
• The total coverage area on the surface of the
earth is given by,
A  2 re2 (1 Cos[ ])
• Ref:
http://www.cdeagle.com/ommatlab/coverage.pdf
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 21
Sample Calculation [Mathematica®]
re = 6378.137; (* km *)
delta = 32.4171; (* degrees *)
area = 2 p re^2 (1 - Cos[delta Degree]);
Print["Area = ", area, “[km^2]"]
Area = 3.98313*10^7 [km^2]
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 22
Ground Coverage Area
• Coverage with satellite altitude,
 re 

r
 sat 
  Sin 1 
 rsat

Sin[ ]  
 re

  Sin 1 
A  2 re2 (1  Cos[ ])
• For satellite radius rsat
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 23
Coverage vs Satellite Altitude
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 24
®
Mathematica
Notebook
re = 6378.137; (* km *)
rs = re + hs;
alpha = ArcSin[re/rs]
ad = alpha/Degree
delta = ArcSin[(rs/re)*Sin[alpha]] - alpha
dd = delta/Degree
A = 2 p re^2 (1.0 - Cos[delta])
Plot[A, {hs, 1000, 2000}, AxesLabel -> "Coverage
[km^2]", Frame -> True,
FrameLabel -> {"Altitude [km]", "Coverage [km^2]"}]
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 25
System Components
•
•
•
•
Satellite(s)
Ground station(s)
Computer systems
Information network
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 26
Basic Satellite Network
Satellite network with earth stations.
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 27
Satellite Components
•
•
•
•
•
•
•
•
•
Receiving antenna
Receiver (uplink)
Processing (decode, security, encode, other)
Transmitter (downlink)
Transmitting antenna (beam shaping)
Possible (de)multiplexing (for rotating satellites)
Power and environmental control systems
Attitude control
Possible position holding (geosynchronous)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 28
Simple Satellite Schematic
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 29
Telemetry Block Diagram
7/7/2015
© 2010 Raymond P. Jefferis III
Lect 03 - 30
Satellite Power Sources
• Solar panels (near-earth satellites)
– Power degrades over time - relatively long
• Radioactive isotopes (deep space probes)
– Lower power over very long life
• Fuel cells (space stations with resupply)
– High power but need maintenance and chemical
resupply
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 31
Solar Panels
Type: GaAs/Ge
Voltage: 53.1 Volts
Power: 1940 Watts
( Effective Load +
Source Resistance:
1.45341 Ω )
Geostationary Operational Environmental
Satellites (GOES) - Ground testing of solar
panels, NASA
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 32
Solar Power
• Power available in orbit: ~1400 watts of
sunlight per square meter
• Conversion efficiency: ~25%
• Useful power: ~350 Watts/square meter
• Panel steering required for maximum power
• Typical power levels: 2 - 75 kW
• Photocell output degrades over time
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 33
Communications Links
•
•
•
•
•
•
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 (“dish”)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 34
Bandwidth/Spectrum
• Frequency band: range of frequencies
• Bandwidth: size or “width” (in Hertz) of a
frequency band
• Channel capacity increases with bandwidth
(see next slide – Slide 29)
• Electromagnetic spectrum: all frequencies
(“DC to light” – see Slide 30)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 35
Cnannel Capacity
• Shannon (BSTJ, Vol. 27,1938)
The capacity C [bits/s] of a channel with
bandwidth W, and signal/noise power ratio
S/N is
S

C  W log 2  1  

N
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 36
Frequency and Wavelength
• Velocity = Frequency * Wavelength
• Wavelength = Velocity/Frequency
where,
velocity ≈ velocity of light in vacuum
( about 3 x 108 meters/sec)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 37
Satellite Communications Frequencies
• Generally between 300 MHz and 300 GHz.
 The microwave spectrum
 Allows efficient generation of signal power
 Energy radiated into space
 Energy may be focused
 Efficient reception over a specified area.
• Properties vary according to the frequency used:
 Propagation effects (diffraction, noise, fading)
 Antenna Sizes
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 38
Millimeter Waves
• Planck space exploration satellite
– Planck is a flagship mission of the European Space Agency (Esa).
It was launched in May 2009 and moved to an observing position
more than a million km from Earth on its "night side".It carries two
instruments that observe the sky across nine frequency bands. The
High Frequency Instrument (HFI) operates between 100 and 857
GHz (wavelengths of 3mm to 0.35mm), and the Low Frequency
Instrument (LFI) operates between 30 and 70 GHz (wavelengths of
10mm to 4mm).
• Johnson noise problems
– Some of its detectors operate at minus 273.05C
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 39
Communications Channel
• Microwave energy at frequency, f (Hertz)
• Moves at velocity, v [m/s]
• With wavelength (distance between peak
intensities),  [m]
• Formula:  = v / f (v = c for space) Note:
The speed of light, c, in a vacuum (space) is
fixed at, c = 299 792 458 [m/s]
v  f
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 40
Microwaves
• Frequencies from 0.3 GHz to 300 GHz.
- Line of sight propagation (space and atmosphere).
- Blockage by dense media (hills, buildings, rain)
- Wide bandwidths compared to lower frequency bands.
- Compact antennas, directionality possible.
-Reduced efficiency of generation
•1 GHz to 170 GHZ spectrum divided into bands with letter
designations (see next slide)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 41
Electromagnetic Spectrum
Wikipedia
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 42
Designated Microwave Bands
Standard designations
For microwave bands
Common bands for satellite
communication are the L, C
and Ku bands.
Wikipedia
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 43
Common Frequency Allocations
• L band
 0.950 - 1.450 GHz
 Note: GPS at 1.57542 GHz
• C band
 3.7 - 4.2 GHz (Downlink)
 5.925 - 6.425 GHz (Uplink)
• Ku band
 11.7 - 12.2 GHz (Downlink)
 14 - 14.5 GHz (Uplink)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 44
Other Frequency Allocations
• Ka band
 18.3 - 18.8, 19.7 - 20.2 GHz (Downlink)
 30 GHz (Uplink)
• V band
 40 - 75 GHz
 60 GHz allocated for unlicensed (WiFi) use
 70, 80, and 90 GHz for other wireless
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 45
Wavelength/Antenna Constraints
• Maximal antenna sizes push satellite radio
wavelengths below 2m.
• Requirements for antenna gain, due to
communication path losses, reduce the
practical wavelengths to below 20cm.
(Diameter, d, of many wavelengths,  )
• Dish-Antenna Power Gain =  (d/ 2
(where  is antenna efficiency)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 46
Antenna Gain Calculation
• Ku-Band antenna
 Diameter 80 cm (d/ = 40),  = 0.6
(about 40 wavelengths at 15GHz)
 Power Gain = 0.6*(3.14*40)2 = 15775
GdB = 10 log10[Power Gain ] = 40 dB
Note: Losses and sidelobe effects can reduce
this gain to 60% or less of its possible value.
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 47
Antenna Gain Efficiency
• From text, p115
d /  = 5.6 (4GHz),  = 0.35
GaindB = 10 log10 (d/ )2 = 20.9 dB
• From text, p116
d = 9m,  = 0.075m (4GHz),  
GaindB = 10 log10 (d/ )2 = 49.3 dB
Note: Smaller antenna has lower efficiency.
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 48
C-Band
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 49
C-Band
• Frequencies: 3.7 - 6.425 GHz ( ~5cm)
• Uses:
– TV reception (motels)
– IEEE-802.11 WiFi
– VSAT
• Features:
– Large dish antenna needed (3m diameter)
– Low rain fade - Low atmospheric atten. (long paths)
– Low power - terrestrial microwave interferences
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 50
Ku-Band
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 51
Ku-Band
• Frequencies: 12 - 18 GHz ( ~ 2cm)
• Uses:
– Remote TV broadcasting
– Satellite communications
– VSAT
• Features:
– Rain, snow, ice (on dish) susceptibility
– Small antenna size - high antenna gain
– High power allowed
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 52
Ka-Band
• Frequencies: 18 - 40 GHz ( ~ 1cm)
• Uses:
– High-resolution radar
– Communications systems
– Deep space communications
• Features:
– Obstacles interfere
– Atmospheric absorption
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 53
V-Band
• Frequencies: 40 to 75 GHz. ( ~ 5 mm)
• Uses:
– Millimeter wave radar research (expensive!)
– High capacity millimeter wave communications
– Point-to-point fixed wireless systems (WiFi)
• Features:
–
–
–
–
1/13/09
Rain fade
Obstacles block path
Atmospheric absorption
Expensive equipment
© 2010 Raymond P. Jefferis III
Lect 01 - 54
Path Loss
• 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
• Atmospheric noise effects that increase the
Bit Error Rate (BER)
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 55
Satellite Communications
Link budget analysis
• Overview
• Antenna gain
• Path loss
• Obstacle loss
• Atmospheric loss
• Receiver gain
7/7/2015
© 2010 Raymond P. Jefferis III
Lect 06 - 56
Antenna Gain and Link Losses
Pt = transmitted power
Pr - received power
At = transmit antenna aperture
Ar = receive antenna aperture
Lp = path loss
La = atmospheric attenuation loss
Ld = diffraction losses
Antenna Gain (t or t):
Gt/r = 4Ae t/r/ 2
Combined Antenna Gain (t + t):
G = GtGy
7/7/2015
© 2010 Raymond P. Jefferis III
Lect 04 - 57
Simple Path Loss Model
• Free-space power loss = (4d /  )2
In dB this becomes,
LossdB  32.44  20 log10 (d)  20 log10 ( f )
where:
d is the path distance in
km
f is the frequency in MHz
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 58
Sample Path Loss Calculation
• Ku band geosynchronous satellite:
f = 15,000 MHz
d = 42,000 km
• LossdB = 32.44 +
20 log10(40,000) +
20 log10(15,000) = 208 dB
• Atmospheric losses must be added to this
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 59
BPSK Bit Error Rate Graph
7/7/2015
© 2010 Raymond P. Jefferis III
Lect 05 - 60
Atmospheric Attenuation
O2
53.5 65.2 GHz
H2O
22.2 GHz
Microwave Attenuation [dB/km] vs Frequency [GHz], Wikipedia
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 61
H2O vs Dry Air Absorption
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 62
Remedies for Path Loss
•
•
•
•
•
•
•
High gain antennas
High transmitter power
Low-noise receivers
Tracking of antennas
Modulation techniques
Error correcting codes
Frequency selection
1/13/09
© 2010 Raymond P. Jefferis III
Lect 00 - 63
Radiation Pattern of Aperture
0.0 3
0.0 2
0.0 1
0.0 0
- 0.0 1
- 0.0 2
- 0.0 3
0.2
0.4
0.6
0.8
1.0
E-field for aperture with D/ = 10
The Mathematica® notebook follows, for D/ = 10:
7/7/2015
© 2010 Raymond P. Jefferis III
Lect 04 - 64
System Example
Intelsat GALAXY-11 at 91W (NORAD 26038)
• 39.1 dBW on C-Band (20W, 24 ch, Bw: 36 MHz)


5945 (+n*20 MHz) MHz Uplink
3720 (+n*20 MHz) MHz Downlink
• 47.8 dBW on Ku-Band (75/140W, 40 ch, Bw: 36 MHz)
 14020 (+n*20 MHz) MHz Uplink
 11720 (+n*20 MHz) MHz Downlink
• Power Supply: 10 kW (Xenon ion propulsion needs)
• Polarization: v (odd), h (even) - Downlink opposite
7/7/2015
© 2010 Raymond P. Jefferis III
Lect 02 - 65
Intelsat Galaxy-11 Specifications
• Location:
• Power:
• Antennas:
– C-Band:
– Ku-Band:
• Transponders:
– 24 channels C-Band:
– 24 channels Ku-Band:
– 16 channels Ku-Band:
1/13/09
91W
Solar, 10.4 KW
2.4m
1.8m
20W each
75W (data)
140W (TV video)
© 2010 Raymond P. Jefferis III
Lect 01 - 66
Intelsat Galaxy-11 C-Band Coverage
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 67
Intelsat Galaxy-11 Ku-Band Coverage
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 68
Conclusions
•
•
•
•
•
•
Limited satellite transmitter power
Significant path losses
High gain antennas needed
Antenna patterns can be shaped as desired
Location and tracking necessary
Atmospheric effects can be significant
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 69
End
1/13/09
© 2010 Raymond P. Jefferis III
Lect 01 - 70