1/13/09© 2010 raymond p. jefferis iii lect 01 - 1 satellite communications introduction general...

1/13/09 © 2010 Raymond P. Jefferis III Lect 01 - 1

Satellite Communications

Introduction • General concepts• Satellite characteristics• System components• Orbits• Power sources• Communications

Frequencies Path losses

GPS Satellite - NASA


Text

• Text– Satellite Communications, Second Edition, T.

Pratt, C. Bostian, and J. Allnut, John Wilen & Sons, 2003.

Other Useful ReferencesIppolito, 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.



General Concepts

• Satellite is in (earth) orbitSpecial 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



• Advantages• Disadvantages• What is involved• Why use space• Frequency spectrum• Satellite components and systems• System design considerations

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


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


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)

•


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



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


Application Examples

• Telecommunications

• Military communications

• Navigation systems

• Remote sensing and surveillance

• Radio / Television Broadcasting

• Astronomical research

• Weather observation

Orbits

• Have particular advantages and disadvantages

• Are determined by satellite mission

• Obey Keppler’s Laws


Types of Orbit


Dr. Leila Z. Ribeiro, George Mason University


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


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”)


The Clarke Orbit

• Arthur C. Clarke, Wireless World, February, 1945, p58.


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


Orbital Features

• Ground image area (instantaneous)

• Ground track coverage (multiple orbits)

• Stationarity (geostationary orbit)

• Space coverage (satellite-satellite)


Orbital Inclination Angles• Equatorial

– Prograde - toward the east – Retrograde - toward the west

• Inclined– Various inclination angles, including polar

• Geostationary

• Sun synchronous


Earth Coverage

By the Law of Sines:

and,

rs

sin(ρ)=

dsin(δ )

ρ =90 +θ

The elevation angle is approximately,cos(θ) =rssin(δ ) / d

• The total coverage area on the surface of the earth is given by,

• Ref:

Earth Coverage (continued)


A =2πre2 (1−Cos[δ ])

http://www.cdeagle.com/ommatlab/coverage.pdf

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]


Ground Coverage Area

• Coverage with satellite altitude,

• For satellite radius rsat


α =Sin−1 re

rsat

⎡

⎣⎢

⎤

⎦⎥

δ = Sin−1 rsat

re

Sin[α ]⎡

⎣⎢

⎤

⎦⎥− α

A = 2π re2 (1− Cos[δ ])

Coverage vs Satellite Altitude


Mathematica® Notebook

re = 6378.137; (* km *)rs = re + hs; alpha = ArcSin[re/rs]ad = alpha/Degreedelta = ArcSin[(rs/re)*Sin[alpha]] - alphadd = delta/DegreeA = 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]"}]



System Components

• Satellite(s)

• Ground station(s)

• Computer systems

• Information network


Basic Satellite Network

Satellite network with earth stations.


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)


Simple Satellite Schematic


Telemetry Block Diagram


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


Solar Panels

Geostationary Operational Environmental Satellites (GOES) - Ground testing of solar panels, NASA

Type: GaAs/GeVoltage: 53.1 VoltsPower: 1940 Watts( Effective Load + Source Resistance: 1.45341 Ω )


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

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”)


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)

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


C =Wlog2 1+SN

⎛⎝⎜

⎞⎠⎟

Frequency and Wavelength

• Velocity = Frequency * Wavelength

• Wavelength = Velocity/Frequency

where,

velocity ≈ velocity of light in vacuum( about 3 x 108 meters/sec)


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

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



Communications Channel

• Microwave energy at frequency, f (Hertz)

• Moves at velocity, v [m/s]

• With wavelength (distance between peak intensities), λ [m]

• Formula: λ = 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 = fλv = fλv = fλ

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)

Electromagnetic Spectrum


Wikipedia

Designated Microwave Bands


Wikipedia

Standard designationsFor microwave bands

Common bands for satellite communication are the L, C and Ku bands.


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)


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


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, λ)

• Dish-Antenna Power Gain = η(πd/λ2

(where η is antenna efficiency)


Antenna Gain Calculation

• Ku-Band antenna Diameter 80 cm (d/λ = 40), η = 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.


Antenna Gain Efficiency

• From text, p115d / λ = 5.6 (4GHz), η = 0.35GaindB = 10 log10η(πd/λ)2 = 20.9 dB

• From text, p116d = 9m, λ= 0.075m (4GHz), ηGaindB = 10 log10η (πd/λ)2 = 49.3 dB

Note: Smaller antenna has lower efficiency.


C-Band


C-Band• Frequencies: 3.7 - 6.425 GHz (λ ~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


Ku-Band


Ku-Band• Frequencies: 12 - 18 GHz (λ ~ 2cm)• Uses:

– Remote TV broadcasting– Satellite communications– VSAT

• Features:– Rain, snow, ice (on dish) susceptibility– Small antenna size - high antenna gain– High power allowed


Ka-Band• Frequencies: 18 - 40 GHz (λ~ 1cm)• Uses:

– High-resolution radar– Communications systems– Deep space communications

• Features:– Obstacles interfere– Atmospheric absorption


V-Band• Frequencies: 40 to 75 GHz. (λ~ 5 mm)• Uses:

– Millimeter wave radar research (expensive!)– High capacity millimeter wave communications– Point-to-point fixed wireless systems (WiFi)

• Features:– Rain fade– Obstacles block path– Atmospheric absorption– Expensive equipment


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)



Link budget analysis• Overview• Antenna gain• Path loss• Obstacle loss• Atmospheric loss• Receiver gain


Antenna Gain and Link LossesPt = transmitted powerPr - received powerAt = transmit antenna apertureAr = receive antenna apertureLp = path lossLa = atmospheric attenuation lossLd = diffraction lossesAntenna Gain (t or t):Gt/r = 4πAe t/r/λ2

Combined Antenna Gain (t + t):G = GtGy


Simple Path Loss Model

• Free-space power loss = (4πd / λ)2

In dB this becomes,

LossdB =32.44 + 2 log1 (d) + 2 log1 ( f )

where:d is the path distance in kmf is the frequency in MHz


Sample Path Loss Calculation

• Ku band geosynchronous satellite:f = 15,000 MHzd = 42,000 km

• LossdB = 32.44 + 20 log10(40,000) + 20 log10(15,000) = 208 dB

• Atmospheric losses must be added to this


BPSK Bit Error Rate Graph


Atmospheric Attenuation

Microwave Attenuation [dB/km] vs Frequency [GHz], Wikipedia

O2

53.5 - 65.2 GHz

H2O

22.2 GHz


H2O vs Dry Air Absorption

Remedies for Path Loss

• High gain antennas

• High transmitter power

• Low-noise receivers

• Tracking of antennas

• Modulation techniques

• Error correcting codes

• Frequency selection



Radiation Pattern of Aperture

0.2 0.4 0.6 0.8 1.0

- 0.03- 0.02- 0.010.000.010.020.03

The Mathematica® notebook follows, for D/λ= 10:

E-field for aperture with D/λ = 10


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


Intelsat Galaxy-11 Specifications• Location: 91W

• Power: Solar, 10.4 KW

• Antennas:

– C-Band: 2.4m

– Ku-Band: 1.8m

• Transponders:

– 24 channels C-Band: 20W each

– 24 channels Ku-Band: 75W (data)

– 16 channels Ku-Band: 140W (TV video)


Intelsat Galaxy-11 C-Band Coverage


Intelsat Galaxy-11 Ku-Band Coverage


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

End


1/13/09© 2010 raymond p. jefferis iii lect 01 - 1 satellite communications introduction general...

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