Prepared By:

Ismail Mehrez - Mohamed Khaled

• 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

Dr. Leila Z. Ribeiro, George Mason University

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

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

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

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.