Satellite Communications

 
Satellite Communications
Satellite communications provide an alternative for creating wireless networks communications
systems. Satellites have been used by government and industry for decades, but it
was not until the last ten years that satellite networks have been commercially used for
wireless communications. This section discusses types of satellites, how wireless satellite
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communication works and the various companies that provide key satellite systems and
services to consumers.
There are three main types of mobile satellite systems—Low Earth Orbits (LEOs),
Medium Earth Orbits (MEOs) and Geostationary Orbits (GEOs) (Fig. 10.10). Their characteristics
include their position from earth, rotation time around the earth and how they
perform mobile communications. [***A. Dornan, The Essential Guide to Wireless
Communications, Prentice Hall (Upper Saddle River, NJ)***]. Low Earth Orbit Satellites
(LEOs) are 100–300 miles above the Earth’s surface and usually require many satellites
(sometimes hundreds) to form a complete network (Fig. 10.10). LEOs are low to the
Earth’s surface which allows them to send and receive transmission signals quickly. LEOs
are placed in a ring pattern that follows the Earth’s curvature in one orbit pattern to overcome
line-of-sight issues. For example, consider a mobile device user in North America
who wants to send a message to a user in Europe. The device sends a signal up to a LEO
which is in the range of the first mobile device. This satellite cannot directly send a signal
to the user in Europe because the signal would bounce off the Earth’s surface. To account
for this, satellites in the LEO system for a particular network pass signals from one station
to another until the signals reach a satellite in the system that can directly communicate
with the user in Europe. LEOs do not have the same rotational time period as the earth—
they circle the Earth every 90 minutes. This means that a user will not always communicate
with the same satellite. LEOs must be replaced every few years because the Earth’s gravitational
pull drags the satellites down from their original orbit. Once they are knocked out
of their orbit, they cannot communicate effectively with other satellites. [***A. Dornan,
“The Last Five Hundred Miles,” Network Magazine March: 58.***].
Medium Orbit Earth Satellites (MEOs) are located 6,000 to 12,000 miles above the
Earth’s surface and have six different orbit patterns (Fig. 10.10). MEOs are similar to LEOs
in that multiple satellites can be used to complete communications, but fewer than required
by LEOs. MEOs are less popular in mobile communications for businesses and average
consumers but are used for government applications such as intelligence gathering and
weather forecasting. Their distance from the Earth requires MEOs to use relay stations or
satellite dishes to send and receive information from the Earth. [***A. Leon-Garcia and
I. Widowed, Communication Networks, Mc Graw Hill (Columbus, OH)***]
Geostationary (Geosync) Orbit Satellites (GEOs) are located 22,282 miles above the
Earth’s surface (Fig.10.10). This distance gives a satellite the same rotational period as the
Earth (one rotation approximately every 24 hours). GEOs need to be replaced less often
than LEOs or MEOs. However, GEOs encounter transmission problems with certain areas
of the Earth’s surface, such as near the equator. GEOs distance from Earth causes the signal
transmission to contain delays and distortion that is noticeable to users.[***A. Dornan,
The Essential Guide to Wireless Communications, Prentice Hall (Upper Saddle River,
NJ)***].
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180 Wireless Communications Technologies I Chapter 10
Fig. 10.10 LEOs, MEOs and GEOs. [***Screen shot taken from
<www.cs.berkeley.edu/~randy/Courses/CS294.S96/MSS.pdf>
Permission requested from Cal State Berkeley 05-14-2001***].
Satellites enable a variety of mobile communications services ranging from voice and
data services to location-based services. In addition, when natural disasters, such as earthquakes,
hurricanes etc. destroy ground network communication infrastructure, satellites
can help restore services and save lives because they are undamaged in such events.
[***P. Brown, “Satellite Telephony A Vital Link,” ViaSatellite January 2001:
42.***]. However, Satellites can be damaged by meteorites which never hit the Earth’s
surface because they burn up in the atmosphere.
Satellite networks which provide voice and data services include Aries, Ellipso,
Odyssey, Iridium (see Iridium Feature) Starnet, Inmarstat, Mobilstat and Teledesic. Globalstar
is a satellite system used primarily with location-based services. Satellite communications
is on average more expensive than cellular service. Location services are
discussed in Chapter 3, Location-Based Services and access technologies are discussed in
Chapter 10, Wireless Communications Technologies Part II.
Iridium: A Second Chance? [***B. Feder, “Iridium Satellite System Is
About to Be Revised,” The New York Times 28 March 2001: C4.***]
In 1997, Motorola invested over five billion dollars to establish a satellite network. This
network provided voice, data, fax and location-based services to its customers. It used
LEO satellites. Originally, designers thought that a total of 77 satellites would be needed
to establish a network to cover the earth’s surface and user population, so the system
was named Iridium after the 77th element in the periodic table of the elements. The final
design of Iridium included only 66 satellites in six different orbits, but the original
name was kept.
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