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32-2 A cell phone can be
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equipped with a modem, allowing portable or mobile access to online computer networks.
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A satellite system is like a huge cellular network with the base stations (repeaters) located in space rather than on the earth s surface. The zones of coverage are large, and they change in size and shape if the satellite moves relative to the earth s surface.
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Geostationary-Orbit Satellites You learned about geostationary orbits in Chap. 25. Geostationary satellites are used in television (TV) broadcasting, in telephone and data communication, for gathering weather and environmental data, and for radiolocation. In geostationary satellite networks, earth-based stations can communicate through a single bird only when the stations are both on a line of sight with the satellite. If two stations are nearly on opposite sides of the planet, say in Australia and Wisconsin, they must operate through two satellites to obtain a link (Fig. 32-3). In this situation, signals are relayed between the two satellites, as well as between either satellite and its respective earth-based station. A potential problem with geostationary satellite links is the fact that the signal path is long enough so that perceptible propagation delays occur. This delay, and its observed effect, is known as latency. This doesn t cause problems with casual communications or Web browsing, but it slows things down when computers are linked with the intention of combining their processing power. Low-Earth-Orbit Satellites The earliest communications satellites orbited only a few hundred miles above the earth. They were low-earth-orbit (LEO) satellites. Because of their low orbits, LEO satellites took only about 90 minutes to complete one revolution. Communication was spotty, because a satellite was in range of any given ground station for only a few minutes at a time. This is the main reason why geostationary satellites became predominant once rocket technology progressed to the point where the necessary altitude and orbital precision could be obtained.
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Satellites and Networks 557
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32-3 A communications
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However, geostationary satellites have certain limitations. A geostationary orbit requires constant adjustment, because a tiny change in altitude will cause the satellite to get out of sync with the earth s rotation. Geostationary satellites are expensive to launch and maintain. When communicating through them, there is always a delay because of the path length. It takes high transmitter power, and a sophisticated, precisely aimed antenna, to communicate reliably. These problems with geostationary satellites have brought about a revival of the LEO scheme. Instead of one single satellite, the new concept is to have a large fleet of them. A good LEO satellite system is launched and maintained in such a way that, for any point on the earth, there is always at least one satellite in direct line-of-sight range. The satellites can relay messages throughout the fleet. Thus, any two points on the surface can always make, and maintain, contact through the satellites. The satellites are placed in polar orbits (routes that pass over or near the earth s geographic poles) to optimize the geographical coverage. Even if you re at or near the north geographic pole or the south geographic pole, you can use a LEO satellite system. This is not true of geostationary satellite networks, where the regions immediately around the geographic poles are not seen by the satellites. A LEO satellite wireless communications link is easier to access and use than a geostationary satellite link. A small, simple antenna will suffice, and it doesn t have to be aimed in any particular direction. The transmitter can reach the network using only a few watts of power. The latency is less than 100 milliseconds (ms), compared with as much as 400 ms for geostationary satellite links.
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Medium-Earth-Orbit Satellites Some satellites revolve in orbits higher than those normally considered low-earth, but at altitudes lower than the geostationary level of 22,300 mi (36,000 km). These intermediate birds are called medium-earth-orbit (MEO) satellites. A MEO satellite takes several hours to complete each orbit. MEO satellites operate in fleets, in a manner similar to the way LEO satellites are deployed. Because the average MEO altitude is higher than the average LEO altitude, each bird can cover a larger region on the surface at any given time. A fleet of MEO satellites can be smaller than a comparable fleet of LEO satellites, and still provide continuous, worldwide communications. The orbits of geostationary satellites are essentially perfect circles, and most LEO satellites orbit in near-perfect circles. But MEO satellites often have elongated, or elliptical, orbits. The point of
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