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.
A communications link that employs two geostationary satellites
You learned about geostationary orbits in previous topics. Geostationary satellites are used in television (TV) broadcasting, in telephone and data communication, for gathering weather and environmental data, and for radio location.
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 (above figure). 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.
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.
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.
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 lowest altitude is called perigee; the point of greatest altitude is called apogee. The apogee can be, and often is, much greater than the perigee. Such a satellite orbits at a speed that depends on its altitude. The lower the altitude, the faster the satellite moves. A satellite with an elliptical orbit crosses the sky rapidly when it is near perigee, and slowly when it is near apogee; it is easiest to use when its apogee is high above the horizon, because then it stays in the visible sky for a long time.
Every time a MEO satellite completes one orbit, the earth rotates beneath it. The rotation of the earth rarely coincides with the orbital period of the satellite. Therefore, successive apogees for a MEO satellite occur over different points on the earth’s surface. This makes the tracking of individual satellites a complicated business, requiring computers programmed with accurate orbital data. For a MEO system to be effective in providing worldwide coverage without localized periodic blackouts, the orbits must be diverse, yet coordinated in a precise and predictable way. In addition, there must be enough satellites so that each point on the earth is always on a line of sight with one or more of the satellites, and preferably, there should be at least one bird in sight near apogee at all times.
At A, a client-server wireless LAN. At B, a peer-to-peer wireless LAN.
A local area network (LAN) is a group of computers linked together within a building, campus, or other small region. The interconnections in early LANs were made with wire cables, but wireless links are increasingly common. A wireless LAN offers flexibility, because the computer users can move around without having to bother with plugging and unplugging cables. This arrangement is ideal when notebook computers (also known as laptops) are used. The way in which a LAN is arranged is called the LAN topology. There are two major wireless LAN topologies: the client-server wireless LAN and the peer-to-peer wireless LAN.
In the client-server topology (above figure A), there is one large, powerful, central computer called a file server, to which all the smaller personal computers (labeled PC) are linked. The file server has enormous computing power, high speed, and large storage capacity, and can contain all the data for every user. End users do not communicate directly. All the data must pass through the file server. In a peer-to-peer LAN (above figure B), all of the computers in the network are PCs with more or less equal computing power, speed, and storage capacity. Each user maintains his or her own data. Subscribers can, and almost always do, communicate directly without the data having to pass through any intermediary. This offers greater privacy and individuality than the client-server topology, but it is slower when a large number of users need to share data.
Client-server LANs are favored by large institutions. Small businesses and schools, or departments within a larger corporation or university, prefer to use peer-to-peer LANs, mainly because they are cheaper and easier to maintain. In these illustrations, only three PCs are shown in the networks. But any LAN can have as few as two, or as many as several dozen, computers.
Home Internet users sometimes employ a modified version of the arrangement shown in above figure A. In place of the file server, a device called a wireless router provides a hub through which the computers can communicate. The router is connected to the Internet by a high-speed interface such as a cable modem, allowing several computers in a household to have Internet access at the same time.