Satellite

1. Satellite Satellite (Short history): In 1947, Arthur C. Clarke presented a paper to the scientific community in Washington, DC. Clarke suggested that if we explored orbits in higher elevations above the earth, we might achieve an orbit at which a satellite would be able to serve as a communications broadcast tool. NASA launched the first experimental satellite in 1963. The first commercial satellite was launched in 1965, so 1965 marked the beginning of the use of satellite communications to support public telephony as well as television, particularly international television. Since then, large numbers of satellites have been launched. At this point, there are more than 250 communications-based satellites in space, as well as hundreds of other specialized satellites that are used for meteorological purposes, defence, remote sensing, geological exploration, and so on, for a total of more than 700 satellites orbiting the earth. And it seems that many more satellites will be launched in the future.

2. Continue………. There are still approximately three billion people who are not served by even basic communications services, and we can't possibly deploy enough wire line facilities in a short enough time frame to equalize the situation worldwide. Therefore, satellites are very important in bringing infrastructure into areas of the world that have previously not enjoyed that luxury. Satellites are beginning to see increased investment and, in fact, other facilities in space are as well. According to US News & World Report ("The New Space Race," November 8, 1999), U.S. companies are expected to invest as much as US$500 billion in space by the year 2010

3. Continue………….. Orbits: The path in which the satellites moves around the earth is called Orbit. It can be Equatorial, Inclined or Polar as shown in the figure.

4. Continue…………. Footprint Satellites process microwaves with bidirectional antennas(line-of-sight). Therefore, the Signal from a satellite is normally aimed at a specific area called the footprint. The signal power at the center of the footprint is maximum. The power decreases as we move Out from the footprint center. The boundary of the footprint is the location where the Power level is at a predefined threshold. Three Categories of Satellites The categories of the satellites based on the location of orbit • Geostationary Earth Orbit (GEO) A GEO satellite is launched to 22,300 miles (36,000 kilometers) above the equator. A signal from such a satellite must travel quite a distance; as a result, there is a delay. It's a 0.25-second delay in each direction, so from the time you say "Hello, how are you?" to the time that you hear the person's response, "Fine," there is a 0.5-second delay. GEO satellites have the benefit of providing the largest footprint of the satellite types. You can cover the entire world with just three GEO satellites. We are beginning to see data rates of up to 155Mbps with GEO systems, particularly in the Ka-band

5. Figure

6. Continue…….. • MEO Satellites MEO satellites orbit at an elevation of about 6,200 to 9,400 miles (10,000 to 15,000 kilometers). MEO satellites are closer to the earth than GEO satellites, so they move across the sky much more rapidly—in about one to two hours. As a result, to get global coverage, you need more satellites (about five times more) than you would with GEO systems. But because the altitude is lower, the delay is also reduced, so instead of a 0.5-second delay, you see a 0.1-second delay. The main applications for MEOs are in regional networks, to support mobile voice and low speed data, in the range of 9.6Kbps to 38Kbps.

7. Continue………… LEO Satellites Low-Earth-orbit (LEO) satellites have polar orbits. The altitude is between 500 and 2000km, with a rotation period of90to 120 min. The satellite has a speed of 20,000 to 25,000 km/h. An LEO system usually has a cellular type of access, similar to the cellular telephone system. The footprint normally has a diameter of 8000km. Because LEO satellites are close to Earth, the round-trip time propagation delay is normally less than 20 ms, which is acceptable for audio communication. An LEO system is made of a constellation of satellites that work together as a network; each satellite acts as a switch. Satellites that are close to each other are connected through Inter satellite links (ISLs). A mobile system communicates with the satellite through a user mobile link (UML). A satellite can also communicate with an Earth station (gateway) Through a gateway link (GWL).

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9. Continue………….. Categories of LEOs LEO satellites can be divided into three categories: • Little LEOs • Big LEOs • Broadband LEOs The little LEOs operate under 1GHz. Little LEOs offer 2.4Kbps to 300Kbps. They are mostly used for low-data-rate messaging and like vehicle location services. The big LEOs operate between 1and 3 GHz. Big LEOs offer 2.4Kbps to 9.6Kbps . Globalstar and Iridium systems Are examples of big LEOs. The broadband LEOs provide communication similar to fiber optic networks. Broadband LEOs offer 16Kbps to 155Mbps and they support data and multimedia files. The first broad band LEO system was Teledesic.

10. Continue………… Iridium System The concept of the Iridium system, a 77-satellite network, was started by Motorola in 1990. The project took eight years to materialize. During this period, the number of satellites was reduced. Finally, in 1998, the service was started with 66 satellites. The original name, Iridium, came from the name of the 77th chemical element. Iridium has gone through rough times. The system was halted in 1999 due to financial problems; it was sold and restarted in 2001 under new ownership. The system has 66 satellites divided into six orbits, with 11 satellites in each orbit. The orbits are at an altitude of 750 km. The satellites in each orbit are separated from one another by approximately 32° of latitude. Since each satellite has 48 spot beams, the system can have up to 3168 beams. However, some of the beams are turned off as the satellite approaches the pole. The number of active spot beams at any moment is approximately 2000. Each spot beam covers a cell on Earth, which means that Earth is divided into approximately 2000 (overlapping) cells. The whole purpose of Iridium is to provide direct worldwide communication using handheld terminals. The system provides 2.4- to 4.8-kbps voice and data transmission between portable telephones. Transmission occurs in the 1.616- to 1.6126-GHz frequency band. Inter satellite communication occurs in the 23.18- to 23.38-GHz frequency band. The altitude is 750 km.

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12. Continue…….. Globalstar Globalstar is another LEO satellite system. The system uses 48 satellites in six polar orbits with each orbit hosting eight satellites. The orbits are located at an altitude of almost 1400 lan. The Globalstar system is similar to the Iridium system; the main difference is the relaying mechanism. Communication between two distant users in the Iridium system requires relaying between several satellites; Globalstar communication requires both satellites and Earth stations, which means that ground stations can create more powerful signals.

13. Continue………. Frequency Allocations of Satellite In satellite communications, the frequency allocations always specify two different bands: one is used for the uplink from earth station to satellite and one for the downlink from satellite to earth station. Many different bands are specified in the various satellite standards, but the most dominant frequency bands used for purposes of communications are C-band, Ku-band, Ka-band, and L-band. C-Band With C-band, you transmit uplink around the 6GHz range and downlink around the 4GHz range. The advantage of C-band, as compared to other bands, is that because it operates in the lower frequency bands, it is fairly tolerant of adverse weather conditions. It has larger wave forms, so it doesn't suffer as much disturbance as do smaller wave forms in the presence of precipitation, for instance. The disadvantage of C-band is that its allocation of frequencies is also shared by terrestrial systems. So selecting sites can take time because you have to contend with what your neighbors have installed and are operating. Licensing can take time, as well.

14. Continue……. Ku-Band Ku-band was introduced in the early 1980s and revolutionized how we use satellite communications. First, it operates on the uplink at around 14GHz and on the downlink at around 11GHz. The key advantage of Ku-band is that this frequency band allocation is usually reserved specifically for satellite use, so there are no conflicts from terrestrial systems. Therefore, site selection and licensing can take place much more rapidly. Second, because it doesn't interfere with terrestrial systems, it offers portability. Therefore, a Ku-band dish can be placed on top of a news van or inside a briefcase, and a news reporter can go to the story as it is breaking to broadcast it live and without conflict with surrounding systems. The disadvantage of Ku-band is that it's a slightly higher frequency allocation than C-band, so you can experience distortions under bad climactic conditions (for example, humidity, fog, rain)

15. Continue……. Ka-Band The new generation of satellite—the broadband satellites—will operate in the Ka-band. The key advantage of Ka-band is that it offers a wide frequency band: about 30GHz uplink and about 20GHz downlink. The difference between 20GHz and 30GHz for Ka-band is much greater than the difference between 4GHz and 6GHz for C-band. This expanded bandwidth means that Ka-band satellites are better prepared than satellites operating at other bands to accommodate tele-education, telesurveillance, and networked interactive games. A disadvantage of Ka-band is that it's even higher up in the frequency band than the other bands, so rain fade (that is, degradation of signal because of rain) can be a more severe issue. Thus, more intelligent technologies have to be embedded at the terminal points to be able to cost-effectively deal with error detection and correction.

16. Continue……. L-Band L-band operates in the 390MHz to 1550MHz range, supporting various mobile and fixed applications. Because L-band operates in the lower frequencies, L-band systems are more tolerant of adverse weather conditions than are other systems. It is largely used to support very-small-aperture terminal (VSAT) networks and mobile communications, including handheld terminals, vehicular devices, and maritime applications.

17. Continue……. VSAT(vary small aperture terminal) : Business enterprises use VSAT networks as a means of private networking, essentially setting up point-to-point links or connections between two locales. A VSAT station is so compact that it can be put outside a window in an office environment. VSATs are commonly deployed to reduce the costs associated with the leased lines, and depending on how large the network is, they can reduce those costs by as much as 50%. Most users of VSATs are enterprises that have 100 or more nodes or locations (for example, banks that have many branches, gas stations that have many locations, convenience stores). VSATs elegantly and economically help these types of enterprises do things such as transport information relevant to a sale made from a remote location to a central point, remotely process information, and process reservations and transactions.

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19. Continue……… VSATs are also useful as disaster-recovery tools. Because you can set up a VSAT system very quickly, it can be very useful in the event of a major disaster, when land-based facilities are entirely disabled. An emerging application for VSAT is broadband Internet access. Products such as Hughes DirecPC provide Internet downloads at up to 2Mbps. Similarly, intranets (that is, site-to-site connections between company locations) could be based on VSATs.Easily accesses remote locations. Rapid deployment— A VSAT system can be installed in two to four hours (as long as you have already secured the license to operate within an allocated spectrum).