Balloon Satellites

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BALLOON SATELLITE
ABSTRACT The main theme involved in developing this paper is to provide a clear understanding of satellites, purpose of satellites and their types, need of satellite communications in day-to-day life. There are two types of communication satellites low earth orbit communication satellites (LEO) and geosynchronous communication satellites. Under LEO communication satellites there comes Echo and Telstar communication satellites, their advantages and disadvantages and advancements of geosynchronous communication satellites compared to LEOs has been discussed. A brief idea on basic communication satellite components has been given. In the years to come, there are many projected plans in the area of satellites. This will be a benefit to the United States and the rest of the world. Some of these advances are in satellite photography, communications, and weather technology. Some of the future advances are in the distant future, while others are being developed right now. The usage of satellites today, new satellite communications, the latest in satellite gear, direct broadcast satellite T.V, satellite advances in television, future global satellite networks. We have also presented some of the launch vehicles used to send satellites into space.

WHAT IS A SATELLITE A satellite is something that goes around and around a larger something, like the earth or another planet. Some satellites are natural, like the moon, which is a natural satellite of the earth. Other satellites are made by scientists and technologists to go around the earth and do certain jobs. Can We Imitate Nature (Artificial Satellites) Very soon after Newton's laws were published, people realized that in principle it should be possible to launch an artificial satellite which would orbit the earth just as the moon does. A simple calculation, however, using the equations which we developed above, will show that an artificial satellite, orbiting near the surface of the earth (R = 4000 miles) will have a period of approximately 90 minutes. This corresponds to a sideways velocity (needed in order to "miss" the earth as it falls), of approximately 17,000 miles/hour (that's about 5 miles/second).

Figure: Aryabhatta (First Indian Satellite)

TYPES OF SATELLITES Boeing 376, built by Boeing Satellite Systems. The Boeing 376 is used mostly for broadcast television and cable television. Boeing 601―which is also built by Boeing Satellite Systems. The Boeing 601 is used for many purposes, including direct broadcast TV, such as DIRECTV. Direct broadcast TV is a system for receiving television using a very small satellite dish. The television signal is relayed by a Boeing 601 satellite. The Boeing 601 also relays telephone, fax, and computer communications. The most powerful commercial satellite in the world is the Boeing 702. Designed and built by Boeing Satellite Systems, this giant has a wingspan of nearly 157 feet more than a Boeing757jetplane. Why Satellites for Communication By the end of World War II, the world had had a taste of "global communications." Edward R. Murrow's radio broadcasts from London had electrified American listeners. We had, of course, been able to do transatlantic telephone calls and telegraphs via underwater cables for almost 50 years. At exactly this time, however, a new phenomenon was born. The first television programs were being broadcast, but the greater amount of information required transmitting television pictures required that they operate at much higher frequencies than radio stations. Television signals however required much higher frequencies because they were transmitting much more information - namely the picture. Both radio and television frequency signals can propagate directly from transmitter to receiver. This is a very dependable signal, but it is more or less limited to line of sight communication. The mode of propagation employed for long distance (1000s of miles) radio communication was a signal which traveled by bouncing off the charged layers of the atmosphere (ionosphere) and returning to earth. The higher frequency television signals did not bounce off the ionosphere and as a result disappeared into space in a relatively short distance. This is shown in the diagram below. Radio Signals Reflect Off the Ionosphere; TV Signals do not in addition, of course, the appetite for transatlantic radio and telephone was increasing rapidly. Adding this increase to the demands of the new television medium, existing communications capabilities were simply not able to handle all of the requirements. By the late1950s the newly developed artificial satellites seemed to offer the potential for satisfying many of these needs. Low Earth-Orbiting Communications Satellites In 1960, communications satellite ever conceived was launched. It was called Echo, because it consisted only of a large (100 feet in diameter) aluminized plastic balloon. Radio and TV signals transmitted to the satellite would be reflected back to earth and could be received by any station within view of the satellite. Echo Satellite Unfortunately, in its low earth orbit, the Echo satellite circled the earth every ninety minutes. This meant that although virtually everybody on earth would eventually see it, no one person, ever saw it for more than 10 minutes or so out of every 90 minute orbit. In 1958, the Score satellite had been put into orbit. It carried a tape recorder which would record messages as

it passed over an originating station and then rebroadcast them as it passed over the destination. Once more, however, it appeared only briefly every 90 minutes - a serious impediment to real communications. In 1962, NASA launched the Telstar satellite for AT&T.

Figure: ECHO communication satellite

Telstar Communications Satellite Telstar's orbit was such that it could "see" Europe" and the US simultaneously during one part of its orbit. During another part of its orbit it could see both Japan and the U.S. As a result, it provided real- time communications between the United States and those two areas - for minutes out of every hour Geosynchronous Communications Satellites: The solution to the problem of availability, of course, lay in the use of the geosynchronous orbit. In 1963, the necessary rocket booster power was available for the first time and the first geosynchronous satellite, Syncom2, was launched by NASA. For those who could "see" it, the satellite was available 100% of the time, 24hours a day. The satellite could view approximately 42% of the earth. For those outside of that viewing area, of course, the satellite was NEVER available.

Figure: Telstar communication satellite

Syncom2 Communications Satellite However, a system of three such satellites, with the ability to relay messages from one to the other could interconnect virtually all of the earth except the Polar Regions. The one disadvantage(for some purposes) of the geosynchronous orbit is that the time to transmit a signal from earth to the satellite and back is approximately ¼ of a second - the time required to travel 22,000 miles up and 22,000 miles back down at the speed of light. For telephone conversations, this delay can sometimes be annoying. For data transmission and most other uses it is not significant. In any event, once Syncom2 had demonstrated the technology necessary to launch a geosynchronous satellite, a virtual explosion of such satellites followed. Today, there are approximately 150 communications satellites in orbit, with over 100 in geosynchronous orbit. One of the biggest sponsors of satellite development was Intelsat, an internationally-owned corporation which has launched 8 different series of satellites (4 or 5 of each series) over a period of more than 30 years. Spreading their satellites around the globe and making provision to relay from one satellite to another, they made it possible to transmit 1000s of phone calls between almost any two points on the earth. It was also possible for the first time, due to the large capacity of the satellites, to transmit live television pictures between virtually any two points on earth. By 1964 (if you could stay up late enough), you could for the first time watch the Olympic Games live from Tokyo. A few years later of course you could watch the Vietnam War live on the evening news. Basic Communications Satellite Components every communications satellite in its simplest form (whether low earth or geosynchronous) involves the transmission of information from an originating ground station to the satellite (the uplink), followed by a retransmission of the information from the satellite back to the ground (the downlink). The downlink may either be to a select number of ground stations or it may be broadcast to everyone in a large area. Hence the satellite must have a receiver and a receive antenna, a transmitter and a transmit antenna, some method for connecting the uplink to the downlink for retransmission, and prime electrical power to run all of the electronics. The exact nature of these components will differ, depending on the orbit and the system architecture, but every communications satellite must have these basic components. This is illustrated in the following drawing. Basic Components of a Communications Satellite Link.

Figure: Syncom2 satellite

DEVELOPMENTINSATELLITECOMMUNICATION Some of the first communications satellites were designed to operate in a passive mode. Instead of actively transmitting radio signals, they served merely to reflect signals that were beamed up to them by transmitting stations on the ground. Signals were reflected in all directions, so receiving stations around the world could pick them up. Echo 1, launched by the United States in 1960, consisted of an aluminized plastic balloon 30 m (100 ft) in diameter. Launched in 1964, Echo 2 was 41 m (135 ft) in diameter. The capacity of such systems was severely limited by the need for powerful transmitters and large ground antennas. Score, launched by the United States in 1958, was the first active communications satellite. It was equipped with a tape recorder that stored messages received while passing over a transmitting ground station. These messages were retransmitted when the satellite passed over a receiving station. Telstar 1, launched by American Telephone and Telegraph Company in 1962, provided direct television transmission between the United States, Europe, and Japan and could also relay several hundred-voice channels. Launched into an elliptical orbit inclined 45° to the equatorial plane, Telstar could only relay signals between two ground stations for a short period during each revolution, when both stations were in its line of sight. Hundreds of active communications satellites are now in orbit. They receive signals from one ground station, amplify them, and then retransmit them at a different frequency to another station. Satellites use ranges of different frequencies, measured in hertz (Hz) or cycles per second, for receiving and transmitting signals. Many satellites use a band of frequencies of about 6 billion hertz, or 6 gigahertz (GHz) for upward, or uplink, transmission and 4 GHZ for downward, or downlink, transmission. Another band at 14 GHZ (uplink) and 11 or 12 GHZ (downlink) is also much in use, mostly with fixed (non mobile) ground stations. A band at about 1.5 GHZ (for both uplink and downlink) is used with small, mobile ground stations (ships, land vehicles, and aircraft). Solar energy cells mounted on large panels attached to the satellite provide power for reception and transmission. GEOSYNCHRONOUS ORBIT A satellite in a geosynchronous orbit follows a circular orbit over the equator at an altitude of 35,800 km (22,300 mi), completing one orbit every 24 hours, in the time that it takes the earth to rotate once. Moving in the same direction as the earth's rotation, the satellite remains in a fixed position over a point on the equator, thereby providing uninterrupted contact between ground stations in its line of sight. The first communications satellite to be placed in this type of orbit was Syncom2, launched by the National Aeronautics and Space Administration (NASA) in 1963. Most communications satellites that followed were also placed in geosynchronous orbit RECENT TECHNICAL ADVANCES Communications satellite systems have entered a period of transition from point-to-point high-capacity trunk communications between large, costly ground terminals to multipoint-tomultipoint communications between small, low-cost stations. The development of multiple access methods has both hastened and facilitated this transition. With TDMA, each ground station is assigned a time slot on the same channel for use in transmitting its communications; all other stations

monitor these slots and select the communications directed to them. By amplifying a single carrier frequency in each satellite repeater, TDMA ensures the most efficient use of the satellite's onboard power supply. A technique called frequency reuse allows satellites to communicate with a number of ground stations using the same frequency by transmitting in narrow beams pointed toward each of the stations. Beam widths can be adjusted to cover areas as large as the entire United States or as small as a state like Maryland. Two stations far enough apart can receive different messages transmitted on the same frequency. Satellite antennas have been designed to transmit several beams in different directions, using the same reflector. A method for interconnecting many ground stations spread over great distances was demonstrated in 1993 with the launch of NASA's ACTS (Advanced Communications Technology Satellite). The satellite uses what is known as the hopping spot beam technique to combine the advantages of frequency reuse, spot beams, and TDMA. By concentrating the energy of the satellite's transmitted signal, ACTS can use ground stations that have smaller antennas and reduced power requirements. The concept of multiple spot beam communications was successfully demonstrated in 1991 with the launch of Aalst, developed by the Italian Research Council. With six spot beams operating at 30 GHZ (uplink) and 20 GHZ (downlink), the satellite interconnects TDMA transmissions between ground stations in all the major economic centers of Italy. It does this by demodulating uplink signals, routing them between up- and downlink beams, and combining and re modulating them for downlink transmission. Laser beams can also be used to transmit signals between a satellite and the earth, but the rate of transmission is limited because of absorption and scattering by the atmosphere. Lasers operating in the blue-green wavelength, which penetrates water, have been used for communication between satellites and submarines. The latest development in satellites is the use of networks of small satellites in low earth orbit (2,000 km (1,200 mi) or less) to provide global telephone communication. The Iridium system uses 66 satellites in low earth orbit, while other groups have or are developing similar systems. Special telephones that communicate with these satellites allow users to access the regular telephone network and place calls from anywhere on the globe. Anticipated customers of these systems include international business travelers and people living or working in remote areas.

BALLOON SATELLITE

Figure: Balloon satellite

Model of a Balloon Satellite A balloon satellite (Also occasionally referred to as a "Balloon", which is a trademarked name owned by Gilmore Schjeldahl's G.T. Schjeldahl Company) is a satellite that is inflated with gas after it has been put into orbit. The Pythagorean Theorem allows us to calculate easily how far a satellite is visible at such a great height. It can be determined that a satellite in a 1,500-kilometer (930 mi) orbit rises and sets when the horizontal distance is 4,600 kilometers (2,900 mi). However, the atmosphere causes this figure to vary slightly. Thus if two radio stations are 9,000 kilometers (5,600 mi) apart and the satellite's orbit goes between them, they may be able to receive each other's reflected radio signals if the signals are strong enough. Optical visibility is, however, lower than that of radio waves, because the satellite must be illuminated by the sun the observer needs a dark sky (that is, he must be in the Earth's own shadow on the planet's twilight or night side) the brightness of a sphere depends on the angle between the incident light and the observer (see phases of the moon) the brightness of a sphere is much reduced as it approaches the horizon, as atmospheric extinction swallows up as much as 90% of the light. Despite this there is no problem observing a flying body such as Echo 1 for precise purposes of satellite geodesy, down to a 20° elevation, which corresponds to a distance of 2,900 kilometers (1,800 mi). In theory this means that distances of up to 5,000 kilometers (3,100 mi) between measuring points can be "bridged", and in practice this can be accomplished at up to 3,000–4,000 kilometers (1,900–2,500 mi).For visual and photographic observation of bright satellites and balloons, and regarding their geodetic use, see Echo 1 and Page’s for further information. Other balloon satellites For special testing purposes two or three satellites of the Explorer series were constructed as balloons (possibly Explorer 19 and 38).Echo 1 was an acknowledged success of radio engineering, but the passive principle of telecommunications (reflection of radio waves on the balloon's surface) was soon replaced by active systems. Telstar 1 (1962) and Early Bird (1965) were able to transmit several hundred audio channels simultaneously in addition to a television program exchanged between continents. Satellite geodesy with Echo 1 and 2 was able to fulfill all expectations not only for the planned 2–3 years, but for nearly 10 years. For this reason NASA soon planned the launch of the even larger 40-meter (130 ft) balloon Page’s. The name is from "passive geodesic satellite", and sounds similar to "Geo", a successful active electronic satellite from 1965.1965-1975: Success with flashing light beacons Bright balloon satellites are well visible and were measurable on fine-grained (less sensitive) photographic plates, even at the beginning of space travel, but there were problems with the exact chronometry of a satellite's track. In those days it could only be determined within a few

milliseconds. Since satellites circle the earth at about 7–8 kilometers per second (4.3–5.0 mi/s), a time error of 0.002 second translates into a deviation of about 15 meters (49 ft). In order to meet a new goal of measuring the tracking stations precisely within a couple of years, a method of flashing light beacons was adopted around 1960.To build a three-dimensional measuring network, geodesy needs exactly defined target points, more so than a precise time. This precision is easily reached by having two tracking stations record the same series of flashes from one satellite. Flash technology was already mature in 1965 when the small electronic satellite Geo (later named Geos1was launched; along with its companion Geos2, it brought about a remarkable increase in precision. From about 1975 on, almost all optical measurement methods lost their importance, as they were overtaken by speedy progress in electronic distance measurement. Only newly developed methods of observation using CCD and the highly precise star positions of the astrometry satellite Hip arcos made further improvement possible in the measurement of distance.

CONCLUSION Looking at the rate of advancement in satellite communication one would foresee the use of satellites in every field where communication is required such as relaying television and radio signals. Special telephones that communicate with these satellites allow users to access the regular telephone network and place calls from anywhere on the globe. Additional satellites are scheduled for launch that will enable new communication systems to be used around the world. Advances in the new Satellite Technology have made people no more than a phone call away. Satellites can send messages from one continent to another and also from one planet to another. Satellite technology brings us the weather, cellular phones, wireless cable, and direct broadcast television. Satellite communication companies are expecting these services to be offered all over the world in the very near future.

REFERENCES: www.msn.encarta.com http://www.amazon.com http://www.electronicsforu.com http://www.odyseus.nildram.co.uk/Systems Comms.pdf http://www.cse.wustl.edu/~jain/cis788-97e_nets.pdf http://www.connected-earth.com. http://www.boeing.com http://www.technology-post.com http://www.info.com

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