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This paper deals with the historical development of satellite communication systems. Then the basic elements of satellite communication system along with their features are discussed. Then the working of a satellite communication system and the use of EM waves in this system is discussed and then finally the applications, advantages and limitations of satellite communication system are discussed.
Electromagnetic wave is a wave of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays..The behaviour of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400Â nm to 700Â nm. Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields.
Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time. At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum,
and our technology can also manipulate a broad range of wavelengths.
Fig. 1 electromagnetic spectrum
A satellite is a physical object that orbits or revolves around some celestial body. In general satellite is an artificial satellite stationed in space for the purposes of communication, military, surveillance, etc.
A satellite communications (sometimes abbreviated to Comsat) is an artificial satellite stationed in space for the purposes of telecommunications using microwave frequencies. Most communications satellites use geosynchronous orbits or near geostationary orbits, although some recent systems use low Earth-orbiting satellites.
Communications satellites provide a technology that is complementary to that of fibre optic submarine communication cables. Unlike fibre optic communication, satellite communication has a propagation delay (also called a path delay) of at least 270 milliseconds, which is the time it takes the radio signal to travel 35,800 km from earth to a satellite and then back to earth. Satellite Internet connections average a 600-800 millisecond delay, about ten times than that of a terrestrial Internet link. This delay is a challenge to deployment of Virtual private networks over satellite internet connections.
HISTORY OF SATELLITE COMMUNICATION
The concept of satellite communications was first proposed by Arthur C. Clarke, based on Herman PotoÄnik's pseudonymous work from 1929. In 1945 Clarke published an article titled "Extra-terrestrial Relays" in the magazine Wireless World. The article described the fundamentals behind the deployment artificial satellites in geostationary orbits for the purpose of relaying radio signal. Thus Arthur C. Clarke is often quoted as the inventor of the communications satellite. The first artificial satellite was the SOVIET SPUTNLK-1 launched on October 4, 1957, and aquipped with an onboard transmitter that worked on two frequencies i.e. 20.005 and 40.002 MHz The first American satellite to relay communications was project SCORE in 1958 which used tape recorder to store and forward voice messages. Telstar was the first active, direct relay communications satellite belonging to AT & T.
USE OF EM WAVES IN SATELLITE COMMUNICATION
The fastest growing and most recent field of communication involves the use of various satellite relays. Let us discuss the space wave communication. In this mode of propagation, electromagnetic waves from the transmitting antenna reach the receiving antenna either directly or after reflections from ground in the earth's troposphere region. Troposphere is that portion of the earth which extends up to 16 km from the earth surface. It means in the former, wave reaches directly from the transmitting antenna to receiving antenna and in later, the wave reaches the receiving antenna after reflection from the ground., where the phase change of 180 degree is also introduced due to reflection at the ground, in the ground reflected wave. Although both the waves leave the transmitting antenna at the same time with the same phase but may reach the receiving antenna either in the phase or out of the phase, because the two wave travel different path lengths. The strength of the resultant waves, thus, at the receiving point may be stronger or weaker than the direct path alone depending upon whether the two waves are adding or opposing in phase. At receiving point the signal strength is the vector addition of direct and indirect waves. Space wave propagation is also called as tropospheric propagation because space wave propagates through troposphere. Space wave propagation is mainly in VHF, and higher frequencies because at such frequencies sky wave and ground wave propagation both fail. Beyond 30 MHz sky wave fails as the wavelength becomes too shorts to be reflected from ionosphere and ground waves are propagating close to the antenna only, as attenuation is very high. Therefore just after few hundred feet ground wave also die due to attenuation and wave tilt. Space wave propagation is also called as the line of sight propagation because at VHF, UHF and microwave frequencies, this mode of propagation is limited to the line of sight distance and is also limited by the curvature of earth. Although in actual particle space wave propagates even slightly beyond the line of sight distance due to the refraction in the atmosphere of the earth. In line of sight distance transmitting antenna and receiving antenna can usually see each other. In fact, the line of sight distance i.e. range of communication can also be increased by increasing the heights of transmitting and receiving antennas. The curvature of earth and the height of the transmitting and receiving antennas determines maximum range of communication through direct waves.
In fact, the line of sight distance has now been extended by what is known as Space Communication or specially Satellite communication which has facilitated trans-oceanic propagation of microwaves with the potentiality of large bandwidth. By space communication we mean the radio traffic between a ground station and satellite or space probe, between satellites or space probes and also between the ground station itself via man made communication satellites or natural space body( e.g. the sun, the moon, the venus etc. ). Earlier it was not possible to propagate beyond the radio horizon and hence it revolutionized the field of communication engineering and it is possible to show that three geosynchronous satellites can establish communication over entire world. Role of electromagnetic waves can be seen by studying the different bands available for satellite communication
Selection of the band
The selection of the band is not something that individual service providers decide, but is rather chosen by large satellite operators based on different factors. These are explained below:
C-band is still the most widely available worldwide. Ku-band is becoming more available recently in regions which were less covered in the past (South America, Asia, Africa).
C-band is more prone to interference from other transmission services that share the same frequencies (adjacent satellites or terrestrial transmissions) than the higher bands.
While the C-band technology is cheaper in itself, it requires larger dishes (1 to 3 m) than Ku- and Ka-band (0.6 to 1.8 m) and therefore imposes relatively higher (installation) costs on the end-user.
Ku- and especially Ka-band make better use of satellite capacity.
Higher frequency bands (Ku- and especially Ka-) suffer significantly more from signal deterioration caused by rainfall: to ensure availability in bad weather conditions, the signal has to be much stronger. Note that 0.1% of unavailability means in fact that the service will be interrupted for almost 9 hours over a 1-year period. 1% unavailability represents 90 hours or almost 4 full days.
Bands of Interest
C-band is the oldest allocation and operates in the frequency range around 6 GHz for transmission (uplink) and between 3.7 and 4.2 GHz for reception (downlink).
Ku-band is the most common transmission format in Europe for satellite TV and uses around 14 GHz for uplink and between 10.9 and 12.75 GHz for downlink.
Ka-band uses around 30 GHz up- and between 18 and 20 GHz downlink frequency.
C-band and Ku-band are becoming congested by an increasing amount of users, so satellite service operators are more and more turning to the use of Ka-band.
Using C-band and K-band
C Band is a name given to certain portions of the electromagnetic spectrum, as well as a range of wavelengths of light, used for communications. The IEEE C band and its variations, in particular, are microwave ranges used for certain satellite television broadcasts, and by some Wi-Fi devices, cordless phones, and weather radars. Typical antenna sizes on C-band capable systems ranges from 7.5 to 12 feet (2.5 to 3.5 meters) on consumer satellite dishes, although larger ones also
can be used. Slight variations of C band frequencies are approved for use in various parts of the world.
C Band Variants Around The World
Extended C Band
Super Extended C
INSAT C Band
Palapa C Band
Russian C Band
LMI C Band
K band is defined as a frequency band between 20 and 40 GHz (7.5-15 mm). The IEEE K band is a portion of the electromagnetic spectrum in the microwave range of frequencies ranging between 18 and 27 GHz. K band between 18 and 26.5 GHz is absorbed easily by water vapour (water resonance peak at 22.24 GHz, 1.35 cm).
The IEEE K band is conventionally divided into three sub-bands:
Â· Ka band: K-above band, 26.5-40 GHz, mainly used for radar and experimental communications.
Â· K-band 18-27 GHz
Â· Ku band: K-under band, 12-18 GHz, mainly used for satellite communications, terrestrial microwave communications, and radar, especially police traffic-speed detectors.
MAIN COMPONENTS OF A SATELLITE COMMUNICATION SYSTEM
Satellite communications are comprised of two basic elements
The ground station
The satellite is also known as the space segment. It is composed of the following separate units; the satellite and telemetry controls and the transponder.
The transponder comprised of the receiving antenna to catch-up signals from the ground station, a broad band receiver, an input multiplexer and a frequency converter that is used to reroute the received signals through a high powered amplifier for downlink.
The main function of satellite is to reflect signals. In case of a telecom satellite, the primary role is to pick up signals from a ground station, which is located, a considerable away from the first. This relay action can be two way, as in the case of a long distance phone call. Another use of satellite is the television broadcasts. Number of programs are first up-linked and then down-linked over wide region. The customer having appropriate devices can receive and watch the programs. One of the modern uses of satellite is getting information along with image (commonly known as space/satellite image) of any desired location on earth.
Fig. 2 diagram showing satellite and ground station
The Ground Station
This is called the earth segment. Earth station is the common name for every installation located on the Earth's surface and intended for communication (transmission and/or reception) with one or more satellites.
A base band processor, an up-converter, high Powered amplifier and a parabolic dish antenna is involved to transmit the terrestrial data to an orbiting satellite. In the case of downlink, the ultimate reverse operation is being down and up-linked signals are recaptured through parabolic antenna.
WORKING OF A SATELLITE
Satellite is mainly working on the basis of Electromagnetic waves. In our daily life EM waves are useful for Radio, Internet, T.V etc. For all these electronic equipments are working on the basis of EM waves. Firstly a satellite is keep in the orbit. Then it rotates along the orbit. From the source station it receives signals and spread them to all the electronic equipments. Satellites easily transfer news with in fraction of seconds it means in microseconds. In order send signals the smallest frequency waves are required. At the station the producers send the microwaves to satellite, because microwaves are waves having short frequency when compare to the other waves (Microwaves are electromagnetic waves with frequency from 30MHz to 1GB) ,they can easily penetrate throw the ionosphere, and reaches to satellite. Satellites provide links in two ways. Firstly a satellite provide point to point communication link between one ground station and the other.
One ground station transmit signal to the other satellite and next ground station receives them from the satellite. Secondly, satellite receives signals from one ground station and transmits to them to the number of ground receivers. It is illustrated in figure 2. Most satellite use frequency bandwidth through from 5.92 to 6.4GHz from transmission of data from earth to the satellite and a frequency bandwidth from 3.7 to 4.1GHz for transmission from satellite to the earth. A satellite can provide service to a certain part of the earth if it is in sight. This can be done only if the satellite remains stationary with respect to the earth.
LOW EARTH ORBITING COMMUNICATION SATELLITE
In 1960, the simplest 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.
Fig. 3 diagram showing 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.
Telstar is the name of various communications satellites; including the first ever such satellite able to relay television signals. The first two Telstar satellites were "Telstar 1", launched July 10, 1962 and operational until February 21, 1963, and "Telstar 2", launched May 7, 1963 and operational until May 16, 1965. They were experimental, and nearly identical. Telstar 1 relayed the first television pictures, telephone calls and fax images through space and provided the first live transatlantic television feed.
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 a few minutes out of every hour. Some of the main advantages of low and medium earth orbit include: (a) the possibility of using hand-held receiver terminals because satellites are closer to the Earth and can therefore provide stronger signals at the receiver and ground stations need to transmit at lower power; (b) the possibility of reusing the frequencies more often than is possible with geostationary orbit because the geographical area covered by low earth orbit satellites is much smaller; (c) the possibility of reduction in transmission delay.
Fig. 4 diagram showing Telstar satellite
Geostationary Communications Satellites
In 1963, the necessary rocket booster power was available for the first time and the first geostationary satellite, Syncom 2, was launched by NASA. For those who could "see" it, the satellite was available 100% of the time, 24 hours 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.
Fig. 5 diagram showing Geostationary satellite
INDIA'S FIRST COMMUNICATION SATELLITE "APPLE"
Apple stands for "Airline Passenger Payload Experiment". It got the name as it was carried as a "Passenger" by the European space agency. Apple the first Indian three-axis stabilized geo-stationary experimental communication satellite, weighing 673kg was successfully launched on June 19, 1981 from Kourou, French Guyana, by the Ariane Launch Vehicle of European Space Agency on its third developmental flight.
After 17 minutes 25 seconds the craft was successfully placed in the transfer orbit. The space craft sub-systems were functioning normally. Test commands have been issued from SHAR to the APPLE space craft successfully.
Fig. 6 diagram showing APPLE satellite
Launch Date: 19.06.1981
Launch Vehicle: Ariane-1(V-3)
Type of Satellite: Geo-Stationary Satellite
Mission: Experimental geostationary communication
Weight: 670 kg
Communication: VHF and C-band
Stabilization: Three axis stabilized (biased momentum) with Momentum Wheels, Torques & Hydrazine based Reaction control system
Mission life: Two years
APPLICATIONS OF SATELLITE COMMUNICATION
The breakthrough provided by satellites in telecommunications resulted in a major research and development effort in all the related technologies. Most of the early work concentrated on international point to point telecommunications applications. Later, the application of satellite communication was extended to the direct satellite broadcasts (1970s), mobile communications (1980s), and personal communications (1990s). In general, satellites are serving the mobile and broadcast.
Radio and Television Broadcasting
Satellites have been used since 1960 to transmit broadcast television signals between the network hubs of television companies and their network members. Sometime, a whole set of programs is transmitted at once and recorded at the affiliate, and then broadcast to the local populace according to the appropriate time. In the 1970's it became possible for private individuals to download the same signals that the network and cable companies were transmitting, using C-band reception dishes. This free viewing of the corporate contents by individuals let to scrambling and subsequent resale of the descrambling codes to individual customers, which started the direct-to-home industry. The direct-to-home industry has gathered even greater response since the introduction of digital direct broadcast service.
Business Radio And TV
Digital television has made it possible to distribute information within organizations and companies that are geographically dispersed, or to deliver distance education. Similarly, digital radio allows for the delivery of radio services to relatively small closed user groups.
Thin Route or Trunk Telephony
Telecom operators have been using satellite communications for many years to carry long-distance telephone communications, especially intercontinental, to complement or to bypass submarine cables. To the end-user this is transparent: the phone calls are routed automatically via the available capacity at any given moment.
Mobile satellite telephony
Mobile telephony allows the user to make telephone calls and to transmit and receive data from wherever he/she is located. Digital cellular mobile telephony such as GSM has become a worldwide standard for mobile communications, but its services lack coverage over areas that are sparsely populated or uninhabited (mountains, jungle, sea), because it is not economically viable or practical for the network operators to build antennas there. Satellite telephony seems to be able to provide a possible solution to the problem of providing voice and data communications services to these other locations
In the marine community, satellite communication systems such as Immarsat provide good communication links to ships at sea. These links use a VSAT type device to connect to geosynchronous satellites, which in turn links the ship to a land based point having respective telecommunications system.
Global Positioning Services
Another VSAT oriented service, in which a small apparatus containing the ability to determine navigational coordinates by calculating a triangulating or the signals from multiple geosynchronous.
Military Satellite System
For military communications Army, Air force and Navy use both fixed and mobile satellite systems. In addition to the normal communications, military communications are also required for tactical communications from remote and inhospitable locations.
The special requirements of military communication terminals are high reliability, ruggedness, compact, operations under hostile environment, immunity to jamming, ease of portability and transportation, etc. Examples of military satellite communications systems are:
DSCS (US AF)
FLTSATCOM (US NAVY)
Because of the special frequency band used in Military satellite system and other special requirements, Military satellite Systems are always much costlier and it takes longer time to design and develop compared to commercial satellite communications systems. Realizing that not all communications are strategic in nature, there is a trend now to use commercial communications system as far as possible. US Department of Defence is one of the major users of commercial Iridium satellite system with their own gateway.
Broadband Satellite System
Broadband satellite service is an emerging service which has caught the fancy of many for meeting the demand of worldwide fibre like access to telecommunications services such as computer networking, broadband Internet access, interactive multimedia and high quality voice. These systems use advanced satellite technology at Ka band or Ku band frequencies to achieve the high bandwidth requirements.
Examples of proposed Broadband Satellite systems are: Teledesic, SkyBridge, Spaceway
LIMITATIONS OF SATELLITE COMMUNICATION
Latency (Propagation Delay)
Due to the high altitudes of satellite orbits, the time required for a transmission to navigate a satellite link (2/10ths of a second from earth station to earth station) could cause a variety of problems on a high speed terrestrial network that is waiting for the packets.
Due to radio spectrum limitations, there is a fixed amount of bandwidth allocable to satellite transmission.
The strength of a radio signal's strength is in proportion to the square of the distance travelled. Due to the distance between ground station and satellite, the signal ultimately gets very weak. This problem can be solved by modulation of carrier wave.
The outer space has always fascinated people on the earth and communication through space evolved as an offshoot of ideas for space travel. The earliest idea of using artificial satellites for communications is found in a science fiction Brick Moon by Edward Evert Hale, published in 1869-70. While the early fictional accounts of satellite and space communications bear little resemblance to the technology as it exists today, they are of significance since they represent the origins of the idea from which the technology eventually evolved. The satellite communication through the EM waves has many applications for the smooth functioning of life and it made the communication with each other very simple. In the area of satellite communications, the technology has been responsive to the imaginative dreams. Hence it is also expected that technological innovations will lead the evolution of satellite communications towards the visions of today.
I would like to express my gratitude to all those who gave me the possibility to complete this term paper. I want to thank department of Electronics and communication of lovely professional university for giving me permission to commence this term paper. I have further more to thank the EMFT faculty member, Mr. Princejeet Singh. I am bound to the physics faculty for their stimulating support.
My friends Amit and Sulabh supported me in this term paper. I want to thank them for their help, support, interest and valuable hints.
Especially I would like to thank my sister who helped me and enabled me to complete this term paper.