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ABSTRACT
A communications satellite ( COMSAT)is an artificial satellite stationed in space for the purpose of telecommunications. we will take a look at the launching of the satellite, its orbits, subsystems, frequencies, earth stations and applications of satellite communication technology in our information technology.
INTRODUCTION
The word SATELLITE from the literary point of view could be used to refer to something or a place far away from a given reference point.eg we speak of satellite towns in lagos which is an area developed for habitation and which is located some distance away from the city center. The moon is a satellite object away from the earth which might be away up to 240,000miles and also the stars which are natural satellites. it has now become possible for man-made artificial satellite to be placed on space for the purpose of relaying information from one point of the earth to another.
WHAT IS SATELLITE?
A Satellite is a solid object which revolves around some heavenly body due to the effect of gravitational forces which are mutual in nature. We can categorize satellites in two types, namely Passive Satellites and Active satellites. Passive satellites are not like active satellites. Even a moon can be a passive satellite. Thus passive satellites are relay stations in space. A passive satellite can be further subdivided into two types, namely Natural satellites and artificial satellites. A moon is a natural satellite of earth. But spherical balloon with metal coated plastic serve as artificial satellites
Active satellites are complicated structures having a processing equipment called Transponder which is very vital for functioning of the satellite. These transponders serve dual purpose i.e. provides amplification of the incoming signal and performs the frequency translation of the incoming signal to avoid interference between the two signals.
HISTORY OF SATELLITES
The first artificial satellite was the Soviet Sputnik1, launched on October 4, 1957, and equipped with an on-board radio-transmitter that worked on two frequencies, 20.005 and 40.002 MHz. The first American satellite to relay communications was Project SCORE in 1958, which used a tape recorder to store and forward voice messages. It was used to send a Christmas greeting to the world from U.S. President Dwight D. Eisenhower. NASA launched an Echo satellite in 1960; the 100-foot (30 m) aluminized PET film balloon served as a passive reflector for radio communications. Courier 1B, built by Philco, also launched in 1960, was the world’s first active repeater satellite. With the launch of Alouette 1 in 1962 Canada became the third country to put a man-made satellite into space. Because Canada did not have any domestic launch capabilities of its own, (and still does not), Alouette 1, which was entirely built and funded by Canada, was launched by the American National Aeronautics and Space Administration (NASA) from Vandenberg AFB in California.
Fig 1. A passive Telsa satellite
Fig 2a. an Active satellite
Fig 2b. An Active satellite
TYPES OF SATELLITE
Communication satellites
Navigation satellites
Weather satellites
Military satellites
Scientific satellites
TYPES OF SATELLITE ORBITS
Johnannes kepler(1630)discovered the laws that govern satellite motion.Kepler first law states that the satellite will follow an elliptical path in its orbit around the primary body.Kepler second law states that for equal time intervals,the satellite sweeps out equal areas in the orbital plane focused at the barycentre.
There are three areas for satellite orbits:
GEO: Geostationary Earth Orbit
MEO: Medium Earth Orbit
LEO: Low Earth Orbit
GEO satellites orbit the earth directly over the equator, approximately 35 400 km (22 000 miles) up. At that altitude, one complete trip (orbit) around the earth takes 24 hours. Thus, the satellite remains over the same spot on the surface of the earth (geo) at all times, and stays fixed in the sky (stationary) from any point on the surface from which it can be "seen." A satellite in a geostationary orbit appears to be in a fixed position to an earth-based observer. A geostationary satellite revolves around the earth at the same angular velocity of the earth itself, 360 degrees every 24 hours in an equatorial orbit, and therefore it seems to be in a fixed position over the equator. The geostationary orbit is useful for communications applications because ground based antennas, which must be directed toward the satellite, can operate effectively without the need for expensive equipment to track the satellite’s motion
MEO is defined simply as the area between LEO and GEO. The primary satellite systems there are the GPS (Global Positioning System) satellite constellations.
LEO is between 200 and 1400 km above the earth. Satellites in LEO rapidly circle the earth and are typically in range of one location for only 90 minutes. Their main advantage is how close they are, providing shorter delays for faster communications. However, for consistent communications they require a constellation of satellites so that communications can be maintained as one satellite moves out of range and another moves within range of the ground station. LEO satellites are less expensive to build, typically less powerful, and have a shorter average life span.
SATELLITE LAUNCH
Communication satellites are put in space by the use of rockets or space shuttle of NASA. these rockets provide the necessary escape velocity required for and object of such size to move away from gravitational pull of the earth. A rockets could weigh up to 250tons .few minutes after blasting from the lunch pad, the rocket begins to separate and the engines are fired one after the other.the satellite is temporarily placed in a parking orbit(an apogee of2,220km) for the sake of determining the time and place suitable for firing satellite. the rocket is fired more to an elliptical orbit with an apogee 36000km and it has an inclination angle of 28° to the equator which is the transfer orbit because the satellite now in a circular orbit and is now separated from the rocket. The ground control station begins to track and control the satellite`s altitude using gas jets called thrusters incorporated in the satellite so as to start changing the satellite until the spin axis is set in parallel with the axis of the earth.with the satellite travelling near the geostationary orbit,the position is adjusted little by little from the ground station eastwards or westwards until the satellite reaches the required stationary position.
Fig 4. Launching of a satellite
Fig 5.photograph of a space station
COMMUNICATION SATAELLITE
A communications satellite (sometimes abbreviated to COMSAT) is an artificial satellite stationed in space for the purpose of telecommunications. Modern communications satellites use a variety of orbits including geostationary orbits, Molniya orbits, other elliptical orbits and low (polar and non-polar) Earth orbits.
Most communications satellites are in GEO. A single geostationary satellite can cover as much as 40 percent of the earth's surface; so, in theory, three such satellites can provide global coverage. To ensure accurate and strong coverage of a specific region, continent or country, the transponders are often “shaped” to focus transmission and increase signal strength for a service area.
A satellite’s job in the communications network is to serve as a repeater. That is, it receives a signal from one location and rebroadcasts it so another station can receive the signal. Reception and retransmission are accomplished by a transponder. A single transponder on a geostationary satellite is capable of handling approximately 5,000 simultaneous voice or data channels. A typical satellite has 32 transponders.
Transponders each work on a specific radio frequency wavelength, or “band.” Satellite communications work on three primary bands: C, Ku and Ka. C was the first band used and, as a longer wavelength, requires a larger antenna. Ku is the band used by most current VSAT systems. Ka is a new band allocation that isn’t yet in wide use. Of the three, it has the smallest wavelength and can use the smallest antenna.
Because of attenuation and business competition, there are far more than three GEO satellites. Satellites of similar frequency can be as close as 3 degrees apart without causing interference. Since there are 360 degrees in a circle, that means 120 satellites of a specific frequency can be placed in GEO orbits.
The combination of individual transponder volumes and the number of transponders in orbit means today's communication satellites are an ideal medium for transmitting and receiving almost any kind of content, from simple data to the most complex and bandwidth-intensive video, audio and data content.
Satellite communication components
A communication satellite system is made up of a transponder subsystem, antenna subsystem, telemetry and command subsystem, control subsystem and power subsystem. The transponder subsystem which consists of a receiver and transmitter units, which receives weak signals from the earth stations, amplifies them and returns them to the earth after frequency conversion. Amplification and frequency conversion are carried out by the transponder. Three types of antennas are provided for satellites, namely; global beam antennas, spot beam antennas and telemetry antenna/command antennas. For communication, there are pair of global beam receiving antennas, a pair of spot beam transmitting antennas. Global beam antennas can cover all earth stations seen from the geostationary satellite. these antennas are of a horn reflector type.
They have a beam width of 17 degrees which is capable of covering a third of earth. Radio frequency signals transmitted from earth stations are received by global beam receiving antenna. The spot beam antenna has a higher gain and a narrower beam width,4.5°,and its direction can be changed on command from earth control station(ground station).The telemetry and command antenna is used to received commands from the earth and send telemetry signals to the earth until the satellite is placed at the stationary position. The telemetry section of the satellite sends down to the ground control station all the information concerning the operating conditions of the ground satellite. The command section receives command signal from the ground control for the control of various equipments in the satellite and send content of the signal to each control unit.
The control subsystem of the satellite comprises the thrusters control unit and the electrical control unit. The thrusters corrects the altitude and orbit of the satellite as instructed from the ground control with command signals. The spin rotation rate, the altitude and orbit are corrected by the operation of the thrusters driven by the thrusters control unit. The electrical control unit, controls switch over between working and standby communication equipment, parallel operation of power supply system and change of radiating direction of the spot beam antenna.
THE EARTH STATION
Earth stations can be grouped into the following categories, namely;
High performance stations
Medium performance stations
Minimum cost station
Earth station comprises then following High power amplifier(HPA),the low noise amplifier(LNA),the ground communication equipment(GCE) and the power supply equipment. We have different types of earth stations and different frequencies.
Space Communications Bands and their Frequency Range
L Band
1.0–2.0GHz Frequency Range
S Band
2.01–4.0GHz Frequency Range
C Band
4.01–8.0GHz Frequency Range
X Band
8.01–12.0GHz Frequency Range
Ku Band
12.01–18.0GHz Frequency Range
K Band
18.01–27.0GHz Frequency Range
Ka Band
27.01–40.0 GHz Frequency Range
Table 2 Space Communications Bands and their Frequency Range
Principle of operation
The parabolic shape of a dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish's focal point is a device called a feed horn. This feed horn is essentially the front-end of a wave guide that gathers the signals at or near the focal point and 'conducts' them to a low-noise block down converter or LNB. The LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the down linked C-band and/or Ku-band to the L-band range. Direct broadcast satellite dishes use an LNBF, which integrates the feed horn with the LNB. (A new form of omni directional satellite antenna, which does not use a directed parabolic dish and can be used on a mobile platform such as a vehicle.
Fig 14.satellite ground station dish or front feed
Fig 14.statellite ground station dish
Fig 15. C band dish or Axial feed
APPLICATION OF SATELLITE COMMUNICATION SYSTEM
Telecommunications
Cellular phone transmission
Television signal
Fixed satellite service(FSS)
Broadcasting satellite service(BSS)
Direct broadcast satellite(DBS)
Direct to home television(DHT)
Marine communication
Space bourn land mobile
Global positioning system
Satellite internet
Satellite radio
Military transmission
Satellite messaging for commercial jet
CONCLUSION
From this technology of satellite communication system, the role of it cannot be over emphasise in the information technology of these days and that has made information to be relayed across the world in lightening rapidity.eg final match between Zambia and Ivory coast 2012.
REFERENCES
Tarnoff, Philip John, Bullock, Darcy M,
Young, Stanley E, et al. "Continuing Evolution of Travel Time Data
Information Collection and Processing", Transportation Research Board
Annual Meeting 2009 Paper #09-2030. TRB 88th Annual Meeting Compendium of
Papers DVD
Mohan, Prashanth, Venkata N.
Padmanabhan, and Ramachandran Ramjee. Nericell: rich monitoring of road and
traffic conditions using mobile smartphones. Proceedings of the 6th ACM
conference on Embedded network sensor systems. ACM, 2008.
Tyagi, V., Kalyanaraman, S.,
Krishnapuram, R. (2012). "Vehicular Traffic Density State Estimation Based
on Cumulative Road Acoustics". IEEE Transactions on Intelligent
Transportation Systems.
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