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communications satellite

communications 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, otherelliptical orbits and low (polar and non-polar) Earth orbits.For fixed (point-to-point) services, communications satellites provide a microwave radio relay technology complementary to that of submarine communication cables. They are also used for mobile applications such as communications to ships, vehicles, planes and hand-held terminals, and for TV and radio broadcasting,

Basic Elements
Satellite communications are comprised of 2 main components:
1. The Satellite
2. The Ground Station
The Satellite

The satellite itself is also known as the space segment, and is composed of three separate units, namely the fuel system, the satellite and telemetry controls, and the transponder. The transponder includes the receiving antenna to pick-up signals from the ground station, a broad band receiver, an input multiplexer, and a frequency converter which is used to reroute the received signals through a high powered amplifier for downlink. The primary role of a satellite is to reflect electronic signals. In the case of a telecom satellite, the primary task is to receive signals from a ground station and send them down to another ground station located a considerable distance away from the first. This relay action can be two-way, as in the case of a long distance phone call. Another use of the satellite is when, as is the case with television broadcasts, the ground station's uplink is then downlinked over a wide region, so that it may be received by many different customers possessing compatible equipment. Still another use for satellites is observation, wherein the satellite is equipped with cameras or various sensors, and it merely downlinks any information it picks up from its vantagepoint.

The Ground Station.
This is the earth segment. The ground station's job is two-fold. In the case of an uplink, or transmitting station, terrestrial data in the form of baseband signals, is passed through a baseband processor, an up converter, a high powered amplifier, and through a parabolic dish antenna up to an orbiting satellite. In the case of a downlink, or receiving station, works in the reverse fashion as the uplink, ultimately converting signals received through the parabolic antenna to base band signal.

And The Various use of Satellite Communication are

1. Cellular
2. Television Signals
a. C-Band
b. Digital
3. Marine Communications
4. Spacebourne Land Mobile
5. Satellite Messaging for Commercial Jets
6. Global Positioning Services

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en.wikipediawiki/Communications_satellite
http://www.cse.wustl.edu/~jain/cis788-97...e_nets.pdf

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Satellite Communication
Introduction
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 Satellite Communication
 USSR launched the first artificial earth satellite, Sputnik in 1957.
 US launched the first GEO satellite, Relay-1 in 1962.
 First trans pacific TV traffic distribution in 1963.
Satellite Communication in the Early Internet
 First satellite packet radio network in 1970.(ALOHAnet).
 Satellites connected the American continent and UK in 1975.(SATNET).
 First demonstration of interconnection between ARPANET, SATNET and PRNET in 1977.
 First TCP specification was released in 1982.
Satellite Communication in India
The first satellite that was used for communication purpose in INDIA was ARYABHATTA and it was launched in 19th April.1975. It was made and assembled by an organization called Indian Space Research Organization (ISRO). In the year 1981, a satellite named APPLE was launched in space which was the first Indian Experimental communication satellite. The unique feature of it was that it was a three axis stabilization geosynchronous satellite and weighed around 645 kg. The term APPLE is an abbreviation for Ariane Passenger Payload Experiment. It consisted of a (6/4 Ghz) processing equipment called Transponder. Various experiments were carried out with APPLE, [SITE, STEP (Other satellite telecommunication experiment projects)] and the results obtained from these experiments provided an impetus for Govt. of India to have its own multipurpose Geosynchronous Earth Orbit satellite under INSAT (Indian National Satellite) program. The first satellite INSAT-1A was launched in the year 1982 which was under this INSAT program, but this effort went in vain as the power house of this satellite consisting of solar cells did not operate properly( failed to open) and this satellite was unused latter on. The average electrical power required by INSAT-1 was approximately 1000W and was provided by the power house subsystem of the satellite. The payload was one C-band transponder and two S-band transponders. Later succession of INSAT-1 series was launched like INSAT-1B, INSAT-C and INSAT-D. After this due to the success of the first generation satellites, INSAT-2 series was launched viz. INSAT-2A, INSAT-2B, INSAT-2C, INSAT-3D and INSAT-2E which provided variety of services.
Components & Working of Satellite Communication
The term Satellite communication is very frequently used, but what is satellite communication? It is simply the communication of the satellite in space with large number of earth stations on the ground. Users are the ones who generate baseband signals, which is processed at the earth station and then transmitted to the satellite through dish antennas. Now the user is connected to the earth station via some telephone switch or some dedicated link. The satellite receives the uplink frequency and the transponder present inside the satellite does the processing function and frequency down conversion in order to transmit the downlink signal at different frequency. The earth station then receives the signal from the satellite through parabolic dish antenna and processes it to get back the baseband signal. This baseband signal is then transmitted to the respective user via dedicated link or other terrestrial system. Previously satellite communication system used large sized parabolic antennas with diameters around 30 meters because of the very faint and weak signals received. But nowadays satellites have become much stronger, bigger and powerful due to which antennas used have become automatically smaller in size. Thus the earth station antennas are now not large in size as the antennas used in olden days. A satellite communication system operates and works in the millimeter and microwave wave frequency bands from 1 Ghz to 50 Ghz. There are various frequency bands utilized by satellites but the most recognized of them is the uplink frequency of 6 Ghz and the downlink frequency of 4 Ghz. Actually the uplink frequency band is 5.725 to 7.075 Ghz and the actual downlink frequency band is from 3.4 to 4.8 Ghz. The major components of a Satellite Communication system is spacecraft and one or more earth earths.
THE EXCITING COMPONENTS OF SATELLITE i.e ITS SUBSYSTEMS
• Attitude & orbit control system:
This subsystem comprises of rocket motors that keeps the correct orientation of the satellite in space by moving it back to the correct orbit. Various external forces cause to change the parking position of the satellite. The primary factors are gravitational forces of sun, moon earth and also other planets of solar system. Other factors include solar pressure on the antennas and solar sails, which is present on the body of the satellite. All these factors are hugely responsible for misbalancing of the satellite and also responsible for changing the parking position of the satellite. Apart from this the earth’s magnetic field is also playing a major role in changing the parking position of satellite. The earth’s magnetic field generates eddy currents in the metallic structure of the satellite as the satellite moves through the magnetic field. Thus the body of the satellite gets rotated called as wobble of the satellite.
Remedy for Misbalancing of the satellite: station keeping:
It is a method of periodically accelerating the satellite in the opposite direction against the forces acting on the body of the satellite like gravitational forces, eddy currents etc. in order to maintain the correct orientation of satellite in space and maintaining its orbit. The two most common methods employed to keep the satellite stable in orbit are: spin stabilization and three axes body stabilization.
• TTC and M SUBSYSTEMS:
These subsystems are found partly on the satellite and partly on the earth stations. Data obtained from the sensors present on the spacecraft are sent by the Telemetry systems through telemetry link to the controlling earth stations. The telemetry system monitors the condition of the spacecraft. Furthermore the Tracking system is present on the earth station which is all concerned about range, azimuth angles and elevation angles of the spacecraft by providing necessary information on it. There are various techniques used for tracking of satellite:
1. Velocity and acceleration sensors on the satellite can be used to establish the change in orbit.
2. Doppler shift of the telemetry carrier from the earth station or beacon transmitter may be measured to determine the rate at which the range is changing.
3. Ranging tones may be used for range measurement.

Satellite Communications

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Introduction

In 1962, the American telecommunications giant AT&T launched the world's first true communications
satellite, called Telstar. Since then, countless communications satellites have been placed into earth orbit, and
the technology being applied to them is forever growing in sophistication.

Basic Elements

Satellite communications are comprised of 2 main components:
The satellite itself is also known as the space segment, and is composed of three separate units, namely
the fuel system, the satellite and telemetry controls, and the transponder. The transponder includes the
receiving antenna to pick-up signals from the ground station, a broad band receiver, an input
multiplexer, and a frequency converter which is used to reroute the received signals through a high
powered amplifier for downlink. The primary role of a satellite is to reflect electronic signals. In the
case of a telecom satellite, the primary task is to receive signals from a ground station and send them
down to another ground station located a considerable distance away from the first. This relay action
can be two-way, as in the case of a long distance phone call. Another use of the satellite is when, as is
the case with television broadcasts, the ground station's uplink is then downlinked over a wide region,
so that it may be received by many different customers possessing compatible equipment. Still another
use for satellites is observation, wherein the satellite is equipped with cameras or various sensors, and
it merely downlinks any information it picks up from its vantagepoint.

The Ground Station.

This is the earth segment. The ground station's job is two-fold. In the case of an uplink, or transmitting
station, terrestrial data in the form of baseband signals, is passed through a baseband processor, an up
converter, a high powered amplifier, and through a parabolic dish antenna up to an orbiting satellite. In
the case of a downlink, or receiving station, works in the reverse fashion as the uplink, ultimately
converting signals received through the parabolic antenna to base band signal.

Television Signals

Satellites have been used for since the 1960's to transmit broadcast television signals between the
network hubs of television companies and their network affiliates. In some cases, an entire series of
programming is transmitted at once and recorded at the affiliate, with each segment then being
broadcast at appropriate times to the local viewing populace. In the 1970's, it became possible for
private individuals to download the same signal that the networks and cable companies were
transmitting, using c-band reception dishes. This free viewing of corporate content by individuals led
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 momentum since
the introduction of digital direct broadcast service.

DBS

DBS (Direct Broadcast Satellite) -The transmission of audio and video signals via satellite direct
to the end user. More than four million households in the United States enjoy C-Band DBS.
Medium-power Ku-Band DBS surfaced in the late 1990s with high power Ku-Band DBS
launched in 1994.

Hybrid Networks

In today's global networking landscape, there are many ways to transmit data from one place to
another. It is desirable to be able to incorporate any type of data transmission media into a network,
especially in networks that encompass large areas. A hybrid network is one that allows data to flow
across a network, using many types of media, either satellite, wireless or terrestrial, transparently.
Since each type of media will have different characteristics, it is necessary to implement a standard
transmission protocol. One that is normally used in hybrid networks is TCP/IP. In addition, much work
is being done to use TCP/IP over ATM for the satellite segments of hybrid networks, about which
more will be discussed later.
One way to get around the need in ARR for the receiver to have to request retransmit via an expensive
and slow satellite link is to use a form of hybrid network. In one form of hybrid network, the reciever
transmits its requests back to the sender via a terrestrial link. Terrestrial link allows for quicker, more
economical and less error prone transmission from the reciever, and the costs associated with the
receivers hardware are greatly reduced when compared to the costs involved if it had to transmit back
over the satellite link. There are products on the market today that allow a home user to get intenet
access at around 400MB via digital satellite, while its retransmit signals are sent via an inexpensive
modem or ISDN line.

ATM Over Satellite

Two qualitites of Asynchronous Transfer Mode (ATM) made it highly desirable for the
implementation of satellite links within hybrid networks. The first is the ATM's asynchrony and the
second is its ability to use variable transfer rates. In addition, ATM fits well into existing networks
with its wide range of upper-layer services and its ability to operate in a wide range of environments.
There are problems, however. ATM's relatively large propagation delays can significantly increase the
latency of feedback mechanisms essential for congestion control. acquisition time

SATIN - Satellite Integrated Terrestrial Network:

The goal of SATIN is to create a fully integrated hybrid network in which the method of
communication, which can incorporate networks of local, metropolitan and wide area scope,
Broadband ISDN, Integrated Network Management, AIN (Advanced Intelligent Networks) and
PCS (Personal Communications Services), in addition to ATM (Asynchronous Transfer Mode)
over satellite, is totally transparent to the user. The difficulties inherent in this are obvious.
Differences in latency, noise, bandwidth and reliability must be equalized in all the media that
will encompass the network.

This report is beneficial for communications satellite.

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