Satellite communications
A communications satellite is an artificial satellite stationed in space for the purposes of telecommunications. Modern communications satellites use a variety of orbits. 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. |
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| Introduction, satcom systems, functions, history |
| What is satellite communications? Satellite communications is radio communications, or radio broadcasting, via a relay station in space, in a suitable orbit around the earth. In the early days of satellite communications the most important functions of satellite systems were to establish long distance communications for telephony and similar services. |
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| The space segment: Satellite orbits |
| Arthur C. Clarke’s idea was to place the relay station or rocket station as he called it, in the geostationary earth orbit, GEO. This orbit is in the orbital plane, and it is circular with a radius of 42164 km. A satellite moving eastwards in this orbit will have the same angular velocity as the earth’s rotation, and it will therefore be stationary relative to an observer on earth. |
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| The space segment: The satellite platform |
| For a "relay station" to operate in space a number of supporting functions are required: power supply, attitude control, thermal control etc. Arthur C. Clarke also addressed some of these functions in his 1945 paper. For the power supply he proposed to use solar energy, as it is done today |
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| The space segment: Transparent, "bent pipe" transponder |
| Let us consider a simple satellite repeater with a single amplifier, or transponder, as shown in the illustration below. The transponder will perform the following functions: |
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| The space segment: Communications satellite description |
| Today’s communications satellites are complicated and contain a large number of transponders feeding different antennas with different polarization. To explain the function of a communications satellite it is more convenient to look at a very early version, like Marisat, which contains all the essential elements, and describe the different functions. |
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| Earth stations |
| In the early days of satellite communications the dominating service was long distance communications between nodes of the global telecommunications network. The earth stations were similar in construction and had many of the same functions. |
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| Radio transmission: Antennas |
| An antenna is a component that couples the signals in electronic circuits to electromagnetic waves, both on the transmitting and on the receiving side. Antenna design is an important topic in satellite communications because of the strong impact on cost, size, weight and not the least on performance. |
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| Free space wave propagation |
| The figure shows two antennas separated by d meters. One could be on the satellite and the other one an earth station. Let us assume for the moment that there is line of sight between the antennas, and that there are no reflected waves. |
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| Polarization |
| Electromagnetic waves are defined by vectors and the electric and magnetic field vectors have defined directions in space. We know this phenomenon from light. Ordinary sun light is un-polarized, meaning that the direction of the field vectors are random and uniformly distributed. |
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| Travelling wave amplifiers |
| A travelling wave tube amplifier (TWTA) uses a vacuum tube device, a travelling wave tube (TWT), invented during World War II. It consists of a glass tube with a cathode, an electron gun emitting electrons at one end of the tube, and an anode collecting the electrons at the other end. |
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| Mobile satellite communications |
| One field where satellites are extremely well suited is mobile communications over a large area and in particular with ships. An international organization, INMARSAT, was established in 1981 to provide such services. |
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| Satellite broadcasting |
| The regulatory basis for satellite broadcasting was established at a conference in Geneva in 1977, the "World Administrative Radio Conference for Broadcasting Satellites", referred to as WARC-BS 77. A number of decisions were made. |
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| Satellite broadcasting: DVB (Digital Video Broadcasting) |
| Around 1990 it became apparent that the time was ripe also for TV, as the last main service, to become all digital. This followed the development of signal compression technique which allowed TV signals with broadcast quality to be quantized to about 10 – 12 Mbit/s, as compared to uncompressed data rates used in studios of 250 Mbit/s and higher. |
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| Satellite broadcasting: Interactive broadcasting, DVB-RCS |
| One of the most attractive data services is access to the Internet, and DVB-RCS is suitable for that purpose. A hub is connected to the Internet and is sending data via DVB-S or DVB-S2 to the remote DVB-RCS terminals. |
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