Project content:
Introduction
3.Satellite communication system
4. Application of satellite communication
5.VSAT technology
7.Mobile satellite communication systems
8. Disadvantages of satellite communications
9. Conclusion
Introduction
Modern realities are already talking about the inevitability of satellite communications to replace the usual mobile ones, and even more so, landlines. The latest technology satellite communications offer effective technical and economic solutions for the development of both universal communication services and networks of direct sound and TV broadcasting. Thanks to outstanding achievements in the field of microelectronics, satellite phones have become so compact and reliable in use that all the demands are made by various user groups, and the rental service satellite devices is one of the most demanded services in the market of modern satellite communications. Significant development prospects, obvious advantages over other telephony, reliability and guaranteed uninterrupted communication - all this is about satellite phones.
Satellite communication today is the only cost-effective solution for providing communication services to subscribers in areas with low population density, which is confirmed by a number of economic studies. The satellite is the only technically feasible and cost-effective solution if the population density is lower than 1.5 people/km2.Satellite communication has the most important advantages necessary for building large-scale telecommunication networks. Firstly, it can be used to quickly form a network infrastructure that covers a large area and does not depend on the presence or condition of terrestrial communication channels. Secondly, the use of modern technologies for accessing the resource of satellite repeaters and the possibility of delivering information to an almost unlimited number of consumers at the same time significantly reduce the cost of network operation. These advantages of satellite communication make it very attractive and highly efficient even in regions with well-developed terrestrial telecommunications. Preliminary forecasts for the development of personal satellite communication systems show that at the beginning of the 21st the number of their subscribers amounted to approximately 1 million, and over the next decade - 3 million. Currently, the number of users of the Inmarsat satellite system is 40,000.
AT last years In Russia, modern types and means of communication are increasingly being introduced. But, if a cellular radiotelephone has already become familiar, then a personal satellite communication device (satellite terminal) is still a rarity. An analysis of the development of such means of communication shows that in the near future we will witness the daily use of personal satellite communication systems (SPSS). The time is approaching for the unification of terrestrial and satellite systems into a global communications system. Personal communication will become possible on a global scale, i.e., the reach of the subscriber anywhere in the world will be ensured by dialing his telephone number, regardless of the location of the subscriber. But before this becomes a reality, satellite communication systems will have to successfully pass the test and confirm the claimed specifications and economic performance and the process of commercial exploitation. As for consumers, what to do right choice, they will have to learn how to navigate well in a variety of proposals.
Project goals:
1. Study the history of the satellite communication system.
2. Familiarize yourself withfeatures and prospects for the development and design of satellite communications.
3. Get information about modern satellite communications.
Project objectives:
1. Analyze the development of a satellite communication system at all its stages.
2. Get a complete understanding of modern satellite communications.
1. Development of a satellite communication network
At the end of 1945, the world saw a small scientific article, which was devoted to the theoretical possibilities of improving communication (primarily, the distance between the receiver and transmitter) by raising the antenna to its maximum height. The use of artificial satellites as repeaters of radio signals became possible thanks to the theory of the English scientist Arthur Clark, who published a note entitled "Extraterrestrial repeaters" in 1945. He actually foresaw a new round in the evolution of radio relay communications, proposing to bring repeaters to the maximum available height.
American scientists became interested in theoretical research, who saw in the article a lot of advantages from a new type of connection:
no need to build a chain of terrestrial repeaters anymore;
one satellite is enough to provide a large coverage area;
the possibility of transmitting a radio signal to anywhere in the world, regardless of the availability of telecommunications infrastructure.
As a result, practical research and the formation of a satellite communications network around the world began in the second half of the last century. As the number of repeaters in orbit grew, new technologies were introduced and equipment for satellite communications improved. Now this way information exchange has become available not only to large corporations and military companies, but also to individuals.
The development of satellite communication systems began with the launch of the first Echo-1 apparatus (a passive repeater in the form of a metallized ball) into space in August 1960. Later, key satellite communication standards (operating frequency bands) were developed and are widely used throughout the world.
1.1 The history of the development of satellite communications and the main types of communications
The history of the development of the Satellite Communication System has five stages:
1957-1965 The preparatory period, which began in October 1957 after the launch Soviet Union the world's first artificial satellite of the Earth, and a month later, the second. This happened at the height of the Cold War and the rapid arms race, so, naturally, satellite technology became the property of the military in the first place. The stage under consideration is characterized by the launch of early experimental satellites, including communication satellites, which were mainly launched into low Earth orbits.
The first geostationary relay satellite TKLSTAR was created in the interests of the US Army and launched into orbit in July 1962. During the same period of time, a series of US military communications satellites SYN-COM (Synchronous Communications Satellite) was developed.
1965-1973 The period of development of global SSN based on geostationary repeaters. The year 1965 was marked by the launch in April of the geostationary SR INTELSAT-1, which marked the beginning of the commercial use of satellite communications. Early satellites of the INTELSAT series provided transcontinental communications and mainly supported backbone communications between a small number of national gateway earth stations providing an interface to national public terrestrial networks.
The main channels provided connections through which telephone traffic, TV signals were transmitted and telex communications were provided. In general, the Intelsat CCC complemented and backed up the submarine transcontinental cable communication lines that existed at that time.
1973-1982 The stage of wide dissemination of regional and national CCC. At this stage of the historical development of the CCC, the international organization Inmarsat was created, which deployed the Inmarsat global communications network, the main purpose of which was to provide communication with sea vessels in navigation. Later, Inmarsat extended its services to all types of mobile users.
1982-1990 The period of rapid development and spread of small earth terminals. In the 1980s, advances in engineering and technology of the key elements of the SSS, as well as reforms to liberalize and demonopolize the communications industry in a number of countries, made it possible to use satellite channels in corporate business networks communications, called VSAT.
VSAT networks made it possible to install compact satellite earth stations in the immediate vicinity of user offices, thereby solving the “last mile” problem for a huge number of corporate users, created conditions for a comfortable and efficient exchange of information, and made it possible to offload public terrestrial networks. The use of “smart” satellites connections.
Since the first half of the 90s, the CCC entered into quantitative and qualitative new stage of its development.
A large number of global and regional satellite networks communications were in the stage of operation, production or design. Satellite communications technology has become an area of significant interest and business activity. During this time period, there was an explosion in the speed of general purpose microprocessors and the volume of semiconductor storage devices, while improving the reliability, as well as reducing the power consumption and cost of these components.
Main types of communication
Given the wide scope, I will highlight the most common types of communication that are currently used in our country and around the world:
radio relay;
high frequency;
postal;
GSM;
satellite;
optical;
control room.
Each type has its own technology and complex necessary equipment for full functionality. I will consider these categories in more detail.
Communication via satellite
The history of satellite communications begins at the end of 1945, when British scientists developed the theory of transmitting a radio relay signal through repeaters that would be at high altitude (geostationary orbit). The first artificial satellites began to be launched in 1957.
The advantages of this type of connection are obvious:
the minimum number of repeaters (in practice, one or two satellites are enough to provide high-quality communications);
improvement of the basic characteristics of the signal (no interference, increased transmission distance, improved quality);
increase in coverage area.
Today, satellite communications equipment is a complex complex, which consists not only of orbital repeaters, but also base ground stations, which are located in different parts planets.
2. The current state of the satellite communication network
Of the many commercial MSS (Mobile Satellite) projects below 1 GHz, one Orbcomm system has been implemented, which includes 30 non-geostationary (non-GSO) satellites providing Earth coverage.
Due to the use of relatively low frequency bands, the system allows the provision of low-speed data transfer services, such as e-mail, two-way paging, remote control services, to simple, low-cost subscriber devices. The main users of Orbcomm are transport companies, for which this system provides a cost-effective solution for the control and management of cargo transportation.
The most well-known operator in the MSS market is Inmarsat. There are about 30 types of subscriber devices on the market, both portable and mobile: for land, sea and air use, providing voice, fax and data transmission at speeds from 600 bps to 64 kbps. Inmarsat is competing with three MSS systems, including Globalstar, Iridium and Thuraya.
The first two provide almost complete coverage of the earth's surface through the use of large constellations, respectively, consisting of 40 and 79 non-GSO satellites. Pre Thuraya went global in 2007 with the launch of a third geostationary (GEO) satellite that will cover the Americas where it is currently unavailable. All three systems provide services telephone connection and low-speed data transmission to receiving devices comparable in weight and size to GSM mobile phones.
The development of satellite communication systems plays a significant role in the formation of a single information space on the territory of the state and is closely related to federal programs to eliminate the digital divide, the development of nationwide infrastructure and social projects. The most significant Federal targeted programs in the territory of the Russian Federation are the projects on "Development of TV and Radio Broadcasting" and "Elimination of Digital Divide". The main tasks of the projects are the development of digital terrestrial television, communication networks, systems of mass broadband access to global information networks and provision of multi-service services on mobile and mobile objects. In addition to federal projects, the development of satellite communication systems provides new opportunities for solving the problems of the corporate market. Areas of use satellite technologies and various satellite communication systems are rapidly expanding every year.
One of the key factors in the successful development of satellite technologies in Russia is the implementation of the Program for the Development of the Orbital Constellation of Civil Communication and Broadcasting Satellites, including satellites in highly elliptical orbits.
Development of satellite communication systems
The main drivers for the development of the satellite communications industry in Russia today are:
launch of networks in the Ka-band (on Russian satellites "EXPRES-AM5", "EXPRES-AM6"),
active development of the segment of mobile and mobile communications on various transport platforms,
entry of satellite operators into the mass market,
development of solutions for organizing backbone channels for cellular communication networks in the Ka-band and M2M applications.
The general trend in the global satellite services market is the rapid growth of data transfer rates provided on satellite resources, which meets the basic requirements of modern multimedia applications and meets the development of software and the growth in the volume of data transmitted in the corporate and private segments.
In satellite communication networks operating in the Ka-band, the greatest interest is associated with the development of services for the private and corporate segment in the face of decreasing cost of satellite capacity implemented on Ka-band satellites with high bandwidth (High-Throughput Satellite - HTS).
Use of satellite communication systems
Satellite communication systems are designed to meet the needs of communication and satellite Internet access anywhere in the world. They are needed where increased reliability and fault tolerance are required, they are used for high-speed data transmission in organizing multi-channel telephone communication.
Specialized communication systems have a number of advantages, but the key is the ability to implement high-quality telephony outside the coverage areas of cellular communication stations.
Such communication systems allow you to work from autonomous power supply for a long time and be in the call waiting mode, this happens due to the low energy performance of the user equipment, light weight and omnidirectional antenna.
There are currently many various systems satellite communications. All have their pros and cons. Additionally, each manufacturer offers users an individual set of services (Internet, fax, telex), defines a set of functions for each coverage area, and also calculates the cost of satellite equipment and communication services. In Russia, the key ones are:Inmarsat, Iridium and Thuraya.
Spheres of use of SSS (Satellite communication systems): navigation, ministries and departments, governing bodies of state structures and institutions, the Ministry of Emergency Situations and rescue units.
The world's first mobile satellite communications system offering full set modern services to users around the world:, and in the spirit.
Satellite communication system Inmarsat (Inmarsat) has a number of advantages:
coverage area - the entire territory of the globe, except for the polar regions
the quality of the services provided
confidentiality
additional accessories (car kits, fax machines, etc.)
free incoming calls
availability in use
online system for checking account status (billing)
high level of trust among users, time-tested (more than 25 years of existence and 210 thousand users worldwide)
The main services of the satellite communication system Inmarsat (Inmarsat):
Telephone
Fax
Data transfer (including high-speed)
Telex (for some standards)
GPS
The world's first global satellite communications system that operates anywhere in the world, including the regions of the South and North Poles. The manufacturer offers a universal service available for business and life at any time of the day.
The satellite communication system Iridium (Iridium) has a number of advantages:
coverage area - the entire territory of the globe
low tariff plans
free incoming calls
The main services of the Iridium satellite communication system (Iridium) :
Telephone
Data transfer
Paging
satellite operator, which provides service to 35% of the globe. Services implemented in this system: satellite and GSM handsets, as well as satellite payphones. Inexpensive mobile connection for freedom of communication and movement.
Thuraya satellite communication system has a number of advantages:
compact size
the ability to switch between satellite and cellular communications automatically
low cost of services and telephone sets
free incoming calls
The main services of the Thuraya satellite communication system:
Telephone
Data transfer
GPS
3.Satellite communication system
3. 1. Satellite repeaters
For the first time years of research, passive satellite transponders were used (examples are the Echo and Echo-2 satellites), which were a simple radio signal reflector (often a metal or polymer sphere with a metal coating) that did not carry any transceiver equipment on board. Such satellites have not received distribution.
3.2 Orbits of satellite transponders
The orbits in which satellite transponders are located are divided into three classes:
equatorial
inclined
polar
An important variation of the equatorial orbit is the geostationary orbit, in which the satellite rotates with an angular velocity equal to the angular velocity of the Earth, in a direction that coincides with the direction of the Earth's rotation.
An inclined orbit solves these problems, however, due to the movement of the satellite relative to the ground observer, it is necessary to launch at least three satellites per orbit in order to provide round-the-clock communication access.
Polar - an orbit that has an orbital inclination to the plane of the equator of ninety degrees.
4.VSAT system
Among satellite technologies, special attention is drawn to the development of satellite communication technologies such as VSAT (Very Small Aperture Terminal).
On the basis of VSAT equipment, it is possible to build multiservice networks that provide almost all modern communication services: Internet access; telephone connection; an association local networks(building VPN networks); transmission of audio and video information; redundancy of existing communication channels; data collection, monitoring and remote control of industrial facilities and much more.
A bit of history. The development of VSAT networks begins with the launch of the first communications satellite. In the late 60s, in the course of experiments with the ATS-1 satellite, an experimental network was created, consisting of 25 earth stations, satellite telephone communications in Alaska. Linkabit, one of the original creators of Ku-band VSAT, merged with M/A-COM, which later became the leading supplier of VSAT equipment. Hughes Communications acquired the division from M/A-COM, transforming it into Hughes Network Systems. Hughes Network Systems is currently the world's leading provider of broadband satellite communications networks. A VSAT-based satellite communications network includes three key elements: a central control station (CCS), a repeater satellite, and subscriber VSAT terminals.
4.1.Satellite repeater
VSAT networks are built on the basis of geostationary repeater satellites. The most important characteristics of the satellite are the power of the onboard transmitters and the number of radio frequency channels (trunks or transponders) on it. The standard trunk has a bandwidth of 36 MHz, which corresponds to a maximum throughput of about 40 Mbps. On average, the power of transmitters ranges from 20 to 100 watts. In Russia, Yamal communication and broadcasting satellites can be cited as examples of repeater satellites. They are intended for the development of the space segment of OAO Gascom and were installed in orbital positions 49°E. d. and 90 ° in. d.
4.2 Subscriber VSAT terminals
Subscriber VSAT terminal is a small satellite communication station with an antenna with a diameter of 0.9 to 2.4 m, designed mainly for reliable data exchange via satellite channels. The station consists of an antenna-feeder device, an outdoor external radio frequency unit and an indoor unit (satellite modem). Outdoor unit is a small transceiver or just a receiver. The indoor unit provides pairing of the satellite channel with the user's terminal equipment (computer, LAN server, telephone, fax, etc.).
5. VSAT technology
There are two main types of access to a satellite channel: two-way (duplex) and one-way (simplex, asymmetric or combined).
When organizing one-way access, along with satellite equipment a terrestrial communication channel (telephone line, fiber optic, cellular networks, radio ethernet) is necessarily used, which is used as a request channel (it is also called a reverse channel).
One-way access scheme using a DVB-card and a telephone line as a reverse channel.
Two-way access scheme using HughesNet equipment (Hughes Network Systems).
Today, there are several significant VSAT network operators in Russia, which serve about 80,000 VSAT stations. 33% of such terminals are located in the Central Federal District, 13% each in the Siberian and Ural Federal Districts, 11% in the Far East and 5-8% each in the other federal districts. Among the largest operators it is worth highlighting:
6.Global satellite communication system Globalstar
In Russia, the operator of the satellite communication system Globalstar is the Closed Joint Stock Company GlobalTel. As the exclusive provider of global mobile satellite communication services of the Globalstar system, CJSC GlobalTel provides communication services throughout the entire Russian Federation. Thanks to the creation of CJSC "GlobalTel", the inhabitants of Russia have another opportunity to communicate via satellite from anywhere in Russia to almost anywhere in the world.
The Globalstar system provides high quality satellite communications for its subscribers with the help of 48 working and 8 spare low-orbit satellites located at an altitude of 1410 km. (876 miles) from the Earth's surface. The system provides global coverage of almost the entire surface of the globe between 700 North and South latitudes with an extension of up to 740. Satellites are capable of receiving signals up to 80% of the Earth's surface, i.e. from almost anywhere on the globe, with the exception of the polar regions and some areas of the central part of the oceans . The satellites of the system are simple and reliable.
6.1. Areas of application of the Globalstar system
The Globalstar system is designed to provide high quality satellite services to a wide range of users, including: voice, short message service, roaming, positioning, facsimile, data, mobile Internet.
Subscribers using portable and mobile devices can be business and individuals working in territories that are not covered by cellular networks, or whose specific work involves frequent business trips to places where there is no connection or poor communication quality.
The system is designed for a wide consumer: representatives of the media, geologists, workers in the extraction and processing of oil and gas, precious metals, civil engineers, power engineers. Employees of state structures of Russia - ministries and departments (for example, the Ministry of Emergency Situations) can actively use satellite communications in their activities. Special kits for installation on vehicles can be effective when used on commercial vehicles, on fishing and other types of sea and river vessels, on railway transport, etc.
7.1. Mobile satellite communication systems
A feature of most mobile satellite communications systems is the small size of the terminal antenna, which makes signal reception difficult. In order for the signal strength reaching the receiver to be sufficient, one of two solutions is applied:
The satellites are in geostationary orbit. Since this orbit is 35,786 km away from the Earth, a powerful transmitter is required on the satellite. This approach is used by the Inmarsat system (whose main task is to provide communications services to ships) and some regional personal satellite communications operators (for example, Thuraya).
7.1. Satellite Internet
Satellite Internet - a way to provide access to the Internet using satellite communication technologies (usually in DVB-S standard or DVB-S2).
Access options
There are two ways to exchange data via satellite:
one-way (one-way), sometimes also called "asymmetric" - when a satellite channel is used to receive data, and available terrestrial channels are used for transmission
two-way (two-way), sometimes also called "symmetrical" - when satellite channels are used for both reception and transmission;
One way satellite internet
One-way satellite Internet implies that the user has some existing way to connect to the Internet. As a rule, this is a slow and / or expensive channel (GPRS / EDGE, ADSL connection where Internet access services are poorly developed and speed limited, etc.). Only requests to the Internet are transmitted through this channel.
Two way satellite Internet
Two-way satellite Internet means receiving data from the satellite and sending it back also via the satellite. This method is of very high quality, as it allows you to achieve high speeds during transmission and sending, but it is quite expensive and requires permission for radio transmitting equipment (however, the provider often takes care of the latter). The high cost of two-way Internet is fully justified due to the much more reliable connection in the first place. Unlike one-way access, two-way satellite Internet does not require any additional resources (other than power, of course).
A feature of "two-way" satellite Internet access is a sufficiently large delay on the communication channel. Until the signal reaches the subscriber to the satellite and from the satellite to the Central satellite communication station, it will take about 250 ms. The same amount is needed for the trip back. Plus, the inevitable delays in signal processing and in order to go "over the Internet". As a result, the ping time on a two-way satellite link is about 600 ms or more. This imposes some specifics on the operation of applications via satellite Internet and is especially sad for avid gamers.
Another feature is that the various manufacturers practically incompatible with each other. That is, if you have chosen one operator working on a certain type of equipment (for example, ViaSat, Hughes, Gilat EMS, Shiron, etc.), then you can only go to the operator using the same equipment. An attempt to implement the compatibility of equipment from different manufacturers (DVB-RCS standard) was supported by a very small number of companies, and today it is more of a "private" technology than a generally accepted standard.
Equipment for one-way satellite Internet
8. Disadvantages of satellite communications
Weak noise immunity
The huge distances between earth stations and the satellite cause the signal-to-noise ratio at the receiver to be very low (much less than for most microwave links). In order to provide an acceptable error probability under these conditions, it is necessary to use large antennas, low noise elements and complex error-correcting codes. This problem is especially acute in mobile communication systems, as they have a limit on the size of the antenna and, as a rule, on the power of the transmitter.
Influence of the atmosphere
The quality of satellite communication is strongly influenced by effects in the troposphere and ionosphere.
Absorption in the troposphere
The absorption of a signal by the atmosphere depends on its frequency. The absorption maxima are at 22.3 GHz (water vapor resonance) and 60 GHz (oxygen resonance). In general, absorption significantly affects the propagation of signals above 10 GHz (i.e., starting from the Ku-band). In addition to absorption, during the propagation of radio waves in the atmosphere, there is a fading effect, the cause of which is the difference in the refractive indices of different layers of the atmosphere.
Ionospheric effects
Propagation Delay
The problem of signal propagation delay, one way or another, affects all satellite communication systems. Systems using a satellite transponder in geostationary orbit have the highest latency. In this case, the delay due to the finiteness of the radio wave propagation speed is approximately 250 ms, and taking into account multiplexing, switching and signal processing delays, the total delay can be up to 400 ms. Propagation delay is most undesirable in real-time applications such as telephony. In this case, if the signal propagation time over the satellite communication channel is 250 ms, the time difference between subscribers' replicas cannot be less than 500 ms. In some systems (eg, VSAT systems using a star topology), the signal is transmitted twice via a satellite link (from a terminal to a central site, and from a central site to another terminal). In this case, the total delay is doubled.
9. Conclusion
Already at the earliest stages of creating satellite systems, the complexity of the work ahead became obvious. It was necessary to find material resources, apply the intellectual efforts of many teams of scientists, organize work at the stage of practical implementation. But, despite this, transnational companies with free capital are actively involved in solving the problem. Moreover, not one, but several parallel projects are currently being implemented. Firms-developers are stubbornly competing for future consumers, for world leadership in the field of telecommunications.
At present, satellite communication stations are combined into data transmission networks. Combining a group of geographically distributed stations into a network makes it possible to provide users with a wide range of services and opportunities, as well as to effectively use satellite resources. In such networks, there is usually one or more control stations that provide operation of earth stations in both administrator-managed and fully automatic modes.
The advantage of satellite communications is based on serving geographically distant users without additional costs for intermediate storage and switching.
SSNs are constantly and jealously compared to fiber optic communication networks. The introduction of these networks is accelerating due to the rapid technological development of the relevant areas of fiber optics, which raises questions about the fate of the SSN. For example, development and planning, most importantly, the introduction of concatenating (composite) coding dramatically reduces the likelihood of an uncorrected bit error, which, in turn, allows you to overcome the main problem of CCC - fog and rain.
12. List of sources used
1Baranov V. I. Stechkin B. S. Extremal combinatorial problems and their
applications, M.: Nauka, 2000, p. 198.
Bertsekas D. Gallagher R. Data transmission networks. M.: Mir, 2000, p. 295.
Black Yu. Computer networks: protocols, standards, interfaces, M.: Mir, 2001, p. 320.
Bolshova G. "Satellite communications in Russia: "Pamir", Iridium, Globalstar..." "Networks" - 2000 - №9. - With. 20-28.
Efimushkin V. A. Technical aspects of satellite communication systems "Networks" - 2000 - No. 7. - With. 19-24.
Nevdyaev L. M. Modern technologies satellite communications // "Bulletin of Communications" - 2000 - No. 12. - p. 30-39.
Nevdyaev L. M. Odyssey at medium heights of the "Network" - 2000 - No. 2. - With. 13-15.
SPC "Elsov", Protocol on the organization and logic of the satellite data transmission network "Banker". – 2004, p. 235.
Smirnova A. A. Enterprise systems satellite and HF communications Moscow, 2000, p.
Smirnova A. A. Personal satellite connection, Volume 64, Moscow, 2001, p.
Mobile satellite communications
Introduction
Any communication system ultimately depends on some basic system parameters that determine the quality of communication.
So, if for cellular communication such a main parameter is the height of the antenna base station, then for satellite communication systems, this is the type of orbit of its space segment and the characteristics of the orbit. In general, any satellite communication system consists of three segments, as mentioned above: space (or space constellation), ground (ground service stations, gateway stations) and user segment (directly terminals located at the consumer).
Figure 1 The structure of the satellite communication system on the example of the VSAT network of the State Enterprise "Cosmic" communication"
According to the type of orbits used, satellite communication systems are divided into two classes: systems with satellites in geostationary orbit (GEO) (altitude 36,000 km; the number of satellites for the GEO constellation is 3, one satellite covers 34% of the earth's surface; communications - 600 ms) and non-geostationary.
Figure 2. Orbits and coverage areas of the earth's surface on the example of the geostationary space constellation of the INMARSAT system
Non-geostationary satellite systems, in turn, are divided into medium-altitude MEO (height - 5000-15000 km; number of spacecraft - 8-12; coverage area of one satellite - 25-28%; delay in voice transmission for global communications - 250-400 ms) and low-orbit LEO (height - 500-2000 km; number of spacecraft - 48-66; coverage area of one satellite - 3-7%; voice transmission delay for global communication - 170-300 ms).
Most of the existing satellite communication systems have geostationary satellite constellations, which is easily explained: a small number of satellites, coverage of the entire surface of the earth. However, a large signal delay makes them applicable, as a rule, only for radio and television broadcasting. For radiotelephone communication systems, a large signal delay is highly undesirable, as it leads to poor communication quality and an increase in the cost of the user segment. Therefore, initially, most satellite communication systems provided mainly fixed satellite communications (communication between stationary objects), and only with the introduction of digital communication methods and the launch of non-geostationary spacecraft did mobile satellite communications become widely developed. Note that modern systems mobile satellite communications, firstly, are compatible with traditional terrestrial mobile communications systems (primarily with digital cellular), and, secondly, the interaction of mobile satellite radio networks with the public telephone network is possible at any level (local, intra-zone, intercity).
The main global mobile satellite communications operators known in Russia
Iridium system (international consortium Iridium lls, Washington). The Iridium global mobile personal satellite communication system was intended to provide communication services with mobile and fixed objects located throughout the globe. The space segment of the system consisted of 66 main (orbit height 780 km above the Earth's surface) and 6 backup satellites (645 km). The system provided subscribers with the following services: voice transmission (2.4 Kbps), data transmission and telefax at the same speed, personal call and location determination.
Being a very expensive project (more than 5 billion dollars), Iridium set ultra-high prices for terminals and traffic at the initial stage of development, erroneously targeting only very rich consumers of the service. In addition, technical and financial problems unforeseen by the project arose during operation, which led the consortium to bankruptcy.
Globalstar system (Globalstar ltd., San Jose, California). The system of global mobile personal satellite communications "Globalstar" is designed to provide communication services with mobile and fixed objects located on the globe between 700 * north latitude. and 700* S
Portable terminals of the "Globalstar" system are produced in several modifications to ensure the possibility of their use both for organizing communication in the "Globalstar" system and in networks of terrestrial cellular communication of the GSM, AMPS, CDMA standards.
The space segment of the system is a constellation of 48 main and 8 standby satellites, weighing less than 450 kg, placed in circular orbits at an altitude of 1414 km above the Earth's surface. The first generation satellites are designed to operate in full load mode for at least 7.5 years.
To cover the populated territory of the globe, it is planned to build about 50 interface stations, providing maximum coverage (up to 85%) of the earth's surface with the space segment of the system. At the first stage of system development, 38 interface stations were built. There are 3 such stations in operation in Russia: in the Moscow region (Pavlov Posad), in Novosibirsk and in Khabarovsk. These stations ensure the provision of mobile services with high quality of service practically throughout Russia south of 700 north latitude. Each of these stations is connected to the Russian public network. The Globalstar system has been in operation in Russia since May 2000.
ICO system (international company ICO Global Communications). The system of global mobile personal satellite communications "ICO" is designed to provide communication services with mobile and fixed objects throughout the globe, including polar regions. The company "ICO Global Communications" was established on the initiative of the international organization "INMARSAT". It is a truly international organization. None of the countries plays a dominant role in it. More than 60 companies around the world are ICO investors.
It is planned that the ICO system will work in conjunction with cellular communication systems, providing services to regions and zones not covered by cellular radio communication systems. According to the project, most of the subscriber terminals of the ICO system will be personal pocket telephones capable of operating in two modes (satellite/terrestrial cellular). The estimated cost of the subscriber terminal of the ICO system is $1,000, one minute of traffic is $1.
The space segment of the system will be represented by a constellation of 10 main and 2 backup satellites in the MEO orbit at an altitude of approximately 10,390 km above the Earth's surface.
A feature of this system will be a specially formed network "IcoNet", which will connect twelve satellite access nodes (SAN) located around the world with "intelligent" communication lines, and will provide a quick connection of public networks with mobile terminals and mobile terminals to each other, regardless from their location. On the territory of Russia, it is planned to build one USD. The infrastructure of the terrestrial segment of the ICO system is based on the proven architecture of GSM networks, as well as standard components used in large quantities to ensure the compatibility of the ICO system with other terrestrial cellular communication standards.
The ICO system plans to provide users with the following types of services: teleservices, transport environment services, services provided in the GSM system, messaging and roaming services.
Teleservices will provide such services as: digital telephony, emergency calls, group 3 fax transmission at speeds up to 14.4 kbps and short message services. At the same time, digital telephony will provide voice quality similar to that provided by existing land mobile radio standards.
In addition, the ICO system plans to provide services for the transmission of low-speed transparent and non-transparent data in asynchronous mode at speeds of 300, 1200, 2400, 4800 and 9600 bps and transparent data in synchronous mode at speeds of 1200, 2400, 4800 and 9600 bps. /With.
Due to the financial problems of the consortium, it was decided to merge ICO Global Communications with Teledesic Corporation, which would delay the start of services until 2003. One USD on the territory of Russia is supposed to be built by the same date. It is expected that 450,000 subscribers will use the ICO system in Russia.
System INMARSAT(company «INMARSAT ltd.», London). INMARSAT owns satellites installed in geostationary orbit at the following positions: 54*W, 15.5*W, 64.5*E, 178*E. This provides an almost global connection between 75 * S.l. and 75* s.l.
More than 50 earth stations operate in the INMARSAT system, providing communication with mobile equipment installed on sea and river vessels, drilling platforms, aircraft, vehicles (practically none in Russia), in business cases.
The following types of mobile stations are used: "INMARSAT-A", "INMARSAT-B", "INMARSAT-M", "INMARSAT-mini-M", "INMARSAT-C", "INMARSAT-D +" (pager with answer), " INMARSAT-aero" (various types). The listed types of stations have different physical and electrical characteristics, which determines a large difference in the price of stations, the tariff for communication and its quality (information transfer rate, voice transmission quality).
Currently, about 170 thousand stations of all types operate in the INMARSAT system, of which about 10 thousand have Russian numbers(are Russian).
ORBCOM system (ORBCOM Global, Dallas, Virginia). The ORBCOM communication system is intended for two-way data transmission and object location determination using low-orbit artificial Earth satellites (from 28 to 48 satellites). Data transmission on the satellite-to-Earth line is carried out at a speed of 4.8 Kbps, and on the Earth-to-satellite line - 2.4 Kbps. The system was developed in the USA by ORBCOM Global to meet the needs for information exchange with areas remote from the existing terrestrial telecommunications infrastructure.
The main drawback of the system is the lack of a telephone service.
News from global satellite operators
One of the most sensational and well-known projects of global satellite communications is the project of the Iridium concern. In November 2000, the US Bankruptcy Court transferred control of Iridium to a venture capital firm. As a result, this seemingly long-lost company was awarded a $72 million project to equip the US Department of Defense with mobile satellite communications. This is all the more interesting because the tender was won against another large and most dynamically developing operator at the moment - the Globalstar company.
This year was generally unsuccessful for Globalstar (despite receiving a large order for equipping bus handsets in Brazil and launching the system in Russia). It began with the refusal of the main shareholders ("Loral Space & Communications Ltd" and "QUALCOMM"), from further participation in the Globalstar projects. However, a little later, the much needed $183 million was found, and the company continued its activities. In November, Globalstar announced its results for the third quarter of 2000. The company's revenues amounted to $1.4 million, losses - $97.5 million. Compared to the same period in 1999, the company's losses per share increased almost five times and amounted to $1 per share (in 1999 - 20 cents per share). share). At the end of the third quarter, the company served 21,300 subscribers, twice as many as at the end of the second quarter of 2000. The company's management believes that this is extremely small for the successful operation of the global satellite communications system, but on the whole evaluates the project as viable and claims that the company has the financial resources necessary for its activities until the end of May 2001.
At the same time, Globalstar's losses did not lead to a deterioration in the financial position of its major shareholder, QUALCOMM (a supplier of satellite data transmission systems, which is a competitor in this business to ORBCOMGlobal with services such as Trackmaile-, "Omni-track" and "Euteltrack"). This was mainly due to other projects of the concern. QUALCOMM owns major technology patents wireless communication CDMA standard, on 3G WCDMA technology (mobile communication third generation, the standard was developed by European companies), on 3G technology of the cdma2000 standard (the standard was developed by QUALCOMM).
American Mobile Satellite Corp continued its course of developing fleet management communication services and data transmission systems over its ARDIS terrestrial network.
The Japanese company NTT DoCoMo provides communications services for the national fleet. The Australian company "Optus" serves more than 9,000 subscribers. The European EMCAT network offers a full range of mobile services, while the Belgian mobile satellite network IRIS provides satellite data transmission.
The project of ICO Global Communications has been suspended. The commissioning of the system is scheduled for no earlier than 2003.
On October 20, 2000, Boeing Satellite Systems successfully launched the Thuraya 1 satellite as part of its own mobile satellite communications deployment project, which is expected to cover the Middle East, North and Central Africa, Europe, Central Asia and India (number of residents - up to 1, 8 billion people).
Mobile satellite operators in Russia. INMARSAT
After the termination of the activities of the Iridium company, two mobile satellite communications operators remained in Russia: INMARSAT and Globalstar.
The INMARSAT system was created in 1979 in the USSR to establish satellite communications with sea vessels and ensure the safety of navigation. INMARSAT currently manages a global satellite constellation that is used to provide voice services, facsimile telex and multimedia communications for mobile users. Satellites of the INMARSAT system are located in geostationary orbit. Guaranteed communication is provided on average from 70°S. up to 70° N Each satellite covers approximately one third of the Earth.
However, although the INMARSAT system has quite a few subscribers in Russia, it cannot be said that its use is widespread. The main reason is the high price of user terminals and the high tariff for communication. For example, the tariff for 1 minute of telephone communication when using various types of subscriber stations is: for "INMARSAT-A" - about 6.0-6.5 dollars, for "INMARSAT-B" - about 4.0 dollars, for "INMARSAT- mini-M" - about 2.5 dollars, for "INMARSAT-aero" - about 6.0-6.5 dollars. The cost of terminals ranges from $3,000 to $15,000. So, the most common standard "INMARSAT-mini-M" has the dimensions of a "laptop", weight is about 2 kg, price is $3000.
Models of satellite portable terminals of the "INMARSAT-mini-M" type, available for sale in the Russian Federation
Figure 3.TT-3060A
The TT-3060A mobile phone of the INMARSAT satellite system is designed to transmit telephone and facsimile messages, data and e-mail. Built-in battery and voltage converter provide non-volatile operation for 48 hours in standby mode and 2.5 hours in talk mode. Handset, 2-wire RJ-11 fax port and Hayes compatible 2.4Kbps data port have personal phone numbers(total number - 4). The ability to protect against unauthorized access is provided by the built-in SIM card reader. It is possible to connect STU-IIB/STU-III cryptographic equipment and use image transfer software. Magnesium alloy body weighing less than 2.2kg.
Rice. 4. World Phone Hybrid
WorldPhone Hybrid provides access to the international telephone network with the ability to send faxes, data and e-mail. Key Features: 4.8Kbps Voice, 2.4Kbps Fax, 3 Hour Talk Time, Backlit LCD Display, Speakerphone, Short Message Service (SMS), Voice/Fax Mail, Call Forwarding, Notebook.
Mobile satellite operators in Russia. "Globalstar"
A subsidiary of GlobalTel (a joint venture between Globalstar and Rostelecom) began to provide its services on the territory of the Russian Federation in May 2000. At the moment it is telephony (voice transmission) and call forwarding. The following services are also provided in the system, but have not yet been implemented: data transmission, facsimile communication, transmission and reception of short messages, global roaming, location determination, voice mail, call emergency services.
The space segment includes a constellation of 48 low-orbit (and 4 standby) satellites providing coverage from 70°N. up to 70° and placed by 6 satellites in 8 circular orbits at an altitude of 1414 km. The system of low-orbit satellites makes it possible to drastically reduce the cost of a subscriber terminal and a minute of conversation.
The user segment consists of portable mobile and stationary terminal devices. Devices can work in several modes (up to three). Dual- and tri-mode devices, in addition to accessing the Globalstar system, can also be used to access terrestrial cellular networks in GSM, AMPS, CDMA standards..
Prices for subscriber terminals: mobile $1000-1900 (depending on the manufacturer), stationary - from $3000. Tariff for 1 min. outgoing traffic within Russia — $1.2-2.0 (including the public network tariff).
Models of satellite portable mobile terminals available on the Russian market that support Globalstar services
Rice. 5. Ericsson portable subscriber mobile terminal
Dual mode Ericsson terminal. The contract for the production of handsets also includes the supply of car and / or stationary subscriber terminals. Working hours - Globalstar | GSM. Dimensions mm - 160 × 60 × 37. Weight - 350g. Talk time Globalstar /GSM hours - ?. The standby time of Globalstar /GSM hours is 5/36.
Rice. 6. Portable subscriber mobile terminal Telit
Telit terminal provides communication in Globalstar modes | GSM and has the following characteristics: dimensions mm - 220 × 65 × 45; weight - 300g; talk time Globalstar /GSM hours - ?; standby time Globalstar /GSM hours - 36/36.
Rice. 7. Qualcomm portable mobile subscriber terminal
Qualcomm Tri-Mode Terminal - Globalstar | AMPS | CDMA. Dimensions mm - 178 × 57 × 44. Weight - 357g. Talk time Globalstar /APMS/CDMA hours - 1/1/3. Standby time Globalstar /AMPS/CDMA hours is 5/7/25. Display 4×16 characters, address book for 99 numbers, fast auto redial, voice mail, message reception, caller ID.
Conclusion
At the moment, despite certain failures (the bankruptcy of the Iridium concern, the suspension of the ICO project, the losses of Globalstar), mobile satellite communications have occupied their (what?) Segment of the global communications market. Sales of user terminals are steadily growing, the number of telecom operators is increasing (satellite launches by Boeing, development of a new generation of small satellites by Intersputnik), and investor interest is not weakening. At the same time, it is necessary to constantly monitor developments in this market segment and keep a “hand on the pulse” so that users of mobile satellite phones in Russia do not find themselves in a situation similar to the one that developed in Russia with the termination of the Iridium concern, when the owners did not know what to do with the pipes, which in an instant turned into a pile of iron. Let's hope that in the foreseeable future such serious cataclysms will not happen again, and the cost of user terminals and traffic will gradually equal the cost of conventional cellular communications.
In contact with
Classmates
Space or satellite communication is essentially a kind of radio relay (tropospheric) communication and differs in that its repeaters are not on the surface of the Earth, but on satellites in outer space.
The idea of satellite communications was first introduced in 1945 by the Englishman Arthur Clark. In a radio engineering journal, he published an article on the prospects of rockets like the V-2 for launching Earth satellites for scientific and practical purposes. The last paragraph of this article is significant: “An artificial satellite at a certain distance from the Earth will make one revolution in 24 hours. It will remain motionless over a certain place and within optical visibility from almost half of the earth's surface. Three repeaters, placed in a well-chosen orbit with an angular separation of 120°, will be able to cover the entire planet with TV and VHF radio broadcasts; I am afraid that those who plan post-war work will not consider this a simple matter, but I consider this way to be the final solution to the problem.
On October 4, 1957, the USSR launched the world's first artificial Earth satellite, the first space object whose signals were received on Earth. This satellite marked the beginning of the space age. The signals emitted by the satellite were used not only for direction finding, but also for transmitting information about the processes on the satellite (temperature, pressure, etc.). This information was transmitted by changing the duration of the messages emitted by the transmitters (pulse width modulation). On April 12, 1961, for the first time in the history of mankind, a manned flight into outer space was carried out in the Soviet Union. The spacecraft "Vostok" with pilot-cosmonaut Yu. A. Gagarin on board was put into orbit as an Earth satellite. Numerous measuring and radiotelemetric equipment were installed on board to measure the parameters of the orbit of the satellite and control the operation of its onboard equipment. For the direction finding of the ship and the transmission of telemetric information, the Signal radio system was used, operating at a frequency of 19.955 MHz. The astronaut's two-way communication with the Earth was provided by a radiotelephone system operating in the ranges of short (19.019 and 20.006 MHz) and ultrashort (143.625 MHz) waves. The television system transmitted the astronaut's image to Earth, which made it possible to have visual control over his condition. One of the television cameras transmitted the image of the pilot from the front, and the other from the side.
The achievements of domestic science in the field of space exploration made it possible to realize the predictions of Arthur C. Clarke. At the end of the 1950s, experimental studies began in the USSR and the USA on the possibilities of using artificial Earth satellites as radio repeaters (active and passive) in terrestrial communication systems. Theoretical developments in the field of energy capabilities of satellite communication lines made it possible to formulate tactical and technical requirements for satellite repeater devices and ground devices, based on the actual characteristics of the technical means that existed at that time.
Given the identity of the approaches, we will present experimental studies in the field of creating satellite communication lines using the example of the United States. The first active radio repeater "Score" was launched on December 18, 1958 into an inclined elliptical orbit with an apogee of 1481 km, a perigee of 177 km. The satellite equipment consisted of two transceivers operating at frequencies of 132.435 and 132.095 MHz. The work was carried out in the slow relay mode. The storage of the signal sent by the ground transmitting station was carried out by recording on magnetic tape. Silver-zinc batteries with a capacity of 45 ampere-hour at a voltage of 18 volts were used as power sources. The duration of communication was approximately 4 min per 1 revolution of the satellite. Retransmission of 1 telephone or 7 teletype channels was carried out. The lifetime of the satellite was 34 days. The satellite burned up on re-entry on January 21, 1959. The second active radio relay "Courier" was launched on October 4, 1960 into an inclined elliptical orbit with an apogee of 1270 km and a perigee of 970 km. The satellite equipment consisted of 4 transceivers (frequency 150 MHz for transmitting commands and 1900 MHz for communication), a magnetic memory device and power sources - solar cells and chemical batteries. Silicon solar cells in the amount of 19,152 pieces were used as the primary power source. Nickel-cadmium batteries with a capacity of 10 amperes - hour at a voltage of 28-32 volts were used as a buffer stage. The duration of the communication session was 5 min per one revolution of the satellite. The service life of the satellite was 1 year. On July 10, 1962, the Telstar active relay was launched into an inclined elliptical orbit with an apogee of 5600 km and a perigee of 950 km, which was intended for active relaying of radio signals in real time. At the same time, it relayed either 600 simplex telephone channels, or 12 duplex telephone channels, or one television channel. In all cases, the work was carried out according to the method of frequency modulation. Communication frequencies: on the satellite-Earth line 4169.72 MHz, on the Earth-satellite line 6389.58 MHz. The duration of the communication session on the US-Europe line via this satellite was about 2 hours per day. The quality of the transmitted television images varied from good to excellent. The project provided for a very significant satellite service life of 2 years, but after four months of successful operation, the command line failed. It was found that the cause of the failure was surface damage due to the action of radiation when the satellite passed through the inner radiation belt.
On February 14, 1963, the first synchronous satellite of the Sinkom system was launched with orbital parameters: apogee height 37,022 km, perigee height 34185, orbital period 1426.6 minutes. The operating frequency on the Earth-satellite line is 7360 MHz, on the satellite-Earth line is 1820 MHz. 3840 solar cells with a total power of 28 W at a voltage of 27.5 volts were used as the primary power source on the satellite. Communication with the satellite was maintained for only 20,077 seconds, after which observations were made by astronomical methods.
On April 23, 1965, the first communications satellite Molniya-1 was launched in the USSR. With the launch of the second communication satellite "Molniya-2" on October 14, 1965, regular operation of the long-distance communication line through satellites began. Later, the Orbita deep space communication system was created. It consisted of a network of ground stations and artificial Earth satellites "Lightning", "Rainbow", "Horizon". Below, in Chapter 7, it will be shown that modifications of the Horizon satellites continue to function in the 21st century. This indicates the high reliability of domestic equipment compared to foreign ones.
The first satellite communication stations were built, tested and put into operation in the town of Shchelkovo near Moscow and in Ussuriysk. By cable and relay communication lines, they were connected, respectively, with television centers and telephone long-distance stations in Moscow and Vladivostok.
The TR-60/120 tropospheric communication equipment, which, as is known, used high-power transmitters and highly sensitive receivers with low-noise parametric amplifiers, turned out to be the most suitable for the equipment of earth stations of the satellite system. On its basis, a receiver-transmitter complex "Horizont" is being developed, which is installed at ground stations of the first satellite communication line between Moscow and Vladivostok.
Specially designed transmitters for communication and command-measuring lines, parametric amplifiers with a noise temperature of 120 K for installation in the antenna under the mirror cabin, as well as completely new equipment that provides docking with local television centers and long-distance telephone exchanges.
In those years, earth station designers, fearing the influence of powerful transmitters on receivers, installed them on different antennas and in different buildings (receiving and transmitting). However, the experience of using one common antenna for receiving and transmitting, obtained on tropospheric communication lines, made it possible in the future to transfer the receiving equipment to the transmitting antenna, which greatly simplified and cheapened the operation of satellite communication stations.
In 1967, through the Molniya-1 communications satellite, an extensive television network of receiving earth stations "Orbita" was created with a central transmitting station near Moscow. This made it possible to organize the first communication channels between Moscow and the Far East, Siberia, Central Asia, to transmit the Central Television program to remote areas of our Motherland and to additionally cover more than 30 million viewers.
However, the Molniya satellites revolved around the Earth in elongated elliptical orbits. To track them, the antennas of ground receiving stations must constantly rotate. It is much easier to solve this problem by satellites rotating in a stationary circular orbit, which is located in the equatorial plane at an altitude of 36,000 km. They make one revolution around the Earth in 24 hours and therefore seem to a ground observer hanging motionless over one point of our planet. Three such satellites are enough to provide communications for the entire Earth.
In the 1980s, communication satellites "Raduga" and television satellites "Ekran" operating in stationary orbits effectively functioned. To receive their signals, complex ground stations were not needed. Television transmissions from such satellites are received directly on simple collective and even individual antennas.
In the 1980s, the development of personal satellite communications began. In this connection, a satellite phone is directly connected to a satellite in Earth orbit. From the satellite, the signal arrives at the ground station, from where it is transmitted to the conventional telephone network. The number of satellites required for stable communication anywhere in the world depends on the radius of the orbit of a particular satellite system.
The main disadvantage of personal satellite communications is its relative high cost compared to cellular communications. In addition, high power transmitters are built into satellite phones. Therefore, they are considered unsafe for the health of users.
The most reliable satellite phones operate on the Inmarsat network, established over 20 years ago. Inmarsat satellite phones are a flip-top case the size of the first laptop computers. The cover of the satellite phone is also an antenna, which must be turned towards the satellite (the signal level is displayed on the phone display). These phones are mainly used on ships, trains or heavy vehicles. Every time you need to make or answer someone's call, you will need to install the satellite phone on some flat surface, open the lid and twist it, determining the direction of the maximum signal.
Currently, in the overall balance of communications, satellite systems still account for approximately 3% of world traffic. But the need for satellite links continues to grow, as satellite links become more cost-effective than other forms of long-distance communication at ranges in excess of 800 km.
Communications satellites launched into space, as a rule, enter geostationary orbits, that is, they fly at the speed of the Earth's rotation and end up in an unchanged position with respect to the planet's surface. Circulating at an altitude of 22,300 miles above the equator, one such satellite can receive radio signals from one third of the planet.
The original satellites, such as Echo, launched into orbit in 1960, simply reflected radio signals directed at them. Advanced models not only receive signals, but also amplify them and transmit them to specified points on the earth's surface. Since the launch of the first commercial INTELSAT communications satellite in 1965, these devices have grown in complexity. latest model a solar-powered satellite handles 30,000 phone calls or four television broadcasts at the same time. The signals come from the antennas of the Earth-LA communication station and are received by the satellite transponder. This electronic device amplifies the signal and switches it to an antenna, which transmits it to the nearest LA-Earth communication station. In order to avoid interference, the up and down signals are transmitted at different frequencies.
Launched into geostationary orbits, three INTELSAT satellites (left) transmit longwave radio signals around the world. Serving the regions of the Pacific, Indian and Atlantic Oceans, satellites make high-speed telephone, television and telegraph communications possible. In this regard, high frequency radio signals lose out as they repel the charged particles that make up the E and F layers of the atmosphere.
This parabolic antenna can receive even very weak signals from the satellite, most of these systems can also serve for Earth-to-aircraft communications.
INTELSAT-6
Radio signals arriving at the satellite gradually weaken over a long journey to such a level that they can hardly be transmitted back to Earth. Satellites such as INTELSAT, modeled above, amplify incoming signals using energy solar panels. Each satellite also has a supply of solid propellant to keep it in its orbit.
Pictured at the top of the article:
- solar power cell
- parabolic reflectors
- parabolic reflectors
- parabolic reflectors
- parabolic reflectors
Like terrestrial antennas, this satellite antenna consists of a tooth-like device called the primary emitter and a reflective parabolic shield. Two elements of this system ensure the reception of incoming radio waves and the destruction of alien waves.
Stations located on the surface of the planet communicate with INTELSAT through huge, 30-foot-wide parabolic antennas, like the one shown in Fig. above.
Although the commercial use of geosynchronous communications satellites began almost 25 years ago, their widespread use in communications networks became possible only in the early 1980s. Television, telephony, broadband data transmission continue to dominate the list of CCC services. Modern satellite communication systems provide unprecedented opportunities for the development of private networks, the organization of point-to-point and point-to-multipoint communication services.
Satellite connection
A satellite is a communication device that receives signals from an earth station (ES), amplifies and broadcasts in a broadcast mode simultaneously to all ES located in the satellite's visibility range. The satellite does not initiate or terminate any user information, except for signals for monitoring and correcting emerging technical problems and signals for its positioning. A satellite transmission starts at some ES, travels through the satellite, and ends at one or more ES.The SSN consists of three basic parts: the space segment, the signal part, and the ground segment (Fig. 1). The space segment covers satellite design, orbit calculation and satellite launch. The signal part includes the issues of the used frequency spectrum, the influence of distance on the organization and maintenance of communication, sources of signal interference, modulation schemes and transmission protocols. The ground segment includes the placement and design of the ES, the types of antennas used for various applications, multiplexing schemes that provide efficient access to satellite channels. The space segment, the signal part, and the ground segment are discussed in the following sections.
Picture 1.
Iridium system.
Advantages and limitations of CCC
CCC have unique features that distinguish them from other communication systems. Certain features provide advantages that make satellite communications attractive for a number of applications. Others create restrictions that are unacceptable in the implementation of some applied tasks.SSS has a number of advantages:
We also highlight a number of limitations in the use of CCC:
The influence of the mentioned advantages and limitations on the choice of satellite systems for private networks is quite significant. The decision to use CCC rather than distributed terrestrial networks must be economically justified every time. Fiber-optic communication networks are becoming more and more competitive for SSNs.
Space segment
Modern communication satellites used in commercial SSS occupy geosynchronous orbits, in which the period of the orbit is equal to the period of the mark on the Earth's surface. This becomes possible when the satellite is placed above a given place on the Earth at a distance of 35800 km in the plane of the equator.The high altitude required to maintain a geosynchronous satellite orbit explains the distance insensitivity of satellite networks. Path length from given point on Earth via a satellite in such an orbit to another point on the Earth is four times the distance on the Earth's surface between its two most distant points.
At present, the most densely occupied orbital arc is 76 degrees (approximately; 67 degrees to 143 degrees west). Satellites of this sector provide communication between the countries of North, Central and South America.
The main components of a satellite are its structural elements; position control systems, power supply; telemetry, tracking, commands; transceivers and antenna.
The structure of the satellite ensures the functioning of all its components. Left to itself, the satellite would eventually go into random rotations, becoming a communication device useless. Stability and desired orientation of the antenna is supported by the stabilization system. The size and weight of the satellite is limited mainly by the capabilities of the vehicles, the requirements for solar panels and the amount of fuel for the life support of the satellite (typically for ten years).
The satellite's telemetry equipment is used to transmit information about its position to Earth. If it is necessary to correct the position, the corresponding commands are transmitted to the satellite, upon receipt of which the power equipment is turned on and the correction is carried out.
signal part
The width of the line
The bandwidth (bandwidth) of a satellite channel characterizes the amount of information that it can transmit per unit of time. A typical satellite transceiver has a bandwidth of 36 MHz at frequencies from 4 MHz to 6 MHz.Typically, the bandwidth of the satellite channel is large. For example, one color television channel occupies a 6 MHz bandwidth. Each transceiver on modern communications satellites supports a 36 MHz bandwidth, with the satellite carrying 12 or 24 transceivers, resulting in 432 MHz or 864 MHz, respectively.
Frequency spectrum
Communication satellites must convert the frequency of signals received from the ES before relaying them to the ES, therefore the frequency spectrum of the communication satellite is expressed in pairs. Of the two frequencies in each pair, the lower one is used for transmission from the satellite to the ES (downstreams), the upper one is used for transmission from the ES to the satellite (upstreams). Each pair of frequencies is called a band.Modern satellite channels most often use one of two bands: C-band (from satellite to ES in the 6 GHz region and back in the 4 GHz region), or Ku-band (14 GHz and 12 GHz, respectively). Each frequency band has its own characteristics oriented to different communication tasks (Table 1).
Table 1.
Most active satellites use the C-band. A C-band transmission can cover a large area of the earth's surface, making satellites particularly suitable for broadcast signals. On the other hand, C-band signals are relatively weak and require developed and rather expensive antennas on the ES. An important feature of C-band signals is their immunity to atmospheric noise. The earth's atmosphere is almost transparent to signals in the 4/6 GHz band. Unfortunately, this same factor also makes C-band signals best suited for terrestrial point-to-point microwave transmissions that corrupt weaker satellite signals. This circumstance makes it necessary to place APs using the C-band during transmission many kilometers from urban centers and densely populated areas.
Ku-band transmission has the opposite properties. The beam in this transmission is strong, narrow, making transmission ideal for point-to-point or point-to-multipoint connections. Terrestrial microwave signals do not interfere with Ku-band signals in any way, and Ku-band APs can be placed in city centers. The inherent high power of Ku-band signals makes it possible to get by with smaller, cheaper ES antennas. Unfortunately, Ku-band signals are extremely sensitive to atmospheric conditions, especially fog and heavy rain. Although such weather events are known to affect a small area for a short time, the results can be quite severe if such conditions coincide with the HHP (busy hour, eg 4 pm, Friday noon).
Voice and data transmission
Frequency Division Multiplexing (FDM) is widely used to multiplex multiple voice or data channels onto a single satellite transceiver.In FDM, the waveform of each individual telephone signal is filtered to limit the bandwidth to the range audio frequencies between 300 and 3400 Hz, then converted. The twelve channel signals are then multiplexed into a composite baseband signal. Each group is made up of telephone signals placed in intervals with a bandwidth of 4 kHz. Then several groups are re-multiplexed and form a large group, which can contain from 12 to 3600 individual speech channels.
Time division multiplexing (TDM) is another technique for transmitting voice and/or data over a single channel. Whereas FDM assigns separate frequency segments within the entire band for voice (or data) transmission, TDM transmits over the entire allocated frequency band. In the outgoing channel, repeating base time periods, sometimes called frames (frame), are divided into a fixed number of cycles, which are allocated sequentially for signaling incoming voice channels and data channels. To protect against possible loss of information, storage devices (buffers) are used.
Aloha system
The impact of the Aloha multiple access protocol (also known as the Aloha system) developed at the University of Hawaii in the early 1970s on the development of satellite and local area networks cannot be overestimated.In this system, APs use packet transmission over a common satellite channel. Each ES can transmit only one packet at any given time. Since the satellite is assigned the role of a repeater in relation to the packets, whenever a packet from one ES reaches the satellite while it is broadcasting a packet from some other ES, both transmissions overlap (interfer) and "destroy" each other. A conflict situation arises that needs to be resolved.
Under an early version of the Aloha system, known as the "pure Aloha system", APs can start transmission at any time. If, after the propagation time, they listen to their successful transmission, they conclude that they have avoided a conflict situation (ie, thus receive a positive receipt). Otherwise, they know that an overlap has occurred (or perhaps some other source of noise has been in operation) and they must retransmit (i.e., receive a negative acknowledgment). If the APs immediately after listening repeat their programs, then they will surely again fall into a conflict situation. Some kind of conflict resolution procedure is required in order to introduce random retransmission delays, and to space the conflicting packets in time.
Another variant of the Aloha system is to divide time into segments - windows, the length of which is equal to the length of one packet during transmission (it is assumed that all packets have the same length). If we now require that the transmission of packets begin only at the beginning of the window (the time is tied to the satellite), then we will get a double gain in the efficiency of using the satellite channel, because overlaps are limited to the length of one window (instead of two, as in clean system Aloha). This system is called the synchronous Aloha system (Figure 2).
Figure 2.
Period of vulnerability for the Aloha system.
The third approach is based on the reservation of time windows at the request of the AP.
Readers familiar with LAN multiple access protocols will recognize that the described Aloha system is a precursor to the Carrier Sense Multiple Access with Collision Detection (CSMA-CD) protocol used on Ethernet networks. A feature of the CDMA-CD protocol is the ability to quickly detect conflicts (within a micro- and even nanosecond) and instantly stop transmission. On the satellite channels Due to the large propagation time, it is unfortunately impossible to promptly stop the transmission of obviously corrupted packets.
Another improvement of the Aloha system can be the prioritization of APs with a high load intensity.
Ground segment
Technological development has led to a significant reduction in the size of the AP. At the initial stage, the satellite did not exceed several hundred kilograms, and the ES were giant structures with antennas over 30 m in diameter. Modern satellites weigh several tons, and the antennas, often less than 1 m in diameter, can be installed in a wide variety of locations. The trend to reduce the size of the AP, together with the simplification of the installation of equipment, leads to a decrease in its cost. To date, the cost of AP is, perhaps, the main characteristic that determines the widespread use of CCC. The advantage of satellite communications is based on serving geographically distant users without additional costs for intermediate storage and switching. Any factor that lowers the cost of installing a new AP will definitely encourage the development of applications oriented towards the use of SSS. The relatively high costs of deploying ES allow terrestrial fiber-optic networks in some cases to successfully compete with SSN.Therefore, the main advantage of satellite systems is the ability to create communication networks that provide new communication services or expand existing ones, while from an economic point of view, the advantage of SSN is inversely proportional to the cost of the ES.
Depending on the type, the ES has transmit and/or receive capabilities. As already noted, virtually all intelligent functions in satellite networks are carried out in the ES. Among them - the organization of access to the satellite and terrestrial networks, multiplexing, modulation, signal processing and frequency conversion. Finally, we note that most of the problems in satellite transmission are solved by the 3S equipment.
There are currently four types of APs. The most complex and expensive are the ESs oriented to high user load intensity with very high throughput. Stations of this type are designed to serve user populations that require fiber-optic communication lines to provide normal access to the ES. Such APs cost millions of dollars.
Stations of average throughput are effective for serving private networks of corporations. The sizes of such ES networks can be very diverse depending on the implemented applications (voice, video, data transmission). There are two types of corporate CCCs.
An advanced enterprise CCC with a large capital investment typically supports services such as videoconferencing, email, video, voice, and data. All APs of such a network have an equally large bandwidth, and the cost of the station reaches $1 million.
A less expensive type of corporate network is the CCC of a large number (up to several thousand) of microterminals (VSAT - Very Small Aperture Terminal) connected to one main ES (MES - Master Earth Station). These networks are usually limited to the reception/transmission of data and the reception of audio-video services in digital form. Microterminals communicate with each other through transit with processing through the main AP. The topology of such networks is star-shaped.
The fourth type of AP is limited by the possibilities of reception. This is the cheapest station option, since its equipment is optimized for the provision of one or more specific services. This AP can be oriented to receive data, audio, video, or combinations thereof. The topology is also star-shaped.
International consortiums in the CCC
Intelsat
The Intelsat consortium (The International Telecommunications Satellite Organization) - the oldest and largest - was formed in 1965 to provide the member states of the consortium (mainly developing countries) with modern communication technologies. Intelsat is an organization with over 120 full member countries and about 60 associate member countries.The first commercial satellite, Early Bird, was launched into orbit by Intelsat in April 1965. By June of that year, the satellite had officially begun transmitting over 240 telephone channels, equivalent to one television channel in bandwidth. Intelsat quickly grew to become the largest STS with 18 satellites spread over the Atlantic, Indian and Pacific Oceans. At present, Intelsat's base satellites are the most powerful Intelsat VIII and Intelsat-K, which are significantly superior in their characteristics to the first Early Bird. So in comparison even with Intelsat VI, equipped with 48 transceivers, Intelsat VIII has 36 C-bands and 10 Ku-bands and supports hundreds of thousands of telephone channels. The price of a satellite for one channel has dropped from $100,000 to several thousand, and the price of a minute of using a channel by a subscriber, which was previously $10, has dropped to $1. The power of Intelsat VIII solar panels is 4 kW, i.e. increased compared to Intelsat VI by 54% and, accordingly, 4 times compared to Intelsat V.
Eutelsat
The Eutelsat Consortium (The European Telecommunications Satellite Organization) was formed in 1977 to transmit phone calls and European television programs on the continent. In 1994, 36 European countries were members of Eutelsat, at present the countries of Eastern Europe are becoming full members of the consortium.Eutelsat's current technology program is based on the powerful Eutelsat II satellites, and will be refocused starting in 1998 on third-generation Eutelsat III satellites, providing enhanced operational capabilities and intended for use in the first decade of the next century.
Inmarsat
The Inmarsat Consortium (The International Marine Satellite Organization) was formed in 1979 at the request of the International Maritime Organization (IMO), headquartered in London, to organize satellite communications for mobile objects (ships and aircraft). The organization includes 64 states, contains 20 large fixed APs located around the world and allows you to simultaneously serve up to 10 thousand moving objects.Technology trends
Recent advances in technology in the field of satellite communications indicate the great potential of CCC in expanding the capacity of transmission channels, developing and implementing new communication services. The future of CCC lies in broadband broadcast applications and mobile satellite systems.New members are actively joining the ranks of large consortiums and organizations focused on geosynchronous satellites, offering mobile communication network services and using low earth orbit satellite systems (LEO - Low Earth Orbit). LEO systems, developed by a number of American firms, use big number light satellites in orbits below 2 thousand km for the organization of services for the transmission of messages and voice, location and urgent communications between mobile terminals. Unlike terrestrial cellular mobile networks, in which the subscriber moves sequentially through adjacent small cells, in the LEO system such a "cell" is limited only by the earth's horizon. The low satellite orbit reduces latency drastically compared to systems oriented to geosynchronous satellite orbits.
One of the most ambitious projects of the LEO system is the Iridium system developed by Motorola, which includes 66 satellites to provide two-way radiotelephone voice communications. In principle, there are no technical barriers to a full deployment of the Iridium system, but the global nature and ability to operate outside of national telephone networks require preliminary study and the establishment of the necessary regulatory barriers. A number of companies have made major investments in the Iridium project, including Motorola, Nippon Iridium, Lockheed/Raytheon, Sprint and China Great Wall Industry.
Among other large projects of LEO systems, we note Globalstar, Odyssey, Ellipso and Aries.
In conclusion, we note that SSNs are constantly and jealously compared with fiber-optic communication networks. The introduction of these networks is accelerating due to the rapid technological development of the relevant areas of fiber optics, which raises questions about the fate of the SSN. Let's advise fans of satellite communications to remain optimistic: evolutionary/revolutionary transformations are also subject to evolutionary/revolutionary transformations, as one would expect, and CCC. For example, the development and, most importantly, the implementation of concatenating (composite) coding dramatically reduces the likelihood of an uncorrected bit error, which, in turn, allows you to overcome the main problem of CCC - fog and rain. Brrr! Efimushkin V.A. - Ph.D., head. Telecommunications Laboratory of the Computing Center of the Peoples' Friendship University of Russia. His email address: