Basics of satellite communications
This chapter introduces satellite communication technology from a non-specialist point of view. The basic principles will be introduced along with a list of generic functions that satellite technology can perform (broadcast, unicast, bi-directional, multicasting). The chapter traces a broad history of satellite technology and discusses the developments currently taking place, covering broadcast type functions and additional applications and services.
It will further introduce general trends within the overall ICT sector that have an influence on the evolution of satellite technology. Although some issues and topics may not seem directly related to the use of satellites in an educational context, it is important to understand the fundamentals of the technology. This chapter is not the easiest part of the report and the reader may consider skipping to the following chapter which discusses the practical applications and return to this chapter at a later stage. Both chapters are self contained and can be read independently of the rest of the report.
A satellite is an object that orbits or revolves around another object. For example, the Moon is a satellite of Earth, and Earth is a satellite of the Sun. In this document, we will examine human-made satellites that orbit Earth. They are highly specialized wireless receiver/transmitters that are launched by a rocket and placed in orbit around the Earth. There are hundreds of satellites currently in operation.
Satellite communication is one particular example of wireless communication systems. Similar and maybe more familiar examples of wireless systems are radio and television broadcasting and mobile and cordless telephones. Systems of this type rely on a network of ground-based transmitters and receivers. They are commonly referred to as 'terrestrial' systems as opposed to satellite systems.
Satellite communication systems differ from terrestrial systems in that the transmitter is not based on the ground but in the sky: the transmitter here consists of a ground-based part called the uplink, and the satellite-based part that 'reflects' the signals towards the receivers. This part is called the transponder.
Satellites come in many shapes and sizes and have many uses. The first artificial satellite, called Sputnik, was launched by the Soviet Union in 1957 and was the size of a basketball. Its purpose was simply to transmit a Morse code signal repeatedly. In contrast, modern satellites can receive and transmit hundreds of signals at the same time, from simple digital data to complex television programmes. They are used for many purposes such as television broadcasting, amateur radio communications, Internet communications, weather forecasting and Global Positioning Systems (GPS).
Communications satellites act as relay stations in space. One could imagine them as very long, invisible poles that relay high frequency radio waves. They are used to bounce messages from one part of the world to another. The messages can be telephone calls, TV pictures or Internet connections. Certain communications satellites are, for example, used for broadcasting: they send radio and TV signals to homes. Nowadays, there are more than 100 such satellites orbiting Earth, transmitting thousands of different TV (and radio) programmes all over the world.
Other applications: remote-sensing satellites
Military, government, weather, environment, scientific, positioning
Remote-sensing satellites study the surface of the Earth. From a relatively low height (480 km) up, these satellites use powerful cameras to scan the planet. The satellite then transmits valuable data on the global environment to researchers, governments, and businesses including those working in map making, farming, fishing, mining and many other industries. Instruments on remote-sensing satellites gather data about features such as the Earth's plant cover, chemical composition and surface water. Remote-sensing satellites are also used to study changes in the Earth's surface caused by human activity. Examples of this kind of observation include investigations into presence of ozone and greenhouse gasses in the atmosphere, the desertification in West Africa and deforestation in South America.
Weather satellites record weather patterns around the world. Almost all countries use the data coming from satellites like TIROS (Television Infrared Observational Satellite) ENVISAT to forecast weather, track storms and carry out scientific research. TIROS is part of a system of weather satellites operated by the National Oceanic and Atmospheric Administration (NOAA). There are two TIROS satellites circling Earth over the poles. They work with another set of satellites in geosynchronous orbit called Geostationary Operational Environmental Satellites (GOES), such as the Meteosat satellites. Using this group of satellites, meteorologists study weather and climate patterns around the world.
Many satellites in orbit conduct scientific experiments and observations. SOHO (SOlar and Heliospheric Observation) for instance is an international project managed by Europe and the United States. Its very sophisticated instruments can measure activity inside the Sun, look at its atmosphere or corona and study its surface. SOHO does not orbit Earth. In fact it orbits the Sun, about a million miles away from Earth. In that position neither the Moon nor the Earth can block its clear view of the Sun.
The military have developed the Global Positioning System (GPS), but now people are using these satellite services to determine their exact latitude, longitude and altitude wherever they are in the world. GPS satellites can be used for navigation almost everywhere on Earth: in an airplane, boat, or car, on foot, in a remote wilderness, or in a big city. GPS uses radio signals from at least three satellites in sight to calculate the position of the receiver.
Military and government institutions make extensive use of satellites for a mixture of communication, remote sensing, imaging, positioning and other services, as well as for more secret applications such as spying or missile guidance. Extremely useful civilian technology spin-offs resulted from developments in this sector, although GPS originated as a military application. The domains of image processing and image recognition also benefited greatly from Military Research and Development.
Although the purpose of this report is not to train future satellite engineers, there are certain parts of a satellite system that are worth knowing about and which can help the reader understand how satellites behave and how they can be used for different purposes. From this point onwards we will focus almost exclusively on communication satellites, particularly those parts and elements that are relevant to satellite communications.
The two most important elements of the communications system are the satellite itself and the Earth station.
The earth station
Earth station is the common name for every installation located on the Earth's surface and intended for communication (transmission and/or reception) with one or more satellites. Earth stations include all devices and installations for satellite communications: handheld devices for mobile satellite telephony, briefcase satellite phones, satellite TV reception, as well as installations that are less familiar, eg VSAT stations and satellite broadcast TV stations. The term Earth station refers to the collection of equipment that is needed to perform communications via satellite: the antenna (often a dish) and the associated equipment (receiver/decoder, transmitter).
Antennas vary in size to match the particular service for which they are designed. For example, a 70 cm antenna may be sufficient for direct reception of satellite TV programmes in the home, but would not be suitable for, for example, transmitting television programmes.
Handheld satellite telephone, antenna for satellite
TV reception, satellite transmitting Earth station
The other part of the Earth station is the application device which, in the case of reception, translates radio signals into information that can be displayed on a TV screen or processed by a computer. In the case of transmission, this device will transform information into a signal that is suitable for transmission via the antenna, using modulation, amplification and other processing techniques. In the case of VSAT- type two-way systems, both send and receive functions can be carried out at the same time.
The parts in the sky
The two main parts in the sky common to all satellites are called the payload and the bus.
Payload: transponders, antennas
The payload represents all equipment a satellite needs to do its job. This can include antennas, cameras, radar and electronics. The payload is different for every satellite. For example, the payload for a weather satellite includes cameras to take pictures of cloud formations, while the payload for a communications satellite includes large antennas to transmit TV or telephone signals to Earth.
The transponder is the key component for satellite communications: it is the part of the payload that takes the signals received from the transmitting Earth station, filters and translates these signals and then redirects them to the transmitting antenna on board. Communications satellites carry a large number of transponders on board (normally from six to more than 24), enabling them to deliver multiple channels of communication at the same time. These channels are called carriers.
There are two main types of transponders. The 'bent pipe repeater' does not actually process the signal at all. The second type of transponder, the 'onboard processor', can introduce digital detection for the uplink signal and subsequent digital switching and modulation for the downlink. Onboard processing is a major step in the implementation of new technologies onto satellites. In the case of Iridium and many of the Internet access satellites, satellites act as mini switchboards in the sky.
Communications satellites carry, as part of their payload, antennas that receive the original signal from the transmitting Earth station and re-transmit this signal to the receive stations on Earth. The antennas that were used in the past to do this were omni-directional (transmitting signals in every direction) and not very effective. They were replaced by more efficient high-gain antennas (most often dish shaped) pointing quite precisely towards the areas they were servicing. To allow for flexibility in services or areas covered, later developments allowed the re-pointing of the so-called steerable antenna to cover a different area or to reshape or reformat the beam. Future developments will allow for a highly precise and efficient reshaping of the transmitted beam in order to cover very small areas (pencil beams). This will greatly facilitate the differentiation of services within large regions. The antennas on board the satellite are typically limited in size to around 2-3 m by the space that is available on the satellite structure.
Bus: physical platform, remote control
The bus is the part of the satellite that carries the payload and all its equipment into space. It is the physical platform that holds all the satellite's parts together and that provides electrical power, navigation, control and propulsion to the spacecraft. The bus also contains equipment that allows the satellite to communicate with Earth, a kind of 'remote control'.
Orbits: GEO, MEO, LEO, elliptical, polar
The most common type of communications satellites, particularly the broadcast satellites like AfriStar, Intelsat, PanAmSat, Eutelsat and ASTRA, are in geosynchronous orbit (from geo = Earth + synchronous = moving at the same rate). That means that the satellite always stays over one spot on Earth. It does this by placing the satellite in a position 35,786 km out in space perpendicularly above the equator. The imaginary ring around the Earth where all geostationary satellites are stationed for their lifetime is called the Clarke belt. The consequence of this type of fixed location is that Earth stations (receive as well as transmit stations on the Earth surface) can almost be permanently fixed because they are constantly pointed to the same point in the sky where they 'see' the satellite.
A medium Earth orbit (MEO) satellite is one with an orbit from a few hundred miles to a few thousand miles above the Earth's surface. Satellites of this type are in a higher orbit than low Earth orbit (LEO) satellites, but lower than geostationary (GEO) satellites. The orbital periods (the time in between two successive passes over one particular place on Earth) of MEO satellites range from about 2 to 12 hours. Some MEO satellites orbit in near perfect circles, therefore they have constant altitude and travel at a constant speed. Others have a more elliptical shaped orbit, which results in different fly-over times according to the place on Earth from where they can be seen. A fleet of several MEO satellites with properly coordinated orbits can provide global coverage. There are several advantages of the use of MEO satellites: because they are closer to the Earth's surface than geostationary satellites, they require less power to transmit. The Earth stations (transmitters and receivers) by consequence can be much smaller and have a small rod-shaped antenna. It is possible to use mobile and even handheld terminals with such systems.
Low earth orbiting satellite system
A low Earth orbit (LEO) satellite system consists of a large number of satellites each in a circular orbit at a constant altitude between 320 and 800 km. Because they orbit so close to Earth, they must travel very fast so gravity does not pull them back into the atmosphere. Satellites in LEOs circle around the Earth at 27,359 km per hour. The orbits take the satellites over the geographic poles. Each revolution takes from less than 90 minutes up to a few hours. The fleet is arranged in such a way that from any point on the surface at any time at least one satellite is in line of sight. The system operates in a cellular network structure (almost like mobile phones). The main difference is that in a mobile telephone network the relay towers or aerials are fixed on the Earth while with satellites these aerials (called transponders or wireless receiver/transmitters) are moving in space. LEO systems may form the space segment of future mobile phone systems (such as S-UMTS) that will allow true mobile, global, broadband multimedia connectivity. But although telecoms experts predicted a bright future for this technology in the beginning of this century, to date only a few systems have actually got off the ground.
Footprints: global, regional, spot beams
The area on Earth that the satellite can 'see' (or reach with its antennas) is called the satellite 'footprint'. A satellite's footprint refers to the area over which the satellite operates: the intersection of a satellite antenna transmission pattern and the surface of the Earth.
Global coverage requires that the pattern of satellite antenna transmission covers the largest possible portion of the Earth that can be viewed from the satellite. For geostationary satellites, the beam width for global coverage is about 17.4 degrees.
No satellite can cover the whole surface of the Earth at one time: to achieve a global coverage, multiple transmission beams from at least 3 different satellites are combined.
Combining footprints from Intelsat APR-1, 511 and 701 providing global coverage
The map above shows examples of how different satellites cover different areas. The combined Intelsat satellite footprints on this map cover the whole Earth. A person in Australia can use this satellite to communicate with anyone in Alaska. In combination with the regional beams from these satellites, communication can be established between many areas simultaneously.
Regional or zone coverage is the result of a partial illumination of the global coverage area. The area may have a simple shape such as a circle or ellipse or may be irregularly shaped (contoured) to cover certain areas most effectively, for example the shape of a continent or sub-continent. Typical regional beams measure around 5 degrees in width.
Regional coverage of the Eutelsat W1
Spot beam coverage towards South Africa from the Eutelsat W1
Spot beam coverage is an area that is much smaller than global coverage. The beam width is reduced to around 2 degrees. Spot beams have the advantage of high antenna gain, but are disadvantaged because they can only cover a smaller area. This drawback can be overcome by the combination of multiple spot beams.
Most geostationary telecommunication satellites cover large regions (continents or sub-continents). Sometimes satellites cover different areas at the same time from where they are positioned. For example: the Eutelsat W1 satellite, a typical broadcast satellite, positioned at 10 degrees East, provides a high-power coverage of Europe with a total of 20 channels. In addition, the satellite provides a high-power steerable narrow coverage carrying another eight channels directed towards southern Africa (see map above).
Being on the edge of the satellite footprint means the curvature of the Earth starts to disrupt transmission. It also means being further away from the satellite and therefore having to transmit or receive over larger distances through the atmosphere than would be required if transmitting/receiving from the centre of the footprint. Antenna size and power by consequence are invariably increased at the edge of the footprint. These values can be deducted from the footprint maps that are published by satellite service operators (see maps above). The numbers on the circles on the maps above indicate the signal strength received at that location expressed in dBW. From tables like the one below, users who wish to receive a transmission can read what size antenna they need. The size varies depending on the meteorological conditions of the location: places with regular heavy rainfall will need the larger dimension.
Antenna Size and Signal Strength
Satellite communications, like any other means of communication (radio, TV, telephone, etc), use frequency bands that are part of the electromagnetic spectrum. The electromagnetic radiation spectrum starts with the longest waves (including those in the audible range) and extends through radio waves and the visible light, which is effectively a very small part of the spectrum, all the way to the extremely short wavelengths such as radioactive radiation. Within this broad range of frequencies, the International Telecommunications Union (the United Nations institution that regulates worldwide use of airwaves) has allocated parts of the spectrum that are suitable for and dedicated to transmission via satellite. Some of these bands are exclusively dedicated to satellite transmission; others are shared with terrestrial transmission services.
Satellite communications spectrum
The satellite transmission bands that are of interest to us are the C-, Ku- and Ka-bands.
C-band is the oldest allocation and operates in the frequency range around 6 GHz for transmission (uplink) and between 3.7 and 4.2 GHz for reception (downlink).
Ku-band is the most common transmission format in Europe for satellite TV and uses around 14 GHz for uplink and between 10.9 and 12.75 GHz for downlink.
Ka-band uses around 30 GHz up- and between 18 and 20 GHz downlink frequency.
C-band and Ku-band are becoming congested by an increasing amount of users, so satellite service operators are more and more turning to the use of Ka-band.
The selection of the band is not something that individual service providers decide, but is rather chosen by large satellite operators based on different factors:
- Availability: C-band is still the most widely available worldwide. Ku-band is becoming more available recently in regions which were less covered in the past (South America, Asia, Africa)
- C-band is more prone to interference from other transmission services that share the same frequencies (adjacent satellites or terrestrial transmissions) than the higher bands
- While the C-band technology is cheaper in itself, it requires larger dishes (1 to 3 m) than Ku- and Ka-band (0.6 to 1.8 m) and therefore imposes relatively higher (installation) costs on the end-user
- Ku- and especially Ka-band make better use of satellite capacity
- Higher frequency bands (Ku- and especially Ka-) suffer significantly more from signal deterioration caused by rainfall: to ensure availability in bad weather conditions, the signal has to be much stronger. Note that 0.1% of unavailability means in fact that the service will be interrupted for almost 9 hours over a 1-year period. 1% unavailability represents 90 hours or almost 4 full days
Satellite control and lifetime
In principle, geostationary satellites occupy a fixed position in space and consequently the ground-based antennas do not need to be constantly redirected to follow the satellite’s movements. The fact that the orientation of ground-based antennas is fixed is a major advantage of the geostationary satellite orbit used by satellite broadcasters.
In practice however, the satellite wanders slightly around its nominal orbital position under the gravitational influence of bodies such as the Sun and the Moon, as well as other influences such as Sun radiation pressure and Earth asymmetry.
It is therefore necessary to take corrective actions in order to keep the satellite within acceptable margins from its ideal position. This is achieved by activating the so-called ‘thrusters’ that are mounted on the body of the satellite as part of its propulsion system.
As long as the satellite has enough fuel left to operate its thrusters, it can be kept in the correct position. As soon as the satellite is out of fuel, it will drift out of control and into space, which brings an end to its operational life. The satellite service operator can decide to save on fuel (and by consequence extend the lifetime expectancy of a satellite) by allowing the satellite to drift a little bit. Although this may bring down the costs for the communication via the satellite considerably, there is a consequence on the Earth station side. These stations have to be equipped for tracking (following the drift of) the satellite. The Earth stations that are used with LEO and GEO systems use omni-directional antennas that make precise pointing of the antenna unnecessary.
However, for this application, the ability to ‘see’ the satellite (line of sight should not be obstructed by walls, roofs, excess foliage) is still required, which means that indoor use is excluded.
The communication functions of a satellite (antennas, processors) are powered by electricity provided through a combination of solar energy and batteries. These batteries automatically take over the power supply from the large wing-shaped solar cell panels at moments when the satellite finds itself in the shadow of the Earth.
LEOs and MEOs spin around the Earth at high speeds in order to resist the Earth’s gravitational forces. They are designed to be cheaper and therefore are smaller and lighter than large GEOs. They take less fuel to correct their flight paths and in most cases have a shorter life expectancy than GEOs. LEO operators expect to renew their satellite fleet between 5 and 7 years. GEO operators estimate the lifetime of their satellites to be between 10 and 12 years.
Applications of Satellite Communications Technology
Satellite communications systems differ from terrestrial systems in one obvious and important aspect - the transmitter is no longer located on the ground but rather in the sky. Because it's positioned in space, it is able to serve a very large geographical area. This has several advantages.
As few as three geostationary satellites can cover almost the whole of the Earth's surface, with the exclusion of the sparsely populated polar regions. To achieve the same coverage by terrestrial means would require a very large and expensive network of ground-based transmitters.
Services can be established quickly, since coverage is available for everyone from the day transmissions start. There is no need for a phased introduction as is the case with ground-based transmissions where antennas need to be added to meet the expansion of the serviced area. With satellite communications, even users in very remote locations enjoy the same level of service as any other user in the coverage area.
Satellites can overcome national boundaries, providing possibilities for truly international services.
Although terrestrial systems may be better suited generally to provide communications services, in many cases the need to be connected can only be met effectively and rapidly by the implementation of satellite services.
Radio and TV Broadcasting
The most familiar use of satellites is television broadcasting. TV satellites deliver hundreds of television channels every day throughout the world. These satellites are even used to supply television signals to terrestrial transmitters or cable-head end stations for further distribution to the home, or to exchange signals between television studios. The bandwidth required to transmit multiple programmes at the same time can easily be provided using satellites. In addition, developments in broadcast technology (digitalisation, multiplexing and compression) allow different types of transmissions to be sent sharing the same satellite signal. To address the largest possible number of viewers, the cost to the viewers must be small, requiring small receive antennas and cheap receivers.
Satellite service operators such as Intelsat, Eutelsat, ASTRA, PanAmSat, NileSat, AsiaSat and AfriStar carry the signals for satellite broadcasters such as BSkyB, CanalPLus, Multichoice, DirecTV and WorldSpace. These in turn bundle programmes from different public and private broadcasters in order to make them accessible for their viewers in an open ('free-to-air') or closed (restricted) way. Some satellite broadcasters bundle special offers into so-called 'bouquets of services' that are offered at additional cost.
The importance of satellite TV broadcasting is enormous: at the moment Eutelsat broadcasts over 900 TV channels and 560 radio stations to more than 84 million satellite and cable homes, the vast majority of them via the five HOT BIRD satellites at 13 degrees East. ASTRA, another European direct-to-home satellite system, transmits more than 1,000 television and radio channels in analogue and digital format to an audience of more than 89 million homes throughout Europe. It does this via 12 satellites at the orbital positions of 19.2 degrees, 24.2 degrees and 28.2 degrees East.
In order to make their offer more attractive to broadcasters, satellite service operators try to place their satellites aimed at the same regional market as far as possible in one single position. This is why we find the Eutelsat HotBird constellation at 13 degrees East or the ASTRA position at 19 degrees East, where in each case a number of satellites are clustered. In consequence, viewers need to point their antennas in one direction only in order to receive a large number of satellite programmes coming effectively from different satellites but looking as if they come from only one.
Satellite TV reception antenna
There are many different applications of satellite TV viewing, depending on the needs and objectives of the broadcaster or the viewers. Direct-to-Home or DTH - also called DBS or Direct Broadcast via Satellite - speaks for itself: the TV programmes are aimed at the consumer and transmitted in such a way that residential customers can buy and install the equipment to receive the programmes at the lowest possible expense. This requires a network of local resellers that offer the hardware (satellite receive equipment), installers (technicians that assist the customer in setting up the receive equipment) and service suppliers (who provide and administer subscriptions).
The Eutelsat HotBird position at 13 Degrees East
Programme suppliers can opt for free-to-air programming, where every viewer with a standard satellite receiver can receive and view the programme without restrictions. However, some programmes contain information that is not for public viewing. To protect these programmes so that only those who are the targeted audience will be able to view the contents, some type of conditional access can be applied. What happens is that programmes are encrypted and must then be unscrambled with a specific device (usually integrated in the receiver and therefore often called an Integrated Receiver Decoder or IRD) to view the contents.
The move from analogue to digital services
The number of analogue channels transmitted and the number of homes receiving analogue continues to decrease. However, analogue takes up a significant portion of the range of frequencies available. In addition, even in space, transmission capacity is limited. Where an analogue signal will occupy a full transponder consuming a bandwidth of 36 or even 72 MHz, digital broadcasting makes it possible to compress signals, vastly increasing the number of channels available by combining multiple programmes onto one single transponder.
Nowadays, most digital TV signals are compliant to the MPEG-2/DVB standard and can be received with standard consumer digital reception equipment that decodes the signal and separates the different types of content out of the data stream. With transmission bit rates between 34 and 38 Mbps, a digital signal can carry a combination of up to 12 television channels, along with numerous radio transmissions and data.
Consequently, it is digital television that is now driving the satellite TV market, aiming at large numbers of consumers equipped with small antennae of typically 50 to 80 cm in diameter in Western Europe and 1.2 to 1.5 m in diameter in other regions. Digital technology has spurred the development of interactivity and aided the convergence of the worlds of television, radio, personal computing and telephony.
It appeals to the end-user by providing better video and audio quality, improved programme and service choice and greater control over content delivery.
Pay TV is a service where the viewer is charged according to the programmes she/he views, selected from the TV programme on offer. Video on Demand and Near Video on Demand enables individual viewers to decide at any given moment (in the case of real Video on Demand) or at a later time to be scheduled (in the case of Near Video on Demand) to view the programme of his/her own choice. IP-TV is a Video on Demand application using compression technologies that allow highly efficient distribution of video and audio using common multimedia formats such as MPEG-1, 2 and 4. Streaming technologies are based on the Internet Protocol, which allows delivery over all kinds of networks including the Internet.
The latest development in advanced television applications including delivery via satellite is the Personal Video Recorder (PVR). These devices are used both to digitally record and play back programmes: the programme provider sends the content the normal way (TV networks, cable, satellite). At the receive end the content is fed into the PVR. Compression such as MPEG-2 and MPEG-4 is used to decrease the bandwidth. The PVR unit is basically a computer that saves the incoming live TV signal from the cable or antenna onto its large internal hard drive. In this way, the viewer can play it back with a few seconds delay. The viewer is then watching off the computer hard drive, instead of straight from the antenna or cable connection. This allows the viewer to rewind, slow down, stop and pause at any point.
Broadcasters and content providers are able to improve their service offers. New satellite facilities that are being offered or under development are Pay TV services, (Near) Video on Demand, IP-TV delivery and Personal TV using devices such as the Personal Video Recorder (PVR) or the Multimedia Home Platform (MHP1), services that are called interactive TV or enhanced TV. While the concept of interactive TV (iTV) is not new (there have been numerous interactive TV pilot services and some limited applications have been rolled in parts of the world), the broadcast world seems to be waiting for final and concrete iTV standard protocols and definitions to give the public uptake the expected boost. Interactive or enhanced services such as electronic programme guides, on-screen games, quizzes, enhanced TV with retrievable background information on demand, recording, rewinding, even Internet services such as mail and web browsing, are all applications that are not only appealing new services for leisure TV but will also find applications within the education domain where the high degree of penetration of TV sets and transmit capacity combined with the low threshold of the television programmes for the end-user, may well become a strong argument for the use of interactive TV in educational communities.
The Multimedia Home Platform (MHP) is a software specification that will be implemented in set-top boxes, integrated digital TV receivers as well as multimedia PCs. The MHP will connect the broadcast medium with the Internet, television, computer and telecommunication and enables digital content providers to address all types of terminals ranging from low-end to high-end set-top boxes, integrated digital TV sets and multimedia PCs.
But even if the broadcasting world is rapidly going digital, analogue TV and radio are set to remain for several years yet. For some services, analogue is still the most attractive option due to the large installed audience base and the widespread availability of consumer equipment that is less expensive than digital equipment. Analogue is particularly popular for free-to-air broadcasting. Moreover, the capacity to transmit several audio sub-carriers on one analogue TV signal allows multilingual TV programmes. As for analogue radio broadcasting, up to eight mono channels or four stereo pairs can be transmitted as sub-carriers of an analogue television signal. However, due to the heavy use of analogue sub-carriers and the decreasing number of transponders used for analogue TV, analogue sub-carriers are becoming less and less available. Service providers considering new audio services should therefore consider digital. When you take into account the cost of transmission and the numerous innovative applications that are becoming available, such as the PVR, encryption and the ability to carry data for multicasting, the appeal of digital broadcasting is hard to resist.
Business radio and TV
Narrowcasting or business TV and radio is a term used for satellite broadcasters who use transmission time to reach a very specific audience. Technically speaking, there is no difference with broadcast satellite TV applications described in the previous section. Digital television has made it possible to distribute information within organisations and companies that are geographically dispersed, or to deliver distance education. Similarly, digital radio allows for the delivery of radio services to relatively small closed user groups.
MPEG-2/DVB technology is the dominant standard for digital television, but other computer-based media coding techniques (such as MPEG-1, Real Video, etc) are also used to embed video and/or audio into data streams, often integrated with other multimedia or Internet services. Transmission via satellite requires there to be digital receivers available at relatively low prices on the consumer market. The advantage is that more advanced or popular audio coding techniques (for example MP3) can also be used and that the same stream can be used for other applications, such as data distribution, outside broadcast hours.
Contribution ('backhaul' and SNG)
Satellite transmission technologies can be used to bring the signal that needs to be broadcast to the place where it can be processed and prepared for re-distribution, for example: to a broadcaster's main studio; to a number of cable-head end stations; to an Internet Service Provider where it can be injected into the Internet; or to a network of local Points-of-Presence for distribution in local networks. These links respond to the need for point-to-point and point-to-multipoint transmission and are often called a 'hop'. The signal can be digital or analogue and can include video, audio, data or multimedia.
When used by news companies this type of contribution link is called Satellite News Gathering (SNG). News and information are sent from a mobile station - a truck equipped with an antenna or a suitcase uplink - through the satellite to a central point, which is in most cases the home studio, equipped with an Earth station with a fixed antenna. The home studio can retransmit it live or record and re-edit it for later use.
Thin route or trunk telephony
Telecom operators have been using satellite communications for many years to carry long-distance telephone communications, especially intercontinental, to complement or to bypass submarine cables. To the end-user this is transparent: the phone calls are routed automatically via the available capacity at any given moment. However, the 74,000 km round trip, even at the speed of satellite signals, takes 250 milliseconds causing a delay that makes telephone conversation rather unnatural, hence the preference for telephony over cable.
In regions where it is not so easy to install terrestrial telephone connections because of the low density of population or because of the nature of the terrain, satellite is still being used to connect the local switchboard to the telephone network. This technology is called thin route or trunk satellite telephone networking. Wireless (microwave, two-way radio, etc) and optical links, however, are replacing satellite increasingly in this area. With the advent of true mobile telephony (cellular systems such as GSM, the Global System for Mobile communication), and new end-user connection technologies such as Fixed Wireless Local Loop where there is no longer a need to wire up each subscriber, satellite thin route telephony is becoming less and less popular. In the future, satellite thin route telephony is expected to only hold a small share of trunk telephony in areas that are otherwise impossible to reach.
Mobile satellite telephony
Mobile telephony allows the user to make telephone calls and to transmit and receive data from wherever he/she is located. Digital cellular mobile telephony such as GSM has become a worldwide standard for mobile communications, but its services lack coverage over areas that are sparsely populated or uninhabited (mountains, jungle, sea), because it is not economically viable or practical for the network operators to build antennas there. Satellite telephony seems to be able to provide a possible solution to the problem of providing voice and data communications services to these other locations.
Inmarsat was the world's first global mobile satellite communications operator, founded in the late 1970s. It focuses on communications services to maritime, land-mobile, aeronautical and other users. Inmarsat now supports links for phone, fax and data communications at up to 64 Kbps to more than 210,000 ship, vehicle, aircraft and portable terminals.
The range of Inmarsat systems includes mobile terminals from handhelds to consoles, with easy set-up mechanisms that allow users wherever they are to connect via a global fleet of geostationary Inmarsat satellites to the terrestrial communications network and to carry out telephone conversations, data transfers, and increasingly multimedia applications and Internet access. Inmarsat is aimed at professionals who need a reliable communications system wherever they are: ship owners and managers, journalists and broadcasters, health and disaster-relief workers, land transport fleet operators, airlines, airline passengers and air traffic controllers, government workers, national emergency and civil defence agencies, and peacekeeping forces. The cost is rather high while the capacity is still rather limited: voice/fax/data systems achieve a maximum data rate of 64 Kbps at connection costs starting at almost US$ 3 per minute. Dedicated mobile IP systems can achieve a maximum download speed of up to 144 Kbps.
Another mobile satellite communications system is the Globalstar satellite telephone network. Globalstar, which was established in 1991 and began commercial service in late 1999, offers service from virtually anywhere across over 100 countries, as well as from most territorial waters and several mid-ocean regions. Globalstar deploys handheld telephone sets that switch between the terrestrial wireless telephone network (GSM) and a LEO-based satellite network in places where no terrestrial GSM network is available.
Globalstar telephony coverage map (April 2003)
Signals from a Globalstar phone or modem are received by one of the 48 LEO satellites and relayed to ground-based gateways, which then pass the call on to the terrestrial telephone network.
Globalstar LEO Satellite Telephone Service
A similar LEO satellite communications system is Iridium. Both Iridium and Globalstar are based on constellations of satellites that can communicate with small handheld telephone sets as well as between themselves, effectively acting as switchboards in the sky. The satellites orbit at approximately 800 km above Earth and provide worldwide mobile telephony and Internet access. Because of the short delay times (thanks to the low height and thus short distance between Earth station and satellite) it is theoretically possible to introduce videoconferencing and interactive multimedia to both fixed (with outdoor antenna) and mobile transceivers at a later stage. It is easy to understand how LEO services would be suited for urban or rural areas that are not connected to a broadband terrestrial infrastructure or that cannot be covered economically using traditional terrestrial infrastructures.
An alternative approach to satellite telephony uses a geostationary satellite instead of the LEO. This results in longer delays (approximately half a second) but switching on board the satellite reduces this inconvenience as much as possible. The Thuraya mobile satellite system was launched in 1991, its satellite maintains a geo-synchronous orbit at 44 degrees East. Thuraya operates effectively in both satellite and GSM environments. Its satellite network capacity is about 13,750 telephone channels. When within reach of a GSM network, Thuraya's mobile phone acts as an ordinary GSM handset. Outside this GSM coverage it seamlessly switches to become a satellite telephone. The system can be used for voice, data, fax, SMS and location determination (GPS-like). Thuraya handsets and subscription services are distributed through service providers (mobile telecom companies) located in 106 countries in Europe, Africa, the Middle East, Asia and India. Through roaming agreements, customers can use their handsets in a number of other countries as well.
Thuraya GSM/Satellite Telephone Service
Satellite based mobile telephony: Conclusion
The deployment of these LEO-based services has not been as successful as had been hoped by the providers. While initially it took a long time before the first service became available, the competitors, in this case cellular mobile telephony, eg GSM, had already won a market share that was lost for the new technology. The receivers were initially too expensive (about US$ 2,000), the communications costs too high (from less than US$ 1 to more than US$ 5 per minute depending on the call destination and the payment programme, Thuraya being cheaper in general) and the service had a reputation of not being very reliable: the technology did not seem to be sufficiently mature and calls were frequently interrupted. The transmission speed was very low (maximally 9.8 Kbps which is comparable to GSM-based transmissions).
Data, broadband and multimedia services
When we consider that TV and radio, telephone and fax nowadays are all being digitised and packaged in datagrams (small data packets) to be transported on any type of network, it is easy to understand that any digital content can be distributed in much the same way. This is obviously the case with data over satellite communications networks.
Normally, data does not suffer from the small delay caused by the long transmission path via satellite. Telecoms and global telecommunications carriers have been using satellite data links to complement existing wire-based data networks for many years. Large, multinational companies or international organisations in particular have exploited the ability to transfer data over satellite networks since the time when smaller and cheaper satellite terminals and more flexible satellite network services became available. Satellite services could support the different services they were interested in, such as data collection and broadcasting, image and video transfer, voice, two-way computer transactions and database inquiries.
The development of common datagram and data transport standards, and the digitisation of voice, image, video and multimedia in general, have led to a shift to Internet Protocol (IP)-based satellite communication systems that integrate seamlessly into the Internet world. It is useful at this point to make a distinction between three different types of applications.
IP over satellite for ISPs
Telecoms and connectivity providers have started using satellite communications to bypass the increasingly clogged terrestrial and submarine networks to complement their backbone connectivity or to supplement them where they are not yet available. This approach takes advantage of the fact that satellite is not a real point-to-point connection like cable, but a connection that allows the delivery to multiple points at the same time. This allows for simultaneous updating of multiple caching, proxy or mirroring servers.
IP via satellite for ISPs
In much the same way, it is possible to push Internet content to and even over the edges of existing networks. When it is necessary to provide large amounts of content to places that are poorly connected to the Internet, it is now possible to push content to local PoPs (Point of Presence) edge servers. These can then in turn serve as ISPs to the local users or user communities.
Although cable may be the preferred way to connect areas with a concentrated demand for access (like cities or densely populated areas), satellite communications can still assist local ISPs especially when there is not yet a reliable wired connection to the Network Access Points or the Metropolitan Access Exchange points on the backbone. This is also practical when a large quantity of content needs to be transported between two particular points and high-capacity cable connections are not available.
Corporate or institutional VSAT networks
A particular application of data via satellite is VSAT (Very Small Aperture Terminal) networks. Organisations with many remote affiliates can create a private high-speed satellite intranet, which links the main office reliably with all local sites. Within and amongst institutions there is an ever-growing need to communicate and to enhance the existing networks, both human and physical. These networks, comparable to the corporate or institutional networks of large multinational companies or international institutions, today need high speed, reliable and cost-effective communications. This is especially true when the locations are dispersed over remote regions and multiple countries, and barely connectable via a terrestrial network infrastructure. In this case, satellite communications are an effective way to provide private or secure data networks. VSAT can provide a complete network capable of connecting all sites and connecting to the Internet, wherever the facilities are located or wherever facilities will be located in the foreseeable future, including the homes of staff, members, students etc.
VSAT stands for Very Small Aperture Terminal and refers to combined send/receive terminals with a typical antenna diameter of 1 to 3.7 m linking the central hub to all remote offices and facilities and keeping them all in constant immediate contact. VSAT networks offer solutions for large networks with low or medium traffic. They provide very efficient point-to-multipoint communication, are easy to install and can be expanded at low extra cost. VSAT networks offer immediate accessibility and continuous high-quality transmissions. They are adapted for any kind of transmission, from data to voice, fax and video.
The great advantage of VSAT is its flexibility. It permits any kind and size of network based around a central hub and remote locations. This makes them particularly useful for corporate networks or, for example, communication between educational, government or health-care institutions. Through a VSAT network, a corporation can communicate freely and constantly with branch offices:
- Voice and fax transmissions
- Local Area Network interconnection
- Data broadcasting
- In-house training
Various network topologies, protocols and interfaces are available to implement VSAT communications applications. It is possible to lease satellite capacity on a carrier-per-carrier basis for any type of VSAT network. VSAT operators offer turnkey solutions including installation, licensing and maintenance.
VSAT networks are generally 'star' networks. This means there is a central location that acts as a hub through which remote locations can transmit and receive data to and from each other. They can be one- or two-directional.
VSAT Star-shaped Networks
A mesh configuration enables remote terminals to contact each other without passing through the hub and is particularly appropriate for large corporations where local facilities need to be in contact with other regions.
VSAT Mesh-shaped Networks
New VSAT technologies and services are being offered to support these demands. Employing one- or two-way satellite communication, IP-compatible solutions enable private network operators to provide their network members with enhanced speed and reliability for institution-wide communication. Networks featuring PC-based user terminals equipped with data cards linked to a receive/transmit satellite dish ensure fast Internet access and fast, simultaneous data broadcast to all user terminals via satellite. Intranets, Extranets, Internet access and email messaging are becoming just as important as the traditional video, voice and data requirements of videoconferencing, business TV and data-file exchange. Different levels of VSAT services can deliver various options depending on the requirements of each network.
Capacity can be booked on a full-time basis with prior reservation for minimum utilisation of any 24-hour contiguous period per occasion or on an ad-hoc basis according to a pre-arranged plan of identical transmissions during specific periods, or for occasional use.
PAMA (Permanently Assigned Multiple Access) means having a permanently assigned frequency channel that provides dedicated bandwidth, through which the network can send data, voice or video. This may be required when larger amounts of data continuously need to be transmitted between each element of the network. This can be the case in mission-critical real-time processes such as process monitoring, distributed processes and data collection, but also in media streaming (as in TV and radio broadcasting).
DAMA (Demand Assignment Multiple Access) provides intermittent communication or managed VSAT services on a pay-per-usage basis. With DAMA, satellite capacity is instantaneously assigned and adapted according to immediate traffic needs. It is available when needed, and users only pay for what they use. DAMA can support changing or intermittent image-based or heavy data transfer needs and is best suited where multiple services are integrated into a single network, since it supports telephony, low- and high-speed data, video and multimedia applications. In order to be cost effective, DAMA requires the network to be designed quite precisely to meet the organisation's needs for data distribution and communications. Peak and minimal usage levels need to be particularly estimated. DAMA is a highly efficient means of instantaneously assigning telephony and data channels in a transponder according to immediate traffic demands.
Advantages of VSAT networks include:
- Wide geographic coverage
- Independence from terrestrial communication infrastructure
- High availability
- Communication costs independent of transmission distance
- Flexible network configuration
- Rapid network deployment
- Centralised control and monitoring
- Any service can be provided from telephony through to ATM, Frame Relay, and of course, high speed broadband Internet
- VSAT services are generally expensive
- VSAT services are not available for single site users, but only to multiple site networks
- The ODU (outdoor unit, antenna) may be prone to vandalism or adverse weather conditions (lightning, storm, etc)
- Requires professional installation, management, monitoring and maintenance
- In some countries VSATs are heavily regulated
- As with all satellite solutions, there is a latency (delay) in the signal, making telephone and videoconferencing services more difficult
End-user services for home or small office
Broadband access for end-users is usually considered a 'wired' solution: fibre optic backbones, cable modems on coax, xDSL and ISDN on twisted copper. ADSL can only be provided up to a distance of between 4 and 6 km from the local telephone exchanges, depending on various factors. The cost to upgrade the existing copper network is very high. This means that many households, particularly those in rural and remote areas, will probably never be able to receive ADSL. Similarly, the cost of laying bi-directional cables for interactive TV and Internet means that cable distribution is also unlikely to be available to those living in small towns or in the countryside.
Satellite broadband connectivity has never been considered seriously, as long as it did not allow for interactivity. However, nowadays satellites can provide interactivity via either the satellite return channel or by using a hybrid solution with narrow-band return path via a telephone line. With Internet via satellite, every user with the correct equipment and living within the satellite footprint can now have a broadband connection.
Satellite has the capability to reach everywhere, thus effectively removing local loop difficulties, especially in areas with poorly developed infrastructure. The subscriber requests (eg the click on a hyperlink in a web page) can still be routed through terrestrial telephone lines, but the downloaded data can now be routed via satellite directly to the Earth station of the end-user. The typical asymmetry of home and small business Internet use opens up the possibility of using a slow, small pipe in one direction and a fast, wide pipe in the other. The average user does not need in-bound high-bandwidth connectivity around the clock and needs even less out-bound high-bandwidth2. So the hybrid of high-speed satellite for in-bound matched to a low-cost, low-speed request path may well be the most cost-effective solution. Using phone lines and a satellite downlink path means that you don't pay for more technology than you need.
Most Internet-type traffic is asymmetric by nature: on average, the downlink (from ISP to end-user) is 20 times greater than the uplink (from end-user towards the Internet). It is worth noting however, that this is not true for certain particular end-users, web builders, content distributors etc, where the ratio is, of course, different.
One-way Satellite Internet Connection
Recent developments have made it possible to send all requests and return data through the satellite, which is ideal for areas with a weak telephone infrastructure.
Two-way Satellite Internet Connection
Current configurations can deliver data at a rate of up to 40 Mbps, but in practice, this means that the hub communicates with the end-user terminals at speeds of up to 4 Mbps. The terminals have a return link to the hub depending on the set-up of the network via a telephone modem connection or via the satellite return system with speeds ranging from 16 Kbps to 512 Kbps. The hub is continually listening for data requests from the terminals so, to the user, the system appears to be 'always on'. It is understood that in order to use public Internet access through the satellite effectively, the hub needs to be well connected to the Internet backbone.
Not only does this system provide a broadband connection via the satellite downlink, but it also means that the inherent advantages of satellites in many applications can be exploited, especially the ability to multicast or broadcast the same data to millions of users over a huge area. By applying intelligent caching techniques and news group feeds, traffic in the networks can be reduced and the relatively high bandwidth cost of the space link becomes insignificant especially when compared to the reach.
This allows not only for 'pull' services, such as high-speed web browsing, where a single user requests a specific item, but more importantly also for 'push' or multicast services, where a file or stream is transmitted to many users at the same time, for example, real-time information or streamed video. This type of push service is much harder to accomplish with traditional wire-based terrestrial Internet connection technologies because it consumes valuable bandwidth on all branches of the network.
With satellite transmission, the number of potential users that can receive and decode broadcast data can be restricted to one user, a group of users or all users. With a point-to-point data transfer, TCP/IP is used to send data addressed to one particular user. With point-to-multipoint file download or video stream, UDP or IP multicasting is used, and the satellite broadcasts data that can be decoded by a specific group of users.
Only authorised users can connect the base station through the Internet and operate in interactive mode (eg initiate an online web session). Conditional access is implemented at the DVB transport level by conventional means, using 'smart cards' or a similar technology. In addition, every user station has a unique station identification (hardware address) that is used at link level for individual addressing of stations. Service operators are able to set access authorisations on a user level so that transmissions may be restricted to a closed user group, for example, for security reasons, or to allow subscription-based services. The PC board can also be used for receiving video and audio broadcasting services already available and transmitted by digital broadcasters on the same satellite.
Satellite Internet connectivity offers possibilities that are becoming commonly accepted in many different end-user communities, in regions that are excluded from access until such time as wired or wireless broadband become available. Because it is impossible to predict when such services will become available, it may be better to opt for satellite telecommunications technology as it is immediately available. While financially it may seem to make sense to wait for the best solution that will become available in the future, in the short to medium term this could mean a high cost of lost (educational) opportunity.
Educational institutions can communicate across countries, regions and cultures, share libraries and databases of research information, or offer distance-learning services that are based on the TCP/IP protocols. Medical institutions can develop networks for telemedicine applications. Government entities can deliver citizen information services. Push services enable in all instances the multicasting of video and audio streams, database downloading and software update distribution. Access to Internet and multimedia becomes available to remote communities, effectively fighting exclusion.
To conclude, the advantages of two-way satellite Internet connectivity for end-users include:
- Reception is possible with a small antenna (one already in use to receive TV can, in many cases, be sufficient but may require adaptation)
- Connection is possible almost anywhere instantly within the footprint of the satellite, with no cabling work or delays dependent on terrestrial infrastructure, thus effectively solving the typical 'last mile' problem
- Consumer equipment is relatively low cost
- Internet connectivity can be combined with traditional broadcast technologies such as digital TV and radio, enabling content providers to select the most appropriate delivery means for particular content
- In addition, multimedia push services via satellite, such as data broadcasting or information streaming, are extremely efficient. In these cases, there is no need for a return link via modem, so there is no additional cost for connectivity to the Internet
Some of the main disadvantages include:
- Satellite Internet is generally more expensive than terrestrial access solutions, at least in regions where they are available
- The outdoor unit (antenna and cabling) are more prone to vandalism and weather conditions
- Bandwidth availibility is somewhat limited
- Requires professional support
- Not the ideal technology for videoconferencing
Mobile data communications
We talk about fixed or mobile services depending on the specific application. Fixed services are aimed at Earth stations that stay in the same place while operating. The antenna does not move during transmission and reception. Mobile services in contrast are aimed at users that need to receive or transmit while moving.
Euteltracs and some Inmarsat applications are examples of mobile satellite data communication services. Euteltracs equips cars, trucks, ships etc, with a small antenna, an on-board terminal with keyboard and LCD, plus software linking the on-board information system via the Euteltracs Network Management Centre based in France with the vehicle's home base. This set-up enables low data-rate services between the mobile vehicle's home base and the vehicle itself while on the move, which allows for:
- Vehicle or vessel localisation with an accuracy of 100 m
- Transmission of alarm and distress messages
- Message exchange between the mobile terminal and base
- Data collection and transmission from the vehicle or vessel
- Access to external databases for example, for weather or traffic conditions
This type of system is extremely rugged but allows only for very limited amounts of data to be transferred. It is therefore not an evident choice for multimedia applications.
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