Col Vikas Samyal
Chief Maintainability Engineer (CME), Maintainability Advisory Group (MAG)
<< Communication satellites have changed the way information used to be shared between the users. It is no longer about exchange of voice data but involves a host of other functions >>
The use of satellites in communication systems is a fact of everyday life. This is evident by the fact that an increased number of houses are now equipped with antennas and dishes for reception of analog/ digital signal via the satellites. Communications satellite systems have entered a period of transition from point-to-point high-capacity trunk communications between large and costly ground terminals to multipoint-to-multipoint communications between small and low-cost stations. Since invention develops with the passage of time, satellite communication has also moved a step ahead from what it was in the past. This has happened due to several reasons such as frequency reuse, interconnecting many ground stations spread over the world, concept of multiple spot beam communications, and lasers which are effectively used these days for transmission through satellites.
Satellites offer a number of important features, which are not readily available with other means of communication. Some of them are enumerated below.
- Very large area of earth is visible from a geostationary satellite (about 42 per cent), that is, communication is possible beyond the earth curvature (beyond the line of sight).
- Satellite offers communication with remote communities/ isolated islands and in sparsely populated areas, which are difficult to access by other means of communication.
- Satellite communication ignores political as well as geographical boundaries.
- Satellite provides communication with moving aircraft from ground control station across the country.
- The combination of three satellites in geostationary orbits, with an ability to relay messages from one to other could interconnect virtually everything on the earth except the polar regions as shown in Figure 1.
Basic components of communication satellite
Every communications satellite in its simplest form (whether polar or geosynchronous) involves the transmission of information from an originating ground station to satellite (the uplink), followed by a retransmission of information from satellite to the ground (the downlink). The downlink may either be to a select number of ground stations or it may be broadcast to everyone in a large area. Hence, the satellite must have a receiver and a receive antenna, a transmitter and a transmit antenna, some way of connecting the uplink to the downlink for retransmission alongwith prime electrical power to run all its electronics. The exact nature of these components will differ, depending on the orbit and the system architecture, but every communications satellite must have the basic components enumerated below:-
- Transmitting and receiving antenna: Functional requirements of transmitting and receiving antenna are markedly different but the directional characteristics apply equally to both the antennas. A single antenna, however, may be simultaneously used for both transmitting and receiving the signal simultaneously. One of the biggest differences between a low earth orbit satellite and a geosynchronous satellite is in their antennas. Transmitting antenna transmits the energy in one direction for reducing the loss and this characteristic decides the gain of antenna. Gain simply tells us how much more power will fall on 1 square meter (or square centimeter) with this antenna than would fall on that same square meter (or square centimeter) if the transmitter power were spread uniformly (isotropically) over all directions.
- Transmitter / power amplifier: The amount of power which a satellite transmitter needs to send out signals, depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. This is because the geosynchronous satellite is at an altitude of 36,000 km, while the low earth satellite is only a few hundred km away. Hence, the transmitting power required by the geosynchronous satellite is more than a low-orbit satellite, if everything else remains the same.
- Control system and electronics: The satellite antennas should always be pointing towards the earth for them to be functional. This is simpler in the case of geosynchronous satellites as the satellite is relatively stationary with respect to the earth. As seen from the earth station, the satellite never appears to move any significant distance and vice-versa. The low earth orbit satellite, on the other hand, as seen from the ground is continuously moving. Likewise, the earth station, as seen from the satellite is a moving target. As a result, both the earth station and the satellite need some sort of tracking capability, which will allow its antennas to follow the target during the time it is visible. Thus precise and accurate control electronics is required to track the motion of satellite. The other alternative is to make that antenna beam so wide that the intended receiver (or transmitter) is always within it.
Frequency allocation for satellite communication
The allocation of frequency for satellite communication is a complicated process. This requires international coordination and planning and is carried out under the auspices of the International Telecommunication Union (ITU). Within the spectrum allocated for satellite communication, the allocation of frequency is carried out based on the type of service provided by the satellite. Over the complete frequency spectrum, the ITU has allocated frequency bands for various types of communication given in Figure 3.
Orbits of satellite
For providing communication links, the following orbits are preferred:-
- Polar orbits: The polar orbiting satellites orbit the earth in such a way as to cover the north and south polar regions. The altitude of polar orbiting satellite is constant over the polar region and is approximately at 1,000 km. The period of the orbit is about 1.5 hrs and the 90°inclination ensures that the satellite passes every region of the earth. Examples of polar orbit communication satellites are IRIDIUM, GLOBAL STAR etc.
- Equatorial orbit: The equatorial orbit has 0° inclination from earth’s equator. The most popular orbit is the geostationary orbit which is present at a height of 35,786 km from the Earth surface. The satellite in geostationary orbit appears to be stationary with respect to earth since the orbit period of satellite is 23 hrs, 56 minutes and 4 second which is equal to the period of the earth rotating around its axis. The disadvantage of the geosynchronous orbit is that it takes a few seconds to transmit a signal from the earth to the satellite and back. For telephone conversations, this delay can sometimes be annoying but for data transmission and other uses, it is not of much significance. Today, there are approximately 350 communication satellites in orbit, with over 250 in geosynchronous orbit. These satellites have the provision to relay from one satellite to another, thus making it possible to transmit millions of phone calls across any two points on the earth. Due to the large bandwidth available, it is now possible to transmit live television pictures between virtually any two points on the earth.
- It is to be noted that there are infinite number of polar orbits across the globe but there is only one geostationary orbit. Large number of communication satellites of various countries are present in geostationary orbit. Communication authorities throughout the world regard geostationary orbit as a natural resource and its use is carefully regulated through national and international agreements.
Indian communication satellites
The Indian National Satellite (INSAT) system which is placed in geostationary orbits is one of the largest domestic communication satellite systems in Asia-Pacific region. Established in 1983 with commissioning of INSAT-1B, it initiated a major revolution in India’s communication sector. India’s space satellite system as on date consists of 24 satellites out of which 10 are in service (INSAT-2E, INSAT-3A, INSAT-4B, INSAT-3C, INSAT- 3E, KALPANA-1, INSAT-4A, INSAT-4CR, GSAT-8 and GSAT-12) India’s space satellite system with a total of nearly 175 transponders in the C, extended C and Ku-bands provides services to telecommunications, television broadcasting, weather forecasting, disaster warning and Search and Rescue operations.
Satellite communication applications
Telephone: The first and historically most important application for communication satellites was in intercontinental long distance telephony. The fixed public switched telephone network relays telephone calls from land line telephones to an earth station, where they are then transmitted to a geostationary satellite. The downlink follows an analogous path. Improvements in submarine communications cables, through the use of fiberoptics, caused some decline in the use of satellites for fixed telephony in the late 20th century, but they still serve remote islands such as Saint Helena and Diego Garcia where no submarine cables are in service. There are also regions in some continents and countries where landline telecommunications are rare or nonexistent, for example, regions in South America, Africa, Canada, China, Russia, and Australia. Satellite communications also provide connection to the edges of Antarctica and Greenland. Satellite phones connect directly to a constellation of either geostationary or low-earth-orbit satellites. Calls are then forwarded to a satellite teleport connected to the Public Switched Telephone Network.
Fixed Service Satellite (FSS): Fixed Service Satellites use the C band and the lower portions of the Ku bands. They are normally used for broadcast feeds to and from television networks and local affiliate stations (such as programme feeds for network and syndicated programming, live shots and backhauls). These are also used for distance learning by schools/ universities, telemedicine applications, business television, videoconferencing, general commercial telecommunications and for cable telecast. Free-to-air satellite TV channels are also usually distributed in the Ku band.
Direct Broadcast Satellite (DBS): A direct broadcast satellite is a communications satellite that transmit signals to small satellite dishes (usually 18 to 24 inches or 45 to 60 cm in diameter). Direct broadcast satellites generally operate in the upper portion of the microwave Ku band. DBS technology is used for DTH-oriented (Direct- To-Home) satellite TV services. Operating at lower frequency and lower power than DBS, FSS satellites require a much larger dish for reception (3 to 8 feet (1 to 2.5m) in diameter for Ku band, and 12 feet (3.6m) or larger for C band).
Mobile satellite technologies: Initially available for broadcast to stationary TV receivers, popular mobile direct broadcast applications made their appearance in 2004, with the arrival of two satellite radio systems in the United States, the Sirius and XM Satellite Radio Holdings. Some manufacturers have also introduced special antennas for mobile reception of DBS television. Using GPS technology as a reference, these antennas automatically re-aim to the satellite, no matter where or how the vehicle (on which the antenna is mounted) is situated. These mobile satellite antennas are popular with news agencies and some recreational vehicle owners. Such mobile DBS antennas are also used by JetBlue Airways for DirectTV, wherein passengers in an aircraft can view television on LCD screens.
Satellite radio: Satellite radio offers audio services in some countries, notably the United States. Mobile services allow listeners to roam in any part of the continent, listening to the same audio programming anywhere and everywhere. A satellite radio or subscription radio (SR) is a digital radio signal that is broadcast by a communications satellite, which covers a much wider geographical range than terrestrial radio signals, for example, radio channels of Worldspace.
Amateur radio: Amateur radio operators have access to a few dedicated satellites that have been designed specifically to carry amateur radio traffic. Most of these satellites operate as space borne repeaters and are generally accessed by amateurs equipped with UHF or VHF radio equipment and highly directional antennas such as Yagis or dish antennas. Due to launch costs, most current amateur satellites are launched into fairly low Earth orbits and are designed to deal with only a limited number of contacts at any given time.
Satellite internet: After 1990s, with the development in broadband satellite communication technology, it has become possible to access broadband internet through the satellite linkages. This has been a boon for users who are located in remote areas, and cannot access a broadband connection.
- Communications satellites are used for military communications like voice, data (including telemetry, imagery, texting, file transfer, remote sensor computer access, paging, email, internet and facsimile),video teleconferencing, digital video broadcast services and broadband IP networking. These can be used for airborne, seaborne and ground based static/mobile communication in the Ku band. Tactical communication is also possible in the UHF and X band.
- With the help of these satellites, messages can be passed to submarines, control signals can be sent to a cruise missile/drone and video received from a drone can be transmitted back to the control station located thousand of miles away. Lightweight backpack terminals can now be linked to a satellite to provide voice/date link between commanders and a soldier/ aircraft/ landing craft operating in an isolated remote locality.
- The satellites can be used to provide television and FM radio broadcasts to aircraft and ships.
- Wideband data can be used to deliver high-rate intelligence, imagery, weather reports, map and video data to tactical forces using small, portable terminals.
- GPS services are now utilised to know the exact geographical location and direction of an object.
- Satellites are being utilised to relay data from Arctic buoys and automated weather stations.
- A satellite communication user can now operate in contested environments (for example, a battlefield or area where signals are being intentionally jammed). These users can work with low to moderate data rates in exchange for protection against detection, interception, jamming, spoofing, and scintillation, as well as effects from nuclear detonations. Such protected systems are being used, for example, the US Air Force Satellite Communications System (AFSATCOM).
- Satellite communication is being used for search and rescue operations, vessel traffic systems, maritime highway monitoring, in situ sensor data collection and dissemination, and surveillance.
- Satellites are being used for signal intelligence. Signals intelligence (or SIGINT) is intelligence gathering by interception of signals between people (communications intelligence or COMINT) or electronic signals not directly used in communication (electronic intelligence – ELINT), or combination of the two. Also, traffic analysis – the study of who is signalling and to whom and in what quantity – can often produce valuable information even if messages cannot be decrypted.
Trends in satellite technology
Communications satellite systems have entered a period of transition from point-to-point high-capacity trunk communications between large, costly ground terminals to multipoint-to-multipoint communications between small, low-cost stations. With TDMA, each ground station is assigned a time slot on the same channel for use in transmitting its communications; all other stations monitor these slots and select the communications directed to them. By amplifying a single carrier frequency in each satellite repeater, TDMA ensures the most efficient use of the satellite’s onboard power supply.
A technique called frequency re-use allows satellites to communicate with a number of ground stations, using the same frequency, by transmitting signals in narrow beams towards each of the stations. Beam widths can be adjusted to cover areas as large as the entire United States or as small as Maldives. Two stations which are far apart, can receive different messages transmitted on the same frequency. Satellite antennas have been designed to transmit several beams in different directions, using the same reflector. The latest trend is the use of network of small satellites in low earth orbit (2,000 km or less) to provide global telephone communication. Special telephones that communicate with these satellites allow users to access the regular telephone network and place calls from anywhere on the globe. Anticipated customers of these systems include international business travellers and people living or working in remote areas.
The trend in GEO satellites is increased power and number of transponders. Satellites with numerous C and Ku-band transponders are becoming a common thing. These increases have not resulted in a proportionate scaling of the weight of these satellites, since the use of shaped antennas eliminates the need for considerable microwave plumbing and the use of lighter structures has helped contain the weight of the satellites. Nevertheless, GEO satellites are becoming heavier and launch capability is increasing to accommodate the additional features of modern satellites. Increased power is driven by the desire to decrease the ground terminal size and cost. The power subsystem is composed of the solar array (solar cells on the supporting structure including pointing devices), batteries, and the power conditioning electronics. While it has long been known that GaAs has an intrinsically higher efficiency than silicon, the difficulty in fabricating GaAs cells that are competitive in cost to silicon, has prevented large scale application in satellites.
Another promising solar array technology is the use of concentrators which focus the light down onto the GaAs cells. AEC-Able Engineering Co, Inc. of Goleta, CA is working on parabolic reflectors that gather 7-8 times the light that would normally fall on a cell. These reflectors would also shield the cells against the high energy particles that degrade them. Such a technology would offer the promise of reducing the number of cells and the weight and thus, the cost of the solar array. The key trends in spacecraft antenna technology are larger effective apertures, significantly higher numbers of beams, and integrating computationally-intensive beam forming and switching activities with other onboard processing functions. These trends are an integral part of universal efforts to raise spacecraft effective radiated powers (EIRP), make communications payloads smarter and more flexible, and make earth terminals smaller and cheaper. A tremendous amount of research is going on in micro-optics, optical memory, optical signal processing, and optical communications throughout the world. Diffractive optical components for use in free-space and bulk microoptical systems are being studied for optical communications, information processing, optical computing and sensor applications. These include high-efficiency blazed micro Fresnel lenses, high-efficiency chirped gratings, Bragg gratings, binary gratings, and arrays and their composites. Integrated optics technologies are expected to play an important role in the development of new devices for future optical memory systems.
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- https://www.seminarprojects.com/ Thread-satellite-communication-pdfreport# ixzz1fwYPJeJ2
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