Applications

Date: 2015-10-07; view: 488.

Satellites can be used for virtually any kind of telecommunication system. They provide a very flexible transmission medium; once a satellite is available in orbit, many kinds of earth station, for many kinds of use, can be put in place and set into operation very quickly. Satellite networks also offer high reliability. However, the costs of satellite systems fall quite differently when compared with the costs of terrestrial radio and cable systems where these are capable of providing equivalent facilities.

In some circumstances there are major differences between the demands which satellite and terrestrial radio systems make on another resource which is in short supply, namely the frequency spectrum. When these two factors, cost and spectrum availability, have been taken into account there are four main areas of applica­tion where satellite communication is already established or is becoming established and is likely to remain in significant use:

1.Trunk telecommunications. The global trunk network, linking fixed points at national centres and provincial centres all over the world, carrying massive amounts of telephone, data and video channels and all the other kinds of telecommunication facilities that can be conveyed by such channels, is routed mainly by terrestrial and submarine cable (increasingly optical fibre) and terrestrial radio relay systems. Such systems are very competitive in cost, relative to the satellite medium, for short distances. However, the distance between link terminals is sometimes large, and for these long links the satellite medium, the costs of which are little affected by distance, may be lower than those of terrestrial media.

2.Thin-route telecommunications. The economies of scale which make terrestrial transmission media so competitive for high capacity, short and medium distance links between pairs of fixed points do not necessarily apply in some other circum­ stances. Public telecommunications facilities may be provided economically by satellite over relatively short distances when the level of demand is low, especially where the intervening terrain is underdeveloped and rugged. Private telecommunica­tions networks covering extensive geographical areas and re­quiring flexibility or non-standard facilities (such as high speed data and point to multipoint communication) are another area of application which is already important in some countries.

3.Communication with mobile stations. There is no terrestrial transmission medium for communication with ships and air­ craft when far out of sight of land, which can be regarded nowadays as satisfactory. Satellite communication for ships is well established as the medium of choice. The use of satellites for communication with air liners on ocean crossings has begun, and it is likely to be extended soon to air traffic control and other aeronautical communication services. Finally, while
both cost and limitations of radio spectrum availability seem certain to ensure that most communication with land mobile stations, such as car and hand portable cellular radiotelephones, will be served by terrestrial radio systems, it is likely that satellite communication will extend the availability of such facilities into areas which would not be economically attractive for the establishment of terrestrial systems.

4. Broadcasting. Three transmission media, namely terrestrial radio, cable and satellite, are competing for domestic, standard definition, television broadcasting at present and each seems to be identifying markets where it can dominate. Studies are well advanced on two additional broadcasting services for the satel­lite medium, namely high definition television and high quality
sound broadcasting suitable for reception on motor cars.

These four areas of application are considered further in the next section.

Exercise 1: Terms to know

Exercise 2 : Answer the following questions:

1.What kind of beam is sufficient to cover the whole disc of the Earth visible from a geostationary satellite?

2.In what cases is frequency coordination of satellites made easier?

3.What do you know about the basic high gain antenna used in satellites?

4.What is frequency modulation most commonly used for?

5.How are the index of modulation and the carrier power level chosen?

6.What is wide deviation FM used for?

7.Is phase shift keying always the same for digital signals?

8.In what situations it may be preferable to disperse the spectral energy of the carrier by using frequency hopping spread spectrum modulation?

9.How many multiple access methods are described in this text?

10.What are the main characteristics of TDMA and FDMA systems?

11.Do CDMA systems structure their use of transponders in frequency and time?

12.What are four main areas of application of satellites?

Exercise 3: Give the Russian equivalents:

1. a beam edge gain

2. global coverage antennas

3. to obtain the highest feasible gain

4. to adopt a more complex antenna design

5. to generate beams

6. to take the form of front fed reflectors

7. to come into use

8. another limitation on down-link power levels

9. to have equal allocation status

10. the use of high order phase shift

11. to tolerate lower pre-demodulator carrier to noise ratios

12. a given information capacity

13. the index of modulation

14. to be exceeded by a few decibels

15. to be fully loaded

16. to exhibit strong spectral lines

17. a pseudo-random sequence

18. to relay several signals simultaneously

19. carriers will be relayed by a transponder

20. some of the carriers assigned frequencies

21. to transmit overlapping bursts

22. to be mutilated

23. to provide a very flexible transmission medium

24. to be very competitive of land

25. to be far out of sight of land

26. to extend the availability of such facilities