Satellite communications

Microwave links can span continents but cannot offer communications across the oceans. In principle, it would have been possible to lay a microwave waveguide under the ocean, but it did not even become a practical proposition on land, let alone under the sea. Was there a third microwave alternative? Yes. It was proposed in 1945, not by an expert in telecommunication techniques but by a renowned writer of science fiction, Arthur C. Clarke. This is hardly surprising considering that satellite communications are closely related to the idea of space travel, of which all boys (and some girls) knew who read Jules Verne and H. G. Wells. But Clarke was more than a science fiction writer. He was perfectly aware of all the principles involved. His paper, published in Wireless World, outlined all the main feature of communicating via manned satellites. The title of the paper was 'Extraterrestrial Relays'. As Clarke wrote later 'the paper met with monumental indifference'. He did his best though to promote the idea the only way he could. There were lots of manned satellite communication systems in his novels Prelude to space (1950) and The exploration of space (1952). The latter book sold about a million copies, so there must have been quite a number of men and women on Earth well aware of the potential of communicating through the skies. But it is a different thing to read science fiction and to think about practical realization.

First there was an obvious need for a sufficiently powerful rocket

6 , to put the satellite into orbit. In the US this was established by a team

6 A year later there were only 33 Pic-

turephones operating in the whole city. led by Dr Wernher von Braun who, after the end of the war, suddenly changed his interest from blasting London Town to smithereens to the peaceful joys of space travel. By 1954 the idea of satellite communications was 'in the air'. To bring it down to earth and back into the sky an influential man was needed, willing to look at all the practical details and mobilize resources for a successful demonstration. Such a man was John Pierce of Bell Laboratories. In Fig. 9.9 he is shown in the company of his colleague Rudi Kompfner: the two objects on the table

Fig. 9.9 John Pierce and Rudi Kompfner in discussion.

Fig. 9.9 John Pierce and Rudi Kompfner in discussion.

in front of them are travelling wave tubes. His first paper on the subject was presented in October 1954. His second paper put the subject on the agenda for commercial exploitation and of course the Defense Department soon recognized that they had a new toy to play with which would be very likely to solve their mobile communications problems for ever and ever.

Once it is accepted that satellites may orbit the Earth the principles involved in communicating, say, across the Atlantic, are simple. A terrestrial station in the US sends a microwave beam up to the satellite. This is then amplified, frequency shifted (in order to avoid interference) and reradiated towards Europe. Does it need a lot of power to do this? Let us remember that Marconi needed about 200 kW to send information across the Atlantic. Short waves could do it with a couple of kW, a saving of a factor of hundred. One would think that satellites would need more power because a lot must be wasted in sending fairly wide beams up and down. However using microwaves and a fairly big ground aerial (say 50 m diameter), the beams are not too spread out, so that a power of about 100 W is sufficient at the Earth terminal. Power is obviously at a premium for the transponder in the satellite since it can rely on solar batteries only. A few watts is just about enough.

Of course the satellite has to be at the right place if it is to relay signals between two points on Earth. The orbit of the satellite depends on the launching conditions. Three different kinds of orbits are shown in Fig. 9.10. Out of these, as already pointed out by Arthur C. Clarke, the equatorial orbits have a particular significance. If the satellite moves in the same direction as the Earth rotates, and the speed of the satellite matches the rate of the Earth's rotation then, for us earthlings, the satellite appears stationary. Unsurprisingly, this is called a geostationary orbit. There is thus no need to wait for the satellite to make its

Fig. 9.10 Satellite orbits around the Earth.

Fig. 9.10 Satellite orbits around the Earth.

periodic appearance. It is always there, available for microwave communications, day and night.

How did it all start? The first satellite was launched in October 1957 by the Soviet Union. They called it a Sputnik (meaning fellow-traveller) introducing thereby a new word into the English language. They beat the US by three months. Echo, the first satellite for communications purposes, was launched by the Americans in i960. As its name implies it was a passive satellite, a big metallized balloon, capable of reflecting the microwave signals. The first active satellite, provided with a transponder, was Telstar (Fig. 9.11) which went into orbit in 1962. It established, for the first time, live television transmission between the New and the Old World. In her 1962 Christmas broadcast to the Commonwealth Queen Elizabeth referred to Telstar as 'the invisible focus of a million eyes'.

The first geostationary satellite, Early Bird, was launched in 1965. It provided one television channel or 240 two-way telephone channels; not a very large number but, at the time, it effectively doubled the transAtlantic telephone capacity. It has to be noted here that geostationary satellites are not ideal for telephone conversations. The reason is the finite speed of electromagnetic waves carrying the message. The geostationary orbit is 36000 km above the Earth hence the waves carrying

Fig. g.n The Telstar satellite.

Fig. g.n The Telstar satellite.

Fig. 9.12 A mobile satellite phone for global communications (courtesy of GloCall Satellite Services).

7 It is most noticeable in television inte-views conducted via satellites. The delayed reaction of the interviewee is clearly visible on the screen.

8 A consequence is that they are not geostationary satellites. They move relative to the Earth hence quite a number of them is needed for global coverage.

the signal have to travel roughly 75 000kms. The speed of light being 300000 km per second, this distance accounts for a delay of a quarter of a second. One might think that anyone is willing to wait for half a second (a quarter second there and a quarter second back) for an answer. In fact, such a delay is quite noticeable.7 The modern trend is to halve this delay by using satellites only in one direction and terrestrial channels in the other one.

Thanks to geostationary satellites it is now technically possible to contact London from the Amazonian jungles or from a construction site in Africa. The equipment needed is transportable (see Fig. 9.12). This does not mean of course that mobile telephone communications can, at the time of writing (May 1998) be established between two arbitrary points on Earth. For that, a new system is necessary in which the satellites orbit nearer to the Earth's surface.8 Several of them have already left the drawing board and are at various stages of implementation. The Iridium system is now nearly functional and the date for the launching of the ICO system is the year 2000. They will be discussed in somewhat more detail in the chapter on mobile communications.

The value of satellites for military communications was spread all over the newspapers of the world in the summer of 1995 when an American pilot shot down by Bosnian Serbs hid during the day and tried to make radio contacts at night. An attempt to pick him up six days later was successful.

Fig. B9.1 The electric field lines in a circular waveguide in two different configurations.

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  • PONTO
    Which electromagnetic wave is needed for satellite communication?
    1 year ago

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