SATELLITE COMMUNICATIONS

Introduction. For many years, the notion of satellite communications was a fantasy produced by fertile mind of Arthur C Clarke; a brilliant idea but rather impractical. As with so many creative ideas, technology eventually caught up and now satellites are commonplace. For the communication engineer, however, they represent challenging and stimulating field of work. Within satellite communication system we find the whole gamut of technologies operating in a strange and demanding as little energy to run as possible, the system must be capable of withstanding the arduous journey from earth to orbit. Consequently a careful balance must be struck between the mechanical, structural, electronic, electrical and electromagnetic engineering requirements of the system. Satellite communications provide opportunities, and pose problems, in communication methods. Their large area of access (footprint) allows a single transmission to cover an enormous number of receivers, thus allowing broadcast signals to be transmitted simultaneously to large number of people. However, this feature itself creates difficulties; partly political and partly economic. National boundaries are no barrier whatsoever, and the charging mechanism required to allow the satellite operator to recover the cost of development and provide continuous support requires a novel solution. Satellites can travel in variety orbits, basically elliptical in shape. A geostationary satellite system which orbit is fixed and essentially circular. It maintains a fixed position relative to points on earth, and this allows a cheap receiving antenna to be set up once and then fixed position.  

The disc will be as small as possible and the amplifier as cheap as possible, consistent with obtaining an adequate signal for most of the year. Outage will be high, compare with that for a better quality receiver, but must still be at reasonable level. The preoccupation in satellite link design is the power budget; how much power can be obtained from transmitter, how much power can be directed towards the receiver, how much power is lost over the link, and at the transmitter and receiver, and how much is left at the detector? What margin is necessary above the minimum detectable signal for the detector being used? Essentially the problem is to ensure that the signal level at the detector is large enough to produce a satisfactory output for a large part of the year. Since the satellite distance is fixed, the attenuation (except for affect of rain) is also fixed, and it is high.

In satellite communications the direction of transmission is indicated by the term used: the downlink is transmission from satellite to earth station and the uplink is transmission in the opposite direction.

As we have mentioned before, communications by satellite impose enormous problems on systems designer. In the uplink we have availability of high intensity transmission beam; because the transmitter is ground based and therefore power is not a problem, coupled with a sensitive receiver on the satellite. While in the downlink we have a low power source in the satellite and a highly sensitive receiver in the earth station. However, this presupposes that the earth station cost is not a constraint. For public utility applications such as telephony, cost will not be a limiting factor; the high cost of an expensive receiving station can be spread over a very large number users. In direct broadcast television however, the story is different. A single user has to pay for receiver and therefore its cost must be low. This means that the earth station will be ‘basic’.

****Source : Telecommunication Engineering ; J Dunlop and D G Smith; Chapman &Hall; London