Communication satellite basics

- an overview of the basic concepts of communications satellites

Satellites are able fulfil a number of roles. One of the major roles is for communications. Here the satellite enables communications to be established over large distances. Communications satellites may be used for many applications including relaying telephone calls, providing communications to remote areas of the Earth, providing communications to ships, aircraft and other mobile vehicles, and there are many more ways in which satellites can be used.

When used for communications, a satellite acts as a repeater. Its height above the earth means that signals can be transmitted over distances that are very much greater than the line of sight. An earth station transmits the signal up to the satellite. This is called the up-link and is transmitted on one frequency. The satellite receives the signal and retransmits it on what is termed the down link which is on another frequency.

Using a satellite for long distance communications

The circuitry in the satellite that acts as the receiver, frequency changer, and transmitter is called a transponder. This basically consists of a low noise amplifier, a frequency changer consisting a mixer and local oscillator, and then a high power amplifier. The filter on the input is used to make sure that any out of band signals such as the transponder output are reduced to acceptable levels so that the amplifier is not overloaded. Similarly the output from the amplifiers is filtered to make sure that spurious signals are reduced to acceptable levels. Figures used in here are the same as those mentioned earlier, and are only given as an example. The signal is received and amplified to a suitable level. It is then applied to the mixer to change the frequency in the same way that occurs in a superhet receiver. As a result the satellite receives in one band of frequencies and transmits in another.

In view of the fact that the receiver and transmitter are operating at the same time and in close proximity, care has to be taken in the design of the satellite that the transmitter does not interfere with the receiver. This might result from spurious signals arising from the transmitter, or the receiver may become de-sensitised by the strong signal being received from the transmitter. The filters already mentioned are used to reduce these effects.

Block diagram of a basic transponder

Signals transmitted to satellites usually consist of a large number of signals multiplexed onto a main transmission. In this way one transmission from the ground can carry a large number of telephone circuits or even a number of television signals. This approach is operationally far more effective than having a large number of individual transmitters.

Obviously one satellite will be unable to carry all the traffic across the Atlantic. Further capacity can be achieved using several satellites on different bands, or by physically separating them apart from one another. In this way the beamwidth of the antenna can be used to distinguish between different satellites. Normally antennas with very high gains are used, and these have very narrow beamwidths, allowing satellites to be separated by just a few degrees.

Separating satellites by position

Another variant of communications satellites is those used for direct broadcasting. This form of broadcasting has become very popular as it provides very high levels of bandwidth because of the high frequencies used. This means that large numbers of channels can be carried. It also enables large areas of the globe to be covered by one delivery system. For terrestrial broadcasting a large number of high power transmitters are required that are located around the country. Even then coverage may not be good in outlying areas.

These DBS satellites are very similar to ordinary communications satellites in concept. Naturally they require high levels of transmitted power because domestic users do not want very large antennas on their houses to be able to receive the signals. This means that very large arrays of solar cells are required along with large batteries to support the broadcasting in periods of darkness. They also have a number of antenna systems accurately directing the transmitted power to the required areas. Different antennas on the same satellite may have totally different footprints.

Satellites have also been used for cellular style communications. They have not been nearly as successful as initially anticipated because of the enormously rapid growth of terrestrial cellular telecommunications, and its spread into far more countries and areas than predicted when the ideas for satellite personal communications was originally envisaged. The systems that were set up used low earth orbiting satellites, typically with a constellation of around 66 satellites. Handheld phones then communicated directly with the satellites which would then process and relay the signals as required.

The main advantage of the satellite system is that it is truly global and communications can be made from ships, in remote locations where there would be no possibility of there being a communications network. However against this the network is expensive to run because of the cost of building and maintaining the satellite network, as well as the more sophisticated and higher power handsets required to operate with the satellite. As a result these networks have not been financially successful.