SATELLITE COMMUNICATION SYSTEMS

A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth.
What are the types of satellite orbits?
There are at least three special types of orbits employed by modern satellites. These are;
 1. the Geostationary Orbits,
 2. Low-Earth Orbits and
 3. Molniya types.
A Geostationary Orbit satellite provides an illusion that it is stationary in a fixed location in space. Actually, it revolves around the planet once a day in the equator. It is useful for telecommunication devices especially those that rely on stationary antennas since there is no need to install special equipments for such facilities just to track the satellite.

Meanwhile, Low-Earth Orbit satellites are located at least 400 Km above the Earth. Because of their lower altitude with respect to the ground, they can circle the Earth in only about 90 minutes. These satellites are less expensive and require less energy to receive and transmit data.

On the other hand, the Molniya Orbit group of satellites operates at a certain inclined position suitable for Northern Altitudes. These Comsats were designed so that they will take more time servicing Northern latitudes. Molniya satellites are usually used for Television broadcast transmissions and telephone relays over the Russian state.

Satellite communications basics

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.
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 superheterodyne radio receiver. As a result the communications 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.
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.