Satellite Communication Components for Communication Satellites Part III

Added: (Thu Sep 02 2010)

Pressbox (Press Release) - The simple basic application of any communication satellite, whether it is low earth orbital or geosynchronous, involves transmission of information from an originating Earth station to the satellite concerned, which is termed as "up-linking," followed by re-transmission of the same information to the designated Earth station. This re-transmission is termed as "down-linking." The downlink of the information may be to one particular Earth station or broadcasted over a selected number of Earth stations, situated at a larger area. In order to perform this up-linking and down-linking, the satellite has a receiver and a receive antenna, a transmitter and a transmit antenna, just like a set of walkie-talkie, which has a receiver and a transmitter with an antenna, though, in here, the "receive" and "transmit" are done through the same antenna. Satellites need antennas separately for its two functions of receive and transmit. Additionally, the satellite has electronic switches. This is used to logically switch the uplink signals, down-linking it to the appropriate Earth stations. It has an electronic black-box to determine the destination or destinations of the signals being down-linked to the Earth Stations. There is that ever important electrical power in a satellite required in keeping alive the electronic circuitry. The exact component structure of a satellite may differ from one to the other, depending on its actual application, but the basic component requirements remain the same.

The electrical power needed by satellites for receiving and transmitting signals greatly depend upon its orbital path, that is, whether it is a low Earth or geosynchronous orbital satellite. Electrical power requirement mostly depend upon the height of the satellite above the Earth. The higher it is, a satellite would need that much power for its basic operation in receiving and transmitting signals On basis of this, a geosynchronous satellite, being at an altitude of 22,300 miles, would require much more electrical power than the low earth orbiting satellite, which is situated at only a few hundred miles from Earth. In theory, a geosynchronous satellite would need 10,000 times the electrical power than the low Earth orbiting satellite. This is an awful lot of power and the satellite is designed in a way to work out a compromise, without losing the application reliability.

A satellite is usually powered from a battery or a solar energy system. In some of the communication satellites, a combination of battery and solar power energy is used, with the batteries supplying power to the electronics circuitry in the satellite, with a change over to solar energy during sunlight cycle, when the batteries are left on charging. The battery is turned on during solar eclipses, when the solar panels become inactive.

The main difference between the satellites in different orbital path is the antenna. This antenna design sets the optimum power requirement of a satellite. There are basically many designs available for an antenna. Some direct their radiation to one particular direction and there are others which are omni-directional, radiating all around. This principle is carried further by a communication satellite. If you consider the height at which the satellite is orbiting, even a large area on this Earth will be a mere spot of an area from that height. With the earth stations located in a comparatively small area, a properly designed antenna will beam its signals within that constricted area and not in any other direction. With a bigger antenna dish diameter, the area of radiation decreases in relation to certain design parameters.

One of the parameter in such a design is called "gain" of an antenna. This gain tells us how much more power would be required to beam the signals on one square mile of an area, with the transmitter power evenly distributed (isotropic distribution) over all directions within that area. This is one of the primary design criteria, which goes into the requirement of less electrical power required for a geosynchronous satellite, compared to what it would, in theory.

The larger difference in the antenna system of a geosynchronous satellite and the low earth satellite is that, the antenna should always look at the Earth. While it is fairly easy for the geosynchronous satellite, being stationary relative to the Earth's rotation, the low Earth orbit satellites zoom past any point on the Earth every 5 to 10 minutes. In this case it becomes difficult to maintain the antenna orientation, as required.

The Earth station is a moving target, when looked at from the low Earth orbiting satellite and some sort of tracking system must be incorporated in the design, so that the antenna tracks the Earth station as passes that spot in its orbital path. The other alternative is to make such a design, such that the antenna can beam at a wider angle covering a wider area of the Earth, so that the receiver or transmitter is always within the reach of the reception and transmission of the signals. In doing so, the gain of the antenna reduces and to maintain the right gain, a lot more power would be required for the transmitter to provide such signal transmission.