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.