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, located 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 located 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 typically 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 signs within that constructed area and not in any other direction. With a larger antenna dish diameter, the area of radiation decrees 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 even 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.
Where do we get that power? One may wonder why the transmitters are not designed that way to provide thousands of watts of power. It is simply that it is not possible to make that kind of power available in a spacecraft. The on-board power is generated either by the series of batteries on the craft or by the huge solar panels, mounted on the satellites. These solar panels have numerous solar cells which generate the electrical energy required from the sunlight, while charging the batteries during sun-light periods. During the time when it is dark, the power generation is switched on to the batteries .. These solar cells are similar to the ones that you find in you calculator. There is a limit of how much these solar panels can generate. This limitation sets the limit of how much power can be generated in a satellite system. In practice, in some of the satellites, these solar panels generate few thousands of watts of electrical power. It is just not conceivable in providing that high power transmitters on-board the satellites as is desired.
The batteries on board are deployed, when the earth passes in between the satellite and the Sun, when the solar panels do not get the sunlight required to produce the electrical energy. Therefore, the batteries have to remain in good charged condition in order to take over the energy generation when required.
The satellite receives and transmits the signals in two different frequencies. In here we find the application of transponders in a satellite system. A transponder is a component of a satellite system that receives the signals from Earth stations and transmits it back to the designated Earth station or stations. The uplink of any signal from Earth station is done using a "dish" antenna pointing towards the satellite. This signal is sent to one of the transponders on board. The transponder amplifies this signal, lifts it to a different frequency and transmits back to Earth. This shift of frequency, from the received signal to the transmit signal, is to avoid any interference between the receiver, and transmitter frequencies. A downlink dish like antenna at the earth station, looking at the satellite, captures this signal from the transponder which is then processed. Satellites downlink the signals received, to many Earth stations at any given moment. The system has the advantage of having the ability to uplink and then downlink to multiple earth stations thousands of miles apart and, with multiple satellites relaying the signals, it can easily cover the whole world.