Quiz 20: Satellite Communication
(a) The repeaters are mainly used for strengthening the signals while transmitting signals for large distances. By using repeaters we can extend the communication beyond the level of line-of-sight propagation for frequencies which are mostly higher than what ionosphere returns back. If the repeaters are placed above the ground level the signal can travel farther distance compared to the repeaters on the ground levels. If the repeaters height is very great than the towers, it permits good transmission over the very large distances, but it is not possible practically. For longer distance communication, more repeaters should be placed for strengthening the signals. Hence it is quite complex and financially difficult. (b) Satellites in Low-Earth-Orbit is placed in the range of 300 Km to 1,500 Km distance from the earth, which has less range of distance as compared to the MEO and GEO's systems. The LEO systems are more complex and expensive because of it requires more number of satellites as compared to MEO and GEO's due to short distance from the earth. The LEO system has a great financial difficult level due to requirement of a number of satellites in it. Practically the large distance communication is possible with these systems, but financially it is difficult. (c) The Geostationary satellites have the circular orbit in equal distance and it should be placed above the equator of the earth with a distance of 35,784 Km away from the Earth, which is a far distance away from the earth, so that it requires less number of satellites in its orbit as compared to LEO and MEO's systems. Hence the cost of using this system is very less as compared to LEO systems. The GEO satellites are moving in a motion as same as the Earth's motion due to the orbital period of the GEO satellite, which is equal to the time taken by the earth to rotate at once. For this reason there is no additional antenna requirement for tracking the satellites in GEO system. Financially, it is better than LEO and MEO's systems.
The polarization of a signal transmitted from the satellite is strictly parallel or completely perpendicular to the Earth surface only at the longitude of the satellite. The more distance between the altitude of the satellite and the altitude of the receiving point is the bigger is the inclination. Therefore, the antenna polarizer has a smaller or a bigger inclination relative to the Earth. If an electric-field vector constantly changes its orientation and the angle of rotation, such polarization is called as circular polarization. Thereby a circular polarization is characterized by high speed of clockwise and reverse rotation of a signal. In communication systems, the signals are transmitted from the transmitter to the receiver. In the case of satellite communication the signal must be propagated through the ionosphere to reach the satellite receiver. For the signals below the range of very high frequency (VHF), the proper communication is not possible in the satellite communication systems. At the VHF and UHF frequencies, the ionosphere propagates the transmitted signals randomly; hence communication is not possible in the satellite communication systems. Due to this randomness, the signal cannot be transmitted to the receiver properly. Hence, for proper transmission circular polarization is necessary below the range of the microwave region of the spectrum. The circular polarization increases the reliability in signal strength and resistance to weather conditions. Circularly polarized wave will radiate energy in the horizontal and vertical plane, as well as every plane in between.
The orbital period of the satellite depends on two parameters namely circumference of the orbit, and the velocity of the satellite. The satellite nearer to the earth's surface is in smaller elliptical orbit compared to the satellite that is farther to the earth's surface, since the farther one is in longer elliptical orbit. So generally the distant satellite takes more time to complete one revolution around the earth. The mathematical representation of orbital period of the satellite is, Here, is the circumference of the orbit, and is the velocity in meters per second. The circumference of the orbit is, Here, is the radius of orbit. Here, is the distance above the earth's surface in km. The velocity of satellite is, When the satellite is in orbit farther from the earth's surface, then it travels with lower speed compared to the satellite in orbit closer to the earth's surface. Because, the satellite in farther orbit takes more time to complete an orbit. If the distance between satellite and earth is more, then circumference becomes more which directly effects the orbital period. If the satellite is farther from the earth surface then the velocity of satellite is less as the distance inversely affects the velocity. Observe this relation from the velocity equation. This velocity in turn effects the period of the orbit for a satellite. Therefore, the orbital period of the distant satellite is much higher compared to the satellite that is nearer to earth's surface.