Importance Of POC Unit In Physics-Kumar Physics Classes
The word space communication refers to sending receiving and processing of information through space.
There are three types of space communication.
Type 1-Ground wave propagation Phenomenon
This mode of propagation can exist when the transmitting and receiving antenna are close to the surface of the earth. For radiating high-efficiency signals, the size of the antenna should be of the order of l/4. l = wavelength of the signal.
The field component of such a launched wave soon becomes vertically polarized as it glides over the surface of the planet.
The electrical fields intensity due to the wave induce large charges on the earth’s surface. The ground wave is weakened as a result of energy absorbed by the earth during its propagation. These losses make ground waves unsuitable for very long-range communication.
Ground wave propagation should be sustained only at low frequencies
(~ 500 kHz to 1500 kHz) or for radio broadcast at long wavelengths.
Type 2-Skywave propagation (Ionospheric propagation)
A transmitted wave going up is reflected back by the ionosphere which forms an ionised layer of electrons and ions around the earth. The ionosphere (including
mesosphere and part of the stratosphere) extends from about 65 km to 400 km above the earth’s surface. Constituent gases are ionised in it by solar radiation. Throughout the ionosphere, there are several layers in which the ionisation density either reaches a maximum or remains roughly constant. These regions are designated as D(65-75 km), E(100 km) and F(130 km-400 km) in order of approximate heights above earth’s surface. During daytime, the F layer splits into separate layers called F1 (170-190 km over earth’s surface) and F2 (250- 400 km over earth’s surface). During night F1 layer usually disappears.
Region of high conductivity is confined to a relatively thin layer at the lower edge of E-region and to the upper part of D-region. Therefore high frequencies are attenuated when they penetrate this region. D and E layer disappears during night and low and medium frequency communication also becomes possible. Ionisation density increases as we go up from D layer to F layer and then decreases, due to which their respective refractive indices vary. Therefore the transmitted wave is reflected back in a way similar to the phenomenon of total internal refraction.
Type 3-Space wave propagation (Tropospherical propagation)
This mode is also known as line of sight communication. To send signals at far away stations, either repeater transmitting stations are necessary or height of the transmitter is increased by locating it in a satellite. If hT is the height of transmitting antenna and dT is the distance to the horizon from it, then dT = Root 2RhT , where R is the radius of the earth. dT is called radio horizon of transmitting antenna.
The maximum line-of-sight distance dM between the two antennas having heights hT and hR above the earth is given by
dM = Root 2RhT + Root 2RhR
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