Unit – II

Earth’s Atmosphere:

Earth is surrounded by an envelope of gases called the atmosphere. The atmosphere plays an important role in the transmission of electromagnetic waves. The earth’s atmosphere is broadly divided into four different layers.

Troposphere:
The layer of the atmosphere extending up to a height of 12 km from the surface of the earth is called the troposphere. This layer mostly contains water vapor which leads to the formation of clouds. The local weather changes in the earth’s atmosphere occur in this layer.

Stratosphere:
The region of the earth’s atmosphere lying between 12 km to 50 km is called the stratosphere. The ozone layer is part of the stratosphere extending from 15 km to 30 km; which absorbs the harmful ultraviolet radiations from the sun.

Mesosphere:
The region of the earth’s atmosphere lying between 50 km to 80 km is called the mesosphere.



Ionosphere:
The outermost layer of the earth’s atmosphere is called the ionosphere extending from 80 km to 400 km. The Ionosphere contains charged particles and plays an important role in space communication.

Space Communication:

1. The process of sending, receiving and processing of information through space without any special communication channel is called space communication.
2. The information to be transmitted is superimposed on a high-frequency electromagnetic wave called the carrier wave.
3. After superimposition, the resultant wave called the modulated wave is transmitted from one place to another by using an antenna.

Ground, Sky and Space Wave Propagation:

The electromagnetic waves emitted by a transmitting antenna can reach receiver antenna by the following three modes:
i. Ground wave propagation
ii. Sky wave propagation
iii. Space wave propagation

Ground Wave Propagation:



1. The electromagnetic waves which travel along the surface of the earth are called ground waves or surface waves and this type propagation is called ground wave propagation or surface wave propagation.
2. When a ground wave propagates over the surface of the earth, eddy currents are induced in the surface of the earth which causes attenuation of these waves. Moreover, as they travel over the earth’s surface they bend along the curvature of the earth which results in energy losses. Thus, ground wave propagation is restricted to small distances.
3. The ground wave transmission becomes weaker as the frequency of the electromagnetic wave increases hence this mode of transmission is restricted to frequencies below 1500 KHz.

A Sky Wave Propagation:

1. The electromagnetic waves which are received after being reflected from the ionosphere are called sky waves and this type propagation is called Skywave Propagation.

Wave Propagation

2. The sky wave propagation occurs due to the total internal reflection of the electromagnetic waves by the ionosphere. The ionosphere consists of free positive and negative ions produced due to the ionization of atoms and molecules present in the atmosphere. The charged density of the ionosphere increases with height which results in the decrease in its refractive index
3. electromagnets waves having frequencies less than 2 MHz are absorbed by the ionosphere whereas those having frequencies greater than 30 MHz pass through it.
4. Electromagnetic waves having frequencies greater than 2 MHZ and less than 30 MHz are total internally reflected by the ionosphere and can be received by an antenna at a large distance.



Space Wave Propagation:

1. The electromagnetic waves which travel directly from the transmitting antenna to the receiving antenna are called space waves and this type of propagation is called space wave propagation.

2. High-frequency electromagnetic waves cannot be transmitted as ground waves due to high energy losses. Moreover, these waves are absorbed by the ionosphere hence they cannot be transmitted via sky wave propagation.
3. Therefore, such high-frequency electromagnetic waves are directly transmitted throng Earth atmosphere using a transmitting antenna As these waves travel in a straight line, the receiving antenna must be in line of sight of the transmitting antenna.

Satellite Communication:

1. Radio Communication uses ground wave, sky wave, and space wave propagation. Ground wave and space wave communication are restricted to small distances, whereas sky
wave propagation uses frequencies ranging from 3 MHz to 30 MHz.
2. Very high frequency and microwave communication over long distances use a Satellite. a Satellite communication is useful in sending a large amount of information in a small time over a large distance.



Wave Propagation

Principle of satellite communication:

1. A communication satellite carrying microwave transmitting and receiving equipment called radio transponders is placed in the geostationary orbit of the earth.
2. A beam of modulated microwaves carrying the Signal is directed towards the satellite. This is known as uplinking. The satellite receives the signal; amplifies and redirects it to a receiving stations on earth by using a different carrier wave. This is I known as downlinking.

Global Communication:

1. A single communication satellite covers about one-third of the earth’s surface. Therefore in order to achieve communication link over the entire globe, we need a minimum of three
communication satellites which are 120Oapart.
2. Microwave signals are transmitted from one satellite to another with each satellite covering one-third of the globe. Thus, interlinking between these satellites cover the entire earth’s surface.

Different Orbits of a Satellite:

communication



1. Equatorial Orbit:
When a satellite move in the equatorial plane (i.e. a plane passing through the equator and perpendicular to the line joining the poles) it is said to rotate in the equatorial orbit. A1l
gee-stationary satellites revolve (circular) in the equatorial orbit.
2.Circular Polar Orbit:
Polar regions are covered by launching a satellite in a circular polar orbit (i.e. plane perpendicular to the equatorial plane). Satellites in this orbit revolve from the North
Pole to the South Pole and are very close to the earth’s surface (about 1000km).
3.Highly Elliptical Inclined Orbit:
All orbits around the earth other than the equatorial or polar Orbit are called inclined orbits. High latitude regions are covered by launching a satellite in a highly elliptical orbit having a high angle of inclination (about 63º)

Uses of a satellite:

1. communication satellites are used for transmitting television signals over long distances.
2. communication satellites are widely used in the field of telecommunication.
3. Communication satellites are used to photograph a particular region of the earth’s surface. These photographs give valuable information regarding impending storms, cyclones, tidal waves etc and thus help in weather forecasting.
4. Satellites are used for the purpose of military surveillance and observation of enemy territory.
5. Satellites serve as repeaters or relay stations for long distance communication.
6. The satellites can be used for monitoring the position of various natural resources such as forests, rivers, mineral deposits.
7. satellite communication is used in guiding and navigation of aircraft, ships, missiles etc

Remote Sensing:

1. The process of acquiring information about a distant object without being in physical contact is called remote sensing.
2. Aerial photography is a kind of remote Sensing. In order to cover large surface areas, aerial photographs of the target object taken from a distance. For this purposed a satellite carrying appropriate sensors and cameras is placed in the earth’s polar orbit at a height of 1000 km above its surface.
3. During its orbital motion, the satellite scans a particular region of the earth and takes pictures.
4. The data collected is transmitted back to a receiving station on earth where it is analyzed and interpreted.



Applications of Remote Sensing:

1. Remote sensing satellites are used to monitor changes in local weather conditions.
2. Aerial photography is used for military surveillance.
3. Infra-red sensors on board a satellite are useful in measuring temperature differences over a particular region.
4. Remote sensing techniques are used in geological and archeological surveys, water resource surveys, urban land use, agriculture, forestry etc.
5. Air pollution, forest fires, oil well fires etc. can be accurately detected by using remote sensing satellites.
6. Remote sensing techniques are used to locate potential fishing zones.
7. Study of the sea surface, the location of icebergs, monitoring of oil pollution etc. can be carried out by using remote sensing satellites.
8. Remote sensing techniques are used to locate missing air crafts, ships etc.
9. Remote sensing satellites are used to locate deposits of natural resources such as ores, minerals, oil and natural gas etc.

Line Communication:

1. In space communication, there is no physical contact between the transmitting and the receiving antenna. It is basically an unguided communication where the transmitted signal spreads in all directions. This results in attenuation of the signal. In order to avoid this, we use line communication.
2. In line communication, the signal is transmitted from the transmitter to the receiver through a connecting wire. This communication system requires solid medium as the communication
channel between the transmitter and the receiver this medium is called a transmission line.
3. There are three types of transmission lines:
a) Two-wire transmission line,
b) Co-axial cable
c) Optical fiber

4. Two-wire transmission line:
In this case, the electrical signal is passed through a pair of conducting wires insulated from one another. The most commonly used two wire transmission lines are:

It consists of two metallic wires arranged parallel to each other inside an insulating coating. The metal wires are hard or flexible depending upon the power transmitted. Hard wires are used for high power transmission. In this transmission line, it is necessary to match the impedance of the wire with that of the receiver to obtain maximum transfer of power.
However, the losses increase with the increase in the length of the wire and the frequency of the transmitted signal. Thus, parallel wire lines are used to send low-frequency electrical signals over small distances. Parallel wires are commonly used to connect the antenna with a T.V. receiver.



It consists of two insulated copper wires twisted around each other at regular intervals of distance to minimize electrical interference.
They are used to transmit both digital and analog signals. Twisted pair of lines are inexpensive and easy to install. They are commonly used to connect telephone systems.

5. Coaxial Cable:

It consists of a Central copper wire surrounded by a PVC installation and then is covered by a copper wire mesh. The copper wire mesh is covered by an outer shield of thick PVC material. The signal is transmitted through the central copper wire while the outer conductor is connected to the ground.
Communication through a co-axial line is more efficient than two wire lines as the attenuation of the signal is low. However, they are expensive as compared to two wire lines.
Co-axial Cables are used to transmit microwave and ultra high-frequency signals.



Optical Communication:

1. The ability of a wave to transmit information depends upon its frequency. Higher the frequency of the carrier wave, greater is the amount of information transmitted in a given time.
2. Radio communication systems use electromagnetic waves of frequencies about 106Hz, whereas satellite communication systems use microwaves of frequency 1011
Hz. The frequency of light waves ranges between 1012Hz to 1016Hz which are very large as compared to that of radio and microwaves. Hence, light waves are better
substitutes for communication of large information in a short interval of time.
3. The mode of communication which uses light waves for transmission of information from one place to another is called optical communication.

Advantages of optical communication over the conventional communication system.

1. Optical communication systems have greater information carrying capacity due to greater bandwidth.
2. In the case of optical fiber communication, the transmission losses are very small as compared to electrical cables.
3. Optical fiber cables are of small size and of light weight as compared to electrical cables.
4 Optical fiber communication provides a high degree of signal security as it cannot be tapped easily.
5. Optical fibre communication is free from electromagnetic interference.
6. Fibre optic cables do not carry high voltages or current. Hence, they are safer than electrical cables.

Basic Optical Communication System:

An optical communication system consists of
three main blocks
a) Optical source
b) Optical fiber
c) Optical signal detector
1.Optical source:
A Light Emitting Diode (LED) or a semiconductor laser diode is used as an optical source. The analog signal to be transmitted is first converted into digital signal pulses by using an encoder circuit. The digital signal pulses then drives the optical source to modulate light waves.
2.Optical fiber:
The modulated light waves are transmitted from one place to another by using an optical fiber.
3.Optical signal detector:
The optical signal detector converts light signals into electrical signals by using a photocell or a photodiode. A decoder circuit then converts the digital signal back into its analog form which is then processed.

Optical Fibre:

1. The optical fiber is a device which works on the principle of total internal reflection by which light signals can be transmitted from one place to another with a negligible loss of energy.



Construction:

1. It consists of a very thin fiber of silica or glass of high refractive index called the core.
The core has a diameter of 10 mm to 100 mm. The core is enclosed by a cover of glass or plastic called cladding.
2. The refractive index of the cladding is less than that of the core. The core and the cladding are enclosed in an outer protective jacket made of plastic to provide strength to the optical fiber.

Working:

1. When a ray of light is incident on the core of the optical fiber at a small angle, it suffers refraction and strikes the core-cladding interface, As the diameter of the fiber is very small this angle of incidence is greater than the critical angle.
2. Therefore, the ray suffers total internal reflection at the core-cladding interface and strikes the opposite interface. At this interface also, the angle of incidence is greater than the critical
angle, so it again suffers total internal reflection.
Thus, the ray of light reaches the other end of the fiber after suffering repeated total internal reflections along the length of the fiber.
3. At the other end, the ray suffers refraction and emerges out of the optical fiber.

Principle of fabricating Optical Fibre:

Fabrication of optical fiber involves the following two steps:
1. A glass rod having a definite refractive index is constantly heated by rotating it on the flame of a burner. Silicon tetrachloride vapors are burnt in the same flame so that an oxidized layer of silicon-di-oxide is uniformly deposited on the outer surface of the glass rod. The glass rod is then gradually cooled from 1400OC to room temperature to form a preformed glass rod having different inner and outer glass compositions.
2. The performed glass rod is then heated in a fiber drawing furnace. The end of the rod is pulled at a constant rate to form a thin fiber containing the core and the cladding. This fiber
is then covered with a protective plastic sheath to obtain a fine optical fiber. A bunch of such optical fibers form optical fiber cable.

Uses of Laser Communication:

1. In an optical communication system, the transmission link between transmitter and receiver is established by an optical fiber. The transmitter consists of a light whose waves are modulated with the signal and then sent through an optical fiber.
2. A high degree of coherence, monochromaticity, and directionality make laser light ideal choice for establishing such communication links.
3. Laser communication links are used in the field of computer networking, telecommunication, medical diagnosis, defense applications etc.

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