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Communication System

Space Communication

There are three main types of space communication.

Ground wave propagation

Ground wave communication is used for low

frequencies (500kHz to 1500kHz). This type of

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communication is used for medium wave radio

transmission, ship communication or radio navigation.

Sky wave propagation

Very long distance

For very low and high values of frequencies of em

waves, the sky waves are either absorbed or escape

from the ionosphere. Hence, following two terms are

important for sky wave communication.2

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Critical Frequency (fc)

It is the maximum value of frequency of the radio

waves, reflected back to the Earth from the ionosphere,

when directed normally to the ionosphere.

It is approximately given as, where

Nmax is the maximum electron density of the

ionosphere. Depending on the properties of

ionosphere, the critical frequency changes from 5MHz

to 10MHz.

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Space wave propagation

The electromagnetic waves

which travel directly from

transmitting antenna to

receiving antenna without

being influenced by the Earth

are called space waves. In

this propagation, the em waves move in Earth’s

troposphere, within about 15km over the surface of the

Earth. Hence, they are also known as tropospheric

waves.

Their frequency range is in between 30MHz to

300MHz. These waves travel in straight line. Hence,

the transmitting and the receiving antenna must be in

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the line of sight. But, due to the curvature of the Earth,

these waves cannot be received beyond horizon.

Hence, the effective reception

or the range of these waves is

up to line of sight only. Hence,

the communication is also

called line of sight

communication.

The figure shows curved surface of the Earth R1 B R2.

At B there is a transmitting antenna of height h (BA). R

is the radius of the Earth, i.e. distance of R1, B and R2

from centre of the Earth O. C is the midpoint of the line

joining R1 and R2.

Hence, R1 and R2 are at distance d from C. Triangles

OR1A and OCR1 are right angled triangles.

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∴ OA2 = OR12 + AR12

But, OA = R + h and

AR12 = h2 + d2

∴ (R + h)2 = R2 + h2 + d2

∴ R2 + 2hR + h2 = R2 + h2 + d2

∴ 2hR = d2

Thus, transmitting antenna is installed at the top of the

mountains to increase height h to increase the range.

For a 100m high antenna, the range is approximately

35km. When these waves are reflected from ground,

there is a phase reversal of 180o. If the direct waves

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and ground reflected waves reach the receiving

antenna in anti phase, they cancel each other.

Satellite Communication

The electromagnetic waves having frequencies beyond

30MHz are very useful in communication because of

their higher band widths. But, these waves cannot

transmitted as ground waves or space waves and also

as sky waves because they escape from the

ionosphere. Hence, satellites are used for

communication by these waves.

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The satellite receives them, amplifies them and sends them back towards the Earth, which are received by the receiving antenna.

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The process of sending the signal from the Earth to the satellite is called uplinking and receiving the signals from the satellite is called downlinking. The uplink and down link frequencies have atleast 2MHz frequency difference between them to avoid confusion. The transmitting and receiving antennae are tuned to the corresponding frequencies. As the geostationary satellite doesn’t change its position relative to the Earth, there is no Doppler’s shift in the downlink frequencies.

Global communication

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Advantages 1. long distance wide spread communication.

2. A 24 hour communication is possible in remote

and hilly areas with excellent quality

3. As the band width is high a large amount of information can be send at a faster rate.

4. It is cheaper and maintenance free as compared to cable communication.

5. It can be used in G.P.S. (Global Positioning System) to decide position of any object accurately.

Application of remote sensing10

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Remote sensing A remote sensing satellite is orbiting

in polar orbit at nearly 1000km from the surface of the

Earth.

Uses

1. In meteorology for weather forecasting, prediction

of storm, snow fall etc.

2. In collection of scientific data such as changes in

Earth’s magnetic field, gravity, ionosphere etc.

3. In geological survey of underground water, oil,

radioactive substances etc.

4. In military operations such as movement of troops,

deployment of tanks etc. and for spying.

5. Aerial survey of flood, storm, draught affected

areas. Survey of crop yields, crop diseases.11

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6. For finding fishing zones in sea, to observe

development of forest.

7. In oceanography to study ocean currents,

temperature of the ocean surface.

• Ask Your Doubts• For inquiry and registration, call 9011041155 /

9011031155.

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