Space Communication:- It refers to sending, receiving and processing of information through space. There are four odes of space communication.

1. Ground wave or Surface wave propagation.
2. Space wave propagation.
3. Sky wave or Ionosphere.
4. Satellite communication.

 

1. Ground Wave Propagation:   The radio waves which travel through atmosphere following the surface of earth are known as Ground Wave  or Surface Wave  and their propagation is known as  Ground Wave Propagation or Surface Wave Propagation.

The maximum range of ground of S.W.P. depends upon

1. Frequency of radio wave
2. Power of transmitter

The G.W.P. is suitable for low and medium range frequency up to 2 MHz . If the frequency of the signal wave increases the loss of power in a ground wave propagation increases and that is why G.W.P. is useful for low frequency waves.

2. Sky Wave Propagation:- The radio waves  of frequency ranging from 1710 KHz to 40 MHz are called sky waves. They are propagated through atmosphere and are reflected back by the ionosphere layer of earth atmosphere. Since these rays travel through sky hence their propagation is known as Sky Wave Propagation. For every  layer of atmosphere there is a certain maximum frequency of electromagnetic waves (critical frequency) above which wave is not reflected back but get refracted through that layer. It is found that electromagnetic waves of frequency more than 40 MHz are not reflected back from ionosphere but gets refracted through it that is why in S.W.P. the  upper limit of radio wave  frequency is 40 MHz.
3. Space Wave Propagation:- The space waves are the radio waves of very high frequency between 4 MHz 4.2 GHz. The space waves can travel through atmosphere from transmitter antenna to receiver antenna either directly or after reflection from ground in earth tropospherically (atmospheric region).That is why S.W.P. is known as atmospheric  propagation.The  space wave propagation also known as line of sight propagation. The range of space wave propagation can be increased by using repeaters at suitable distance from each other on the surface of earth.

Note:- If the frequency of radio wave is greater than 54 MHz than these waves cannot travel along the surface of earth as they get absorbed by the earth. Hence, their propagation can’t be like ground wave propagation . Moreover, these  waves can’t e reflected by ionosphere. Hence , their propagation can’t be like sky wave propagation.