The phenomenon of sending, receiving and processing information through space is called space communication.
Radio, Television and Satelite communication come under space communication. In space communication the electromagnetic waves carry information over long distances through space.
Radio wave propagation is classified as follows:
(i) Ground Wave or Surface Wave Propagation
(ii) Sky Wave Propagation
(iii) Space Wave Propagation
Ground Wave or Surface Wave Propagation:
In ground wave propagation, the radio wave travels clode to the earth surface. This wave is generally useful at low frequencies due to absorption.
But there is also ground reflected waves. It travelsfrom transmitting aerial to the earth and is reflected to the receiving aerial.
Below 500 KHz frequency, reliable communication can be obtained over distances upto 1500 KM by ground waves.
Sky Wave Propagation:
The radio waves are radiated away from the surface of the earth. This waveis reflected from ionosphere and moves back to earth. So ionosphere act as radio mirror. Transmission of sky wave is elusive and trircky business. It is subject to fading and eratic changes with seasons, night and day and atmospheric conditions. This wave is not so reliable.
Space Wave Propagation:
At frequencies above 30 MHz, radio transmission cannot be carried out by ground wave or sky waves. So we use space or direct wave transmission of radio waves having frequencies above 300 MHz.
This wave travels directly in an almost straight line from the transmitting antenna to the receiving antenna.
This wave is generally used in FM and TV signals, radar, microwave relays, air navigation etc. Such type of propagation is possible over a distance of 80-100 KM only. Cell phone and mobile phone use are much mode of propagation.