An artificial satellite revolving in a circular orbit round the earth in the equatorial plane, in the same sense of the rotation of the earth and having same period of revolution as the period of rotation of the earth (i.e. 1 days = 24 hours = 86400 seconds) is called geo-stationary satellite.
As relative velocity of the satellite with respective to the earth is zero, it the satellite stationary from the earth's surface. Therefore it is known as geo-stationary satellite or geo-synchronous purposes so it is called communication satellite.
Another class of satellites are called are called the polar satellites. These are at low altitude (500 km to 800 km) satellites. Since its time period is 100 minutes it crosses any latitude many times a day.
Uses of Satellites
i) For the transmission of Television and radiowave signals over large areas of the earth's surface.
ii) For broadcasting telecommunication.
iii) For military purposes.
iv) For weather forecasting and meteorological purposes.
v) For astronomical observations.
vi) For study of solar and cosmic radiations.
vii) To relay distress signals from ships.
viii) To transmit cyclone warnings to coastal villages.
ix) For Geopositions system (GPS).