Communication with Satellites

Global wireless communication relies on satellites. Here, ground stations transmit to orbiting satellites that amplify the signal and retransmit it back to earth. Satellites will move across the sky unless they are in geosynchronous orbits, where the time for one revolution about the equator exactly matches the earth's rotation time of one day. TV satellites would require the homeowner to continually adjust his or her antenna if the satellite weren't in geosynchronous orbit. Newton's equations applied to orbiting bodies predict that the time

for one orbit is related to distance from the earth's center

as

where

is the gravitational constant and

the earth's mass. Calculations yield

, which corresponds to an altitude of 35700 km. This altitude greatly exceeds that of the ionosphere, requiring satellite transmitters to use frequencies that pass through it. Of great importance in satellite communications is the transmission delay. The time for electromagnetic fields to propagate to a geosynchronous satellite and return is 0.24 s, a significant delay.