<< Chapter < Page Chapter >> Page >
Satellites may be used to transmit information over large distances.

Global wireless communication relies on satellites. Here, ground stations transmit to orbiting satellites that amplify thesignal 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 geosynchronousorbit. Newton's equations applied to orbiting bodies predict that the time T for one orbit is related to distance from the earth's center R as

R 3 G M T 2 4 2
where G is the gravitational constant and M the earth's mass. Calculations yield R 42200 km , which corresponds to an altitude of 35700 km . This altitude greatly exceeds that of the ionosphere, requiring satellite transmitters to use frequenciesthat pass through it. Of great importance in satellite communications is the transmission delay. The time forelectromagnetic fields to propagate to a geosynchronous satellite and return is 0.24s, a significant delay.

In addition to delay, the propagation attenuation encountered in satellite communication far exceeds whatoccurs in ionospheric-mirror based communication. Calculate the attenuation incurred by radiation going to the satellite(one-way loss) with that encountered by Marconi (total going up and down). Note that the attenuation calculation in theionospheric case, assuming the ionosphere acts like a perfect mirror, is not a straightforward application of the propagation loss formula .

Transmission to the satellite, known as the uplink, encounters inverse-square law power losses. Reflecting offthe ionosphere not only encounters the same loss, but twice. Reflection is the same as transmitting exactly what arrives,which means that the total loss is the product of the uplink and downlink losses. The geosynchronous orbit lies at an altitude of 35700 km . The ionosphere begins at an altitude of about 50km. The amplitude loss in the satellite case is proportional to 2.8 -8 ; for Marconi, it was proportional to 4.4 -10 . Marconi was very lucky.

Got questions? Get instant answers now!

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Fundamentals of electrical engineering i. OpenStax CNX. Aug 06, 2008 Download for free at http://legacy.cnx.org/content/col10040/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Fundamentals of electrical engineering i' conversation and receive update notifications?

Ask