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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
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
. 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
. The ionosphere begins at an altitude of about
50km. The amplitude loss in the satellite case is
proportional to
;
for Marconi, it was proportional to
.
Marconi was
very lucky.
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Source:
OpenStax, Fundamentals of electrical engineering i. OpenStax CNX. Aug 06, 2008 Download for free at http://legacy.cnx.org/content/col10040/1.9
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