n 1945 Arthur C. Clarke proposed that a man-made Earth SATELLITE could be used for communication by radio microwaves between distant
locations on Earth. The satellite would be positioned in space at an
altitude of about 35 790 km so that its speed of revolution around the
Earth would be the same as the speed of the Earth's rotation. This
synchronous satellite would always appear in the same place in the sky.
It would be in geostationary orbit.
Characteristics
A
typical satellite consists of a number of repeaters (transponders),
each of which provides a large-capacity communication channel. Each
transponder has a receiver tuned to a frequency range that has been
allocated for uplink communication signals from Earth to the satellite.
Following the receiver, each transponder consists of a frequency shifter
to lower the received signals to a downlink frequency, a filter tuned
to the frequency of the transponder and a power amplifier to transmit
signals back to Earth.
The communication capacity of a satellite is
determined by the number of transponder channels and the volume of
communication that can be transmitted on each channel. Although this
varies from one type of satellite to another, the most commonly used
satellite in 1995 had 24 transponders. Each can carry a colour TV signal
(or 6 digitally compressed TV signals) or at least 1200 telephone voice
signals in one direction. Each new generation of satellites tends to
have increased communication capability.
The transmitting and
receiving stations on Earth (earth stations) range in size from
sophisticated, expensive stations that send and receive all types of
communication signals to relatively simple and less costly stations
(dish-shaped TV antennas) used only to receive television signals. The
size and cost of Earth stations depend upon the power built into the
satellite, as well as the frequencies used. The stronger the signal from
the satellite, the smaller and less costly the receiving station.
Direct Broadcast Satellites are designed specifically to minimize size
and cost.
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