This is an especially important consideration of communications satellites because of their size and weight
limitations. If several ground systems are required to share a satellite spectrum, the power is distributed in each
of the several signals. The more ground systems that try to use the satellite at the same time, the less power will
be transmitted from the common satellite for each system. Consequently, there is a further deterioration of the
S/N. Also, overloading by any one system may cause crosstalk between the several systems. It is obvious that
the technique of system operation becomes a controlling factor in the S/N.
LEARNING EVENT 11: COMMUNICATIONS VIA SATELLITES
1. Communications satellite systems offer the advantage of global communications in the microwave
spectrum. These radio relay systems will apply the wideband operating characteristics of microwave equipment
to long-distance radio circuits. The practical result of this combination will be large increases in the amount of
traffic the long-distance circuits can handle, as well as greater reliability due to LOS transmission and reception.
2. Several types of communications satellite systems are being studied. One system would utilize huge
aluminized plastic spheres of 100-foot diameter or greater. This type of satellite communications system is
called passive because the messages are sent through radio waves bounced from the reflecting surfaces to the
receiving antennas. One advantage of the passive satellite is the freedom from complicated instrumentation in
the satellite package. However, the disadvantage appears in the complexity of ground tracking equipment.
3. Another system would employ active-repeater satellites. Such satellites are equipped with radio receivers
and transmitters and other apparatus to receive, store, amplify, and retransmit messages. They serve, in effect,
as microwave repeaters in the sky.
LEARNING EVENT 12: PRINCIPLES OF SATELLITES
1. A satellite is an attendant body that revolves about another body, usually with reference to the solar system.
An example of a celestial satellite is the moon, which revolves about the Earth. An artificial satellite is simply a
manmade moon. In revolving about the Earth, it must obey the same laws the natural moon and planets obey in
revolving about the sun.
2. Any satellite, whether natural or artificial, remains in orbit through the action of two forces-the satellite's
inertial force and the force of gravity.
3. If a satellite is to be placed in orbit, the vehicle must be accelerated to orbital velocity. The force imparted
to the satellite counteracts the gravitational attraction of the Earth. Since this gravitational attraction decreases
with the distance from the center of gravity of the Earth, a different orbital velocity is required for each distance
from the Earth. In every case, the satellite must attain a velocity that will balance the attraction
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