Most operational military satellite earth terminals are housed in transportable vans. These can be
loaded into cargo planes and flown to remote areas. With trained crews these terminals can be put into
operation in a matter of hours. Worldwide communications can be established quickly to remote areas
nearly anywhere in the free world.
Limitations of a satellite communications system are determined by the technical characteristics of
the satellite and its orbital parameters. Active communications satellite systems are limited by two things.
Satellite transmitter power on the down links and receiver sensitivity on the up links. Some early
communications satellites have been limited by low-gain antennas.
The amount of power available in an active satellite is limited by the weight restrictions imposed on
the satellite. Early communications satellites were limited to a few hundred pounds because of launch-
vehicle payload restraints. The only feasible power source is the inefficient solar cell. (Total power
generation in the earlier satellites was less than 50 watts.) As you can see, the rf power output is severely
limited; therefore, a relatively weak signal is transmitted by the satellite on the down link. The weak
transmitted signal is often reduced by propagation losses. This results in a very weak signal being
available at the earth terminals. The level of signals received from a satellite is comparable to the
combination of external atmospheric noise and internal noise of standard receivers. Special techniques
must be used to extract the desired information from the received signal. Large, high-gain antennas and
special types of preamplifiers solve this problem but add complexity and size to the earth terminal. (The
smallest terminal in the defense communication systems network has effectively an 18-foot antenna and
weighs 19,500 pounds.) Development of more efficient power sources and relaxation of weight
restrictions have permitted improved satellite performance and increased capacity.
Powerful transmitters with highly directional antennas are used at earth stations. Even with these
large transmitters, a lot of signal loss occurs at the satellite. The satellite antenna receives only a small
amount of the transmitted signal power. A relatively weak signal is received at the satellite receiver. This
presents little problem as the strength of the signal received on the up link is not as critical as that
received on the down link. The down-link signal is critical because the signal transmitted from the
satellite is very low in power. Development of high-gain antennas and highly sensitive receivers have
helped to solve the down-link problem.
The availability of a satellite to act as a relay station between two earth terminals depends on the
locations of the earth terminals and the orbit of the satellite. All satellites, except those in a synchronous
orbit, will be in view of any given pair of earth stations only part of the time. The length of time that a
nonsynchronous satellite in a circular orbit will be in the ZONE OF MUTUAL VISIBILITY (the satellite
can be seen from both terminals) depends upon the height at which the satellite is circling. Elliptical orbits
cause the satellite zone of mutual visibility between any two earth terminals to vary from orbit to orbit.
These times of mutual visibility are predictable. Figure 4-14 illustrates the zone of mutual visibility.