4-9
Solar cells are installed around the outside surface of a spin-stabilized satellite. This gives you a
large number of solar cells exposed to the sun at all times (except when the satellite is in eclipse). The use
of omnidirectional antennas causes a small part of the total radiated energy to be directed toward the earth
at all times.
Ominidirectional antennas radiate only a small amount of energy toward the earth. Many techniques
have been tried to achieve an earth-oriented antenna system. One system developed uses spin stabilization
for orientation of the satellite. It uses a stationary inner platform for mounting remote controlled antennas.
The satellite is constructed in two parts with both parts having radial jets. The inner platform contains the
communications antennas and the communications package. After the satellite is stabilized in space, inner
radial jets spin the inner platform. The inner platform is stationary with respect to earth and is oriented to
such a position that the communications antennas point continuously toward the earth. This arrangement
allows the use of high-gain directional antennas that concentrate the majority of the radiated energy in the
direction of the earth.
The latest versions of communications satellites use a stationary platform with four high-gain
antennas. Two steerable narrow beam antennas are used for communications between and within regions
of high traffic density. Two horn antennas provide for earth communications between facilities outside
the narrow beam coverage. The antenna arrangement for these types of communications satellites is
shown in figure 4-7.
Q5. What was the major operational limitation of early communications satellites?
Q6. Satellite orientation in space is important for what two reasons?
EARTH TERMINAL CHARACTERISTICS
Communications satellite earth terminals are usually located in areas remote from the actual users of
these communications. This is necessary to minimize rf interference to the satellite.
Locating the terminals in these remote locations requires interconnecting communications links.
Links permit communications flow to and from the users of the satellite systems. Interconnect links are
usually connected via telephone cables or microwave radio links with normal terminal equipment.
Earth satellite communications terminals generally have a single, large antenna; a highly sensitive
receiver; a powerful transmitter; multiplex equipment; modulating-demodulating equipment; and
telemetry equipment. Each of these elements will be discussed later in this chapter.
Antennas
Earth terminal antennas are highly directional, high-gain antennas capable of transmitting and
receiving signals simultaneously. Generally, large, high-gain, parabolic antennas are used.
Generally speaking, three sizes of parabolic-type antennas are currently in use at earth terminal sites.
These are a parabolic antenna sixty feet in diameter, a parabolic antenna forty feet in diameter, and a
cluster of four parabolic antennas, each ten feet in diameter. These four in combination are equal to a
parabolic antenna eighteen feet in diameter. They are shown in figures 4-10, 4-11, and 4-12, respectively.
