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.

Figure 4-10. - Typical satellite earth terminal with sixty-foot antenna.

Figure 4-11. - Forty-foot antenna and pedestal.

Figure 4-12. - Parabolic antenna cluster.

Receivers

All satellite communications earth terminals are equipped with specially designed, highly sensitive receivers. These receivers are designed to overcome down-link power losses and to permit extraction of the desired communications information from the weak received signal. The terminals currently in use have specially designed preamplifiers mounted directly behind the antennas.

Transmitters

All earth terminal transmitters generate high-power signals for transmission to the communications satellites. High-powered transmitters and highly directional, high-gain antennas are combined in this configuration. This is necessary to overcome up-link limitations and to ensure that the signals received by the satellite are strong enough to be detected by the satellite. Each transmitter has an exciter/modulator and a power amplifier. The modulator accepts the input signal from the terminal equipment and modulates an IF carrier. The exciter translates the IF signal to the up-link frequency and amplifies it to the level required by the power amplifier.

Transmitters used in earth terminals have output power capabilities that vary from 10 watts to 20 kilowatts, depending on the type used and the operational requirements.

Telemetry Equipment

Telemetry equipment is included in all communications satellite systems. This permits monitoring of the operating conditions within the satellite. Telemetry can be used also for remote control of satellite operations, such as energizing axial jets for changing the spin axis of the satellite.

Q.7 What type of antennas are generally used at earth terminals?
Q.8 Why do earth terminals require highly sensitive receivers?
Q.9 What is the range of earth terminal transmitter output power?