d. Low- and medium-altitude orbits minimize the possibility of physical destruction of the satellite by a
potential enemy.
e. Satellites pass through the shadow of the Earth, and must therefore have temperature control circuits and
storage batteries to keep the on-board communications equipment operating properly.
LEARNING EVENT 17:
RADIO LINKS
BETWEEN GROUND TERMINALS
AND A
SYNCHRONOUS SATELLITE
Two ground terminals (A and B) and a synchronous-type communications satellite are illustrated in Figure 1-9.
Five major frequencies serve four basic subsystems aboard the satellite-communication, beacon, command, and
telemetry.
1. Communication. Two directions of transmission (A to B and B to A) are used to achieve full-duplex
operation. Each direction of transmission uses a different set of two discrete frequencies, making a required
total of four frequencies to achieve full-duplex operation. This results from the fact that a different frequency is
required for the uplink and downlink in each direction of transmission.
a. A to B. The communications radio receiver in the satellite receives the uplink signal (f1a) from ground
terminal A and converts it to a second frequency (f1b). The communications radio transmitter on board raises
the power level of f1b and relays it over the downlink to ground terminal B. A second frequency is needed
because if the received signal were relayed without conversion, the transmitter signal would block the receiver.
b. B to A. The communications radio receiver in the satellite receives the uplink signal (f2a) from ground
terminal B and converts it to a second frequency (f2b). The communications radio transmitter on board
amplifies the converted signal and sends it to terminal A over the downlink. Two discrete frequencies are also
needed for this direction of transmission to avoid receiver blocking.
2. Beacon. The same downlink beacon signals are used by both ground terminals to give the antenna tracking
systems a target. The beacon signal may be generated by a separate radio transmitter or the tracking equipment
in the ground terminals may use the received communications signal as its source of target information,
depending on the type of communications signal used.
a. Beacon transmitter. A beacon radio transmitter is needed when the communications signal is analog
(continuous signal). The beacon transmitter in this case must operate on its distinct RF (f5). It normally
operates at a lower frequency than that of the communications channel. This arrangement normally requires the
use of two receivers--one for tracking and one for communications.
SS0031
1-32
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