b. Digital signal. When a digital communications signal is transmitted, the beacon-identifying signal bits
can be inserted within the digital structure of the communications signal. Under this condition, a separate
beacon signal is not required and only one receiver is needed for automatic tracking and communications.
3. Command. The function of the command feature is to permit the ground stations to control the "stunts"
required of the satellite. These "stunts" include firing of the apogee rocket, firing of the gas nozzles that spin
the satellite, switch from one communications receiver or communications transmitter to the other, or switch
from one power supply to the other. The command signal is uplink only. The telemetry channel serves as the
downlink feedback function for command.
a. Issuance of commands. Commands are extremely important for a synchronous-type satellite because it
is through these commands that the satellite is "parked" in its synchronous orbit. Only one ground station is
authorized to issue commands to the satellite.
b. Command radio subsystem. The command transmitter in the ground station develops a sequentially
multiplexed RF signal for transmission on the uplink to the command receiver in the satellite. After
demodulation of the command signal, an on-board decoder decodes the sequentially multiplexed radio signal
and sends the command signals to the appropriate circuit.
4. Telemetry. Telemetry has many functions. One important function, mentioned previously, is that of
providing a command system feedback channel to display to the ground station control operator the extent of
the satellite's reaction to command. Another important function is to relay on-board conditions of the satellite
to the ground control station. An additional function of telemetry is to relay signal information to the ground
control station regarding the environmental conditions surrounding the satellite. Telemetry in communications
satellites is normally downlink.
5. Responsibilities. The responsibility of command and telemetry is given by the DoD to the Air Force, while
the responsibility for communications is given to the DCS.
LEARNING EVENT 18:
BLOCK DIAGRAM OF A SYNCHRONOUS COMMUNICATIONS
Although the block diagrams discussed in this paragraph represent one type of current synchronous satellite,
they are likely to become outdated in a short period of time. Use the diagrams only as a guide for further study
and a better understanding of future satellites. The circuits are generally divided into functions for telemetry
and command, and for communications.