TT&C SubsystemThe TT&C subsystem performs several routine functions aboard thespacecraft. The telemetry, or telemetering, function could be interpretedas measurement at a distance. Specifically, it refers to the overall oper-ation of generating an electrical signal proportional to the quantitybeing measured and encoding and transmitting this to a distant station,which for the satellite is one of the earth stations. Data which are trans-mitted as telemetry signals include attitude information such as thatobtained from sun and earth sensors; environmental information suchas the magnetic field intensity and direction, the frequency of meteoriteimpact, and so on; and spacecraft information such as temperatures,power supply voltages, and stored-fuel pressure. Certain frequencieshave been designated by international agreement for satellite teleme-try transmissions. During the transfer and drift orbital phases of thesatellite launch, a special channel is used along with an omnidirectionalantenna. Once the satellite is on station, one of the normal communi-cations transponders may be used along with its directional antenna,unless some emergency arises which makes it necessary to switch backto the special channel used during the transfer orbit.Telemetry and command may be thought of as complementary func-tions. The telemetry subsystem transmits information about the satellite212 Chapter Sevento the earth station, while the command subsystem receives command sig-nals from the earth station, often in response to telemetered information.The command subsystem demodulates and, if necessary, decodes the com-mand signals and routes these to the appropriate equipment needed to exe-cute the necessary action. Thus attitude changes may be made,communication transponders switched in and out of circuits, antennasredirected, and station-keeping maneuvers carried out on command. It isclearly important to prevent unauthorized commands from being receivedand decoded, and for this reason, the command signals are often encrypted.Encrypt is derived from a Greek word kryptein, meaning to hide, and rep-resents the process of concealing the command signals in a secure code.This differs from the normal process of encoding which converts charac-ters in the command signal into a code suitable for transmission.Tracking of the satellite is accomplished by having the satellite trans-mit beacon signals which are received at the TT&C earth stations.Tracking is obviously important during the transfer and drift orbitalphases of the satellite launch. Once it is on station, the position of a geo-stationary satellite will tend to be shifted as a result of the various dis-turbing forces, as described previously. Therefore, it is necessary to beable to track the satellite’s movement and send correction signals asrequired. Tracking beacons may be transmitted in the telemetry chan-nel, or by pilot carriers at frequencies in one of the main communica-tions channels, or by special tracking antennas. Satellite range from theground station is also required from time to time. This can be determinedby measurement of the propagation delay of signals especially trans-mitted for ranging purposes.It is clear that the telemetry, tracking, and command functions arecomplex operations which require special ground facilities in additionto the TT&C subsystems aboard the satellite. Figure 7.11 shows inblock diagram form the TT&C facilities used by Canadian Telesat forits satellites explain
Question
TT&C SubsystemThe TT&C subsystem performs several routine functions aboard thespacecraft. The telemetry, or telemetering, function could be interpretedas measurement at a distance. Specifically, it refers to the overall oper-ation of generating an electrical signal proportional to the quantitybeing measured and encoding and transmitting this to a distant station,which for the satellite is one of the earth stations. Data which are trans-mitted as telemetry signals include attitude information such as thatobtained from sun and earth sensors; environmental information suchas the magnetic field intensity and direction, the frequency of meteoriteimpact, and so on; and spacecraft information such as temperatures,power supply voltages, and stored-fuel pressure. Certain frequencieshave been designated by international agreement for satellite teleme-try transmissions. During the transfer and drift orbital phases of thesatellite launch, a special channel is used along with an omnidirectionalantenna. Once the satellite is on station, one of the normal communi-cations transponders may be used along with its directional antenna,unless some emergency arises which makes it necessary to switch backto the special channel used during the transfer orbit.Telemetry and command may be thought of as complementary func-tions. The telemetry subsystem transmits information about the satellite212 Chapter Sevento the earth station, while the command subsystem receives command sig-nals from the earth station, often in response to telemetered information.The command subsystem demodulates and, if necessary, decodes the com-mand signals and routes these to the appropriate equipment needed to exe-cute the necessary action. Thus attitude changes may be made,communication transponders switched in and out of circuits, antennasredirected, and station-keeping maneuvers carried out on command. It isclearly important to prevent unauthorized commands from being receivedand decoded, and for this reason, the command signals are often encrypted.Encrypt is derived from a Greek word kryptein, meaning to hide, and rep-resents the process of concealing the command signals in a secure code.This differs from the normal process of encoding which converts charac-ters in the command signal into a code suitable for transmission.Tracking of the satellite is accomplished by having the satellite trans-mit beacon signals which are received at the TT&C earth stations.Tracking is obviously important during the transfer and drift orbitalphases of the satellite launch. Once it is on station, the position of a geo-stationary satellite will tend to be shifted as a result of the various dis-turbing forces, as described previously. Therefore, it is necessary to beable to track the satellite’s movement and send correction signals asrequired. Tracking beacons may be transmitted in the telemetry chan-nel, or by pilot carriers at frequencies in one of the main communica-tions channels, or by special tracking antennas. Satellite range from theground station is also required from time to time. This can be determinedby measurement of the propagation delay of signals especially trans-mitted for ranging purposes.It is clear that the telemetry, tracking, and command functions arecomplex operations which require special ground facilities in additionto the TT&C subsystems aboard the satellite. Figure 7.11 shows inblock diagram form the TT&C facilities used by Canadian Telesat forits satellites explain
Solution
The TT&C subsystem performs several routine functions aboard the spacecraft. The telemetry function involves generating an electrical signal proportional to the quantity being measured and transmitting it to a distant station, which is typically one of the earth stations for the satellite. Telemetry signals include attitude information, environmental information, and spacecraft information such as temperatures and power supply voltages.
During the transfer and drift orbital phases of the satellite launch, a special channel and omnidirectional antenna are used for telemetry transmissions. Once the satellite is on station, a normal communications transponder and directional antenna can be used, unless there is an emergency that requires switching back to the special channel used during the transfer orbit.
Telemetry and command are complementary functions. The telemetry subsystem transmits information about the satellite to the earth station, while the command subsystem receives command signals from the earth station, often in response to telemetered information. The command signals are demodulated, decoded if necessary, and routed to the appropriate equipment to execute the necessary action.
Tracking of the satellite is achieved by having the satellite transmit beacon signals that are received at the TT&C earth stations. Tracking is important during the transfer and drift orbital phases, as well as when the satellite is on station. Correction signals can be sent to adjust the satellite's position as needed. Tracking beacons can be transmitted in the telemetry channel, through pilot carriers in the main communications channels, or using special tracking antennas. Satellite range from the ground station can also be determined by measuring the propagation delay of signals transmitted for ranging purposes.
The telemetry, tracking, and command functions are complex operations that require special ground facilities in addition to the TT&C subsystems aboard the satellite.
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