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Design of Telecommand Software for the Mars Orbiter Mission Figure 7. Telemetry EBC logic block diagram. commands are differential time tagged commands. Both event and action are programmable. For example, if one of the star sensors in the loop goes bad, then the event is generated by AOCE to utilize the redundant star sensor in the loop. Therefore, if MTcP gets this particular event, then it executes the commands to bring the redundant star sensor into operation. Programmable Automatic Thermal Control Thermal management plays an important role in proper functioning of the subsystems. The subsystems generally have a requirement to maintain the temperature within certain limits. The software continuously monitors the temperature for all of the subsystems onboard the spacecraft and operates the heaters to maintain spacecraft temperature within certain temperature limits. The temperature limits and sensor information are programmable. The saturation check logic is also incorporated to take care of the any sensor failure. The programmable automatic thermal control (PATC) logic has an enable/disable feature. Also, each heater has an enable/disable feature. Snap Logic The snap signal gets generated when the spacecraft gets separated from the launch vehicle. After the snap signal is detected, some predefined operations have to be done. These operations are loaded in the TT stack at the launch pad. The trigger to these commands is through the snap signal. The snap logic handles the snap signal and takes the proper action after the spacecraft is separated from the launch vehicle. For example, a snap sequence consists of actions like solar panel deployment, thruster selection, etc. When the spacecraft gets separated from launch vehicle, the snap sequence is loaded as TT commands get activated. Macro TIME-TAGGED COMMANDING Differential Time Tagged Command Execution The differential TT commands belong to the class of time tagged commands. The TT commands are useful when a particular sequence of action and delay is required between the commands. 16 Each TT command has a differential delay associated with it. Both command and delay are programmable from the ground station. The TT command execution is shown in Figure 8. When the TT execution start is given, a certain number (N) of commands are executed one by one according to differential delays associated with them. The TT execution start consists of start command number and number of commands to be executed. Macro is a block of differential TT commands which can run in parallel independently. The commands and delays are programmable. The Macro TT is slightly different from the conventional differential TT as it has two level controls. The Macro TT becomes active when it is enabled and initiated. IEEE A&E SYSTEMS MAGAZINE AUGUST 2016

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