Aerospace and Electronic Systems Magazine August 2016 - 17

Bhilawe et al. OBT commands are extremely useful when the spacecraft is not visible or in scenarios where direct commanding is not possible. For example, OBT commands are used for Liquid Apogee Motor (LAM) firing in case of visibility constraints. FAULT DETECTION, ISOLATION, AND RECONFIGURATION FDIR is important in this mission. The ground station will come to know any problem onboard the spacecraft after quite a large time as the distances involved are huge. Therefore, an effective mechanism is required to identify the faults, which can cause mission loss. As we have seen, the general purpose functions in MTcP software can be programmed to take care of failures in the other subsystems. But what will happen if the telecommand subsystem itself has a problem is addressed in this section. Figure 8. Differential TT command execution. The two level control is an extremely useful feature in design and planning of autonomy. For example, in case of Travelling Wave Tube Amplifier (TWTA) autonomy, if the TWTA needs to be switched on at a particular OBT but only when the battery voltage is above a particular value, then the OBT event can enable a Macro TT and an EBC (checking for battery voltage) can initiate it. Also, the macros are useful for payload operations. The Mars color camera has been operated by OBT commands triggering macros to take pictures of the Mars. On Board Time Tagged Command Execution The OBT tagged commands also belong to the class of time tagged commands. The onboard timer reference is from telemetry system, which is running continuously from system power on till end of the mission. The OBT commands are executed when the time running onboard matches with the OBT associated with the command. The uplinked commands are sorted according to the OBT associated with them. The OBT logic compares the OBT of a command having the least OBT stamp with the onboard running OBT. The OBT command is executed when the on board running OBT matches with the command OBT. The OBT software block diagram is shown in Figure 9. The uplinked commands are first validated by OBT validation logic. The validated commands are then processed by OBT sorting logic. OBT processing logic computes OBT difference between main and redundant TM OBTs to take care of telemetry auto-changeover. The computed difference is used to correct OBT after TM auto-changeover. The software also has the features like OBT command deletion and OBT stack reset. The OBT stack reset deletes all the OBT commands on board. AUGUST 2016 Telemetry Auto-Changeover Both main and redundant telemetry are available onboard. The MTcP works on selected telemetry; if the selected telemetry onboard goes into a problem state, then telemetry auto-changeover happens. For example, the parameters for TM auto-changeover are frame pulse failure, OBT errors, calibration voltage errors, TM frame cyclic redundancy check errors, etc. As both main and redundant telemetry are asynchronous, features like PATC, EBC, and OBT dependent on TM data are designed to function properly after TM auto-changeover. For example, in the case of OBT, generally the ground station selects one of the telemetry (let's say TM1) and uplinks all the OBT commands with respect to the OBT in the selected telemetry. If the selected telemetry goes into problem, then telemetry autochangeover happens. As the TM1 OBT reference is lost, the MTcP derives the TM1 OBT from TM2 OBT after TM auto-changeover as the uplinked commands have TM1 OBT tagged with them. Therefore, the OBT modules work even after the TM auto-changeover happens. MTcP Auto-Changeover The MTcP auto-changeover makes the TC subsystem robust. If there is a problem in the selected TC subsystem, then the autochangeover happens to the non-selected/redundant subsystem and MTcP operation continues. So the system can withstand a TC subsystem failure until the ground station intervenes. The basic idea used for MTcP auto-changeover is as follows. The commands are uplinked to both the processors simultaneously IEEE A&E SYSTEMS MAGAZINE 17

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine August 2016