Sensing the need for a reliable and robust system for autonomous survival of SWAYAM in space , Onboard computer(OC) system is entrusted with the electronic control of the picosatellite. Over the course of mission, OC collects crucial data about the health of the various sytems within the satellite and stores it in a non-volatile memory, only to be sent to the ground station on request. The major role of OC relies in handling the set of commands received from the ground stations through the communication system. OC is also responsible for prompt action in case of occurence of fault in any of the power lines. All these functions are handled by the ARM7 microcontroller(uC) from ATMEL(AT91SAM7X).
The power and space constraints in the satellite deem the OC system to be small, light weight and power efficient. Also, the radiation prevalent in the space may induce bit flips in the data present. Hence, the OC system has to detect and correct the radiation induced upsets.
The Hardware systems in OC include:
1) A central micro-controller(ARM7), with on-chip program and data memory. The software resides in these memories.
2) An external persistent memory(microSD card) for payload and satellite health data storage.
3) An external Watchdog timer for fault recovery.
4) A battery backed real time clock(RTC) to maintain the mission time.
5) An I2C interface which interfaces the sensors(temperature, voltage,current etc.) with the central uC.
The Software for OC is written in ARM assembly and in C.They include:
1) The modules that form the lowest-level system software – the boot-up code and the exception handlers.
2) Device and interface drivers.
3) File system for messages and the satellite health data.
4) The modules that execute OC’s functions which are actually present as separate subroutines which are executed only on an interrupt to the uC either from an internal or an external peripheral or fault line.
OILS (OnBoard Computer In-Loop Simulation)
OC also develops OILS, which includes systems for in-house testing of the functional components. OILS is a simulation system where the hardware is present as it is, but the input to the hardware and its environment is simulated. The OILS system is developed to validate the precise operation of the onboard computer in space.
The picosatellite that we develop is a space experiment on passive stabilization. The OC’s interface with the Gyroscope helps determine whether passive ACS is established. And OC is responsible for conveying the information about the same to the ground station. It is also responsible for initiating antenna deployment after the establishment of passive stabilization.It also initiates the kill command when requested for by the ground station or after the expiry of the mission period.
The performance of OC will be crucial in determining the performance of the satellite in space, and also tracking it from the earth’s ground station.