Abstract:
More than 21000 objects fly in outer space and are exposed to the harsh space environment. The size of space objects considerably varies. Our research focuses on nanosatellites, such as CubeSats, which have to respect time, spatial and energy constraints. To tackle this issue, this paper presents and evaluates two fault tolerant online scheduling algorithms: the algorithm scheduling all tasks as aperiodic (called ONEOFF) and the algorithm placing arriving tasks as aperiodic or periodic tasks (called ONEOFF&CYCLIC). Based on several scenarios, we analyse how much the performance (in terms of both the rejection rate and the scheduling time) of ordering policies are influenced by the system load and the proportions of simple and double tasks to all tasks to be executed. The “Earliest Deadline” and “Earliest Arrival Time” ordering policies for ONEOFF or the “Minimum Slack” ordering policy for ONEOFF&CYCLIC reject the least tasks in all tested scenarios. The paper also deals with the analysis of scheduling time to evaluate real-time performance of ordering policies and shows that ONEOFF requires less time to find a new schedule than ONEOFF&CYCLIC. Finally, it was found that the studied algorithms perform well also in a harsh environment and provide the same availability as systems based on triple modular redundancy with very much less system power consumption.
