Software controlled memory scrubbing for the Van Allen Probes Solid State Recorder (SSR) memory

Abstract

The Van Allen Probes mission which was designed and built by the Johns Hopkins University, Applied Physics Laboratory (APL) is also being operated by the APL mission operations team in Laurel, Maryland. The two Van Allen Probes spacecraft have been successfully collecting data on orbit since they were launched on August 30, 2012. These twin probes are providing unprecedented insight into the physical dynamics of the Earth's radiation belts and are giving scientists the data they need to make predictions of changes in this critical region of space, by sampling the harsh radiation belt environment where major space weather activity occurs and many spacecraft operate.[1] Shortly after launch, radiation induced anomalies were reported on both spacecraft and investigated by the hardware and software engineering teams. The most significant of these anomalies was associated with single-event effects experienced by the spacecraft Solid State Recorder (SSR) memory. The radiation effects resulted in correctable memory errors but with a secondary effect of data corruption within the SSR. The investigation into these anomalies ultimately identified a subtle hardware design defect, which was reproducible in a controlled ground test environment. This paper details the nature of the memory upsets and hardware mitigation interactions, and the effect these had on the spacecraft science and housekeeping data. It also describes the subsequent investigation and testing to uncover the hardware shortcomings and the implementation and upload of modified flight software to the operational spacecraft. Lastly this paper will provide on-orbit performance data for the SSR since the modified software was made active. Altogether, this will provide insight into not only the challenges of designing for the harsh radiation environment, but also into the need for flexibility when designing systems.