Observation and prediction of SEU in Hitachi SRAMs in low altitude polar orbits

In-orbit SEU (single event upset) data from three microsatellites are separated into galactic cosmic ray (GCR), South Atlantic anomaly (SAA) and solar flare upsets. Heavy ion and proton testing of the same devices is reported, and predictions using LET (linear energy transfer)-dependent ion cross sections and a two-parameter fit to proton cross section data are compared with in-flight data. SEU trends in memory devices from a single manufacturer, from 16 kb to 4 Mb, are identified. The increasing density of the memories is found to be accompanied by an apparent decreasing sensitivity to the heavy ion environment in polar orbit. Although the LET threshold decreases, the decrease in asymptotic cross section has a greater effect. However, the sensitivity to proton-induced upsets is nearly similar in all memories, so the proton effects become dominant by an order of magnitude. This has important implications for future spacecraft with large onboard memories and high performance processors