In-Orbit Radiation Effects Monitoring on the UoSat Satellites

Since the summer of 1988, radiation effects, in particular single event upset (SEU) phenomena, have been monitored in the three mass semiconductor memory systems onboard the University of Surrey's UoSAT-2 satellite, which is in a 700 km, near-polar low-Earth orbit. Almost 5000 events have now been logged by the spacecraft, and these have been assembled into a database for analysis by ground-based software. This software has allowed the SEU sensitivity of different device types to be analyzed, and the relative performance of the Mostek 4116, Texas 4416 NMOS DRAMs, the Toshiba TC5516, Harris 6564 and 6516, and the Hitachi 6264 and 6116 CMOS SRAMS, has been evaluated. The logs of in-orbit SEUs on UoSAT-2 have provided several interesting statistics, allowing direct comparison of static and dynamic RAMs ranging in capacity from 4 kilobits to 64 kilobits per chip. An unexpected result has been that none of the 288K bytes of 8-bit wide static RAM on UoSAT-2 have shown multiple-bit (per byte) upsets over the monitoring period. Also, the orbital analysis of UoSAT-2 SEUs shows an overwhelming concentration of events in the South Atlantic Anomaly for all device types. Further studies are now underway with the University of Surrey's UoSAT-3 satellite. This spacecraft carries over 4M bytes of semiconductor memory as part of its store-and-forward communications payload, together with the Cosmic Particle / Total Dose Experiments (CPE / TDE), providing direct measurements of the space radiation environment.