SEEs do not cause permanent damage to the device like TID does, but they do cause unwanted logic level transitions. SEE faults refer to electron/hole pairs caused by high energy particles and heavy ions striking the diffusion regions of a device. TID exposure causes a gradual degradation of the part as opposed to instantaneous failure. When this trapped charge occurs in the isolation regions between devices, it can cause leakage current that consumes excessive power and can ultimately destroy the device. When this trapped charge occurs in the gate oxide of a transistor it causes the threshold voltage to be altered, which puts the device into a state where it is either always on or always off. TID failure is caused by lower energy protons and electrons (<30 MeV/amu) striking the substrate and creating electron/hole pairs that are trapped in the insulating materials of the electronic devices. Each of these failure mechanisms are caused by ionizing radiation striking the integrated circuit substrate and depositing unwanted energy. Radiation effects are separated into two broad categories: TID (Total Ionizing Dose) and Single Event Effects. Radiation Effects Computer Electronics: Space computers must operate in a harsh radiation environment that leads to multiple types of failures. With the prevalence of computer systems in all future NASA missions, improving the capability of space computers has significant relevance and broad-scale impact across all NASA programs. Simultaneously, the NASA Strategic Plan calls for "transforming NASA's missions and the Nation's capabilities by maturing cross-cutting and innovative space technologies" (Objective 1.7), particularly those that decrease cost and thus expand opportunities for future space activities. 3) The TA11 roadmap also calls for innovative computing architectures to meet the needs of both science and engineering and emphasizes the need for scalable processing platforms that include intelligent fault-tolerant technologies to increase the robustness of computing platforms for long-duration missions. Additionally, the problem statement for the flight computing needs within the NASA TA11: Technology & Processing Roadmap is "ultra-reliable, radiation hardened platforms which, until recently, have been costly and limited in performance". Project Relevance: The NASA Earth Science Decadal Survey states the need for on-board processing and power efficiency that far exceeds existing computer systems in order to meet NASA's future science goals. This technology has been advanced to TRL-7 through a series of demonstrations and is now being prepared for an orbital mission to achieve TRL-9. When the fault is detected, the impacted region can be re-programmed in the background to flush out any errors and restore the region to its original operational state. In this approach, a redundant architecture is able to continue foreground operation in the presence of faults. MSU (Montana State University) of Bozeman, MT, has been developing a radiation tolerant computer technology for the past decade that exploits the re-programmability of an FPGA to mitigate SEEs. FPGAs (Field Programmable Gate Arrays) are emerging as a potential platform to implement novel computer architectures for space due to their inherent flexibility however, they are uniquely susceptible to SEEs due to storing their configuration in on-board SRAM. ![]() This is especially of concern when flying commercial off-the-shelf (COTS) parts that are attractive from a cost perspective but do not have any intentional radiation "hardening". ![]() Radiation effects on space computers are becoming more of a concern as feature sizes of modern transistors continue to shrink, which in turn increases their susceptibility to SEEs (Single Event Effects) caused by ionizing particles. Mitigation Approach Spacecraft Mission Concept Launch Ground Segment References RadSat (Radiation-tolerant SmallSat Computer System)
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