AIAA - American Institute of Aeronautics and Astronautics
Scope
"Introduction Increasing concerns over trapped radiation effects on microelectronics@ coupled with the availability of new data@ long-term changes in the Earth's magnetic field@ and variations in the trapped radiation fluxes@ have generated the need for better@ more comprehensive tools for modeling the Earth's trapped radiation environment and its effects on space systems. The objectives of this guide are to describe the current status of those efforts and to review methods for attacking the issues associated with modeling the trapped radiation environment in a systematic@ practical fashion. The ultimate goal will be to point the way to increasingly better methods of testing@ designing@ and flying reliable spacecraft systems in the Earth's radiation environment. To set the stage for these discussions@ a review of the key concepts associated with modeling the radiation environment and its effects will be presented first. The review will include a description of the principal models of the trapped radiation environment currently available. Recent results from radiation experiments on spacecraft such as CRRES@ SAMPEX@ and Clementine will then be described. The report will close with a detailed discussion of the current status of the modeling of the radiation environment and recommend a long-range plan for enhancing capabilities in this important environmental area. Because of the increasing sophistication and high level of physical integration of electronics and electronic components@ radiation effects have taken on a new significance in spacecraft design. For example@ the rapid drop in power and voltage levels and the associated drop in feature size for integrated components (ICs) have greatly enhanced the ICs' sensitivities to single event effects (SEES). The push toward commercial off-the-shelf parts has often led to parts that are much less radiation tolerant although this is not always true. Overall@ the requirements for ""cheaper@ better@ faster"" spacecraft have acerbated this trend toward parts that are increasingly more radiation sensitive. The result is that@ far from going away with time@ radiation effects??both total ionizing dose and single event effects??are increasingly coming to dominate the design concerns for satellite manufacturers across the board. Add to this the desire of many new multisatellite communications providers to place their constellations in the middle of the harshest part of the radiation belts@ and accurate modeling of the trapped radiation environment and its effects becomes a very real@ long-term problem for the spacecraft community in general. Solving the problems of trapped radiation effects on spacecraft is not as simple as just developing better models or more shielding. Although these are solutions in many situations@ in general@ most commercial spacecraft designers cannot afford either the large uncertainties in the current models or the extra mass necessary to cover the required design margins. Rather@ proper design of radiation resistant systems requires complex trade-offs among parts@ shielding@ software@ operations@ redundancy@ and orbit configuration. Each of these ""solutions"" is subject to uncertainty and has a cost impact on the final design. Unfortunately@ the key component@ the trapped radiation environment itself@ is not well defined (observations and predictions that vary by factors of two for 5-11 year missions in Earth orbit are considered to be in excellent agreement; for shorter missions@ factors approaching 10-100 are easily possible). Even given an accurate ""average"" description of the environment@ short-term variations of several orders of magnitude in dosage and single event upset (SEU) rates have been seen in the span of hours (e.g.@ the 1989 solar proton events). Complicating the practical application of the radiation environment to spacecraft design@ radiation transport codes and estimates of the effects of radiation damage are often inaccurate. Comparisons between ground tests and in situ measurements have shown significant disagreement. Furthermore@ the parts used on the spacecraft can show variations in sensitivity of factors of 2-10@ even within the same parts lot. Often@ how a system is actually used can mask@ or hopefully limit@ the effects of radiation damage. Thus@ to a degree@ mitigating radiation effects is a black art and@ increasingly@ a very expensive art for which any imprecision in the knowledge of the trapped radiation environment becomes a critical component. However@ the ultimate solution is a comprehensive process that treats all uncertainties. "