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AIAA G-083 Document Information:
Title
Guide to Modeling Earth´s Trapped Radiation Environment
American Institute of Aeronautics and Astronautics
Publication Date:
Jan 1, 1999
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.
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