Important Limitations of the TRP Trapped Proton Module

The TRP program module has a number of significant limitations. Most of these limitations are inherent in the AP8 models and/or reflect the CREME software's primary focus on single event effects. Other limitations arise from the way in which the TRP calculations are presently implemented. The user should be fully aware of these limitations.

1. No Trapped Electrons: Trapped electrons do not cause single event effects. Since single event effects are the primary focus of the CREME software, no trapped electrons are included in the TRP module. However, trapped electrons often provide a significant contribution to total ionizing dose, at least for devices deployed under less than ~300 mils (~2 g/cm2 Al) of shielding. Therefore, CREME/TRP outputs are not recommended for dose calculations. Better software for calculating dose due to trapped radiation, including both electrons and protons, is available elsewhere.

2. No Precession Terms in the Orbit Generator: The orbit generator currently used in the CREME TRP and GTRN modules is purely Keplerian (that is, based on a simple two-body calculation.) The present orbit generator does not take into account orbital precession caused by perturbations, such as the non-spherical Earth, the Moon, and the Sun. These precession terms are almost always small for orbits below 1000 km. At higher altitudes, however, the orbital precession may be significant, especially for long-duration missions in highly elliptical orbits. The present orbit generator also neglects perturbations due to atmospheric drag and solar radiation pressure.

   Installation of an improved orbit generator, which includes the most important precession terms, is planned.

3. No Solar Cycle Variability: Trapped protons, particularly at low energies and at low altitudes, show significant solar cycle variation, primarily due to changes in the atmospheric scale height. The AP8 models, however, provide flux estimates only for times near solar minimum and solar maximum. The AP8 models provide no procedure for interpolating to intermediate points in the solar cycle. Moreover, both AP8 models are long-term averages, based on data accumulated over several years, and may therefore not necessarily represent true solar-cycle extremes.

   The AP8MIN fluxes are generally higher than those from AP8MAX. AP8MIN is therefore recommended as a default for conservative design calculations.

4. Limited Accuracy at Low Altitudes: The AP8 proton models are known to have their greatest uncertainty at low altitudes (250-1000 km and McIlwain L < 1.5). See further information on AP8 accuracy.

5. Static Proton Belts: The TRP/AP8 calculations are completely static and provide no information on disturbed belt conditions, similar to those associated with the 24 March 1991 event observed on CRRES.

6. Omnidirectional Fluxes Only: Known anisotropies in the trapped proton fluxes are neglected. These anisotropies may be important for orientation-stabilized spacecraft and for some applications. See Omnidirectional Trapped Proton Fluxes.

7. Unreliable Geographic Maps: Due to changes in the geomagnetic field since the AP8 database was accumulated, the AP8 model cannot accurately predict the current geographical distribution of trapped protons. As a result, calculations may be highly inaccurate in the case of orbits designed to re-trace a narrow groundtrack.

8. Unreliable at Very Low Energies: The AP8 proton models are widely believed to be unreliable at energies below ~3 MeV. CREME/TRP outputs should not be used for estimating surface effects, such as paint and thermal surface degradation.

9. No Trapped Heavy Ions: TRP calculation also do not include trapped heavy ions. Trapped heavy ions are known to exist in Earth's inner magnetosphere, but primarily at energies too low to cause single event effects.