APEXRAD

APEXRAD is a new low-altitude dose model for solar minimum, developed at the USAF Phillips Laboratory. It is an empirical model, based on direct measurements from the APEX satellite (70^o inclination, 362 km perigee, 2544 km apogee) in 1994-1996. APEXRAD results should be significantly more accurate than those based on the AP8MIN/AE8MIN models, which are outdated and known to have limited accuracy at low altitudes.

For more information on APEXRAD, see:

M.S. Gussenhoven et al. "APEXRAD: Low Altitude Orbit Dose as a Function of Inclination, Magnetic Activity, and Solar Cycle", IEEE Transactions on Nuclear Science 44, 2161-2173 (1997).

The APEXRAD code runs on a PC; it is not presently available over the WWW. Users interested in obtaining a copy of APEXRAD are instructed to contact Dr. E.G. Mullen by mail [Geophysics Directorate, Phillips Laboratory, Hanscom AFB, MA 01731 USA], by e-mail [mullen@plh.af.mil], or by FAX [617-377-3160].

The CREME/TRP software is not recommended for dose calculations (see limitations), especially since the CREME software does NOT include trapped electrons. Nevertheless, as a test of the AP8MIN implementation in CREME/TRP, we compared dose predictions from APEXRAD and from CREME96/TRP. The tables below show these comparisons for various orbits and depths of shielding.

• The CREME results were calculated using the TRP routine to determine the orbit-integrated proton flux, the TRANS module to transport through Al shielding, and the DOSE module to convert the proton flux into a dose value.
• The APEXRAD results shown below were read from the figures given in the above-cited paper and then multiplied by two, since the APEXRAD doses are for a hemispherical acceptance (2-pi steradians) and the CREME calculations are for a spherical (4-pi steradians) acceptance.

Discrepancies can be very large for light shielding (the "no shielding limit"), where the trapped electrons (included in APEXRAD but neglected in TRP) dominate.

In most other cases, AP8MIN predicts a dose which is only ~50-70% of that predicted by APEXRAD. Part of this discrepancy may also be due to energetic trapped electrons. However, the discrepancy primarily reflects the systematic error in the AP8 model at low-altitudes:

• Comparable discrepancies have been reported in comparisons between AP8/AE8 predictions and dose measurements on LDEF and aboard the Space Shuttle.
• The AP8 models also typically underpredicts SEU-rates in low-altitude Earth orbits by a similar factor. [See Figure 7 of E.L. Petersen, IEEE Trans. Nucl. Sci. 44, 2174-2187 (1997).]