d be viewed only as working approximations aimed at describing the major features of the external environment with the recognition that both the subtleties and major features of the environments are the concern for numerous space-borne experiments and ongoing modeling efforts. 2.1 Environment Description and Issues With regard to the range of proton environments and the factors affecting them, the basic models cited above are the predictive tools of the environment specialists. The 1997 Short Course by Janet Barth [Bart-97] offers an excellent discussion of these and several other models, their applicability, issues affecting their accuracy, and the proton environments as they change with orbital position and solar cycle period. Detailed treatment is outside the scope of this material, but in addition to those notes, interested readers may wish to locate the 1988 review article "The Natural Radiation Environment Inside a Spacecraft," [Stass-88] or this conference's Short Course notes on "Radiation Environments in Space" [Stass-90]. For quick reference, the general character of the trapped proton belts external to the spacecraft is provided here as figure 1. AP8MIN EQUATORIAL OMNIDIRECTIONAL RADIAL PROFILES DIPOLE SHELL (L) Figure 1. The AP-8 model for solar minimum conditions at 0 degrees inclination indicates the higher energy protons at lower altitudes [after Stass-88]. The orbit altitude in km is related to dipole shell as [(L X 6370 km)-6370 km] where 6370 km is the Earth's radius. There is a document in development to supplement the various radiation models with practical considerations for satellite applications and make general trends more accessible to design engineers. The IEEE (Draft) Standard 1156.4 [IEEE-1156.4] document is aimed at establishing generic descriptions of four orbit categories: Low Earth Orbit (LEO) below about 10,000 km, Medium Earth Orbit (MEO) from 10,000 to 20,000 km, Geostationary Orbit (GEO) at 36,000 km, and transfer or Highly Elliptical Orbit (HEO). This document identifies examp