 is important in proton susceptibility characterization that adequate attention be paid to establishing valid measurement of the cross section in this segment of the cross section versus energy curve. In the case of heavy ion single event upset estimation, the declining population of particles with increasing LET in the saturation region results in the most significant contribution to SEU rate coming from the larger population of particles having LET in the region slightly above threshold. Device single event upset characteristics are usually expressed in terms of the parameters for standard equations used as best-case fit to the measured data. In the case of heavy ion characterization, the Weibull curve and the four parameters which define this shape are most frequently used. There are other representations such as the cumulative log normal distribution which also provide good fit to measured data. An exponential distribution has also been shown to provide reasonable fit to the device characteristic for some devices. In the case of proton characterization and single event upset rate prediction, the semi-empirical approach of the two parameter Bendel model is used. While it is important to achieve the best possible statistics in measurements of device characteristics, it is also important to consider that the primary contributor to uncertainty in the rate estimation is the particle environment definition for both heavy ions and protons. The space environment models are often ascribed to be factor of two models. 5.3.2 Environment Models[Barth97][Dyer98][Daly96][Huston98][Tylka97] [Gussenhoven97] [Gussenhoven93] [Chenette94] [Majewski95] Most analysis of the effects of trapped particles on earth orbiting satellite systems has been performed using NASA trapped proton and electron models, AP8 and AE8, respectively. The AP8 model was released in 1976, reflecting the analysis of data obtained from satellite measurements between 1958 and 1970. The AE8 model was released in 1983, reflecting the analysis of data obtained 