parated bits in a data stream as the ratio of failed bits to the particle fluence incident on the device during the interval in which the failures are measured. Our objective is to understand the error cross section dependence on environmental factors such particle flux and also the particle energy and angle of incidence, which impact the effective linear energy transfer (LET). Also, for a given receiver design, we measure the cross section dependence on the data link characteristics including the data rate and the optical power incident on the photodiode. The result is a data set that can be readily analyzed with existing descriptions of the expected environment to produce estimates of link performance in orbit. In [Mars-94a] the case is made for treating link bit errors as arising from direct as opposed to the indirect upset mechanism. Several indications point to this interpretation including the angular dependence of measured cross section data, the relation between ionization induced charge and electrical signal size, the relation between device physical size and the magnitude of the error cross section, and the particle and LET dependence of the measured cross section. Realizing that direct ionization causes upsets has two important implications for rate predictions for proton-induced errors. First, the traditional Bendel approach does not apply, and second, the proper approach should more closely follow the approach developed for heavy ion induced upset based on LET. Figure 15, and the discussion found in [Mars-94a], show that the proton-induced error data can in fact be usefully viewed as dependent on the effective particle LET, even though the errors are due to protons. The solid lines in the curve follow the customary Weibull form (equation 2), where a, b and C are fitted parameters and Osat is the saturation cross section of the cross section versus LET relation. The family of Weibull curves corresponds to different levels of optical power used in the operating link. It is important to note that the da