The remainder of this chapter provides a brief background of the development of LATRA and describes the organization of this report.
To assist commanders in making decisions on whether to launch a rocket given the weather conditions at the time of launch, the Air Force developed an atmospheric dispersion computer model, the Rocket Exhaust Effluent Diffusion Model (REEDM), which simulates the dispersion of a rocket's emissions under prevailing weather conditions. Specifically, REEDM predicts an isopleth, or ''footprint," of the concentrations of specific emissions at ground level downwind of the specific launch site.
Initially, the Air Force compared the exposure concentrations predicted by REEDM for each of the emissions with acceptable human exposure levels, called tier limits. Three different tier limits were developed for military and civilian base personnel and for the communities located around the launch centers. The derivation of those tier limits is described in more detail in Appendix A. If REEDM predicted that specific populations would be exposed at concentrations higher than the appropriate tier limits, the commander would be advised to hold the launch. The Air Force later decided that acceptable human exposure levels should not exceed one tenth of the National Institute for Occupational Safety and Health (NIOSH) immediately dangerous to life and health (IDLH) levels, for occupational exposures on base, and short-term public emergency guidance levels (SPEGLs) developed by the NRC, at the breathing zone for the public (U.S. Air Force 1994). (See Appendix B for definitions of IDLH and SPEGL values.)
That policy remained in effect until November of 1994, when a Peacekeeper launch was delayed several times, and then postponed, because REEDM predicted, based on forecasted lift-off weather conditions, that a nearby town would be exposed to HCl at concentrations that would exceed the SPEGL. That cancellation cost the Air Force hundreds of thousands of dollars. The Air Force subsequently estimated that the use of the 1-hr SPEGL of 1 ppm as a maximum allowable concentration (i.e., a ceiling limit value) for HCl reduced the probability of