(systems definition and critical supporting investigations), preliminary environmental and societal assessments, and comparative analyses of the SPS concept and other national energy ventures. For the most part, the Reference System is based on fully-matured engineering precepts (methods, materials, practices, etc.) and realizable projections of future improvements. However, it is by no means an optimized engineering design, and does not account for newly emerging technologies which might become standard practices in the post-2000 era. Continuing systems definition undoubtedly will change many of the current characteristics of the Reference System. Some of those changes can already be reasonably perceived, but others most likely will occur that cannot yet be appreciated. Thus some potential problems associated with the present Reference System may subsequently become moot, and new ones will be recognized as development continues. Despite its current limitations, the Reference System is an important tool for identifying and evaluating significant side effects which conceivably could accompany SPS. The general features of the current microwave-based SPS Reference System which are of concern to astronomy are: • The Reference System consists of 60 satellites each of whose solar collecting area is 55 km^2 . • The energy would be transmitted to earth as microwaves at a frequency of 2.45 GHz.
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