The possibility of generating large quantities of electrical power in space and transmitting it to earth using satellites was first suggested in 1968. During the following years several studies of the concept were conducted by NASA and industry. The energy shortages of 1973 spurred interest and in early 1976 the ERDA and NASA initiated an SPS Concept Development and Evaluation Program, a joint plan covering the period from 1977 to 1980. The plan states that the Reference System selection milestone "will focus the evaluation effort in what is considered to be at that time the optimal direction." This report is submitted in response the the first milestone: Reference System Definition. The SPS Reference System is largely a product of studies conducted by Boeing under contract to the Johnson Space Center from December 1976 to December 1977 and Rockwell International under contract to the Marshall Space Flight Center from March 1977 to March 1978.