Later studies of photovoltaic systems (ref. 2) achieved symmetry via a dual system of MPTS. This eliminated the need for any interference between the microwave power beam and the structure. Structural configurations were sized for dynamic stability and incorporated tiered triangular trusses with cylindrical or geodetic cylindrical elements. The significance of construction on the system and structural configurations was recognized and attention was directed from structural efficiency to ease of construction. Further studies of the SPS (ref. 5) suggested the incorporation of the modular unit concept for photovoltaic, as well as the thermal system. More detailed considerations of operational loads and dynamics of a photovoltaic SPS with aluminum triangular trusses made from beam builders are developed in ref. 6. The study of space shuttle orbiter delivered erectable structures resulted in the use of a planar truss based on a tetrahedron. This structure (called a tetratruss) was employed in a two-tiered MPTS antenna (ref. 10). A concept for an efficient space-fabricated, graphite-composite geodetic cylindrical element has also been developed (ref. 10). A first order analysis for a 1/15 scale similitude space test of the SPS structure and its major systems under appropriately scaled loadings and environmental factors has also been performed. Further studies (ref. 8) of the SPS structure addressed the potential of an erectable structure via an efficiently packaged graphite-composite, "dixie cup" element for establishing firm cost estimates of the structural system. Recent innovative structural concepts for the MPTS and photovoltaic array have been developed (ref. 11). Ref. 11 provides a good discussion of structural element and configuration trades and addresses control/structure trade-offs. 2. Rectenna Structure and Materials The rectenna is the ground unit of the SPS which receives the beamed microwave power and converts it to grid compatible electric power. The receiving area is elliptical due to the location of the receiving station relative to the equatorial orbit plane of the MPTS antenna. As now envisioned conceptually the rectenna is composed of rows of panels oriented normal to the incoming beam.
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