Fig. 5. Expanded rectenna needed to capture full microwave beamwidth at oblique angles of power reception (=£60°).-------- Longitudinal variation from Earth diurnal rotation. --------Area required by latitude eccentricity of lunar orbit. angles of incidence up to 60° from zenith (Fig. 5), and requirements to steer both the lunar transmitting antenna and ground rectenna surfaces in order to avoid excess reflected power scattering. SPS studies indicated the power scattered or reflected by a rectenna increased substantially for incident angles greater than five degrees from rectenna boresight. Thus the LPS rectenna requires an active pointing system in contrast to the SPS rectenna which received the power beam from a stationary source. Each of these complications increases the cost of the LPS rectenna by factors of from two to four as detailed in Table 1. COST COMPARISON In summary, features of a lunar-based power station include: • 5 GW power delivered to three Earth stations • 8 Hours reception per rectenna • Two lunar power stations (including solar arrays, power distribution systems, microwave antennas), each active 10-14 days monthly • 5% Efficient solar cells made from lunar materials (at */a the production cost as compared to SPS, '/io the production cost for array structure) • Doubled solar collection surface area (triangular mounds) • Steered antenna surfaces pointing to Earth low over lunar horizon (zero primary structure cost, 'A secondary structure cost, 7,220 mechanical pointing systems) • 78.5 km2 Power transmission area (100 times SPS); billboards dispersed across an extended land area to form coherent circular beam front • 7,220 Fully retrodirective 10.4 meter subarrays • 1.07 Million parasitically phased subarrays with klystrons or magnetrons (Vio transportation costs, 'A production costs)
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