6. An attractive equatorial orbit may be a 120000 km which provides good ground coverage (6 hr/day) and not have disadvantages of either LEO or GEO. Further, the rectenna size is smaller than that required for a GEO platform. 7. Platforms of the 10 MW class or smaller have a general problem of energy flux at the rectenna location. For the previous orbit (20000 km), it was found necessary to construct a concentrating antenna aligned parallel with the velocity vector of the orbiting equatorial platform. 8. Platform topology is dependent on at least orbit and application. For the small demonstrators, a rigid planar array is adequate. For the higher orbit, a prismatic structure has advantages. With advanced integrated technologies in which both solar array and the phased array transmitting antenna are in plane, large rigid planar arrays may be desirable. This will also reduce the size of the receiving rectenna and increase the energy flux. 9. For orbit raising and plane change, the available high power levels make electric propulsion (for example, xenon ion with an Isp of 5000 s) attractive. The dV in order to make the maneuvers is of the same order as going to low lunar orbit, though in both cases requires relatively little propellant (order of 101). This may be combined with an initial chemical propulsion stage to avoid excessive radiation degradation of the solar cells during the transit through the Van Allen belts. 10. In the near term, both the mass and particularly the costs of the phased array transmitting antenna utterly dominate the platform design. In order to construct the first platform element for 1 billion US dollars, it is necessary for the antenna costs to drop between 10- 100 times. Using technologies which are developing rapidly this decade, this may be possible. To a lesser extent, the solar arrays also contribute significantly to the cost, but a decrease of 10 times with the expected improvements in efficiency will also contribute to the feasibility of the platforms. Finally, advances in electric propulsion technology should reduce the unit costs of each element by a significant amount. 11. Eventual technology growth expected in this decade is expected to significantly reduce the size and cost of critical subsystems, making the overall concept more commercial viable (though launch costs will remain as the dominant economic factor for true commercial applications). The costs for the platforms may drop to a 1 MW, 1-billion class facility, which would be capable of beaming 6 MW-h/day of electricity to an equatorial ground station.
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