arrays and can operate quite efficiently in MEO or GEO. The case study results clearly demonstrate this relationship of performance efficiency (cost per kW) to delivered power and orbit altitude. Electronic beam steering plays a particularly important role in the implementation of the two relay-based concepts. In both concepts, the power satellite is situated in LEO sun-synchronous orbit, while the relays are in either MEO or GEO. In order to reduce both the drag on the LEO-based system and the crosssection exposed to micro-meteoroid impacts, it was decided to keep the transmitting array of the power satellite stationary in the orbital plane, and to use electronic beam steering rather than mechanical rotation to track and transmit to the relays in their higher orbit. However, the consequence of this approach, even with wide-angle beam steering capability, is a severe limitation of the available contact time between power satellite and relay. This strongly affects the cost effectiveness of the relay concepts. For example, with a maximum beam slew of +/- 30 degrees, twelve MEO relays or seven GEO relays are needed to make up for the short contact time per relay and fully utilize the power collected by the single SPS in sun- synchronous orbit. Storage of the collected power at the SPS during periods of non-contact was dismissed as an option early in the study, as preliminary calculations determined that space-based energy storage would be too massive and expensive for this application. The relay system selected for the case studies also makes extensive use of electronic beam steering. In both the LEO to MEO Relay and LEO to GEO Relay concepts, the assumed relay is basically a planar, RF power reflecting array, called a ReflectArray, that uses a phase-shifting approach for beam-forming and redirecting illuminating RF power. The frequency of the incoming and outgoing power remains the same, although frequency conversion is an available but expensive option. Technical details of this concept, prepared by Richard Dickinson, can be found in the appendix. The advantages offered by the ReflectArray over the more traditional approach requiring a separate receiver and transmitter, are primarily efficiency and pointing/operational simplicity. However, a relay system that offered frequency down-conversion would have provided the opportunity to use higher frequency space to space power transmission, and thus produce higher beam-coupling efficiency between the power satellites and relays. Beam-coupling efficiencies between the power satellite(s) and relays is a major factor in the performance of the relay-based concepts. Because of the large number of relays needed and their location in higher orbits, it is highly desirable to keep their individual cost and mass as low as possible. Since the power receiver/reflector array is the most significant mass and cost driver for the ReflectArray relay system, there is a powerful incentive to keep the array diameter small This, however is detrimental to beamcoupling efficiency, since an even smaller fraction of the transmitted beam is then captured by the array. The ability to improve performance by increasing the power and diameter of the transmitting array on the LEO satellite (thereby narrowing the transmitted beam) is an option, but is limited by economics and the 1 GW space-to-space power transmission constraint established by the case studies. Power transmission from space to Earth is also adversely affected by small ReflectArray diameters since they produce larger spot sizes on the ground and therefore require larger rectennas to collect the power. Because the longer transmission distance produces larger spot sizes, this problem is particularly serious for GEO-based relays. For all the above reasons, the modeled performance of the two SSP relay concepts in the case studies was very disappointing. In discussing the Space Segment Model in Section 4, one of the features described is the ability of the model to size the system so that it provides a specified minimum, maximum or average power across a selected set of ground sites. The groundrule used in the case studies was to use the minimum size/cost
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