today, most SPS interference questions have focused on 2.45 GHz in the Industrial, Science and Medicine (ISM) frequency band. Some SSPP interference issues that must be addressed include: • How to limit out of bandwidth power transmission levels (side-lobe energy)? • How to minimize the interference effects of transmission frequency harmonics? Spacecraft Control-Structure Interaction The basic issue facing the structural and control engineers is the control-structure interaction problems observed in large space structures. These are due to the overlapping between the attitude control and structural frequency spectra, resulting in controllability and observability spillover effects. Presently, ground based experiments are under way in order to counteract the high mode excitation, however, further experiments in orbit will have to be carried out in order to confidently deploy large space structures. Space Construction Space construction will be imperative in order to deploy large space structures in orbit with the present state-of-art in launch vehicle technology. A feasibility study will have to be carried out in order to demonstrate deployment and robotic techniques. Would manned EVA construction be necessary? How would reliability in construction techniques be insured? Spacecraft Orbit Selection The issue of orbit selection is a fundamental one. For demonstration purposes, should a LEO be selected; and if so, which is the best orbit for the proposed design constraints, with considerations to launch windows. Space Transportation System It will be necessary to reduce weight of payload as much as possible. This could affect the design and deployment of a SSP satellit. For example, it may be necessary to increase the use of inflatables. Solar Cells Development It is necessary to make more efficient and compact individual solar cells. This relates to a SPS's dimensions and weight. If more efficient and compact solar cells, total cost will be reduced for a SPS. Also, if we can make fewer solar cells for SPS, then reliability will increase. Therefore, we need to determine what energy conversion technology should be used: For example, solar array technologies like thin films vs. crystalline, Si vs. GaAs, thermodynamic system or perhaps a combination of photovoltaic systems should be addressed. Technical Aspects of Long Term Commercial Opportunities The long-term technical problems of SSP are divided into four categories: single SSP satellite design, entire SSP network design, resources and manufacturing, and maintenance and operatibns. We areassuming that at the end of Phase B, the viability of small SSP systems will have already beendemonstrated, but at that time that there will not be any large-scale use of space solar power. Thus, the challenges that must be answered in the next phase involve implementing space solar power on a large scale to the point where it will provide a significant fraction of the Earth's total energy needs- as well as the energy needs of systems in space. The most important problems associated with the single SSP satellite are those involving scaling. Atthe end of Phase B, we should have small SSP satellites in operation providing power on a limited basis. Presumably, the final SSP satellites will be expanded and enlarged versions of these initial demonstration satellites if they are to provide power for much of the Earth. There will be several problems with such scaling. These include: maintaining control of the beam, maintaining both attitude and orbit control, ensuring stability of the entire structure, and reducing vibrations, particularly in the vicinity.of the transmitter. Also, there are other types of scaling problems, such as adapting the engineering failsafes needed to prevent misdirection and overamplification in Phase B to Phase C satellites without overly disturbing transmitting flexibility. In addition to scaling, orbit selection of single SSP satellites will be a major problem. In Phase B, orbits will have been selected for demonstration purposes. But in Phase C', orbits for single satellites will have to be selected within the context of the entire SSP network, regarding not only physical
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