angle ~29° about the nadir which should be readily accomplished by electronic phase control alone. This surely will be both more reliable and of much smaller mass than the universal joint required between antenna and solar collector array on a geostationary SPS. Both antennas and rectennas must be redesigned to accomodate this scanning as well as circular polarization. An obvious problem is how to use the power generated by a satellite which is temporarily out of sight of any load center. Within these areas special load centers can be established to convert sea water to hydrogen fuel (or methane in the Sargaso Sea) for instance. A detailed study of these possibilities is needed. The low orbits do experience a higher gravity gradient, but with some forms of solar power satellites this can be used to advantage. The low orbits experience a smaller tidal effect than do geosynchronous satellites and they experience far less drift toward the east-west stable points at 76°W longitude and 108° E longitude. A detailed study of orbit perturbations and potential accidents needs to be made. The IPS orbits have little advantage or disadvantage in regard to transportation from the LEO staging/pre-assembly orbits. Electric propulsion would carry partially constructed satellites up to geosynchronous orbit or over to the retrograde sun-synchronous orbits. There also may be little advantage or disadvantage in relation to Van Allen belt and solar flare radiation. These issues need study. The primary technological issue in regard to reliability is the fact that the IPS orbits chosen do not enter the earth's shadow and hence these satellites do not experience the very great thermal shock which must be repeatedly experienced by geostationary satellites during Spring and Fall equinox. The economic impact of relaxation of this severe engineering requirement should be studied. Environmental Problems. The first experimental indications of the u^derdense thermal self-focusing instability were presented at this conference. * The instability growth rate observed at Platteville^ was too slow to allow the moving power beam from an IPS orbit to significantly stimulate it. Extended experimental studies of this instability should be made. Social and Political Problems. These problems have received very little comparative study for the low vis-a-vis geosynchronous orbits. The mainspring of the difference is that the low orbit, IPS is inherently also an Interregional Power System. In order to be economically efficient, the system must serve regions covering most of the earth's surface. It favors latitudes 36 to 56°. Acknowledgement. The vital criticism and encouragement received from Dr. Kraft Ehricke is gratefully acknowledged. References. (1) W.C. Brown, Spectrum, Mar. 1973, p.40. (2) J.E. & R.N.Drummond, Solar Power Satellite, Congressional Subcommittee Hearings, 20 Feb. 1976, p.637 & Apr. 12, 1978, p.416. (3) K. Ehricke, ''Space Flight”, 1963. (4) K.K. Reinhartz, DOE SPS Prog.Rev., Apr. 20-25, 1980. (5) C. Rush, ibid. (6) D. Phelps et al, ibid. (7) V. Chobatov, J. Spacecraft Rockets, Apr. 1977, p.249.
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