COMPARISON OF LOW EARTH ORBIT AND GEOSYNCHRONOUS EARTH ORBIT J.E. Drummond Power Conversion Technology, Inc. 11588 Sorrento Valley Rd. #18 San Diego, Ca. 92121 The potential advantages of Solar Power Satellites are attenuated by the costs of transmitting power from geosynchronous orbit to load centers on earth. The capital cost of the transmitting facilities is dependent on the areas of the antenna, A-j, and rectenna, AR. These two areas are connected together by the requirement of high efficiency power transmission^: 2 2 AR - 3X R /cos(6) (90% transmission efficiency) (1) where X = 0.12m is the wavelength of the power radiation, R is the distance between antenna and rectenna, and 0 is the angle between the beam and local zenith at the rectenna. The area AR used here does not include the public safety exclusion area which will have to be many times larger. In an attempt to greatly reduce this initial cost, proposals have been made2 to decrease R by a factor of ~5. According to Eq(l) this would allow both At and Ar to be greatly reduced. Since the power transmission subsystem represents about half the capital cost of the total SPS reference system, it is worthwhile to consider the low orbit alternative at an early stage so that its technological, environmental, social and political problems and advantages may be assessed in comparison with those of the geosynchronous forms. It is the purpose of this paper to point out the salient features of a low orbit system in regard to these issues. Technological Problems. In order to remain in sunshine all the time, these orbits must be sun synchronous; they must prescess 360°/year (as a result of the torque exerted on them by the equitorial bulge of the earth). This imposes a relation between their inclination angle, i, relative to the equatorial plane, and their semi-major axis, a:$ a - 12,351 km x [cos(i) x (1 + 2e2)]2/7 (2) where e is the eccentricity of the orbit. It is also necessary that the major axis not rotate in the orbital plane or rotate with a period of one year in order that the largest distance of the satellite from the earth occur at winter solstice. This will allow the orbit to always clear earth's shadow. The condition that no rotation occur determines i = + 63.4°. These two orbits alone (with minimum eccentricity, e = 0.012) would be adequate to supply the base load needs of centers between latitudes 40 and 60° with rectenna areas an order of magnitude smaller than those required to receive power from an antenna of given area at geostationary orbit. (This result allows for 360° variation in arrival directions of the power beam during each 6 hour period). The condition that the major axis rotate in the same direction as the orbital plane prescesses determines i = ± 73.1°. Four such orbits are shown in Fig.2. The condition that the major axis rotate opposite to the orbital plane prescession determines i = ± 46.4° which are shown in Fig.3. The rectenna areas required are given in Fig.4. Now e must be determined so that the largest distance of the satellite from the earth, (l+e)a, extends beyond the winter solstice shadow. This determines e > 0.38 for i = ± 46.4°. These are iso-insolation orbits; the power system based upon them is abbreviated IPS. It is apparent that this system is complimentary both to an earth-born solar power system and to the geostationary SPS which both favor low latitudes. As Reinhartz has pointed out at this conference4, the enormous rectenna and safety exclusion area required by geostationary SPS sorely impacts SPS viability in Europe. This problem is substantially alleviated by the IPS system. The antennas in the IPS satellites need to scan only within a cone of half
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