defined for this study’s purpose as an orbital altitude below about 3,000 km LEO sun-synchronous, circular orbits have a near-polar inclination; the exact inclination angle depends on the altitude selected. The inclination is set so that Earth oblateness perturbations will advance the longitude of the ascending node by approximately 0.985 °/day, thus keeping the solar power collectors in continuous sunhght. The Middle Earth Orbit (MEO) circular orbit, utilized for either SunTower or relay power beaming concepts, has low-to-mid values of inclination (nominally 30°) and altitudes in the range 6,000-12,000 km Note that the lower end of this altitude range lies in the Van Allen radiation belt region and could degrade electronics and solar cell performance. Table 4-1 Candidate Orbit Characteristics Selection of the best operational altitude is not apparent a priori', among the key tradeoff factors that would need to be examined are the power beaming efficiency and contact time, in-space transportation requirements from LEO to MEO, and amelioration of radiation effects. The third orbit type considered here is Geosynchronous (GEO), with application to either SolarDisc or SunTower-Relay concepts. Earth gravitational perturbations also affect the nominal geostationary orbit and will require periodic orbit maintenance maneuvers to control position drift relative to beamed ground sites. Solar occultation by Earth does occur for MEO and GEO orbits with daily variability through the year, but the effect on power beaming performance is not too significant Maximum shadow time per orbit is about 40 to 70 minutes, but the average daily shadow fraction is less than 10% in MEO and only 1% in GEO. Figure 4-5 shows the defining geometric parameters, namely range and angles, of the orbit-to-ground and orbit-to-orbit power beaming links. With the use of a planar transmitter array, the basic operational assumptions for power beaming to the ground are that the array is oriented to the nadir direction and that electronic beam steering is employed to access ground site receivers. Nominally, the steering angle (a) is constrained to be less than 30°. A planar rectenna is assumed at the ground site; this is laid out level for
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