at least ten times this figure, and since the several dozen present synchronous satellites are all in nearly circular orbits, with eccentricities generally less than 0.001, the possibility of collision cannot be neglected. This is particularly true because the SPS program would be the first instance of a large number of satellites being separated by distances only ten or 20 times the dimensions of the satellites. Probabilities of collision have not been calculated, but the possibility should be considered that eccentric orbits for the SPS's would require all other synchronous satellites within the conflicting range of longitudes to be physically removed from orbit and their functions performed by equipment incorporated (on a "piggy-back" basis) into one or more SPS's. Whether the eccentricity of the SPS orbit is held at zero or permitted to vary, it appears probable that all orbits, SPS and others alike, must be actively controlled to a common eccentricity in order to achieve an acceptably low risk of collision. Inclination Consideration of the effects of an inclined orbit has been limited in this study to a single inclination of 7.3°, based on the assumption that the orbit will either be held at zero or, to save propellant, placed in the 7.3°, constant-inclination orbit discussed in IV-A-3-a. The major effect of an inclined orbit is a daily variation in the angle of incidence of the microwave beam on the rectenna, amounting to roughly 16° (figure IV-A-3-7). This increases rectenna cost in two ways. First, since the major axis of the elliptical area illuminated by the beam is equal to the beam diameter divided by the cosine of the maximum angle of incidence, the land area to be acquired is greater. Second, if the rectenna utilizes the sawtooth configuration commonly assumed (for simplicity of construction, maintenance, etc.), a varying angle of incidence requires more rectenna elements than would a constant angle of incidence, because some of the elements are shadowed by other elements during part of the daily cycle (see figure IV-A-3-8). The magnitude of these effects is summarized in figure IV-A-3-9 as a function of rectenna latitude. These curves indicate only the additional ground area and rectenna elements necessary to intercept the beam as the angle of incidence varies. Variations in azimuth of several degrees will occur and will add still more to the area required, but this has not been evaluated numerically. Variations in power density at a given location on the rectenna will adversely affect efficiency; this has also not been evaluated. The variation in range, and hence in power output, because of the eccentric orbit is increased somewhat if the inclination is not zero. At a rectenna latitude of 40° and eccentricity of 0.04, for example, total variation in output is 1.9% at zero inclination and 2.0 to 2.6%, depending on perigee orientation, at 7.3° inclination.
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