Antenna joint accelerations are also slightly higher for inclined orbits, but the increase over that for a zero-inclination, equal-eccentricity orbit is not significant. Longitude Offset Ideally, the average longitude of the SPS (i.e., the longitude at perigee and apogee) is the same as the longitude of the rectenna. In practice, however, the satellites must be spaced in longitude as uniformly as possible, while rectenna locations will be determined by energy consumption distribution, land availability, etc. Thus, some longitude offset will in general be unavoidable. As with orbit inclination, the principal impact of a longitude offset is on the angle of incidence at the rectenna. This is illustrated in figure IV-A-3-10 for 40° latitude and 0.04 eccentricity. The increase is small up to about 10° offset, and increases rapidly thereafter. The variation in angle of incidence (due to eccentricity) also becomes greater, so that the rectenna not only covers more area but also requires more dipole elements for the same reason discussed under inclination, although the effect is not as severe. The increases in both angle of incidence and variation of angle become rapidly greater at higher latitudes. Hence, high-latitude rectennas should be given preference in SPS longitude assignments. Range also increases with longitude offset, causing a slight power loss if the SPS and rectenna are held constant. However, the loss is only about 1% at an offset of 40° (see figure IV-A-3-11). L. E. Livingston Spacecraft Design Division c. Eclipses An SPS in synchronous equatorial orbit experiences solar ecplises by the earth, the moon, and other SPS. Eclipses by the earth and by other satellites differ in several respects; both must be considered during design and are discussed below. Eclipses by the moon are less important. They are similar in character to eclipses by the earth, but are generally shorter in duration, less severe, and much less frequent. A detailed examination has, therefore, not been made, although it is conceivable that a combination of exceptional circumstances could occasionally produce an eclipse by the moon that exceeded the range of effects of earth and satellite eclipses. Eclipse by Earth These eclipses occur daily when the sun is sufficiently close to the orbit plane for the earth’s shadow to fall on the orbit. Assuming a zero-inclination orbit, this happens at the equinoxes. There are approximately 43 eclipses centered around the spring equinox and
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