types whose grating lobes generally do not coincide, the combined lobes will have considerably less power than those associated with the subarrays. The overall grating lobe pattern for phase control to the power module level is shown in Fig. 10 and should be compared to Fig. 5 with phase control to the subarray level. The result is a reduction in both the amplitude and quantity of grating lobes incident upon the Earth. 5. ANTENNA TILT The actual SPS microwave system will have electrical and mechanical errors and component failures which distort the ground microwave beam pattern. An error budget has been developed through computer simulations to allow 2% tube failures, 0.1 dB amplitude error across each power module, 10° RMS phase error for the phase control system, random subarray misalignments of 3 arc-min due to surface warpage, and antenna tilt of 1 arc-min due to attitude control system errors. These errors scatter energy from the mainbeam into the sidelobes, and, consequently, reduce the power transmission efficiency between the antenna and the ground rectenna from 95.18% (fora perfect antenna) to 88%. The antenna tilt error has its most dramatic effect on the grating lobes, which repeat the mainbeam before being greatly attenuated by subarray pattern nulls. A tilt in the antenna plane causes the subarray pattern (Fig. 7) to shift across the Earth in the direction of the tilt. The grating lobes do not move because of the pilot beam retrodirec- tive phasing technique. As a result, the lobe peaks will no longer coincide with nulls in the subarray pattern as shown in Fig. 11. The magnitude of the antenna tilt and the polar tilt direction determine which grating lobes will be enhanced and to what extent their magnitudes will increase. Even a small tilt can greatly increase the magnitude of the grating lobes because of the steepness of the subarray pattern near its nulls. The condition for an antenna tilted along its X axis is shown in Fig. 12. A tilt of 3 arc-min in the 0° or A axis direction raises the peak power density to the first X axis lobe by a factor of 330, from 0.000421 to 0.139 m W/cm2. It is interesting to note the differences in the subarray pattern off-axis as compared to
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