whereas < 10 m/sec for the typical geosynchronous satellite. Even for deep space probes with very large values of [], aberration is not too serious. For example, for missions to the outer planets, []. In order to hold pointing loss to less than 1.0 dB, the pointing error for a uniformly illuminated circular aperture must be less than X/ttD. On this basis, apertures up to in diameter are permissible. The maximum v^ for inner planet missions may be somewhat larger than [] m/sec, but such huge apertures hardly seem necessary for distances less than 2.0 AU. c) Propagation Medium Effects Even if the points A and B are fixed, the required symmetry of phase delay fails if the phase velocity, [], for propagation from A to B, is different from [], the B to A phase velocity. This may occur if the phase velocity varies with either time or frequency (since [] ). In the case of the SPS, the iono- sphere will be the important time varying and dispersive component of the propagation medium. For more distant spacecraft, the solar plasma may be the dominant medium. The analysis of the effect of the ionosphere on SPS retrodirectivity is greatly complicated by the fact that the ionosphere itself will be significantly affected by energy absorbed from the downlink power beam [11]. d) Effect of Transmission Line Mismatches Our previous analysis of central phasing (Subsection IIA) assumed that the phase shift produced by a transmission line of length £ is simply [], for a signal of frequency [], where the phase velocity, [], is assumed to be independent of []. But this result does not take into account the effect of
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