Further studies of the SPS side lobe levels should be carried out with '.Il realistic estimates of the antenna and transmitter properties. The side lobe levels in the 100 to 1000-km range are probably determined in large part by the number of out-of-service power modules. (If we estimate individual klystron 4 lifetimes at <10 hours, we can expect >10 klystrons to fail per hour in each satellite.) Reduction of side lobe levels would ease the problem of siting rectennas, which we recommend should be studied further. We foresee considerable difficulty in designing sensitive receivers which are not adversely affected by such high interfering power fluxes. Further study of this problem is also required. In addition to the satellite signal, an unknown amount of fundamental and harmonic power and wideband noise would be reradiated by the rectennas. Studies should be carried out to determine the nature of this secondary radiation. This radiation may be reflected by aircraft and by unusual meteorological conditions to distances of the order of 100 km from the rectenna site. The severity of the direct radiation problem will depend on local topography. The aircraft problem can be reduced by extending the restricted airspace which will presumably be required around the rectenna site. At the side lobe levels given in the Reference System Report, we expect that the minimum separation distance between rectenna and observatory must exceed several hundred kilometers, and that grating lobes must also be kept far from any observatory. ON MOVING RADIO ASTRONOMY TO THE LUNAR FAR SIDE The suggestion has been made that if the SPS makes radio astronomical observations impossible from the surface of the earth, all radio observatories should be relocated to the far side of the moon. While the far side of the moon is a highly advantageous site for certain, specific investigations which cannot be conducted from the surface of the earth, the capital expense and, particularly, the annual operating costs of a general program of astronomical research on the lunar far side may be so high as to be prohibitive in comparison with the cost of installing proper mitigative measures in the SPS that
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