Clearly, annual variations in climate and temperature represent potential losses of several tens of millions of watt-years of energy during years of large excursions from mean conditions. Scattering by refractive index fluctuations, termed "angel echoes" by radar meteorologists, are likely to be the most prevalent scatter effect at more arid SPS sites (Southwest U.S.). This is likely to be particularly true in view of the heating of the rectenna causing thermal instability in the layer of atmosphere directly over it. A common appearance of these refractive-indexed-caused angel echoes is as a band or layer on a radar scope. Hence, they are often termed "turbulent layers." In general, the systems most vulnerable to SPS emissions are those that operate in the vicinity of the main beam frequency and its harmonics. The main offenders would be: Included in the band from 2.29-2.45 GHz are allocations to radio location and to space research. The radio location assignments have fairly strict Federal Communications Commission (FCC) requirements as far as radio interference is concerned. The amount of energy from SPS, possibly as far out as 200 km from the rectenna site, may exceed these FCC requirements (Ref. 4.1.10). The equipment itself could easily give erroneous information as to location due to this interference. The space research assignments are used for telemetry from missile tests, the space shuttle, space platform and other space related tests. These are vulnerable to SPS emissions because of high gain-directive antennas that will be looking skyward. Receiver sensitivity and bandwidth considerations indicate unacceptable "noise" effects for the space research class of energy receivers within 150 km of a rectenna site. Special filters could allow operation within about 100 km for the reduced bandwidth instrumentation where the band of interest was at least 100 MHz from the SPS frequency (primary and second harmonic). These interference effects apply to the radio astronomy band from 2.69-2.70 GHz.
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