involved. Failure Related Signals With 6 x 106 tubes in orbit, a tube lifetime of 220,000 hours results in 26 failures per hour. Some failures may be associated with increased noise, development of parasitic oscillations or phase-lock failure. Klystrons are believed to be much less likely to produce such unwanted emissions than other types of microwave-generating devices. However, since so many units are involved, relatively rare failure modes will occur. Life testing of a large number of units is required to evaluate this effect. Rectenna Radiation Some incident power will be reflected from the rectennas (Arndt and Leopold 1978) and noise and harmonics will be generated in the rectification process. The mean distance between 60 rectennas within the U.S. will be about 350 km. Choice of rectenna sites must make use of mountain ranges to obtain adequate isolation of observatory sites. Synthesis arrays in which the signals from many antennas are combined in pairs to produce maps of the sky with high angular resolution are less susceptible to radio interference than single-antenna radio telescopes by factors that range from 10 to 40 dB depending upon the frequency, antenna spacing, bandwidth and other observing parameters (Thompson 1979). Observations using very long baseline interferometry (VLBI) are the least susceptible of all to radio interference (Burke 1979). Thus in Figure 1 the area between the lines marked VLBI and CCIR 224-4 represents the range of harmful interference thresholds for the various types of radio astronomy instruments. Unfortunately synthesis arrays and VLBI systems are not applicable to all types of astronomical investigations. The principal effects of the SPS on radio astronomy can be summarized as follows. (1) For any type of radio astronomy system there will be an angular distance from the satellites within which harmful interference will occur. The width of this precluded zone depends upon various parameters of the observing instrument and is estimated to vary from 20° for a single antenna to a few degrees for a VLBI system. (2) For the 2.69-2.70 GHz and 4.99-5.00 GHz radio astronomy bands sufficient filtering to prevent overloading by the power signal or its second harmonic may not be achievable without significant impairment of sensitivity resulting from filter insertion loss. (3) Conflicts between site requirements for observatories and rectennas are likely to occur. The above three effects represent the minimum likely interaction with radio astronomy, and are sufficient to cause significant restrictions. The effects of intermodulation products and failure-related signals discussed above could be much more serious, but should be more amenable to mitigation. The quality of SPS engineering and maintenance appears crucial to the coexistence of radio astronomy. Radar astronomy, like radio astronomy, uses large antennas and highly sensitive receivers. Interference effects (David 1979) differ from those for radio astronomy chiefly in the following ways. (1) Bandwidths are usually much less than in radio astronomy, resulting in harmful thresholds 10-40 dB higher than the CCIR 224-4 levels. (2) 2380 MHz is an important frequency, particularly at the National Astronomy and Ionosphere Center, Arecibo, Puerto Rico. This is so
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