January 23, 1979 Solar Power Satellite - Assessment of Potential for Interference to Radio Astronomy Observations by National Academy of Sciences - National Research Council Committee on Radio Frequencies The National Aeronautics and Space Administration (NASA) and the Department of Energy (DOE) are currently exploring the feasibility of satellites placed in synchronous orbit to convert solar energy to microwaves. These microwaves, beamed to a receiving site on the earth’s surface, would be converted to 60 Hz alternating current and fed into the nation’s power grid. In one conceptual design (NASA Publication CR-2357), each such satellite would transmit 6 gigawatts of power at a frequency near 3 GHz; eventually the system would grow to about 100 such satellites. The power densities which would be produced in the atmosphere and on the ground by such systems are very much higher than those encountered in presentday communications and radar. This gives rise to concern about the harmful interference that could be caused to both terrestrial and space services. The Radio Astronomy Service is particularly sensitive to such interference, so much so that even a low-powered prototype satellite might cause serious problems at North American radio observatories. Radio astronomy is the study of the universe by reception of natural electromagnetic waves in the wavelength range of about 1 mm to 1 km. The radiant power detected by radio telescopes from remote cosmic sources is typically 10 "W or less, many orders of magnitude less than the man-made signals in common use in the communications industry. Despite the low-power levels, proper frequency management, combined with the siting of radio observatories in remote rural locations, has been successful in maintaining coexistence of these two human activities. The level of interference which can be tolerated in radio astronomy depends in a complex way on naturally occurring sources of radio noise, the kind of observation being pursued, and the bandwidth in use. The International Radio Consultative Committee (CCIR) Report 224-4 summarizes the results of studies of this problem (Tables 1 and 2, Appendix). The CCIR tables of harmful interference levels as a function of frequency have been used, for example, as guidelines in planning satellite operations that could affect radio astronomy observations in the 2690-2700 MHz radio astronomy band. The NASA study includes a discussion of the radio noise levels to be expected from the solar power satellite. In the initial proposal the satellite’s transmitter would consist of 10$ Amplitron tubes. Each tube would produce about 7000 watts of power at a frequency near 2.5 GHz. Current tubes, according to the study, can achieve a noise power, in a 1 MHz band, 55 dB below the signal; with such tubes a broadband noise power of 43 dBW/MHz would be generated by the satellite in a band two hundred MHz wide centered on the signal frequency. Outside this band the noise power density would drop by 15 dB per hundred MHz.
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