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Overarching Issues The preceding sections have dealt with specific agenda items. In this section, we wish to provide some additional comments that are relevant to all agenda items. These comments are intended to provide a broader understanding of the issues unique to the Radio Astronomy Service (RAS) and the Earth Exploration-Satellite Service (EESS). Radio frequency interference is a substantial concern to the passive scientific services, RAS and EESS. The 2010 U.S. National Research Council report, Spectrum Management for Science in the 21st Century, 1 found that “[i]mportant scientific inquiry and applications enabled by the Earth Exploration- Satellite Service (EESS) and the Radio Astronomy Service (RAS) are significantly impeded or precluded by radio frequency interference (RFI). Such RFI has reduced the societal and scientific return of EESS and RAS observatories and necessitates costly interference mitigation, which is often insufficient to prevent RFI damage.” RADIO ASTRONOMY SERVICE Radio astronomy deals with exceedingly weak signals. These signals can be a million times smaller than the internal receiver noise, and their measurement, or even just their detection, can require bandwidths of many gigahertz, and integration times of a day or more. This requirement puts a premium on operating in a very low noise environment. It should be emphasized that serious interference can result from weak transmitters even when they are situated in the sidelobes of a radio astronomy antenna. This state of affairs has been recognized by the International Telecommunications Union (ITU) internationally and by the FCC in the United States, and various spectral bands have been allocated to the RAS for “exclusive” or “shared” use of these bands. However, “exclusive” does not mean that there is zero emission in the protected bands. It is a fundamental fact that any information-carrying signal can contain out-of-band emission, which spreads across a wide radio spectrum. The regulation of this out-of- band emission from a licensed transmitter involves controlling the intensity of the emission, and the FCC definition leads to an allowable level that, unfortunately, can cause serious interference with radio astronomy observations. It is likely that this situation will become worse in the future, as the RAS requirements become stricter with the study of weaker sources, and at the same time the active services are proliferating. Recommendation ITU-R RA.769 discusses interference protection criteria for the Radio Astronomy Service and defines threshold levels of emissions that cause interference detrimental to radio astronomy. Our knowledge of the chemical makeup of the universe comes though measurements of narrow spectral lines arising from quantum mechanical transitions. Radio spectroscopic observations require measurements at frequencies determined by the physical and chemical properties of individual atoms and molecules, so it is important to protect the frequencies characteristic of most important atomic and molecular cosmic constituents. However, the necessary parameters are not known for all possible species of interest. The situation with continuum (broad band) observations of radio emission from cosmic thermal and nonthermal sources, however, is different from that for spectral lines. There are no preferred 1 National Research Council, Spectrum Management for Science in the 21st Century, The National Academies Press, Washington, D.C., 2010. 40
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frequencies, but observations at multiple strategically-spread frequencies are required to define the properties of stars, galaxies, quasars, pulsars, and other cosmic radio sources. Moreover, due to the expansion of the universe, even the spectroscopic lines may be Doppler shifted as much as a factor of five or more, and broad bandwidths are also employed to simultaneously study many spectral lines. Historically narrow bands spaced throughout the spectrum have been given various levels of protection to enable these important studies. However, improvements in antenna and receiver design now permit instantaneous bandwidths up to about 30 percent to be used in the new generation of radio telescopes. This results in up to an order of magnitude improvement in sensitivity over earlier narrow band systems suggesting that a new paradigm for spectrum management will be needed to enable further advances in radio astronomy. Transmissions from satellites and aircraft for the purposes of communications and operations, and their dramatic growth in recent years, are prime concerns for RAS. For cost and technical reasons, these transmissions must be powerful enough to be usefully received by small omnidirectional antennas on Earth. Thus, high transmitter powers are necessary, but have the potential to create interference to RAS if unwanted emissions outside the necessary bandwidth are not sufficiently limited in the RAS observing bands. Because aircraft and satellites, in particular, know no geographical boundaries, the remote location of the telescope sites provides no protection from such sources when in direct line of sight above the horizon. Future progress in radio astronomy at these frequencies may largely depend on national and regional protection of large frequency bands in the vicinity of major radio telescopes along with global regulations of transmissions from satellites and aircraft. EARTH EXPLORATION-SATELLITE SERVICE Recommendation ITU-R RS.1029 provides the protection criteria for EESS. The high radiometric accuracy and sensitivity achieved by current EESS systems results in commensurately high sensitivity to RFI that can cause errors in the retrieved geophysical parameters. The ultimate impact of such emissions on a specific EESS geophysical measurement depends on the sensitivity of the geophysical parameter to changes in brightness temperature, as discussed in §2.2 of Spectrum Management for Science in the 21st Century. The maximum signal-power contamination that can exist without impacting the information contained in the EESS measurement has been derived by EESS scientists for each of the EESS allocated bands and is documented in Recommendation RS.1029. Over the last decade the rate-of-occurrence of harmful interference in EESS allocations between 1.4 GHz and 18.7 GHz has increased. When compared with historical data, the level and rate of interference appears to be on the rise. Specifically, satellites observing within the allocations at 1400 MHz, 10.65 GHz, and 18.7 GHz receive harmful interference on daily to weekly basis. Thus, both interference from unwanted emissions and from transmissions in shared allocations by both ground and space-based sources are of concern to EESS. Spurious and Out of Band (OOB) transmitter emissions from commercial devices typically are neither precisely controlled during manufacture nor essential to the devices’ intended purposes. Even when false measurements due to RFI are detected and eliminated, measurements and their use are degraded by the loss of data. 41
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