Radio frequency interference is a substantial concern to the passive scientific services, the Radio Astronomy Service (RAS) and the Earth Exploration-Satellite Service (EESS). The 2010 National Research Council report Spectrum Management for Science in the 21st Century1 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.” In particular, even when false measurements due to RFI are detected and eliminated, scientific measurements and their use are degraded by the loss of data. Of particular concern to both RAS and EESS are spurious and out-of-band transmitter emissions from commercial devices. Such emissions are typically neither precisely controlled during device manufacturing nor essential to the devices’ intended purposes.
Strategies to minimize RFI for RAS and EESS include cooperative agreements, respecting primary and secondary spectrum allo-
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NOTE: Portions of this text are adapted from National Research Council, Views of the NAS and NAE on Agenda Items at Issue at the World Radiocommunication Conference 2012, The National Academies Press, Washington, D.C., 2013.
1National Research Council, Spectrum Management for Science in the 21st Century, The National Academies Press, Washington, D.C., 2010.
cations, placement of facilities in remote locations, establishment of radio quiet zones, and application of sophisticated RFI excision algorithms. In concert, these mitigation techniques allow continued scientific advancement in some areas, but not all are applicable to all scientific services. For example, many EESS programs include observations of the entire globe and thus cannot rely on local shielding or remote locations to reduce RFI. Conversely, many RAS programs require observations at frequencies not allocated specifically to the passive services and thus rely on local shielding to minimize RFI at these frequencies. It is important to note that, by their very nature, passive services do not interfere with other users of the spectrum. In consideration of these varied approaches to RFI mitigation for the passive services, comments on agenda items include discussion of spectral regions not allocated specifically to RAS or EESS and to geographic regions outside the United States.
For context, it is important to understand the exceedingly weak nature of the typical signals detected by radio telescopes. They 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 Federal Communications Commission (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. Regulation of this out-of-band emission from a licensed transmitter involves controlling the intensity of the emission, but the allowable level of out-of-band emission may still cause harmful interference to radio astronomy observations. It is likely that spurious and out-of-band emissions will be an even greater problem in the future as the active services continue to proliferate and scientific advances drive radio astronomers to observe weaker and weaker sources. Recommendation ITU-R RA.769 discusses interference protection criteria for the RAS and defines threshold levels of
emissions that cause interference detrimental to radio astronomy. Interference protection is often specified separately for spectroscopic observations and continuum observations.
Radio spectroscopic observations require observations at frequencies determined by the physical and chemical properties of individual atoms and molecules. In particular, our knowledge of the chemical makeup of the universe comes through measurement of spectral lines arising from quantum mechanical transitions, so it is important to protect the frequencies characteristic of the most important atomic and molecular cosmic constituents. However, the necessary parameters are not yet known for all possible species of interest. Moreover, due to the expansion of the universe, even known spectroscopic lines may be Doppler shifted by more than a factor of five. For reference, the apparent Doppler shift associated with the expansion of the universe is characterized by the parameter z, such that the observed frequency is lower than the emitted frequency by a factor of 1 + z. Therefore, detection of molecules in distant sources may require observations at frequencies well below the characteristic frequency measured in the laboratory. Thus, observations at spectral frequencies well outside the bands allocated to RAS on a primary or secondary basis are often conducted in order to search for new molecular species and to detect Doppler shifted spectroscopic lines from both nearby and distant sources and the early universe.
The situation with continuum observations of radio emission from cosmic thermal and nonthermal sources, however, is different from that of spectral lines. There are no preferred frequencies, but observations at multiple frequencies are required to define the properties of stars, galaxies, quasars, pulsars, and other cosmic radio sources. 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 of 50 percent or more to be used in the latest generations of radio telescopes. This results in an improvement in sensitivity over earlier narrow band systems by up to an order of magnitude; furthermore, broad bandwidths are also employed to study many spectral lines simultaneously. Unfortunately, receivers can become nonlinear as a result of RFI at neighboring frequencies, and intrinsically weak emission can be easily overwhelmed by RFI. Thus, the advent of routine observations over broad bandwidths by radio telescopes will require even more vigilance in RFI mitigation to enable further advances in radio astronomy. In particular, while improved RFI mitigation and exci-
sion techniques have expanded the scientific return of many facilities, they are an inferior option relative to a clean, interference-free spectrum. This relies on the shared responsibility of users to make sure all are making effective use of the electromagnetic spectrum.
Emissions from satellites and aircraft for the purposes of communications and operations are a prime concern for RAS because satellites and aircraft have no geographical boundaries and are in the direction in which radio telescopes observe. Thus, the remote locations chosen for telescope sites provide no protection from such sources when they are in direct line of sight above the horizon. Future progress in radio astronomy may largely depend on national and regional protection of large frequency bands in the vicinity of major radio telescopes, along with the global regulation of transmissions from satellites and aircraft.
EARTH EXPLORATION-SATELLITE SERVICE
Satellite remote sensing is a uniquely valuable resource for monitoring the global atmosphere, land, and oceans. Passive instruments are particularly vulnerable to man-made emissions within the EESS bands because they rely on very small signals emitted naturally from Earth’s surface and atmosphere and because they monitor globally. In many cases having global coverage is essential to the application (e.g., soil moisture as a parameter for understanding the global water, energy. and carbon cycles). This means worldwide controls are necessary.
Passive remote sensing from satellites provides information that is critical to understanding Earth’s environment. This includes information to predict weather and climate and to understand climate change. Examples are parameters such as ocean temperature and salinity, needed to understand ocean circulation and the associated global distribution of heat. Passive remote sensing is also important for monitoring soil moisture, a parameter needed for monitoring and predicting agricultural productivity for food security; land use; for the assessment, adaptation and risk management of hydrological extremes such as drought and floods; for weather prediction (heat exchange with the atmosphere); and even for defense (planning military deployment). Passive sensors also provide temperature and humidity profiles of the atmosphere, used for weather forecasting, and gather information to monitor changes in the polar ice cover and information needed in assessing hazards such as hurricanes, wildfires, and drought. For many applications, satellite-based micro-
wave remote sensing represents the only practical method of obtaining atmospheric and surface data for the entire planet.
Recommendations ITU-R RS.1029 and RS.2017 provide criteria for protecting applications of the EESS. The high radiometric accuracy and sensitivity needed to accomplish the measurements of modern EESS systems results in commensurately high sensitivity to RFI that can cause errors in the retrieval of geophysical parameters. A description of the impact of such emissions on a specific EESS geophysical measurement is discussed in §2.2 of Spectrum Management for Science in the 21st Century.2 The maximum signal-power contamination that can exist without impacting the information contained in the EESS measurement has been derived by scientists for each of the EESS allocated bands and is documented in Recommendations RS.1029 and RS.2017. However, as technology improves and is more able to meet the requirements of science for better resolution, these limitations also become more restrictive. Hence, more protection will likely be needed in the future.
Furthermore, 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 the rate of interference appear 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 a daily to weekly basis. Thus, interference both from unwanted emissions and from transmission in shared allocations by ground- and space-based sources are of concern to the EESS.
The committee advocates for effective use of the electromagnetic spectrum across all services. This shared use of the spectrum is coupled with a shared responsibility to ensure continued availability of this critical resource for effective use by all.
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2National Research Council, Spectrum Management for Science in the 21st Century, The National Academies Press, Washington, D.C., 2010.
