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Spectrum Management for Science in the 21st Century 3 The Radio Astronomy Service Over the past 75 years, astronomical observations at radio frequencies have transformed the understanding of the universe. They have allowed fundamental scientific issues to be addressed—these include the creation and ultimate fate of the universe, the distribution of matter and primordial energy in the universe, and the environment and manner in which stars and planets form. Many astronomical discoveries that have captured the imagination of astronomers and the public alike were made accidentally with radio telescopes; a list of such discoveries would include that of the primordial cosmic microwave background (CMB), celestial masers, and pulsars—the latter being the dense, fast-rotating, radio-emitting remnants of massive stars. With powerful new facilities such as the Atacama Large Millimeter Array (ALMA), the potential for unexpected discoveries will grow substantially. As was fittingly said in this context years ago by two famous radio astronomy pioneers, “We cannot discuss plans to discover the unsuspected …,”1 but the parade of new, unexpected discoveries has been continuous since the beginning of radio astronomy in the 1930s. With the unprecedented regimes of sensitivity that will arrive with new and planned instruments, one can expect that further remarkable discoveries will be made. Astronomical discoveries have been made possible by the steady and enormous improvement in sensitivity that is shown in Figure 3.1. In this graph the ordinate represents sensitivity and is on a logarithmic scale; there has been an improvement 1 Pawsey and Bracewell, Radio Astronomy, Oxford, United Kingdom: Clarendon Press, 1955, p. 296.
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Spectrum Management for Science in the 21st Century FIGURE 3.1 The minimum detected or detectable signal in flux density versus the year of measurement. The sensitivity is proportional to the temperature of the receiver system and inversely proportional to the collecting area and the square root of both bandwidth and integration time. For measurements after year 1990, an integration time of 12 hours is assumed. The rapid improvement over time is due to system improvements, including the decrease in system temperature (solid-state technology), the increase in collecting area (cost and construction efficiency), and the increase in bandwidth and integration time (electronic and digital technology). The improvement from 1933 to 1983 is about 10 orders of magnitude, a halving time of less than 2 years: a performance improvement similar to that described by Moore’s law. Acronyms in the figure are defined in Appendix F. Figure adapted and updated from J.M. Moran, “Peter Mezger and the Development of Radio Astronomy in the U.S. and Germany, and the Discovery of Radio Recombination Lines,” pp. 475-488 in The Nuclei of Normal Galaxies, Lessons from the Galactic Center, Proceedings of the NATO Advanced Research Workshop, NATO Advanced Science Institutes Series C, Vol. 445, A. Harris and R. Genzel, eds., Kluwer, Dordrecht (1994). of 10 billion in 70 years, and there will be another improvement by a factor of 1,000 from the Very Large Array (VLA) to the Square Kilometer Array (SKA) when it is built. (See Table 3.1 in §3.2, “Radio Observatories and Radio Telescopes,” for the characteristics of the newer instruments.) The current scientific questions that are motivating the construction of these new telescopes are no less exciting than those that were resolved in the past. Obvious
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Spectrum Management for Science in the 21st Century BOX 3.1 Nobel Prizes in Physics for Developments and Discoveries in Radio Astronomy Radio astronomy has been internationally recognized for its fundamental contributions to knowledge. The Nobel Prize in physics has been awarded to eight radio astronomers. The names of the scientists who led the teams to these discoveries, the year of award, and a brief description of the prize-winning science, are listed below. Sir Martin Ryle, 1974, for the development of aperture synthesis, and Antony Hewish, 1974, for the discovery of pulsars; Arno A. Penzias and Robert W. Wilson, 1978, for the discovery of the cosmic microwave background radiation; Russell A. Hulse and Joseph H. Taylor, Jr., 1993, for establishing the emission of gravitational waves by close binary pulsar systems, as predicted by general relativity; and John C. Mather and George F. Smoot, 2006, for demonstrating that the cosmic microwave background radiation has a blackbody spectrum and for discovering spatial fluctuations in the radiation. examples include the exploration of planetary systems in formation around other stars, measurements of neutral hydrogen in the early universe, and the study of star formation in distant galaxies. Furthermore, it is through radio observations that the discovery of life-indicating molecules in other planetary systems might be made. The scientific and technical advances of radio astronomy have been internationally recognized, as listed in Box 3.1. The Nobel Prize in physics has been awarded to eight radio astronomers in the past 40 years. 3.1 THE SCIENTIFIC IMPACT OF RADIO ASTRONOMY What follows is a summary of the scientific advances made possible in a few areas by radio astronomy. A discussion of some advances expected in the near future is also provided. Origin of Planets and the Solar System Speculations concerning the origin of the solar system stretch far back in the science and philosophy of humans. During the coming decade, the capability of understanding the origins and evolution of other planetary systems and thereby coming to understand the origin of our own planetary system will exist: ALMA and the Expanded Very Large Array (EVLA), both coming online in a few years,
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Spectrum Management for Science in the 21st Century will make it possible to detect planets in formation around other stars. ALMA and EVLA will enable the study of the structure, dynamics, and temperature of the material from which planets are forming. The planned Square Kilometer Array will enable detailed studies of such disks. The key strengths of radio measurements that will enable these studies are their ability to trace the distribution of gas and dust throughout the disk, to study the dynamics and temperature of the material involved in planet formation, and to follow the accretion of material as it develops from the tiny, submicron dust particles characteristic of the interstellar medium to centimeter-sized clumps, the first critical step in the formation of terrestrial planets. These radio capabilities are unique in enabling scientists to learn about the physical and dynamical processes that govern the planet-formation process and its outcome—a planetary system. They will be able to “see” the formation of giant planets through the gravitational and thermal influence of these planets on the surrounding gas. Scientists will see disks with gaps and inner clearing zones that are caused by planets. They will be able to follow the orbits of the planets by how they sculpt the disk and to study characteristics of the planets by probing their interaction with the disk material. At present, search techniques for extrasolar planets, or exoplanets, are strongly biased toward finding large planets close to their host star; and correspondingly, the 358 planetary systems known as of November 2009 are very different from our own solar system.2 They typically contain one or more Jupiter-like giant planets in orbits closer than that of Earth, and with eccentricities exceeding those of any planet in our solar system. There is no well-accepted theory for how such planets form or why they should be common. Prior to the discovery of exoplanets, our solar system was thought to be typical, and a template for all planetary systems. This is now known not to be true, and our understanding of the diverse outcomes of formation is significantly incomplete. So that this formation problem can be properly addressed, observations of many young stars are needed. These observations will lay the groundwork for an understanding of the many possible outcomes of the planet-formation process and how terrestrial planets fit into the general picture. The new knowledge of the existence of other planetary systems gives rise to many intriguing questions. Does life exist elsewhere, or is it unique to the solar system? Could there be a common starting point for life? The abundant and complex chemistry of the interstellar medium and of protoplanetary systems possibly provides an answer. More than 140 molecules have been discovered in the interstellar medium. Those with more than four atoms are dominated by carbon, nitrogen, oxygen, and hydrogen. The 31 molecules with seven atoms or more are nearly all organic molecules. They include glycoaldehyde (a simple sugar), and urea and glycine (the latter being a simple amino acid common to life) may have 2 Data from http://exoplanets.org/, accessed November 24, 2009.
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Spectrum Management for Science in the 21st Century been detected. Clearly, the carbon-nitrogen-oxygen chemistry that dominates life on Earth also dominates the complex chemistry of space. The radio spectrum is the place to pursue a connection between astrochemistry and prebiotic terrestrial chemistry because it gives access to the wealth of spectral lines. With the sensitivity and resolution of the coming generation of radio telescopes, it will be possible to search for sugars and amino acids and to follow the flow of chemistry from molecular clouds into protoplanetary systems. Is there a strong interstellar heritage to the chemical compounds that comets and other bodies delivered to early Earth? What is the dominant chemistry of a protoplanetary nebula and how does that change the chemical composition of the planet? Was life on Earth seeded by interstellar molecules? In addition to these questions, others arise because the molecular composition of interstellar and protoplanetary material is strongly impacted by the physical processes that act on the gas. Selected molecules can act as tracers to follow specific physical processes. For example, silicon monoxide (SiO) is commonly used as a tracer for strong shock waves associated with outflow activity, because silicon is heavily depleted onto dust grains, which are readily destroyed by shocks. That destruction liberates silicon into the gas phase, and this silicon is quickly incorporated into SiO. Methanol is a similar tracer for weak shocks, which evaporate ices. These tracers, and others presumably yet to be discovered, will provide important insights into the processes that shaped our solar system and that shape other planetary systems. Now that many planetary systems are being discovered, the search for signs of extraterrestrial life is becoming more compelling. The many planets that will be discovered in the “habitable zone” in the coming years are obvious targets. Searching in the radio band is thought to be the optimum strategy, and some limited searches have already been made with the telescope at the Arecibo Observatory and with other smaller telescopes, but no results have yet been achieved. The Allen Telescope Array (ATA), a dedicated instrument for searching for extraterrestrial signals, is completing its first stage of construction as of this writing and will begin work soon. It will be a multibeam telescope, able to look at many stars simultaneously. This search for an extraterrestrial civilization, while a “long shot,” is seeking an answer to a basic and profound question: Are we alone in the Galaxy? Origin and Evolution of the Universe In the past few decades, cosmology, the study of the origin and evolution of the universe, has been revolutionized. Whereas 30 years ago only a few broad facts in this field were known, today cosmology is a quantitative science with specific, testable hypotheses. This revolution stemmed from advances in astronomical techniques that broadened astronomy from its origin in the optical wave band to the entire electromagnetic spectrum. This expansion across the spectrum was
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Spectrum Management for Science in the 21st Century BOX 3.2 Redshift The continual expansion of the universe stretches electromagnetic waves so that they are received on Earth at a frequency lower than the frequency that they had when emitted. This effect is known as redshift, because light is shifted toward the red end of the spectrum as the distance is increased. Also, because the velocity of light is finite, more-distant galaxies are seen as they had been at earlier times. Looking at distant galaxies, one sees the universe at an early epoch. pioneered by radio astronomy, which has been essential to the study of cosmology because radio astronomy alone can detect the bulk of the coldest matter in the universe, and can detect it at enormous distances and early times (see Box 3.2). We now know that the observable universe has expanded from its origin in a Big Bang some 14 billion years ago. It cooled as it expanded, and nuclei of hydrogen and helium were formed in dense opaque plasma. With further cooling, nuclei and electrons combined into atoms, and the universe became transparent but now dark, since as yet there were no stars. In subsequent evolution, the higher-density regions were able to collapse under their own gravity, giving rise to the first stars and galaxies (see Figure 3.2). Fifty years ago the space density of bright radio galaxies was found to increase with distance faster than expected from the expansion, demonstrating the evolution of the universe and revealing a remarkable epoch of galaxy formation some 10 billion years ago. It was through this simple observation that radio astronomy ruled out the rival, steady-state theory of a non-evolving universe and favored evolutionary models in which the universe has expanded from a compact, hot origin. Radio astronomy also provides the strongest evidence for the Big Bang through the discovery of the cosmic microwave background (CMB) radiation in 1965. This background radiation fills space and has an accurately measured blackbody spectrum with a temperature of 2.725 K and a broad peak at about 100 GHz. This radiation was emitted some 400,000 years after the Big Bang, at a time when the universe had a temperature of about 3000 K and was becoming transparent. Since that time, the radiation has been stretched by a factor of about 1,000 through the expansion of the universe, and the temperature has decreased by the same factor. Because this radiation is so weak and so highly isotropic, it is difficult to distinguish from local sources of noise. Only very careful observations have been able to demonstrate its existence. The CMB has proved to be a gold mine of information about the early universe. The radiation comes from early times when the universe was nearly homogeneous,
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Spectrum Management for Science in the 21st Century FIGURE 3.2 Artist’s conception of the history of the universe. Time runs from left to right. The universe was born in an explosion popularly called the “Big Bang,” which perhaps came from a “quantum fluctuation,” a phenomenon well known in physics. After a period of hyper-expansion (“inflation”), the universe settled to a nearly steady expansion rate. As the plasma became neutral, the afterglow died out, and the universe became dark. After hundreds of millions of years, gravitational contraction of the material in the original density fluctuations produced the first stars, which gave off light, and so the “Dark Ages” ended. Further generations of stars formed, and galaxies and black holes coalesced from the stars. The universe became more complex and now is evolving rapidly, with many varieties of stars and galaxies and exotic objects, including a planet containing sentient beings who are able to contemplate this vast universe. Results from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite (shown in the figure) were used to make the afterglow pattern. Image courtesy of NASA/WMAP Science Team. but even then there were small density and temperature fluctuations that became the seeds of stars and galaxies. After extensive searches, the Cosmic Background Explorer (COBE) satellite found these fluctuations in 1992, at a level of 1 part in 100,000 of the background temperature. The fluctuations appear to be random on the sky, but they have a characteristic angular scale of approximately 1 degree, which reveals properties of the plasma from which they were emitted. Measurements of the angular power spectrum of the fluctuations have fixed the conditions of the universe at the emission time, when the plasma changed to a neutral
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Spectrum Management for Science in the 21st Century gas of hydrogen and helium. Along with observations in other wave bands, radio observations of the CMB have revealed that most of the material in the universe cannot be “normal matter”; it must be something that does not emit or absorb electromagnetic radiation: “dark matter.” In addition, 70 percent of the density is made up of “dark energy,” which has a repulsive antigravity effect, causing the expansion of the universe to accelerate.3 The fluctuations of the CMB have immense cosmological significance, and they are being studied with many instruments. The emission is broadband but peaks at a few hundred gigahertz, where atmospheric emission is a serious contaminant. Hence, the instruments are located on high mountain sites, on balloons, or on satellites. Very wide bandwidths are needed to detect the tiny signals. The CMB fluctuations are linearly polarized at about the 10 percent level, and this provides further insights into the early universe. CMB studies provide a testing ground for theories of fundamental physics and theories on the nature of space and matter, at energies that cannot be reached by experiments on Earth. Between the epoch of recombination, when the universe became transparent and the CMB was emitted, and the epoch of galaxy formation, when stars first began to light up the universe, lies the “Dark Ages” of the universe (see Figure 3.2). This period cannot be studied by optical astronomy, but radio provides a window by way of emission from neutral hydrogen. Over the next decade this study will be one of the major thrusts in radio astronomy. The emission, redshifted from 1.4 GHz, will be detected at much lower frequencies, 200 MHz and below. It will be very faint, and radio interference will be a serious concern. Such observations will have to be made from remote sites and will require careful attention to the mitigation of radio frequency interference (RFI). Pulsars and General Relativity Pulsars are ultradense collapsed cores of heavy stars in the form of neutron stars that have completed their nuclear burning and exploded. Pulsars have a very strong magnetic field and generate a radio beam that, because the neutron star is spinning, produces radio flashes in the same manner that a lighthouse generates optical flashes. In some cases, the pulsar, remarkably, is spinning at about a thousand times a second, leading to the term millisecond pulsars. Because a pulsar is ultradense, its gravity is ultrastrong, and it provides a natural laboratory for the testing of Einstein’s theory of general relativity (GR). One predic- 3 D.N. Spergel, L. Verde, H.V. Peiris, E. Komatsu, M.R. Nolta, C.L. Bennett, M. Halpern, G. Hinshaw, N. Jarosik, A. Kogut, M. Limon, S.S. Meyer, L. Page, G.S. Tucker, J.L. Weiland, E. Wollack, and E.L. Wright, “First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters,” Astrophysical Journal Supplement, 148: 175-194 (2003).
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Spectrum Management for Science in the 21st Century tion of GR is that the orbit of a pulsar in a binary stellar system slowly decays due to the emission of gravitational waves (Figure 3.3). The measurements accurately fit the prediction and prove that gravitational waves do exist. For this demonstration, Hulse and Taylor were awarded the Nobel Prize in physics in 1993 (see Box 3.1). However, this orbital decay is a “weak-field” effect, and GR has not yet been tested in the “strong-field” case. This leaves a fundamental question in physics: Is Einstein’s theory the final word in our understanding of gravity? Important questions are unanswered: Can GR correctly describe the ultrastrong field? Are its predictions for black holes correct? Is the cosmos filled with a stochastic gravitational-wave background? Radio observations of pulsars now approach these questions, and the largest radio telescopes, including the Green Bank Telescope (GBT) and the Arecibo Observatory, and especially the SKA, should give some answers. These telescopes offer the possibility of probing the strong-field realm of gravitational physics by finding and timing many pulsars. The ultimate goal is to obtain extremely tight limits on deviations from GR, to a level a thousand times better than present solar-system limits. In the coming years, radio observations will identify hundreds of millisecond pulsars across the sky. Timed to high precision (~100 ns, the time that it takes light to travel 100 feet), these pulsars will act as multiple arms of a cosmic gravitational-wave detector. This “telescope” will be sensitive to gravity waves at frequencies of nanohertz and will complement the much higher frequencies accessible to direct gravitational-wave detectors such as the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO, ~100 Hz) and the Laser Interferometer Space Antenna (LISA, 1 mHz). The largest radio telescopes will be crucial for these observations. Galactic Nuclei and Black Holes The first stars and galaxies formed out of the fluctuations in the early universe. A detailed understanding of how these processes unfolded will probably be one of the major achievements of astronomy in the coming decades. Astronomers have concluded that most galaxies have a giant black hole in their nuclei, with mass between a million and a billion times the mass of the Sun (see Box 3.3). It is not known whether the black holes formed first and galaxies of stars formed around them or the galaxies formed first and the black holes later condensed from the inner core. A remarkable correlation, however, has been found between the mass of black holes in galaxies and the mass of the halo of stars that surrounds them.4 This relation implies the existence of some regulatory or feedback process linking the black hole and its halo of stars. Over cosmic time, a galaxy grows through mergers 4 L. Ferrarese and D. Merritt, “A Fundamental Relation between Supermassive Black Holes and Their Host Galaxies,” Astrophysical Journal, 539(1): L9-L12 (2000).
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Spectrum Management for Science in the 21st Century FIGURE 3.3 The results of 30 years of observations at the Arecibo Observatory of the radio-emitting pulsar B1913+16. The pulsar is in orbit around a companion neutron star. General relativity (GR) predicts that the orbits of the two stars will shrink as orbital energy is lost to gravitational radiation. This figure shows the first detection of this effect: measurements of the orbital phase (the data points) exactly match the prediction (solid line) calculated with GR. SOURCE: J.M. Weisberg and J.H. Taylor, “The Relativistic Binary Pulsar B1913+16: Thirty Years of Observations and Analysis,” Astronomical Society of the Pacific Conference Series, Vol. 328, F.A. Rasio and I.H. Stairs, eds., 2005.
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Spectrum Management for Science in the 21st Century BOX 3.3 Black Holes Einstein’s theory of gravity (general relativity) predicts that when matter is compressed sufficiently, it contracts into a region of space where gravity is so strong that nothing, not even light waves, can escape. Hence, this ultimate compression forms a dark region that is called a black hole. However, matter falling into a black hole must release some of its energy before it goes “inside.” Thus there can be a bright region near the black hole. Further, the mass of the black hole still produces a gravitational effect. Radiation from infalling material and gravitational effects on the motions of nearby bodies can reveal the presence of a black hole and can give a measure of the mass that the black hole contains. In this way, black holes have been found with masses from a few times to a billion times the mass of the Sun. It has been shown that the center of the Milky Way contains a black hole with a mass of about 4 million solar masses.1 1A.M. Ghez, S. Salim, S.D. Hornstein, A. Tanner, M. Morris, E.E. Becklin, and G. Duchene, “Stellar Orbits Around the Galactic Center Black Hole,” Astrophysical Journal,, 620: 744-757 (2005). with nearby galaxies, and the disruptive forces of these events trigger episodes of star formation. Meanwhile, the central black hole grows episodically by accreting material from the inner parts of the galaxy. The accretion disk that forms during such periods can sometimes produce more radiant energy than all the billions of stars in the galaxy combined—the black hole and disk in this condition is called an active galactic nucleus, or AGN. An early result from radio astronomy was the realization that most of the bright sources of radio radiation lie outside our own Galaxy, the Milky Way, and have high redshifts, so they must be at “cosmological” distances. These objects lie in the nuclei of galaxies and are created as material swirls into giant black holes at the centers of the galaxies. Much of the radiation is emitted anisotropically in two narrow jets along the rotation axis of the black hole (Figure 3.4). The brightest objects—quasars—are those in which the jets are pointed almost directly toward Earth. The discovery and study of these powerful “radio galaxies” in the 1960s provided the first evidence for the existence of supermassive black holes—evidence that was based on the energy conversion required. A major discovery from radio astronomy, made by the technique of very long baseline interferometry (VLBI), was that the jets are flowing at relativistic speeds—close to the speed of light—and that the radiation is beamed by the effects of special relativity. The best-studied supermassive black hole is the one in the center of the Milky Way; it has a mass of about 4 million times the mass of the Sun. Attention was first drawn to it as an important astronomical object in 1974, when radio emission from its envelope was seen. This radiation comes from relativistically excited gas that is
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Spectrum Management for Science in the 21st Century ment, 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 Telecommunication 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 must be zero emission in the protected bands. It is a fundamental fact that any information-carrying signal must contain out-of-band emission, which spreads across a wide radio spectrum. The regulation of this necessary 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. ITU-R Recommendation RA.769 discusses interference protection criteria for the Radio Astronomy Service and defines threshold levels of emissions that cause interference detrimental to radio astronomy. However, for modern measurements these levels are unrealistic, because they are not based on the current state of the art. The levels are calculated as 10 percent of the noise fluctuations, but the noise is calculated with a bandwidth of the allocated channel. However, bandwidths hundreds of times wider than this are routinely used. In fact, much of radio astronomy would no longer be possible if observations were restricted to the allocated channels. The other factor in the noise calculation, the integration time, is assumed to be 2,000 seconds, whereas in modern practice the integration times often are 10 or 50 times longer. Again, if observations were limited to 2,000 seconds, much of radio astronomy, especially the new realms projected for the coming decade, would be impossible. Hence, the limits set by ITU-R Recommendation RA.769 are inadequate today, and they will become more so in the future. This means that unwanted emissions that are legal can be damaging to the RAS measurements. Another facet of the interference problem comes from emissions that essentially are unregulated. Cordless telephones, garage-door openers, and other unlicensed devices are allowed to have some low level of emissions, and at radio observatories an attempt is made to restrict the use of such consumer devices. But in fact they are powerful by RAS standards, as seen by the example of the garage-door opener on the Moon in §3.4, and will cause serious RFI if they are in the near sidelobes of a large antenna, even if they are far away. This problem also is worsening with the availability of new devices and their more widespread use. The incipient wide-spread use of automotive anticollision radar, operating at K-band, is a cause for concern in this regard.
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Spectrum Management for Science in the 21st Century A further cause of harmful RFI comes from transmitters that are operating illegally, either by producing excessive spurious or out-of-band transmissions or by operating at an unassigned frequency. The 1400-1427 MHz band is allocated to the RAS on an exclusive primary basis, but strong RFI has been seen in this band at many radio observatories around the world. Better monitoring of the radio spectrum and allocations would provide a better understanding of actual interference levels. Radio observatories are located in remote sites, often behind mountains, to reduce human-made noise, which is roughly proportional to the local population density. But the problem is particularly severe with aircraft and satellite transmissions, from which there is no escape. Observations of transient phenomena are especially vulnerable to RFI because of the highly variable nature of both the phenomenon and the RFI. Finding: The rules for out-of-band and spurious emissions in the primary allocated Radio Astronomy Service (RAS) bands (e.g., 1400-1427 MHz) do not provide adequate interference protection for RAS purposes. The FCC rules that pertain to the above finding are given in Appendix D. Finding: Geographical separation of radio telescopes from transmitters (e.g., through the establishment of radio quiet zones and the remote siting of observatories) is currently effective in avoiding much radio frequency interference, but the proliferation of airborne and satellite transmissions and the widespread deployment of mobile, low-power personal devices threaten even the most remote sites. Examples of Interference in a Protected Band Figure 3.10 shows interference in the band 1610.6-1613.8 MHz, which is allocated to the RAS on a shared primary basis—the interference was received in a 12 m antenna when an Iridium satellite passed through the beam. The satellite operates in the Mobile Satellite Service (MSS) band 1618.25-1626.5 MHz and, as seen in the figure, emits spurious radiation at 1612 MHz. During the measurement for Figure 3.10, careful attention was paid to ensure that the radiation was from the Iridium satellite itself and not from a GLONASS satellite, and that the RFI was not due to intermodulation in the receiver. Figure 3.11 shows the effect of similar satellite interference on an image made with the VLA in the same protected band, 1610.6-1613.8 MHz. The image made in the presence of the RFI is useless. The 1610.6-1613.8 MHz band is most commonly used by radio astronomers for studying the OH radical that exists in stellar atmospheres and in clouds in the Milky Way; the studies are conducted in a spectroscopic mode in which many
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Spectrum Management for Science in the 21st Century FIGURE 3.10 Showing radio frequency interference due to spurious emission from an Iridium satellite in the band 1610.6-1613.8 MHz, which is allocated to the Radio Astronomy Service on a primary basis. This measurement was made in Leeheim, Germany, in November 2006, with a 12 meter parabolic antenna. Careful attention was paid to eliminating the possibility of unwanted interference from intermodulation products in the receiver. Time runs down in the graph, over a total of 14 minutes, and frequency is horizontal. The motion of the satellite can be seen in the changing Doppler shift of the signals as the satellite passes through the beam of the antenna. The peak is about −85 dBm, substantially higher than the value recommended by the International Telecommunication Union, when it is converted to the standard model using an isotropic antenna. When converted to standard radio astronomy units, the flux density during the short bursts is about 2,500 Jy. Image courtesy of CEPT and BNetzA.
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Spectrum Management for Science in the 21st Century FIGURE 3.11 The effect of radio frequency interference (RFI) on an astronomical image made at the Very Large Array. (Left) Image of a faint “OH/IR star” made in a narrow band at 1612.22 MHz, within the band 1610.6-1613.8 MHz that is allocated to the Radio Astronomy Service on a primary basis. (Right) The same field of observation of the image made when an Iridium satellite was 22 degrees from the star. This image is made useless by the RFI. Images courtesy of G.B. Taylor, University of New Mexico. narrow bands are measured simultaneously. The RFI depicted in Figure 3.10 could adversely affect OH observations made when the satellite is well outside the main beam of the antenna, even for a large antenna like the GBT that has a forward gain of 63 dBi at 1612 MHz. The potential for harmful RFI is high, especially considering that the Iridium constellation contains 66 satellites. Mitigation “Unwanted” emission is of two kinds: “out-of-band” and “spurious.” Out-of-band emission is unwanted emission on a frequency or frequencies immediately outside the transmitter’s necessary bandwidth; it results from the modulation process. This type of emission is different from spurious emission, which results from harmonics or intermodulation products generated in the transmitter. When considering the regulation of signals that may spill into science service bands, account should be taken of how such signals will affect the scientific instruments in question. Simple excision techniques, in both time and frequency, have long been used to mitigate the effects of interference. More sophisticated procedures using statistical methods are currently under investigation, as described in Chapter 4. The present period is one of increasing sensitivity in the radio astronomy systems and increas-
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Spectrum Management for Science in the 21st Century ing use of the spectrum by other users, particularly the low-power wireless applications. These needs are conflicting, and the interference problem will undoubtedly increase. Focusing more attention on mitigation possibilities is important. At the same time, the radio astronomy enterprise must be protected by increased vigilance over its protected bands. The approach to reducing the impact of RFI at radio observatories occurs at several different levels, depending on the resources at each observatory. These approaches are briefly described in the following subsections. See Chapter 4 for additional discussion on this topic. Regulatory and International Meetings Only the largest observatories (e.g., the National Astronomy and Ionosphere Center [NAIC] and the National Radio Astronomy Observatory [NRAO]) are normally able to provide continuous staff attendance at international meetings, such as regular ITU Working Party 7D (WP7D) meetings and the World Radio-communication Conference (WRC). However, smaller, university observatories are kept informed of events in the international arena by regular teleconferences among the observatories, and by attendance at the U.S. WP7D teleconferences. The NRAO has a spectrum manager, an astronomer who pursues his own astronomical research but who spends a significant fraction of his time on spectrum management activities, including responding to the FCC on NRAO’s behalf and contributing to and attending international ITU meetings. Quiet Zones Around Observatories Only two observatories on U.S. soil benefit from Quiet Zone protection: NRAO (Green Bank, West Virginia) and NAIC (Arecibo, Puerto Rico). In addition, the United States is a major partner in the ALMA project being built in northern Chile. The Chilean authorities, through the Subsecretaría de Telecomunicaciones (SUBTEL), have agreed to a considerable level of protection from interference from other services around the ALMA site. The administration of these Quiet Zones requires resources. For example, in the case of the National Radio Quiet Zone (NRQZ) at Green Bank, West Virginia, all applications for fixed transmitters within the NRQZ are examined by NRAO staff, who make comments to the FCC on the basis of a technical analysis, usually including propagation predictions over the specific path. Often, some compromise as to power, frequency, and, in particular, precise location of the new transmitter, is agreed to between the parties concerned. The administration of an NRQZ by a radio observatory requires a significant, continuing effort. However, this effort is usually very well rewarded. For example,
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Spectrum Management for Science in the 21st Century at the Green Bank observatory in West Virginia during 2007, 538 requests for coordination within the Quiet Zone were processed. They involved 850 sites within the Quiet Zone and 872 transmission frequencies. In 13 cases, a site inspection was carried out. For about a dozen of the requests, a power restriction was eventually placed on the applicant’s FCC transmitter license. However, in a far greater number of cases, a solution agreeable to both parties, one that did not necessarily restrict the transmitter power, was negotiated. The negotiations usually resulted in alternative transmitter sites and/or directional antennas pointed away from the observatory, with a compromise in capability for the transmitter operator, while still providing adequate protection for the observatory. Local Radio Frequency Interference The NRAO engineering staff at Green Bank includes a team that tracks down instances of RFI that appear at the observatory. The team’s equipment includes a portable interference system, which can trace interference originating within a few miles of the observatory. If it is possible technically to suppress the interfering source, by simple technical means or perhaps by negotiation with the relevant party, this is done. In very rare cases, where the aforementioned methods fail, the FCC may be called on to intercede. Local Engineering The observatories themselves take all practical engineering precautions in the design and construction of equipment in order to provide adequate filtering and dynamic range so as to make equipment as immune as possible to interference from out-of-band signals. Special techniques are sometimes used, such as a dedicated antenna to monitor a particular source of interference; the data gathered by such monitoring can then be subtracted from the astronomical data by some means. This is more a research than an operational area at present, with few such systems currently in use. NRAO, for example, is investigating several mitigation possibilities, including active RFI cancellation, ways of extending dynamic range, and high-performance filtering using the latest technology. Data Processing RFI mitigation using software processing techniques is in routine use at most observatories. This approach includes data excision based on time or frequency; it is often carried out automatically, with some manual input. Other techniques are active research areas at a number of observatories, as described elsewhere in this report. See Chapter 4, in particular.
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Spectrum Management for Science in the 21st Century Finding: While unilateral radio frequency interference mitigation techniques are a potentially valuable means of facilitating spectrum sharing, they are not a substitute for primary allocated passive spectrum and the enforcement of regulations. Finding: Important scientific inquiry and applications enabled by 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 RAS observatories and necessitates costly interference mitigation, which is often insufficient to prevent RFI damage. 3.6 IMPORTANCE OF RADIO ASTRONOMY TO THE NATION The science of radio astronomy started in 1932, with the accidental discovery of radio waves from the Milky Way by Karl Jansky. Little happened in this field during the 1930s, but during World War II the United States mobilized a huge development effort in radar technology. The instrumentation and techniques resulting from this work fueled modern research in radio astronomy.13 Since then radio astronomy has continuously benefited from new technological developments; many of these have come from government and commercial sources, but some have been forthcoming from the development laboratories within radio astronomy itself. This section outlines some of the important benefits to the nation provided by radio astronomy. Radio Interferometry The development of interferometry has had widespread applications in fields in addition to radio astronomy and provides attendant benefits to society. The underlying principle of interferometry is the measurement of the relative time of arrival of signals from a radio source, among a group of antennas called an array. Triangulation then gives the direction of arrival of the radiation, meaning that the angular position of the radio source can be measured precisely. Furthermore, comparison of the arriving signals provides a method of imaging the source—that is, of determining the angular structure of the emission, which reveals the structure and dynamics of the source. These two applications—precise positioning and imaging—are important, as noted above, in fields beyond radio astronomy. For example, the radio technique of combining observations from different configurations of an array of antennas has formed the underlying principle of back projections, which is mathematically very closely related to all the techniques of medical imaging, such at computed axial tomography (CAT) and magnetic reso- 13 R. Buderi, “The Invention That Changed the World: How a Small Group of Radar Pioneers Won the Second World War and Launched a Technological Revolution,” Touchstone (March 1998).
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Spectrum Management for Science in the 21st Century nance imaging (MRI). There has been much cross-fertilization in the development of these techniques.14 The highest angular precision by radio interferometers has been achieved through the use of networks of telescopes distributed around the world and linked through a technique called very long baseline interferometry. To provide the time-of-arrival information, receiving stations must be equipped with precise clocks. The best technology for this purpose is the hydrogen maser frequency standard, developed originally at Harvard University and since perfected primarily for VLBI, radar astronomy, and space tracking needs. The positions of a set of very distant radio sources have been determined with VLBI networks, providing a stable, precise reference frame for a wide variety of applications. For example, with this established reference frame the relative motions of antennas on Earth can be tracked to an accuracy of a few millimeters per year. This capability led to the first measurement of the contemporary motions of tectonic plates. Fluctuations in the rotation rate of Earth and the orientation of its spin axis are continuously monitored this way and provide information useful to the understanding of the composition and motions of Earth’s molten core and the annual changes in polar-ice loading. These techniques of radio surveying based on triangulation formed the intellectual and technical basis for the development of the GPS and other terrestrial navigation systems. In the radio astronomy case, a distant radio source acts as a transmitter whose signal is received by a number of antennas, so that its position can be determined by relative time-of-arrival methods. In the GPS case, a user at an unknown location on Earth receives signals from an array of satellites, from which the user finds his or her position through triangulation based on a similar time-of-arrival analysis. GPS thus is highly analogous to the earlier VLBI, even to their both using atomic frequency standards as clocks. Communications Disruptions Energetic particles from the Sun, released in bursts called coronal mass ejections, arrive at Earth and can cause a disruption in radio communications, interference with GPS operation, surges on power grids, damage to Earth-orbiting satellites, and hazards to astronauts. The prediction of such events is important so that measures can be taken to ameliorate their effects. Amelioration, for example, might be achieved by shifting communications to less-affected frequencies and by placing satellites in standby mode. Hence the advance knowledge of the onset of these disruptive events is beneficial, just as the prediction of the arrival of meteorological events is important to the reduction of property damage and loss of life. 14 National Research Council, The Decade of Discovery in Astronomy and Astrophysics, Washington, D.C.: National Academy Press, 1991, pp. 129-130.
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Spectrum Management for Science in the 21st Century Coronal mass ejections originate from disturbances on the Sun, generally in the form of prominences and flares. Because CMEs consist of charged particles, they continuously emit radio emission as they travel outward from the Sun, and they can be tracked by radio telescopes, allowing 1 to 2 days of warning. Flares often are associated with huge bursts of radio emission, which have been known to seriously interfere with GPS operations, as described in §3.1. One of the goals of solar physics is to make long-term predictions of flares and CMEs by studying the emissions from the Sun (see Figure 3.7). Fundamental Physics The recent discovery, based on astronomical observations, that normal matter (baryonic matter) constitutes only 4 percent of the mass of the universe, while the rest is in the form of dark matter and dark energy, is transforming the understanding of physics. Radio astronomical observations of the rotation of galaxies have proved to be an excellent way to trace the distribution of dark matter. Meanwhile, laboratory experiments are underway in an attempt to identify the particle nature of dark matter. This combined effort in astronomy and laboratory physics can be expected to lead to a major step forward in the understanding of the universe. The measurement by radio astronomers of the timing of the rotations of pulsars in tight binary orbits about companion neutron stars, with exquisite precision, has been providing physicists with the strongest affirmative answer yet to the century-long question, “Was Einstein right?” (see Figure 3.3). Technology Development Radio astronomy has advanced the limits of technology as it has opened up spectral bands at progressively higher frequencies. For example, the best technology for low-noise receivers at frequencies above 100 GHz and into the terahertz range is based on quantum devices known as superconductor-insulator-superconductor (SIS) mixers. These devices were first developed by radio astronomers at the University of California in the 1980s and independently at AT&T Bell Laboratories. They have now been perfected, primarily for use in radio astronomy, to operate with noise levels at a few times the quantum limit. As military and telecommunications applications move into this band, they will undoubtedly make use of this technology. Precision Antennas The need for high sensitivity has led radio astronomers to develop the technology of building highly efficient, large, parabolic antennas, which have extensive
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Spectrum Management for Science in the 21st Century application in the telecommunications and military communities. Radio astronomers first developed the theory of how to design large, fully steerable antennas that maintain high surface accuracy in the presence of gravitational deformations. They invented an electronic surveying technique, known colloquially as radio holography, which enables reflector surfaces to be set to an accuracy of a few microns. Methods that they developed for measuring antenna efficiency from observations of standard radio sources and solar system bodies are in wide use. Distributed Network Computing The Search for Extraterrestrial Intelligence (SETI) project, a search carried out in radio bands, was faced with an enormous computational problem in analyzing its voluminous data to find nonrandom signals that might be of extraterrestrial origin. The computing resources needed to sort through the collected data were far beyond those available to the SETI researchers. The solution was to enlist the aid of interested people, who would download an analysis program and a section of data and would do the analysis in their computer’s background. More than 5 million people in 226 countries responded and are part of the SETI@home project. The SETI@home researchers went on to develop the Berkeley Open Infrastructure for Network Computing (BOINC). BOINC’s open-source volunteer computing platform currently engages the public in 42 scientific supercomputing projects, including climate modeling and global warming studies (ClimatePrediction.net); drug research for HIV, malaria and cancer, and protein folding (Predictor@home); gravity waves (Einstein@home); particle physics (LHC@home); as well as SETI@home. BOINC volunteers provide about 2 petaflops of computing power to the various projects, more than the world’s most powerful supercomputer. Education and Public Outreach Radio astronomy requires a broad spectrum of technically trained people, from theorists and observers with doctoral degrees to technicians with much less education, perhaps even trained primarily on the job. Theory, observations, and analysis are usually done by small teams consisting of one person or several senior people along with junior people, students, and postdoctoral researchers. Only a small fraction of the people trained for this field actually stay in radio astronomy; the majority go into other fields, usually still in a technical capacity. They form a valuable pool of people with a wide range of skills who readily find technical jobs in industry or government laboratories. The general public is greatly interested in astronomy, perhaps more than in other sciences. There is a steady stream of astronomy stories and images in the press. At the nation’s colleges and universities, it is the most common subject taken
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Spectrum Management for Science in the 21st Century as a science requirement, with thousands of students per year in elementary astronomy classes at the larger universities. All the major radio observatories have well-attended visitor programs, with the Arecibo Observatory in Puerto Rico drawing 120,000 visitors annually, of whom 30 percent are children. This interest translates into an appreciation of science and technology, and draws students into technical subjects, helping to provide the personnel resources needed in today’s world. Finding: In addition to the intellectual benefits that they provide, radio astronomy studies provide many technological benefits to American society. Finding: Radio astronomy provides a diverse and valuable set of educational opportunities.