Until approximately 60 years ago, our knowledge of the universe outside of the Earth came almost entirely from visual, optical astronomical observations. The science of astronomy had made rapid advances after the invention of the optical telescope in the early seventeenth century and the application of photographic methods in the last century.(1) However, all these astronomical observations were in the visible portion of the electromagnetic spectrum. The discovery of cosmic radio emissions in 1932 ultimately led to the science of radio astronomy. Astronomical observations at radio wavelengths spawned a new branch of astronomy called radio astronomy which has developed into a recognized scientific field of research during the past 65 years. The older astronomy in the visible spectrum is now often called optical astronomy to distinguish it from the newer branch.
Radio astronomy and the radio astronomy service are defined in the NTIA Manual as being astronomy based on the reception of radio waves of cosmic origin.(2) This radio service is relatively new, having been proposed and established in 1959.(3) Consequently, many individuals do not fully understand the spectrum requirements of the radio astronomy service. Unlike many other radio services with which it shares the radio frequency spectrum, radio astronomy is a passive service and deals only with the reception of radio wavescosmic radio waves. A passive service is a receive-only radiocommunication application and does not have any influence on the transmitting source nor on the frequency upon which the transmitting source is likely to transmit. All other radiocommunication services, called active services, control both the transmitter and receiver, which can both be manipulated by the operator. The operational constraints on passive services, and the relatively low levels of received signals are the major causes of the vulnerability of radio astronomy to interference from active services.(4)
Radio astronomy has made humanistic, educational, and technical contributions to American society. The most basic contribution of radio astronomy is that this science provides modern answers to questions about our place in the universe. Scientists can now give quantitative answers to questions about which philosophers and scientists in the past could only speculate. Besides satisfying our curiosity about the universe, radio astronomers help nourish astronomical research, delivering to our country a dividend in the form of education. The educational benefit is the instruction taught in schools, colleges, and universities. The American public also benefits informally through scientific television programs, popular magazines, and planetarium shows. Radio astronomy successes attract the younger generation to scientific and technical careers. Radio astronomy innovations have been adapted to computer-aided tomographic (CAT) scans and magnetic resonance imaging (MRI), communications components for the communications industry, and the applications of millimeter and infrared wavelength technology for defense purposes.(5)
The science of radio astronomy has adopted many of the past discoveries in the fields of science, such as physics, chemistry, and optics. Now, it has come full circle where the observations and discoveries of radio astronomy are expanding and aiding discoveries in these and other scientific fields.
Radio astronomy is a relatively new science that has helped to revolutionize the concepts of space, our Galaxy, and the entire universe. Since virtually all knowledge about space comes from our observations of electromagnetic waves, a good understanding about the nature of light is needed to appreciate how discoveries were made by radio astronomy. Scientists had earlier observed that starlight passing through a prism spreads into its component colors. From this spectra, scientists over the years discovered an incredible amount about stars, including their masses, surface temperatures, diameters, chemical compositions, and motions toward or away from Earth. Though radiant energy cannot directly be measured quantitatively, these discoveries have helped scientists to convert radiant energy into some other measurable forms such as thermal, electrical, or chemical.
Stars with different surface temperatures emit electromagnetic radiation with different wavelengths and intensities. Because different chemicals also emit different wavelengths, astronomers can determine the chemical compositions of stars and interstellar clouds by studying their electromagnetic radiation.
Electromagnetic radiation can be emitted, transmitted through a transparent medium (glass, water, space, etc.), reflected (which is how we are able to see the Moon and planets), and absorbed. The last three processes are not strongly affected by temperature so long as you do not boil the water or heat the Moon. However, the first process, emission of electromagnetic radiation, is strongly dependent on temperature.
Scientists have identified the relationship between the temperature of an object, the energy emitted, and the resultant frequency (wavelength) radiated. First, as the object heats and increases its temperature, its color gets brighter because it emits more electromagnetic radiation. Second, the color, or dominant wavelength of the emitted radiation, changes with temperature. A cool object emits most of its energy at long wavelengths (e.g., lower frequencies such as infrared or red). A hotter object emits most of its energy at shorter wavelengths, such as blue, violet, or even ultraviolet.(6)
Stars produce their own electromagnetic radiation rather than reflecting, re-radiating, or absorbing light from some other source. A star's surface temperature can be measured from its dominant color (dominant wavelength).(7) Scientists also established a mathematical relationship between the energy of a photon and its wavelength. Basically, the energy carried by a photon of light is inversely proportional to it wavelengthlong wavelength photons like radio waves carry little energy, while short wavelength photons, like x-rays and gamma rays, carry more energy.(8)
Scientists discovered that, when magnified, the rainbow-colored spectrum of sunlight coming through a prism revealed hundreds of fine dark lines. These lines became known as spectral lines.(9) Scientists also discovered that when a chemical substance is heated and vaporized, the resulting spectrum exhibits a series of bright spectral lines. Further, each chemical element produces its own distinctive pattern of spectral lines.(10) This discovery allowed scientist to determine the chemical composition of distant astronomical objects by identifying the spectral lines in its spectrum.
Dark spectral lines were also observed among the colors of the rainbow and bright spectral lines were noticed against a dark background. The former are called absorption lines and the latter are called emission lines. The conditions under which these different types of spectra are observed and their descriptions are today called Kirchhoff's laws:
a. A hot object or a hot, dense gas produces a continuous spectruma complete rainbow of colors without any spectral lines. The emitted energy changes smoothly from one wavelength to another.
b. A hot, rarefied gas produces an emission line spectruma series of bright spectral lines against a dark background.
c. A cool gas in front of a continuous source of light produces an absorption line spectruma series of dark spectral lines among the colors of the rainbow.(11)
Quantum theory has helped in the understanding of various physical concepts, such as the constituents of atoms, what holds them together, and that atoms have energy levels. Suffice it to say that most people today accept the concept that electrons orbit the nucleus of an atom and that they are held together by the electromagnetic force of attraction between the electrons and the positively charged nuclear protons.(12)
Scientists have discovered that each chemical element has a unique set of energy levels, and when an electron jumps from its orbit to another orbit, energy is gained or lost. When an electron jumps from an inner orbit to an outer orbit, the atom gains (absorbs) energy; an electron jump from an outer to an inner orbit releases (emits) energy. For a particular element, the absorbing or emitting energy produces a unique set of spectral lines. These wavelengths have been measured in the laboratory, and radio astronomers can identify the radiating atoms in a star as hydrogen, helium, carbon, etc., by measuring the wavelengths at which they emit or absorb energy, and comparing these wavelengths with those measured in the laboratory. In this way, radio astronomers determine the chemical composition of distant stars just by analyzing their electromagnetic radiation.
Each chemical element produces its own unique set of spectral lines when its electrons jump from one energy level to another. Spectral lines (wavelengths of electromagnetic radiation) are known to be affected by the motion of their source, just like the sound of an emergency vehicles siren increases or decreases in pitch as it moves toward or away from the observer, respectively. Sound waves compress when the source approaches and stretch out when the source moves away. Spectral lines in the spectrum of an approaching source also shift toward the short-wavelength end of the spectrum and shift to the long-wavelength end when the source is moving away. The shift of the spectral line towards the short-wavelength is called a blueshift and the shift towards the long-wavelength is called redshift. This effect in the shifting of wavelengths was discovered by a mathematics professor, Christian Doppler, and is named in his honor as the Doppler shift. Radio astronomers can thus deduce the motion of a source (stars, galaxies, gases, etc.) by observing whether the emission or absorption lines in its spectrum are shifted in wavelength relative to what they would expect their wavelengths to be if the source were at rest.(13)
Radio astronomers, like many other scientists, use the scientific method of observing, creating mathematically-based theories, and experimentation to explore and explain physical reality. Essentially, the scientific method requires that the scientist's theories about the natural world agree with what they actually observe.
The process of radio astronomical discovery is much more than locating, collecting, archiving, and analyzing electromagnetic radiation. The research activities of radio astronomers today can roughly be separated into three categories: 1) observing and analyzing observations; 2) theorizing; and 3) computer modeling. Many people think that a radio astronomer spends his or her time observing the skies, spending long days and nights directing powerful receivers to reveal the secrets of the cosmos. In fact, most radio astronomers consider themselves very fortunate to get even a week's worth of data at radio astronomy observatories each year. Planning their data collections and analyzing their data take up the majority of their time.
Since radio astronomy was first officially recognized as a radio service, several frequency bands have been allocated for the radio astronomy service. In the table of frequency allocations, frequency bands that provide the greatest protection to radio astronomy are those for which the radio astronomy service has a primary allocation shared only with other passive services. Next in the level of protection are the bands for which radio astronomy has a primary allocation but shares this status with one or more active services. Less protection is afforded where bands are allocated to radio astronomy on a secondary basis to active services.
For many frequency bands, the protection for the radio astronomy service is by footnote rather than by direct listing in the National Table of Frequency Allocations. Footnotes are of several types. For exclusive bands allocated only to passive services, the footnote indicates that all emissions are prohibited in the band. Other footnotes are used when radio astronomy has an allocation in only part of the band appearing in the table. A different form of footnote is used for bands or parts of bands which are not allocated to radio astronomy, but which are nevertheless used for observations important to radio astronomy. It urges administrations to take all practicable steps to protect radio astronomy when making frequency assignments to other radio services. Although such footnotes provide little legal protection, they have often proven valuable to radio astronomy when coordination with other services is required. The frequency bands allocated to the radio astronomy service as shown in the National Table of Frequency Allocations are listed in TABLE 11 below.
|Frequency Band||Allocation Status||Comments|
|1336013410 kHz||Primary||Shared with fixed service (Footnote (FN) 533, G115)|
|2555025670 kHz||Primary||Shared with fixed, maritime mobile, and land mobile services. (FN: 545, US74)|
|37.538 MHz||Secondary||Secondary to non-government land mobile service. (FN 547)|
|3838.25 MHz||Primary||Shared with fixed and mobile services. (FN: 547, US81)|
|7374.6 MHz||Primary||Primary in Region 2. (FN: US74)|
|406.1410 MHz||Primary||Shared with fixed and mobile services. (FN: US74, US117)|
|608614 MHz||Primary||Primary in Region 2TV Channel 37. (FN: US74, US246)|
|14001427 MHz||Primary||Shared with Earth exploration-satellite (passive) and space research (passive) services. (FN: 722, US74, US246)|
|1610.61613.8 MHz||Primary||Shared with aeronautical radionavigation, radiodetermination satellite, and mobile satellite services. (FN: 722, 734)|
|16601660.5 MHz||Primary||Shared with aeronautical mobile satellite (R) service. (FN: 722, 736)|
|1660.51668.4 MHz||Primary||Shared with space research (passive) service. (FN: 722, US74, US246)|
|1668.41670 MHz||Primary||Shared with meteorological aids service. (FN: 722, 736, US74)|
|1718.81722.2 MHz||Implied Secondary||Footnote US256 applies. (FN: 722)|
|26552690 MHz||Secondary||Secondary to fixed, and broadcast satellite services. (FN US269)|
|26902700 MHz||Primary||Shared with Earth exploration-satellite (passive) and space research (passive) services. (FN: US74, US246)|
|4825.04835.0 MHz||Implied Secondary||Footnotes 778 and US203 apply.|
|4950.04990.0 MHz||Implied Secondary||Footnote US257 applies.|
|49905000 MHz||Primary||Footnotes US74 and US246 apply.|
|10.6810.7 GHz||Primary||Shared with Earth exploration-satellite (passive) and space research (passive) services. (FN: US74, US246)|
|14.4714.5 GHz||Implied Secondary||Footnotes 862 and US203 apply.|
|15.3515.4 GHz||Primary||Shared with Earth exploration-satellite (passive) and space research (passive) services. (FN: US74, US246)|
|22.2122.5 GHz||Primary||Shared with Earth exploration-satellite (passive), fixed, mobile (except aeronautical mobile), and space research (passive) services. (FN: 875)|
|42.543.5 GHz||Primary||Shared with fixed, mobile (except aeronautical mobile), and fixed satellite services. (FN: 900)|
|48.9449.04 GHz||Primary||Shared with fixed, mobile, and fixed satellite services. (FN: 904, US264)|
|51.454.25 GHz||Primary||Shared with Earth exploration-satellite (passive) and space research (passive) services. (FN: US246)|
|58.259 GHz||Primary||Shared with Earth exploration-satellite (passive) and space research (passive) services. (FN: US246)|
|6465 GHz||Primary||Shared with Earth exploration-satellite (passive) and space research (passive) services. (FN: US246)|
|72.7772.91 GHz||Primary||Allocated primary status in the United States per Footnote US270. Shared with fixed, mobile, fixed-satellite, and mobile-satellite services.|
|93.0793.27 GHz||Implied Secondary||Footnote 914 applies.|
|97.8898.08 GHz||Primary||Primary status per Footnote 904. Shared with mobile, mobile-satellite, radionavigation, and navigation-satellite services.|
|140.69140.98 GHz||Primary||Allocated primary status per Footnote 918. Shared with mobile, mobile-satellite, radionavigation, and navigation-satellite services.|
|144.68144.98 GHz||Primary||Allocated primary status per Footnote 918. Shared with the radiolocation service.|
|145.45145.75 GHz||Primary||Allocated primary status per Footnote 918. Shared with the radiolocation service.|
|146.82147.12 GHz||Primary||Allocated primary status per Footnote 918. Shared with the radiolocation service.|
|150151 GHz||Secondary||Allocated secondary status per Footnote 919. Secondary to the fixed, mobile, fixed-satellite, Earth exploration-satellite (passive), and space research (passive) services.|
|164168 GHz||Primary||Shared with Earth exploration-satellite (passive) and space research (passive) services. (FN: US246)|
|174.42174.5 GHz||Secondary||Allocated secondary status per Footnote 919. Secondary to the fixed, mobile, and inter-satellite services.|
|174.5175.02 GHz||Secondary||Allocated secondary status per Footnote 919. Secondary to the fixed, mobile, Earth exploration-satellite (passive), space research (passive) and inter-satellite services.|
|177177.4 GHz||Secondary||Allocated secondary status per Footnote 919. Secondary to the fixed, mobile, and inter-satellite services.|
|178.2178.6 GHz||Secondary||Allocated secondary status per Footnote 919. Secondary to the fixed, mobile, and inter-satellite services.|
|182185 GHz||Primary||Shared with Earth exploration-satellite (passive) and space research (passive) services. (FN: US246)|
|250251 GHz||Primary||Allocated primary status per Footnote 923. Shared with Earth exploration-satellite (passive) and space research (passive) services.|
|257.5258 GHz||Secondary||Allocated secondary status per Footnote 924. Secondary to mobile, mobile-satellite, radionavigation, and radionavigation-satellite services. (FN: US74, US211)|
|262.24262.76 GHz||Primary||Allocated primary status per Footnote 923. Shared with mobile, mobile-satellite, radionavigation, and radionavigation-satellite services. (FN: US211)|
|265275 GHz||Primary||Shared with fixed, mobile, and fixed-satellite services. (FN: 926)|
|278280 GHz||Implied Secondary||Footnote 927 applies.|
|343348 GHz||Implied Secondary||Footnote 927 applies.|
Until the middle of the 20th century, astronomical observations had been limited to visible light. The discovery by Heinrich R. Hertz of radio transmission and reception, followed by the development of radio technology for communications, provided the capability to extend our vision of the universe to the radio portion of the electromagnetic spectrum. The discovery of cosmic radio emissions in 1932 by Karl G. Jansky, a radio engineer, led ultimately to the science of radio astronomy.(14) In the late 1940's, following Grote Reber's pioneering research and aided by wartime advances in radio and radar technology, radio astronomy began its development into a major subfield of astronomy and astrophysics. Within a few years, radio astronomers had detected dozens of cosmic radio sources in addition to the Moon, Sun, and stars of the Milky Way Galaxy.(15) Recent improvements in sensitivity and in angular resolution have been critical to this growth of radio astronomy.(16) Enhanced sensitivity, achieved in using low-noise receivers and large radio telescopes, allows detection of extremely weak radio sources and measurement of their strength, which is extremely weak relative to man-made radio signals
Cosmic radio waves are of two general classes: 1) continuum emissions, that have large bandwidths, extending over large portions of the spectrum; and 2) spectral-line emissions that are narrowband with bandwidths as low as a few kilohertz. Continuum emission is radiation that cosmic sources emit at radio frequencies and may be observed over a broad frequency range. The exact frequency observed is not important. Spectral-line emissions, on the other hand, occur at nuclear, atomic, or molecular transitions and, since they are fixed in frequency by the laws of physics, must be observed at the frequency of occurrence where radiation intensity peaks.
The range of all electromagnetic waves is called the electromagnetic spectrum. As cited earlier, radiation can be represented as electric and magnetic fields vibrating with a characteristic frequency or wavelength. In the electromagnetic spectrum, long wavelengths correspond to the radio frequency spectrum, intermediate wavelengths to millimeter and infrared radiation, short wavelengths to visible and ultraviolet light, and extremely short wavelengths to x-rays and gamma rays. Figure 11 depicts the electromagnetic spectrum.
Part of the electromagnetic spectrum consists of the radio frequency spectrum and the optical spectrum. As mentioned earlier, the radio frequency spectrum is completely allocated to one or more radio services and consists of the radio waves with frequencies from 1 Hertz to 300 GHz. The optical spectrum has a frequency range of 300 GHz to 300,000 THz and consists of infrared, visible, and ultraviolet lights.
The visible and radio portions of the electromagnetic spectrum occupy positions coincidental with two important transparent bands in the Earth's atmosphere and ionosphere. These transparent bands are commonly referred to as the optical and radio windows, and are depicted in Figure 11.(17) The optical and radio windows are important because they allow these electromagnetic waves to pass through the atmosphere and be received on the Earth's surface.
Radio astronomy observatories are usually sited at locations specially chosen to minimize radio frequency interference from other radio services. Interference can be the result of frequency sharing of the same band, out-of-band emissions, or of spurious emissions. Radio interference is also the result of unintentional electromagnetic radiation from automobile ignitions, lighting, computers, and many other devices. Radio observatories are generally located a considerable distance away from major terrestrial sources of radio frequencies and usually have terrain shielding resulting from nearby high ground. TABLE 12 list major radio astronomy sites located in the United States.
Even though radio astronomy observatories and telescopes are sited at great distances from major centers of populations, interference is still received. Ironically, radio astronomy has produced many technical innovations that have been adopted by other services and has increased the general level of radio technology. This has led to an expanding scope of radio use that now threatens radio astronomy with increased interference.
|Arecibo Observatory National Astronomy and Ionosphere Center||Arecibo, Puerto Rico|
|Berkeley Illinois Maryland Association (BIMA) Interferometer||Hat Creek, CA|
|Caltech Millimeter Array||Owens Valley, CA|
|Caltech Submillimeter Observatory||Mauna Kea, HI|
|Five College Radio Astronomy Observatory||New Salem, MA|
|Haystack Observatory||Westford, MA|
|James Clerk Maxwell Telescope||Mauna Kea, HI|
|NASA Deep Space Network||Goldstone, CA|
|National Radio Astronomy Observatory (NRAO): Green Bank Telescope||Green Bank, WV|
|NRAO 12-Meter Telescope||Tucson, AZ|
|NRAO Very Long Array (VLA)||Socorro, NM|
|NRAO Very Long Baseline Array Station||Brewster, WA|
|NRAO Very Long Baseline Array Station||Ft Davis, TX|
|NRAO Very Long Baseline Array Station||Hancock, NH|
|NRAO Very Long Baseline Array Station||Kitt Peak, AZ|
|NRAO Very Long Baseline Array Station||Los Alamos, NM|
|NRAO Very Long Baseline Array Station||Mauna Kea, HI|
|NRAO Very Long Baseline Array Station||North Liberty, IA|
|NRAO Very Long Baseline Array Station||Owens Valley, CA|
|NRAO Very Long Baseline Array Station||Pie Town, NM|
|NRAO Very Long Baseline Array Station||St Croix, VI|
|Ohio State University Radio Observatory||Delaware, OH|
|Owens Valley Radio Observatory||Owens Valley, CA|
|Peach Mountain Radio Observatory||Stinchfield Woods, MI|
|Submillimeter Telescope Observatory||Emerald Peak, AZ|
Since its beginning, radio astronomy has contributed much to the science of astronomy and has produced numerous technical innovations that have benefitted radiocommunications and humankind in general. It has provided information on the atmospheric absorption of radio waves, important in the area of telecommunications and communications technology. Its initial innovation has led industry to the continuing development of low-noise amplifiers, extending to progressively higher and higher frequencies in the design and operation of radiocommunications systems and services. The discovery of pulsars and certain stars have led to techniques for precision navigation for military and civil purposes. Numerous astronomical techniques were used for many environmental applications ranging from the study of ozone depletion, the design of fusion reactors, and to the study of global climatic changes. More applications of astronomical techniques and devices that benefit the public are listed in Appendix B.
(1)John D. Kraus, Radio Astronomy, 2nd ed., 1986, at 11.
(2)National Telecommunications and Information Administration, Manual of Regulations and Procedures for Federal Radio Frequency Management § 6.1.1, at 611 (September 1995).
(3)International Telecommunication Union, International Telecommunication Convention (Geneva, 1959) (Geneva: ITU, 1959).
(4)For radio astronomy, the received signals from cosmic radio sources are of the order 109 times weaker than usual active service applications.
(5) National Research Council, The Decade of Discovery in Astronomy and Astrophysics, 1991, at 121135.
(6)R.C. Bless, Discovering the Cosmos, University Science Books, 1996, at 180.
(7)Physicists Josef Stefan and Ludwig Boltzmann in the late 1800's established the intensity-temperature relationship and it is named the Stefan-Boltzmann law in their honor. Also, in the late 1800's, the German physicist, Wilhelm Wien, discovered the mathematical relationship between color peak and temperature with the dominant wavelength, that is, the hotter the object, the shorter the dominant wavelength. This relationship is called Wien's law.
(8) Around 1900, another German physicist, Max Planck, discovered a mathematical relationship between the energy of a photon and its wavelength that is today called Planck's law.
(9)A German optician, Joseph von Fraunhofer, is credited with its discovery.
(10)A German chemist, Robert Bunsen, noted that chemicals could be identified by its distinctive color when sprinkled over his Bunsen gas burner. Bunsen's colleague, a physicist named Gustav Kirchhoff, suggested that the light from these colored flames could best be studied by passing it through a prism. Bunsen and Kirchhoff collaborated in inventing the first spectroscope that consists of a prism and several lenses that magnify the spectrum so it could be examined closely. After photography was invented, scientists began producing permanent photographic records of spectra. The device for photographing spectra is called a spectrograph.
(11) Bless, supra note 14, at 180. An example of Kirchhoff's first law is exemplified by the filament (a hot solid) in an incandescent light bulb, when heated, produces a continuous spectrum with no absorption or emission lines. A sodium-vapor street light is an example of the second law where the high-temperature, low-pressure sodium gas radiates two strong emission lines in the yellow-orange region of the spectrum. The third law is demonstrated by shining the light from a hot tungsten filament (continuous radiation is produced) through cooler sodium vapor resulting in two absorption lines in the yellow-orange, at exactly the same wavelengths at which the emission lines had appeared. See Bless, supra note 14, at 187190.
(12) Bless, supra note 14, at 189190.
(13) Motion of spectral lines that are perpendicular to an observer does not affect the wavelengths of spectral lines. See Bless, supra note 14, at 234235.
(14)In 1932, Karl G. Jansky was a radio engineer at the Holmdel, New Jersey, field test site of the Bell Telephone Laboratories and had been assigned the problem of studying the direction of arrival of thunderstorm static. Although Jansky discovered that some radio emissions were concentrated in the direction of the center of our Milky Way Galaxy, it was Grote Reber, a young radio engineer and amateur astronomer, who pursued and advanced this phenomenon further. Using a 9-meter dish antenna he constructed in his backyard in Wheaton, Illinois, Reber devoted considerable attention towards equipment improvements and undertook a systematic survey of the sky and in 1944 published the first maps of the radio sky. See Kraus, supra note 9, at 19.
(15)Stephen L. O'Dell, Radio Sources, Emission Mechanism, The Astronomy and Astrophysics Encyclopedia, at 595 (Stephen P. Maran ed., 1992).
(17)Source for these transparent bands came from John D. Kraus. See Kraus, supra note 9, at 14. These transparent bands are the result of the resistance (opacity) of Earths gases to certain electromagnetic radiation. The opacity of air to visible light is very small; hence, we have no difficulty seeing each other. The water vapor contained in the air is opaque to various wavelengths in the infrared frequency ranges.Go to: Cover / Exec Summary / Foreword / Chapt 1 / 2 / 3 / 4 / Appendices