Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses (2007) / Chapter Skim
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2 The Scientific Background
2 The Scientific Background
Pages 10-37
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From page 10... ...
With the discovery of new astronomical objects and the development of better equipment and techniques, radio astronomers regularly use frequencies from the lowest allocated radio astronomy band at 13.36-13.41 MHz to frequencies above 1000 GHz. However, much needs to be done to protect the current allocations and to meet the needs of modern research.
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From page 11... ...
ments, which depend on averaging for noise reduction.2 The Doppler shift of spectral lines that is due to the expansion of the universe also necessitates going outside of these allocated bands. • Most bands are shared with active services.
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From page 12... ...
2.1.2 Types of Radio Emission The spectrum of the celestial radio waves reaching the surface of Earth contains a broad continuum that covers the whole range of frequencies that can penetrate Earth's atmosphere, together with a large number of atomic and molecular spectral lines, each of which is confined to a narrow frequency range. 2.1.2.1 Radio Continuum The discovery of radio sources and the bulk of current knowledge about their nature and distribution and about the processes responsible for the radio emission from them have come through observations of the continuum radiation.
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From page 13... ...
Observations of continuum intensity determine the characteristic spectra of sources, but because the distribution of continuum radiation with frequency is relatively smooth, observations of this kind do not need to be made at specific or closely adjacent frequencies. Radio-frequency bands spaced at intervals of about an octave of the spectrum are normally satisfactory.3 The spectrum (strength)
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From page 14... ...
. VLBI relies heavily on the continuum bands, and, in general, observations of the most distant radio sources made by any radio astronomy instruments depend on continuum radiation.
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. Radio techniques allow spectral lines to be observed with very high frequency resolution that cannot be attained by techniques commonly employed at other wavelengths.
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From page 16... ...
molecule at 115, 230, and 345 GHz provide fundamental information about the nature and distribution of the densest forms of the interstellar medium. Many spectral lines from a variety of atomic species and from a large number of molecules have been found in interstellar space and in the atmospheres of stars, planets, and comets.
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From page 17... ...
Alwyn Wootten, National Radio Astronomy Observatory, http://www.cv.nrao.edu/~awootten/; accessed March 1, 2006. aA lower-case "c" indicates a cyclic structure; a lower-case "l," a linear structure; and a "?
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From page 18... ...
effect. The ionized gas that collects in the central region of a cluster of galaxies scatters the cosmic background radiation, producing a depression in the intensity of the background radiation at the location of the cluster.
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nuclear black holes are believed to be ubiquitous in nearby, normal galaxies and a natural part of massive galaxy formation. The most powerful radio sources known are quasars, which are distant, compact objects that emit radio energy at a prodigious rate.
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20 HANDBOOK OF FREQUENCY ALLOCATIONS AND SPECTRUM PROTECTION FOR SCIENTIFIC USES FIGURE 2.6 The history of the universe, the earliest epoch based in part on the Wilkinson Microwave Anisotropy Probe image of the cosmic background fluctuations that illustrate the "tiny density fluctuations." Courtesy of S.G. Djorgovski et al.
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. Courtesy of Chris Carilli, National Radio Astronomy Observatory, and Fabian Walter, Max Planck Institute for Astronomy.
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Courtesy of National Radio Astronomy Observatory/Associated Universities, Inc.
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Millimeter and submillimeter radio telescopes and interferometers are leading astronomers to a new era in the understanding of the star-formation process. At the other end of the stellar life cycle, radio astronomers study the circumstellar envelopes of evolved, post-main-sequence stars, which shed material into the interstellar medium during the latter stages of their evolution.
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From page 24... ...
2.1.5 Very Long Baseline Interferometry The technique of very long baseline interferometry allows radio astronomers to link radio telescopes many thousands of kilometers apart. VLBI provides highly accurate measurements of the structure of distant radio sources and molecular emission regions in space.
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From page 25... ...
. Originally developed as a radio astronomical technique for the high-resolution imaging of astronomical objects, VLBI has found many applications in Earth-based science, a notable example being the sensitive monitoring of crustal motions on Earth.
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2.1.6.2 Prebiotic Chemical Evolution The role of interstellar prebiotic chemistry in the origin of life will be better understood when there is a more complete inventory of complex organic molecules. These will be extraordinarily difficult to detect because they are probably much less abundant as compared with simple molecules and because their emission is spread over a much larger number of spectral lines.
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From page 27... ...
Spaceborne sensors are the only tools that can provide environmental data repetitively on a global scale. Use of active and passive microwave sensors removes the need for sunlight and permits penetration of clouds, thus providing nighttime as well as allweather coverage.
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From page 28... ...
Microwave sensors can detect variations in Earth parameters not readily detected at optical wavelengths. They can measure temperature, humidity, cloud, and trace gas profiles; surface soil moisture; ocean and estuarian salinity; sea-surface temperature; land-surface roughness and biomass; oceansurface wave height and sea state; and the moisture content and melt character of ice and snow.
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From page 29... ...
Atmospheric molecules of H2O, for example, have resonances at 22.235 and 183.10 GHz; frequencies near these resonances are needed for the measurement of water vapor by passive spaceborne sensors. Other atmospheric constituents with resonant frequency bands in the microwave spectrum include O2, O3, CO, NOx, and ClO.
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From page 30... ...
Radio sources on Earth, particularly those transmitting toward space and which may be benign to radio astronomers, are typically incompatible with spaceborne EESS sensors. Another difference arises owing to the rapid motion of satellite-based sensors through space, which limits the integration time available for sensor measurements to seconds, compared with the longer integration times used by stationary radio telescopes.
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Necessary bandwidths and sensitivities defined in terms of ∆Te for various measurements are found in Recommendation ITU-R RS.1028. The ITU-R recommends that in shared frequency bands (except absorption bands)
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From page 32... ...
They can be found in Recommendation ITU-R RS.577. 2.2.3.2 Active Sensors Current and planned Earth remote sensing systems include active spaceborne sensors, such as scatterometers, image-forming radars such as synthetic aperture radars, radar altimeters, precipitation radars, and cloud-profiling radars.
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From page 33... ...
Radio astronomy has been a copious source of transferable technology, algorithms, and trained individuals interested in applying remote sensing and receiver expertise in a variety of sectors, and especially in telecommunications.6 Some examples are listed below: • Originally developed as a radio astronomical technique for the high-resolution imaging of astronomical objects, very long baseline interferometry was used for applications in Earth science -- for example, the determination of geophysical parameters used in studying plate tectonics, polar wandering, 6National Research Council, Working Papers: Astronomy and Astrophysics Panel Reports, National Academy Press, Washington, D.C., 1991, p.
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From page 34... ...
• The VLBI reference frame of celestial coordinates, based on extremely distant radio sources, is basic to the periodic calibration of the GPS reference clocks. • Pulsar observations, VLBI, and applications of Einstein's general theory of relativity play a major role in precision navigation and geodesy -- including that of spacecraft -- and timekeeping.
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From page 35... ...
provide the only means of obtaining accurate assessments of land- and sea-surface and atmospheric conditions on a global scale. Satellite remote sensing has long been used to estimate sea-surface temperature through passive radiometry.
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From page 36... ...
utilities approximately $1 million per degree Fahrenheit daily as a result of an impaired ability to match energy supplies with demand.10 While it is difficult to ascribe forecast errors to interference occurring within any specific microwave band, it is noted that undetected interference in any passive microwave band can seed the growth of large errors in numerical weather-prediction models. (See also §3.7.1 and §3.9.1.)
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From page 37... ...
Landsat imagery and Shuttle Radar Topography Mission digital elevation models, as well as weather data and much data from the Earth Observing Satellite system, and in fact much of NASA's and NOAA's remote sensing data, are provided free of charge to U.S. researchers.12 Within the framework of the Global Earth Observation System of Systems (GEOSS)
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