REPORT 224-4 CHARACTERISTICS OF THE RADIOASTRONOMY SERVICE, AND INTERFERENC E PROTECTION CRITERIA (Question 5-1/2) (1963 - 1966 - 1970- 1974- 1978) A. CHARACTERISTICS OF THE RADIOASTRONOMY SERVICE I. Introduction The science of astronomy concerns itself with the study of the Universe. With very few exceptions, such as meteorites, particles ejected by the Sun and space probes, all the available information about the Universe is conveyed by electromagnetic waves. Radioastronomy and the radioastronomy service are defined in Article 1, Nos. 74, 75, and 75A of the Radio Regulations as being astronomy based upon the reception of radio waves of cosmic origin. Since it uses receiving techniques only, the radioastronomy service does not cause interference to any other service. Radio- astronomy at present utilizes the electromagnetic spectrum ranging from I MHz to about 300 GHz. Radar astronomy, which involves the transmission of a signal at a high power-level and the detection of that signal after reflection from celestial bodies, man-made satellites, or meteor trails; is a quite different service, and is covered in Question 6-1'2. Radioastronomy began with the discovery in 1932, by Karl Jansky, of radio waves of extra-terrestrial origin [Jansky, 1935]. The cosmic emissions with which the radioastronomy service is concerned constitute the "cosmic background noise" of communications engineering. Since Jansky's original observations, remarkable progress has been made in identifying the nature of these emissions, and radioastronomy is now firmly established as an important branch of astronomy. It is a new field of science, but it has already made important contributions to our knowledge of the measurement of atmospheric absorption at radio frequencies and also to our knowledge of the composition and nature of the Sun, the planets, interplanetary space and, in particular, the major disturbances in the solar atmosphere which are often the forerunners of interruptions to radiocommunication circuits and of radiation hazards to man in space. Further afield, studies of individual sources over a range of frequencies, and of the "line” emissions at precise frequencies resulting from transitions within certain atoms and molecules, provide information basic to our understanding of the physical processes responsible for the emissions of plasmas, and of the structure and evolution of galaxies and of the Universe as a whole. The radioastronomy service offers means for studying magnetic fields in distant regions of the Universe, and much of the information it provides is unique in that it is unobtainable by optical or other methods; one of its most spectacular characteristics is the ability to probe even further into the depths of space than is'possible with the largest optical telescopes. In addition to providing new knowledge and understanding of great significance to astrophysics and cosmology, radioastronomv is repaying, in a practical way, some of the investment of specialized radio techniques that helped to bring it into being. It supplied a major stimulus to the development of maser and parametric amplifier techniques, and hence to an increase, by orders of magnitude, in the sensitivity attainable in radio receivers. It has also made, and is continuing to make, significant contributions to the design of large steerable antennae and feed systems. The techniques of very long base-line interferometry (VLBI) are becoming of increasing importance for geodetic measurements of global distances. Radioastronomy methods are now being employed for radionavigation and are Ending applications in the medical field. The cosmic emissions with which the radioastronomy service is concerned are characterized by low power flux levels at the Earth. Most emissions show no modulation, other than random noise; an exception is the pulsars, which emit pulses of radio energy at extremely regular repetition rates. For many sources, the best times for observation are dictated by natural phenomena over which the observer has no control, and so radio astronomers are not generally able to observe over any chosen limited time interval at their own convenience. Furthermore, the radio astronomer is unable to change the character of the “signal” he wishes to receive; he cannot increase the transmitter power nor code the transmitted signal to increase its detectability. Recent discoveries of discrete line emission from molecules at frequencies other than those currently allocated to radioastronomy cause radio astronomers to be faced with interference situations over which they have no control. Studying radiation, the radio astronomer observes and measures all the properties of the electromagnetic emission. These are: — intensity, — frequency, — polarization, — the position in the sky, and - the variation of these parameters with time. The results are combined in order to gain an understanding of the physical processes in the Universe.
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