JOURNAL OF RESEARCH of the National Bureau of Standards-D. Radio Propagation Vol. 67D, No. 2, March- April 1963
b The Protection of Frequencies for Radio Astronomy 1
R. 1. Smith-Rose
President, International Scientific Radio Union
(R eceived November 5, 1962) The International T elecommunications Union in its Geneva, 1959 R adio R egulations r recognises the Radio Astronomy Service in t he two following definitions: N o. 74 Radio A st1" onomy: Astronomy based on t he reception of waves of cos mi c origin. No. 75 R adio A st1"onomy Se1"vice: A service involving the use of radio astronomy. This service differs, however, from other r adio services in two important respects. 1. It does not itself originate any radio waves, and therefore causes no interference to any other service. L 2. A large proportion of its activity is conducted by the use of reception techniques which are several orders of magnit ude )]).ore sensitive than those used in other ra dio services. In order to pursue his scientific r esearch successfully, t he radio astronomer seeks pro tection from interference first, in a number of bands of frequencies distributed t hroughout I t he s p ~ct run:; and secondly:. 1~10r e complete and s p ec i~c prote.ction fOl: t he exact frequency bands III whIch natural radIatIOn from, or absorptIOn lD, cosmIc gases IS known or expected to occur. The International R egulations referred to above give an exclusive all ocation to one freq uency band only- the emission line of h ydrogen at 1400 to 1427 Mc/s. In all other cases, allocations are ma de on a basis of sharing wi th other radio services . . Considering the expensive nature of the ~ qu.ipm e ll t in u ~e a nd under development b.v radIO astronomers t hroughout t he world, it IS Important t hat t he protection of certain bands of frequencies for this science should receive serious attention by all t hose engaged in both lo cal a nd worldwidE' radio services. 1. Introduction It has long been quite unnecessary to justify the purs ~i t of Lhe science of astronomy, for its many The science of astronomy usin g optical telescopes applwatlOns have become so obviously necessary in to explore the Ileavens is some cell Luries old : the the co nduct of modern civilization. The derivation classical works of Copernicus, Galileo, Kepler, and and main tenance of time standards; the navigation Newton form lll.iles tolles in its development in tile services at sea, on land, and more r ecently in the 15th to 18th ce nturies. At first the scientific ob air, are all dependent in the long run on astronom se~·v~tio n al t~c hniques were limited to wavelengths ical observations; and on the knowledge built up in wlthlll the vlSua.l range, since they were dependen t the past by the work of astronomers all over the upon th~ d~ tect lll g power of the hUlNtn eye. But world. But there still r emains the fact that the ~he applicatIOn of electrical re?ording and measuring basic range of wavelengths available for use by the r mstruments allowed an extenSIOn to both the shorter optical astronomer is limited to approximately the I and longer wavelengths comprising the ultraviolet band 1000 to 30,000 A, a m ere fiv e octaves (coITe and infrared portions of the spectrum : and by and sponding to wavelengths from 10 - 7 to 3 X 10 - 6 m; 15 14 I large the photographic camera has now repl aced or frequencies from 3 x 10 down to 10 c/s). As t he eye of the human observer. will be seen, the development. of radio astronomy In order to ensure that the electromagnetic waves has added another 12 octaves of the electromagnetic I radiated from t he galaxy, stars, and planets, suffer wave spectrum to the performance of the tools and a m1 11]]11Um of absorption in traveling through the apparatus available for scientific research. ear th:s atmosphere, it has become an accepted I pra cLlCe ilutt astronomical observatories are situated 2 . Radio Astronomy in. l?cn,tioll s where clouds in Lite aLmospllere are a 111llllml!m ; and, y sually .the observfttory is in stalled The science of radio i\,stronomy is now over 20 at as lnglt an altItude as possible in t he areH, selected . years old, for it was in 1931 tht1.t Karl G. Jansky of ·Moreover witll the growth of our civilization, ft nd the Bell T elephone Laboratories, US.A., first de the corresponding increase of ar tificial illumill aLion tected radio waves received from the Milky Way; ou~doo rs, the ftstronomer is facrd with tlH\ quite later Lo be known in the radio field as "gnJactic senous problem of locating his observatory at a noise." Some 10 years lftter, during the second place where the background illumin ation from World ' Var, two distinct types of radio emission terrestrial sources is reduced to a minimum. from the sun were observed: H ey and his colleagues [1946] detected intense and variable radiation on 1 Presented at the Join t Meeting or U llSI (Canadian and ·Onited Stat es l\"ational Committees) and III E held in Ottawa 15-17 October 1962. meter wavelengths; while Southworth [1945] observed 99. steady eI11lSSlOn on centimeter wavelengths. At to be detected. Two years later, in 1951 , Ewen about the same time Reber [1942, 1944] observed and Purcell of Harvard annoUllced their successful radiation from the sun on a frequency of 160 Mc/s detection of this radiation from the Galaxy; and - (corresponding to a wavelength of abou t 1.9 m). this was immediately confirmed by the independent A little later, ano ther branch of radio astronomy observations of Muller and Oort [1951) in Leiden, sometim es designated nowadays as "radar astron and of Pawsey [1951), and Christensen and Hindman omy"- started following the experimental demon [1952] in Australia. In the past decade, a great stration by Hey and Stewart [1946) that radar deal of research has been con ducted by radio echoes could be received from meteor trails. astronomers on the hydrogen band radiation received Since these early discoveries, the new science of from various parts of the Universe; and this is radio astronomy has developed to an enormous ex undoubtedly making a considerable and worthwhile tent in all parts of the world; and a correspondingly contribution to our lmowledge of the space around extensive literature has developed in books and sci our earth. The need for this work to be protected entific journals. It has been demonstrated that from interference was recognized at the Geneva, radio waves from cosmic sources- or from the con 1959, meeting of the International TelecommUl1ica tinuum as it is termed- can be received over a fre tion Union, which assigned the band of frequencies quency range from a few megacycles per second to 1400 to 1427 Mc/s (wavelength 21 cm) on a primary more than 30 Gc/s, covering over 12 octaves of the basis for the exclusive use of the radio astronomy electromagnetic wave spectrum. Apart from obser service . This is referred to in a later section of vations on such powerful sources as the sun, the the paper. radio astronomer is confronted with the recording and study of very weak radiations, the strength of 2.2. Deuterium Line at 322 to 329 Mc/s which is several orders of magnitude less than those used in the practice of radio communications. To Following the research leading to the discovery of achieve some measure of success the radio ash'on tbe hydrogen line radiation, radio astronomers omer has, in the first place, had to design and con predicted the existence of radiation from deuterium struct large and highly directive antenna systems gas- an isotope of hydrogen of twice its atomic in order to collect enough energy to detect, and to weight- in a band of frequencies 322 to 329 Mc/s. ( enable him to lo cate as accurately as possible the It is to be expected from the much lower concen direction of the source of the radiation. Secondly, tration or "abundance" of this gas, that the radiation he has had to use the most advanced techniques in from it will be more difficult to detect and identify; the development of the highly sensitive receiving but it is natural that the radio astronomer shouid equipment required for his work. expect to be encouraged to do so. Shldowsky in Followrng the early work referred to above, two 1952 suggested that radiation from deuterium at a further discoveries have emerged as playing a frequency of 327 Mc/s (wavelength 91.6 cm) should fUlldamental part in the pursuit of radio astronomy. be detectable even though it was to be expected that After the observation by Hey an d his colleag ues of the concentration of interstellar deuterium is only fluctuations in the intensity of cosmic radio waves about 1/1000 that of hydrogen. Experimpnts made from the con stellation of Cygnus, it was sho'wn by in 1954 in Australia by Stanley and Price (described Bolton and Stanley [1948) that the fluctuation s were in 1956), appeared to confirm that the concentration associated with an intense radio source which, was less than this value. although in the same region, did not appear to be E arly in 1957, the position was reviewed by Adgie identifiable 'with any optical source. It thus became and Hey, who referred to the measurements made clear that there were discrete sources of cosmic during 1954-55 by Gemantzev, Stankevitch, and -1 radio waves, from which the visual radiation was Troitsky, indicating a deuterium concentration negligible, and these have become designated as relative to hydrogen of about one part in 300, an Ifradio stars." abundance ratio some 20 times greater than the The second discovery was that of the atomic terrestrial value. But new experiments conducted hydrogen spectral line in emission from tbe Galaxy. by Adgie and Hey, with equipment estimated to have greater sensitivity than that used by Stanley and 2.1. Hydrogen Line at 1400 to 1427 Mc/s Price, led them to conclude that the abundance of deuterium does not exceed 1/ 2000 that of hydrogen. For more than half a century, astronomers have Two further attempts were made in 1959 by Adgie been interested in the radiation from interstellar using the 250 ft radio telescope at Jodrell Bank, space, and particularly in the emission and absorp England, to detect the 327 Mc/s radiofrequency line tion properties of the gases which, with a luminous of galactic deuterium ; but even when the bandwidth belt of stars, form the Gala,xy or Milky Way, the of the receiver was reduced to 6 kc/s, the experiments average brigb tness of which is about twice that of gave a negative result. An investigation conducted the remaining portion of the sky. at the U.S. National Radio Astronomy Observatory In 1945, van de Hulst suggested the possibility at Green Bank, W.Va., led S. Weinreb [1962] to tbat hydrogen gas in tbe galactic clouds might be conclude that the ratio of deuterium to hydrogen a source of radio frequency waves; and this was is about 1/13000, or about half the terrestrial value. endorsed by Shldovsky [1949] who concluded that Further critical experiments will become possible tbe radiation would be of sufficiently high intensity when new radio telescopes of greater sensitivity have 100 -----~-~~~~--~-~~ been built; all d when the deuteriuDl line can then be use of radio for communication by ionospheric and sought as an absorption band in the continuum of tropo pheric scatter, for aerial and marine naviga radiation from a distant discrete source. It is of tion, and. 1'01' scientific purposes including radio as fundamental importance that such experiments tronomy ftnd communication with, and control of, should be carried out, and radio aslrOllomers should artificial earth satellites and space vehicles. In ad ::ne encouraged to do so by giving them. tbe utmost clition, the demand for radiofrequency channels for protection from inlerference by other users of this fixed and mobile services and for all forms of long ball d of frequen cies. distance communications had steadily increased. Al though with improved techniques, many users were 2.3. Radar Astronomy able to reduce the width of Lhe frequency channel required to carry the inform a LiOl1 , this reduction in Tn concluding this section, a brief reference may be spectrum space was more than offset by the great ~ made to the use of radar techniques in astronomy. increase in demand for further channels. Such techniques have been successfully used in The Geneva conference [1959] was attended by various parls of the world for the scientific investiga upwards of 700 people, including delegates from tion of aurora and meteor trails. For this pUTpose some 90 member countries of ITU, and represent the astronomer takes his place with other users of the atives of international organizations including the spectrum in that he needs to use some bands of International Scientific Radio Dnion (DRSI), the frequencies in which to emit radiation so that he may International Astronomical Union (lAD) and ' study the echoes reflected from the n atural phe the International Committee on Space R esearch nomena mentioned above. There is, however, no (COSPAR). The new frequency allocation table accepted in ternational defmiLion of the term "Radar which resulted from this confel'ence covered the Astronomy;" and indeed there is some difference of range 10 kc/s to 40,000 Mc/s (40 Gc/s), nearly four opinion among the astronomers themselves as to Lhe Limes that of the 1947 ALlanLic City table. . need for such a term. It is likely that proposals will Before referring to the fl'equencies which are of ' come before the forthcoming Plenary assembly of direct interest to radio asLronomers iL will be con CCIR in 1963 which may clarify this aspect of the venient here to refer to some othor organizations subject. associated with lTD. 3 . International O rganiza tion of Radio 3.2. Interna tiona l Ra dio Consultative Committee As we have seen above, the science of radio astron omy has developed rapidly, particularly over the The International Radio Consultative Committee )past decade or so. But over the same period, other (CCIR) wa established in 1927 for the pUTpose of users of Lhe radiofrequency spectrum have also been studying technical questions which are of interest in active in demanding increased facilities for all Lypes the development and practice of radio communica of communications, private and public, civil and tions and which are submittcd by the national military, sound and television broadcasting, and for administrations and private operating agencies con such associated applications as radar and naviga stituting its membership. It deals by separate study tional aids for ships and aircraft. In order to avoid groups with the technical problems of sending and a state of confusion, and possibly clJftos, which would receiving equipment, with the propagation of radio otherwise arise in the radio field, intemational waves of all frequencies and under all conditions, I organization s confer at intervals on both the tech with interference front natural and manmade dis nical and administrative problems involved. turbances and with the specification of conditions and tolerances appropriate to all applications of 3.1. International Tele communication Union radio. The function of CCIR is limited to the expression of an opinion on the various technical It became clear in the early years of the present questions studied and the transmission of conclu century, that the then new service of radio commu sions and recommendations to lTD. In brief, it nications would need to be protected at both national may be said that the object of its work is to see ,and international levels, and rules drawn up to gov- that the radiofrequency spectrum is divided among ern its use. Following earlier international meetings, the users in the most economical and efficient manner it was the Atlantic City Conference of 1947 that possible, to facilitate the conduct of the individual formed the present constitution and practice of the services required with the minimum of intel'ference. International Telecommunication Union (ITU) , which had then become one of the specialized agencies of the 'United. Nations, and had attained a membership of 3.3. Inter-Union Committee on Frequency Alloca 72 countries. At this conference a frequency alloca- tions for Radio Astronomy and Space Science tion table was drawn up for the three regions into which the world was divided for radio communica Although at meetings of CCIR in the past, tion purposes. DRSI , lAD, and more recently, COSPAR have been The next full administrative R adio Conference was able to attend in an observing capacity, it was at held in Geneva in 1959; and in the intervening 12 the General Assembly of URSI in London in 1960, years, major developments had taken place in the that a more definite step was taken to coordinate 101 the future requiremen ts of frequency channels for 4.1. Line Frequencies or Bands Arising from Natural both radio ast,ronomy and space science. At this Phenomena meeting, an Inter-Union Committee on Frequency Allocations for Radio Astronomy and Space Science In the first place, the radio astronOln er seeks full known more colloquially as IUCAF, was formed protection in those bands where naturally occurring with eq ual 11 Ie mbership from the constituen t bodies, radiation exists, and which therefore canno t be \. URSI , JAU, and COSPAR. Dr. J. F. D enisse, the changed by international discussion. As already French astronom er at the :Meudon Observatory, mentioned in section 2.1, the 1959 R adio Regulations was appointed Chairman, and the present writer give an exclusive allocation of the band 1400 to 1427 has under taken the services of Secretary-General Mc/s (the hydrogen line) to radio astronomy; [S mitJI-Rose, 1961}. The constitution and terms of although in certain European countries and the reference of the comm ittee include the coordination U .S.S.R. this band is also allocated to the fixed of the requirements of the three constituent bodies, service and the mobile, except aeronautical mobile, the fo rmulation of proposals for frequency allocations service. The next band in which there are strong 1 to meet th ese requirements, and cooperation with scientific reasons to expect the occurrence of natural CCIR with the view of ensuring t.hat these proposals radiation is the deuterium line at 322 to 329 Mc/s. are placed on the agenda of ITU. In this case the frequencies are shared with fixed, This Inter-Union Committee has already met in mobile, and aeronautical radionavigation services Brussels, London, and Amsterdam, and it will in all parts of the world. Efforts are being made convene again in Geneva ill N ovem ber. The in the European region to seek the cooperation of , Chairman and Secretary-General attended theinteri lll administrations using these other services so that meeting of CCIR Study Group IV in Washington in radio astronomers may conduct observations in this March 1962, and active steps are being taken to band free from harmful interference. ensure that the subject of frequency all ocations for Apart from these two bands (hydrogen and radio astronomy is included in the agenda of the deuterium) a number of other bands in which E xtraordinary Administrative Radio Conference naturally occurring radiation is to be expected have scheduled for the autumn (fall ) or ] 963. aroused the interest of radio scientists. These range from the OH line at 1645 to 1675 Mc/s (1.645- 1.675 Gc/s) , to those associated with oxygen, ozone, 4 . Frequencies for the Radio Astronomy and other gases at cer tain frequencies between about 30 and 100 Gc/s. It is also desirable that there Service should be opportunities to investigate the "windows" in the earth's atmosphere where the absorption of At the Intel'llational Administrative Radio Confer extrenwly high frequency radio waves passes through ence, ] 959, tw o definitions relevant to radio as a m inimum. Such windows are expected to be tronomy were drawn up and printed in chapter 1, found at about 30 and 90 Gc/s, and at higher "Terminology," or thc Radio Regulations [1959]. frequencies. These are: 74. Radio astronomy: Astronomy based on the 4 .2. Frequencies in the Continuum Background recep tion 0 f rad 10 waves of cosmic origin. 75. Radio astronomy service: A service involving For the second and more general class of research, the use of radio astronomy. radio astronomers have stressed their need to use a I From these definitions, it is clear that radio minimum of about 10 or 12 bands of frequencies astronomy differs from all other radio services in distributed throughout the spectrum. The approxi that it does not make use of radio emissions from mate positions of some of these have been indicated manmade sending stations. Its work is based on as fo llows: 40, 80, 160, 640, 2560, 5120, and 10,240 I the recep tio n and study of the natural emissions of M c/s. While the claim to bands near most of these sources in outer space. Thus the radio astronomer is recognized by footnote references in the 1959 can cause no interference to other services; but he Radio Regulations, in all cases the frequencies are justifiably seeks the exclusive use of certain bands allocated on a basis of sharing with the operators of of frequencies in the spectrum, so that he may other radio services. This is very unsatisfactory to pursue his studies free from interference from the the radio astronomer, who is naturally very anxious transrn itting station s of other services. Further to press his case for a more substan tial m easure of more, since the radiation he is seeking to measure is, pro tection. in many cases extremely weak, he must necessarily A very helpful move in the r ight direction was use advanced techniques- large aerial systems and made at the European Broadcasting Conference at very sensitive receivers. But the expense and effort Stockholm in 1961, where it was agreed that so far required to install and operate such systems can be as the European region was concerned, no major justified only if he is assured of adequate protection television broadcastin g s tations should be planned from interfering emissions. to operate in the band 606 to 614 Mc/s, so that this Details of all the frequencies allocated to radio might be kept freely available for the radio astronomy astronomy in the R adio Regulations, Geneva, 1959, service. This agreement was a logical consequence are given in the table for 111 ing appendix A. They of recommendat ion No. 32 (par. 3) of the R adio fall into two distinct classes. R egulations. Paragraph 2 of the same recommenda- 102 tion refers to the poss ibili ty of' maki ng a firm illlo observatories, the strength of interilli tten t radio cation in Lhe r ange 37 to 41 :\l1c/s, where frequencies signals from manmade stations not infrequw! tly at 38 and 41 Yl c/s are being used, or proposed for exceed this value by at least 10 or 20 db. As a use, by radio astronomers. comparativc figure, it may be mentioned that the power fiu x bcill g received in England on the trans 4.3. Degree of Protection Required atlantic radio link by way of Telstar is of the order of 10- 17 w m - 2 (c /s) - I. It has already been stated that the radio astrono What the radio astronomer is seeking is better pro mer carries out his l'esem'ch by receiving radio tection for his obscrvation s made at the very low emission from natural sources. Apart from certain fiux density he has to work with. It is suggested high power radiators such as the sun, the observAt ion that in ternational bodies, such as URSI and CCIR, of these emi ssions r equires the use of receiving should work towards the achievcmcnt of a tolerable techniqu es of the utmost sensitivity. [t is natural lcvel of interference at observatories of not more to find that the radio astroDom er, like his colleague than 20 rlb below 10- 24 W IlC 2 (C/S)-I. In this way who uses optical in struments, endeavors to locate the radio astronomer will be able to pmsue h is obser his observatol'Y where radio in terference frolll lIl an vations and research to the ulLim aLe benefit o[ our Illade sources "is expec ted to be a mi n imum. For knowledge o[ the space around om earth, and the example, thc si te for the U.S. National RH.dio establishmcnt of conLinuously expandmg splwe Astronomy Observatory was carefull y chosen Lo be com lllunications in the futurc. as free as possible from l'adio interfcrence. Fi nella:v 'With flo view to cnsUl'i ng lhat lhis maLLeI' recciv es (1958) lHls desc ribcd the rcs ulls of measmemcnts and adequate l1,ttcn Lion n.t in lCl'llaLional con[e }'(, nces, the prccautic ns taken to sccure proLcc Lion for thc thc Intcr-Union Com mi tLee referred Lo in sec Lion fu ture. Similarly Blulll, Dcnissc, and S tci nbcrg 3.3 has drawn up the Lwo rcco illm endn.tions gi,'cn in (1958) h ,lVC desc ribed Lhe sit e of Lhe NA~CY I'1ldio append ix B. It is hoped LhaL l hc CCIR wi 11 be astronom y fi eld station of the l\leudo n Obsel' vlLLory able to cndorse the c rccolll lll clldnLion s so Lhn.L Lhey in France as lin " ing been 10c,Ltecl 120 111 iles fr om may bc broughL bcfore thc next Administratlvc Radio P aris to secure th e ut ll lOs l reducLion in man lliad e Conference to be held in the a utumn of 1963. in terference. R adio as Lron Olll ers arc sleadily eO lll piling sys 5 . Appendix A. Review of Frequencies tematic meaSUl'e lll enls of the flu x densiLy received Assigned to Radio Astronomy in Radio from a variety of SOLll'CeS; and a few figurcs may be Regulations, Geneva, 1959 quoted to indicl1te the order of se nsitivity at \dlich H their eq uiplll enL is opern.ting. For eXll mplc, in Rcfetcnrc in radio lecture published in 19 58, ~I. R:vle gavc t \\~ O tables )<'0. Frequency b,UHl rlppliC'ah lc to rc~wlation s regions He(;. = Recommendation of results of radio flux: dell siLY as rccei"cd frolll 1" = Footoote galactic a nd cxLmgalactic sources respecL ively. TJle form cr ra ll ged from 1.1 Jor a Supernova to 220 for "'{cis 1 Slandard fn.'q Ut' Il C,\' 1,2, and 3 ______R ee. No. 31 F 20[ 2 t Cassiopeia A, the unit in each casc bein g 10- . \\'lltts gwu d tmnds 2.5. fi, per squitre mcler PCl' cycle pel' sceond . For thc lO, 15, 20, and 2.\. latter- the cx tragalaclic so urces- lhe values quolcd 2 \I' ithin I hc ran ge 3,- 1,2, ilnd 3_~ ~ Hee. Xo. 32 Jo' 23~ 2 2 41 "' leis: (a) 38.0 range from 0.04 to 133 X 10 - .[ WIll- (C/S) - I. For ± 0.25 (b) 40.63 eorrcspondin g mel1suremcnts aL Lhc H anarcl R
104 ------
Shklowsky, 1. S. (1952), Radiospektroskopiia Galaktiki, Astro 'Wells, H. 'vV. (1958), Flux measurements of Cassiopeia A and nomicheskii zhurnal 29,144- 153 (Mar- Apr). Cygnus A between 18.5 Mc and 107 Mc, Proc. IRE 46, Smith-Rose, R. L . (1961), The allocation of radio frequencies 205- 208. for scientific research, ICSU Review 3, 61 - 66. Internationa l T elecommunication Convention, Geneva, Southworth, G. C. (1945), Microwave radiation from the sun (1959). R adio RegUlations, Geneva, Published by the J. Franklin lnst. 239, 285- 297. General Secretariat of the I nternational Telecommunica Stanley, G. J . and R. Price (1956), An investigation of mono t ion Union, Geneva. chromatic radio emission of dcuterium from the galaxy, Final Acts of the E uropean VHF/UHF Broadcasting Con Nature 77, 1221- 22. ference, Stockholm, Published by the ITU, Geneva, 1961. van de Hulst, H. C. (1945), R adio waves from space, ~ed . Tijdschr. Natuurk. 11, 201 and 210. Weinreb, S. (1962), A new up-per limit to t he galactic deu terium-to-hydrogen ratio, atme 195, 367- 368. (Paper 67D2- 247)
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