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A search strategy for finding extraterrestrial radio beacons
ICARUS 20,
187--199 (1973)
A Search Strategy for Finding Extraterrestrial Radio Beacons R O B E R T S. D I X O N The Ohio Ntate (Tniversity Radio Ob~.ercatory, 2015 Nell Avenue, Columbus, Ohio 43210 Received April 23, 1973; rovised May 29, 1973 Tim I)rinciph; of Anti-Cryptography, together with our knowlcdgo of the structure and properties of the universe, lead to tim suggestion that ex~ratt~rrestrial ra(lio beacons will transmit continuously and omni(tirectionally, using binary sense-~witch(;d circular polarizaiton modulation. The expected frequency of transmission is the rest frequtmcy of hydrogen, relative t:o the galactic center. Probable signaling rates are within two orders of magnitude of one second. Meth()ds of implementing tho strategy are suggested, from both the receiving and transmitting viewpoints. THE PROBLEM
Our level of technology has reached the point whcre it is now possible to realistically consider searching for signals sent out b y extraterrestrial civilizations, in fact, a n u m b e r of limited searches have already been carricd out (I)rakc, 1961; Troitskii, 1.()71 ; Verschuur, 1973 ; Zuckerman, 1973) ; others are in progrcss or being planned (Zuckerman, 1973; lcaru,s. 1972). Before a large-scale search is begun, it seems appropriate to (leveh) I) a logical and consistent search strategy which will hopethlly maximize the i>robability of success. This work is an o u t g r o w t h of t h a t bcgun in P r o j e c t Cyclops (NASA, 1971), a feasibility and dcsign study of the subjcct. For general background discussions s e e . C~meron (1963), K a p l a n (1971 ), and Shklovskii and Sagan (1966). E x t r a t e r r e s t r i a l signals of intelligent origin may be of two types: those which are intended tbr the internal use of the sender but are incidentally radiated into space (e.g., radio, television, radar, etc.), and those, which are sent out for the express purpose of" initiating interstellar contact (e.g., beacons). The incidental signals are probably more difficult to detect, because they will be much weaker (they represent wasted power from the sender's viewpoint) Copyright ~5 1973 by Academic Press, Inc. All rights o f reproduction in any form reserved. Printed in Great Britain
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and because we cannot make m a n y assumptions a b o u t their characteristics. For beacons, however, we van assume the Principle of Anti-Cryptography, which encompasses the idea t h a t a beacon will bc designed and operated in such a way as to maximize its probability of discovery, both bv intcntiona l searches and by accidental observation (perhaps by astronomers). Another ihcet of this idea is t h a t since all choices of beacon characteristics must be made b y the sender alone, he will choose those which minimize the n u m b e r of unknown dimensions to be searched b y the recil)ient. I f there is a continuous range of choices bounded by definite limits, the principle can be implemented b y choosing one extreme or the other, r a t h e r t h a n b y making an indefinite "mi(hlle" choice. I f we neglect the incidental signals for the present, we can t)roceed to construct a search strategy based on this principle. We can also use the Assumption of Mediocrity (Shklovskii and Sagan, 1`()66), wherein we assu me t h a t we are just average among the inhabitants of the universe. Since beacon operation l)robably is a characteristic only of civilizations more socially a(ivaneed than ours let us assume tbr the present t h a t their technological capabilities also surpass ours, but only to the e x t e n t t h a t we can tbresee our own
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future capabilities. We assume t h a t a more advanced civilization will in some philosophical sense " a i m " their beacon toward being detected b y " e m e r g i n g " civilizations. W e can probably also assume their knowledge of physical laws and properties of the universe to be at least equal to ours. What, then, are the u n k n o w n dimensions t h r o u g h which wc must search~ T h e y are
:
1, Distance 2, ])irection 3. Time 4. Polarization 5. Signaling rate, bandwidth, modulation, all interrelated 6. Frequency.
and
L e t us consider each of these dimensions individually, and a t t e m p t to minimize their e x t e n t b y applying the anti-cryptog r a p h y and mediocrity ideas, as well as our own knowledge of the universe.
DISTANCE
Of all the unknowns to be searched, distance most lends itself to a progressive approach. I t is our n a t u r e to begin projects on a small scale, and e x p a n d with time, using the knowledge and experience gained at each step to guidc the n e x t step. Our technology will always set the ultimate distance limit to a beacon search at any given time. As distance increases, the only factor which improves the probability of beacon detection is t h a t the n u m b e r of possible stars increases. The n u m b e r of stars is proportional to distance cubed out to a b o u t 1000 L.Y. (NASA, 1972), and b e y o n d t h a t it increases more slowly.
The factors which reduce tile probability of beacon detection with increasing distance include : 1. The signal intensity decreases with distance, t h e r e b y increasing the power requirements of the t r a n s m i t t e r and the sensitivity requirements of the receiver. 2. Our knowledge of detailed stellar characteristics falls off rapidly in the 25100L.Y. range, although we can still make reasonable estimates of the fraction of inhabitable stellar systems beyond t h a t distance. 3. The longer light travel time requires greater cultural longevity. 4. The longer light travel time decreases the chances for and perhaps the a d w m t a g e s of an ultimate two-way exchange. For these reasons and others mentioncd in the Project Cyclops report, let us somewhat arbitrarily set our distance limit at a b o u t 1000L.Y. There are ample stars within this range to occupy us for some time, and it is sufficiently large to provide challenges to our receiver sensitivity and the available beacon power. Figure I shows thc size of a 1000L.Y. sphere relative to our g a l a x y (adopted from Schmidt, 1956), and giw~s perspective to the fact t h a t we are still only searching in our own " b a c k y a r d . "
DI[{ECTIO_N I t is generally agreed (Cameron, 1963; Shklovskii and Sagan, 1966) t h a t the planets of the civilizations we seek orbit stars having certain observable characteristics. A search for beacons should therefore perhaps be made in the directions of
.10 0
I
I [0,000 L Y.
F[(;. 1. Sizo and position of a 1000 L.Y. radius sphere (arrow) eent(,red oil tile Sun, relative to our galaxy. The contours show total mass density, relative to that near tile Sun.
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BEACON SEARCH STRATEGY
those stars. For relatively nearby stars (100 L.Y.), beacons are individually searchable (there is of the order of 1 candidate star per 25 square degrccs of sky), but beyond th at our knowledge of detailed stellar characteristics is poor, and the stellar density is such that a continuous survey might be profitable (at 1000L.Y. there are of the order of 40 candidate stars per square degree of sky). Which search mode to use is related directly to the beam size of the rccciving antenna. A continuous survey has the advantage of avoiding any errors or assumptions made in choosing the candidate stars. Such a survey should perhaps concentrate on those directions where the stellar density is highest. At 1000L.Y., the total number of candidate stars in the direction of thc galactic poles is about 75% of t hat on thc galactic e(tuator (calculated from Cyclops Fig. 6-1). Thus the stellar distribution is relatively isotropic and no directions can be ignored. TIME
A beacon operator could adopt one of two time strategies. He could transmit continuously and omnidirectionally, or he could transmit periodically in specific directions. The latter method provides for stronger signals in the desired directions, and these directions might be chosen to coincide with likely nearby stars. On the other hand, if thc receiving civilization is looking in a diflbrent direction at the time the sender is beaming toward them, the signal will be missed, regardless of the fact th at it may be a strong one. The situation is anah)gous to two needles trying to find one another in the proverbial haystack. I t is better for one nccdlc to stand still while the other nccdle systematically searches thc haystack, since this method will a l w a y s lead to ultimate success. The alternative approach, where they search tbr each other simultaneously, may never be successful. The advantages of omnidirectional beacon transmission havc previously been shown from an information-theoretical standpoint (Townasyan, 1964) and from a systems engincering standpoint (NASA,
1971, p. 61). The operator of a very longrange beacon would perhaps not transmit truly omnidirectionally but would weight his transmitted powcr more heavily toward the galactic plane, in proportion to the integrated stellar density in each direction, out to his assumed range limit.
BANDW1DTI-I
A continuous range of values exists, hence anti-cryptography leads us to zero or infinity, with zero being the easier and more practical. Infinite bandwidth cannot be really approached in practice because it implies either infinite power or zero powcr spectral density, whereas zero bandwidth can be easily approached. From an ease-of-searcil stan(tpoint, any bandwidth " n e a r " zero is equivalent to exactly zero. A bandwidth near zero (rather than infinity), although requiring a frequency search, allows the transmitted signal to be placed where it is most likely to be ibund, and less likely to be interfcrcd with. From our own communications experience, we know t hat when it is desired to attain the utmost range with a given power, the optimum system uses a near-zero bandwidth. SIGNALING RATE
A continuous range of rates exist, hence anti-cryptography leads us again to zero or infinity. The dispersion in the interstellar medium (Kaplan, 1971) places an upper limit on signaling rate of a few thousand [)its per second. Since the infinite limit is unattainable, anti-cryptography leads us to the lowcr limit of zcro. As with bandwidth, any value " n e a r " zero is equivalent in case-of-search to exactly zero, from a practical standpoint. How " n e a r " to zero should be used is open to conjccture but several considerations may enter. It should probably not require more than some small fraction (say 1%) of the recipients lifetime to receive at lcast a simple messagc. Taking our lifetime to be 100yr, we guess l y r as the longcst reasonable time from our standpoint. By Mediocrity we surmise then
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t h a t other beings might have lifetimes within an order of m a g n i t u d e of ours, lea(ling to a message rate estimate of one every~ 0.1-10yr. From the s t a n d p o i n t of an emerging communicative civilization such as ours, it might t)e difficult to receive complete messages if t h e y required more t h a n a b o u t half our planetary rotation period to coral)let e, since multiph" receiving stations might be require(l. Assuming they take this into account, we estinmte a greater message rate of one ever), 1.2-120 hr, by the same reasoning as before. Considerations of the " r a t e - o f - t h i n k i n g " may., give_• some insight, to the signaling rate, as opposcd to the message rate. We regar(l signaling rates of say 0.1 [)it per second to bc as slow as it is practical to go. Slower rates lead to little practical iml)rovement. Thus, wc might estimate their rate to be 0.01 to l bit per second, or having a period of l to 100sec. Coincidentally the periods of known pulsars lit; in the range 0.01-10 sec, which adds attractiveness to t h a t choice because it would increase the chances of accidental discovery by i)ulsar searchers. An a r g u m e n t against too slow signaling is the fact t h a t if the modulation m e t h o d is binary, and only one state happens to be detecte(l, the signal direction might be observed entirely during the undetecte(1 state and hence missed completely. An a r g u m e n t against a high signaling rate, with its a c c o m p a n y i n g high bandwidth, is t h a t it is more difficult to detect. The tra(le off between intbrmation content and detectability must be hcavily weighted toward the detectability end because it is far b e t t e r to send a little infi)rmat.ion and have it be received by someone, than to send lots of information and have it. be received b y no one. '.l'he signalin~ rate might easily be increased after the initial acquisition by sending information as to how and where a second, higher signaling rate c o m m m d c a t i o n channel mi,,ht be found. Once a signal is detecte(I, the receiving systcnl can be reoptimize(I from a search mode into a s t u d y mode. This poses no great problem. The central problem is t h a t of the initial signal acquisition.
POLARIZATION
A narrow band signal is inherently polarized, with the most general polarization being elliptical. Linear polarization (along with all other elliptical polarizations e x c e p t circular) contains an additional unknown, namely the position angh; of the field vectors. ()ne coul(t argue t h a t the vectors could be aligne(l p a r a l M or perpendicular to the galactic plane (a " n a t u r a l " Mignment for inhabitants of our galaxy), b u t f a r a d a y rotation of the vectors in the interstellar medium would tend to destroy a n y such alignment. Thus, linear polarization seems unlikely. The remaining polarization, circular, contains the least n u m b e r of unknowns an(t is therefore the most likely one. There are only two possible senses, left-hand and right-hand, whereas all other polarizations have an infinite n u m b e r of possible position angles. There seems to be no preferred characteristic of nature to in(licate which sense might be chosen. This will be discussed f u r t h e r tamer modulation. MODULATION
I t is reasonable to assume t h a t the beacon signal will be m o d u l a t e d in some way so as to carry intelligence. T h a t modulation ean be accomplished b y intro(tucing time variations into one of the four characteristics which completely describe a signal. a. b. c. (1.
Amplitude Phase Frequency Polarization.
From our own experience, we know t h a t the most effective modulation methods for m a x i m u m range arc two-state (binary) methods. It is also most effective if the t r a n s m i t t e r is on continuously, but switched from one state to the other (i.e., the states should be - l , + l r a t h e r t h a n o, +l). Since one cannot t r a n s m i t a negative amplitude, and since a (0, + l ) method is undeteetable (luring the zero state and thus might be missed, wc can probably rule out amt)litude modulation.
BEACON S E ~ R C g S T R A T E G Y
Binary frequency modulation is obtained by switching the transmitter frequency between two fixed frequencies at appropriate times. This modulation method introduces another unknown (the other frequency) into the reception problem. Since a frequency search of some sort seems inescapable at the receiving end, it seems undesirable to use a mo(tulation method which complicates the search by changing the transmitter frequency. Binary phase modulation may be achieved by introducing a 180 ° phase shift into the transmitted signal at appropriate times. From an information carrying standpoint, phase modulation is superior to any of the other three methods, since it requires only half the signal-to-noise ratio of the others to send the same inibrmation (Stein and Jones, 1967). The signal as a whole, however, is not any easier to detect. On the other hand, phase modulation is least likely to be detected accidentally, since astronomical observations rarely measure phase. Thus while phase modulation is likely to be used where information transfer is of greatest importance, it offers no particular advantage to detecting the initial signal. Binary polarization modulation may he achieved by changing between two orthogonal polarizations at appropriate times, such as between two perpendicular linear polarizations, or between left- and righthand circular polarization. In the discussion of polarization above, linear polarization was ruled out, and there was no good reason to choose between left- or right-hand circular i)olarization. If one chose to modulate, by reversing the sense of circular polarization, not only would the difficulties of other modulation methods be avoided, but the choicc of which sense of circular polarization to use wouhl bc
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obviated, because both senses would then be used. Thus this choice seems preferable, from the modulation and polarization standpoints, as well as from the anticryptograt)hic standpoint of avoiding arbitrary choices between e(tuMly likely alternatives. Polarization modulation is quite uncommon to us, probably because most o four communications are terrestrial, where depolarization effects make it less useful. Nevertheless it is used in laser range-finders (Mitton, 1972, p. 140), it is discussed by Beckman (1968), and could find wide application in space communications. A serendipitous side benefit of senseswitched circular polarization is that it could bc searched for using a sense-switched l)icke radiometer system which wouhl strongly discriminate against any linearly polarized signal. Since most terrestrial transmitters use linear polarization, interfcrence due to these transmissions wouM be reduced. Similarly, it would discriminate against unpolarized signals and hence remove most natural radio sources. In addition, the Dicke system would take full adwmtage of the sense reversals, giving a receiver output as shown in Fig. 2. In a binary modulation system, there still remains the unknown duty factor, or what average percentage of the total time is spent in each state? Clearly one cannot choose the extreme vah|es of 0%, 100% in either sense bccause then there is no modulation at all. The only remaining nonarbitrary allocation is 50%, 50%. This would allow for maximum integration time in each state, and seems the most likely choice. It should be noted that the anti-cryptograi)hy principle was applied indepen(tentlv to the tmknowns of bandwidth, signai ing rate and modulation, which are
RIGHT CIRCULAR
ZERO SIGNAL
LEFT CIRCULAR
FT('. 2. Output of a circular polarization sense-switched Dicke radiometer, in response to a binary circular polarization modulated signM.
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of course not independent. Nevertheless the results obtained are consistent with one another. FREQUENCY
I t is generally agreed (Cameron, 1963; Shklovskii and Sagan, 1966 ; Kaplan, 1971 ; NASA, 1971) that the optimuln frequency range for interstellar communications lies between approximately 1 and 10 GH z. Within t h a t range there are several spectral lines, notably hydrogen at 142o MHz and hydroxyl at 1665MHz. Since neither end of the frequency range is well defined, anti-cryptography leads us to choose one of the spectral line frequencies. The hydroxyl line can be ruled out because it is in fact multiple, consisting of several lines closely spaced. Another unknown would be introduced (that of which component to use) ifa multiple line were chosen. The remaining line, hydrogen, has a number of positive features that make it the best choice (most in keeping with the antieryptographic idea of choosing extremes wherever possible). It is the lowest frequency line within the microwave "window." It is the strongest emitting line in the window and hence most likely to be known and studied by mstronomcrs everywhere. It is perhaps the most astrophysically interesting line bccause the emitting hydrogcn is more widely dispersed throughout our galaxy than othcr emitting substances, and thercfore provides more inibrmation about the galaxy itself than other substances. Hydrogen is the lightest anti most abundant element in the universe. The emitting hydrogen is the simplest molecule among the other emitting molecules. Thc hydrogen line was originally suggested by Cocconi and Morrison (1963) as being a likely choice. Since then, however, several arguments have been advance(t as to why it might n o t be a good choice. The hydrogen emission itself might tend to drown out intelligent transmissions. Absorption by interstellar hydrogen at that frequency might prevent intelligent signals from reaching here. An): intentional transmissions near the hydrogen line might cause
serious interference to astronomers on the same planet who were trying to receive hydrogen emissions. On Earth, we do not allow any transmissions at all near the hydrogen line, for just t h a t reason. While all of these arguments against using the hydrogen line seem valid at first glance, a deeper look show's t h a t none really are. The local astronomer interference problem could be avoided in a number of ways, for example by transmitting from the far side of a satellite, or by transmitting on a frequcncy somewhat different from astronomical reception frequencies (a desired feature to be expanded upon later). The astronomical community of the beacon society may consider hydrogen line observations "pass6" by that time. In the ultimate analysis, it is only a societal value judgment to decide which is more vahmble to them---hydrogen line observations or an active search for other intelligent beings. Our sense of values today is such that we would probably choose the former, but that may not always be the case. And regardless of o u r sense of values, others may have a different set. HYDROGEN EMISSION INTERFERENCF
The deleterious effect of a particular noise added to a communications channel must be evaluate(| in comparison with other noises already present in the channel, rather than by itself in absolute terms. Thus it is of interest to compare the relative total noise level of the hydrogen line frequency with that of a nearby hydrogenfree frequency. The best receiving systems we have today for this frequency range have system noise temperatures of about 50°K. We can perhaps predict future improvement to 10°K. The galactic continuum radiation in this frequency range is shown in Fig. 3, which was compiled from Westerhout (1958), Altenhoff et al. (1970), Mathewson et al. (1962), and Hill (1968). The dotted lines indicate an area not covered by the above authors, and that area is subt.raeted from the total sky area in all calculations to follow. There is no reason to believe the missing area is funda-
BEACON SEARCH STRATEGY
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FIG. 3. Continuum brightncss temperature near the hydrogen line frcquency. mentally different from the othcr areas so its omission should not aflhct these results. The hydrogen line emission radiation is shown in Fig. 4, which was compiled from ])ieter (1972), Wannicr et al. (1972), Venugopal and Shuter (1970), Kerr and Hindman (1972), Kerr (1969), Vcldcn (1970), Kuilenburg (1972), McGee et al. (1963, 1966), Lindblad (1966), an<] Raimend (1966). This figure shows the radiation at the hydrogen line rest frequency, relative to our local standard of rest. That
,50°K
/
frequency is where the hydrogen radiation is strongest, and thus provides the "worst casc" results. In order to combine system temperature with the sky brightness temperaturcs shown in Figs. 3 and 4, the lattcr should be multiplicd by the receiving antenna beam cfficiency (typically 0.7), assuming the antenna bcamwidth is smaller than the angular extent of the emitting region, which is in general true. However, for the purpose at hand this factor can be neglected, thereby making the sky emis-
?
180"
FI(}. 4. Hydrogen line brightness temperature at zero velocity relative to the local standard of rest,.
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sion interference appear somewhat worse t h a n it actually is. The sky areas of interest are t a b u l a t e d in Table I. The systein noise and the continuum radiation noise are present at both frequencies under comparison, whereas the hydrogen radiation noise is present at only one. Qualitatively, let us say t h a t if the combined sky noise is less t h a n the system noise (thereby degrading the system b y less t h a n a factor of 2), then serious interference is not present. Conversely, if the combined sky noise exceeds the system noise, wc will say t h a t serious interference is present. F r o m Table I, the sky noise exceeds 50°K only in a very small fraction of the sky, even if hydrogen emission is included. Thus the hydrogen emission is no problem tbr the 50°K receiving systems of today. F o r a 10°K system, however, interference will be found over a b o u t 40% of the sky, principally from hydrogen emission. Thus hydrogen emission would ultimately be the limiting factor at some time in the future, if the local standard of rest hydrogen frequency were to bc used. E v e n t h o u g h one m a y decide to use the hydrogen-line fl'equcncy, t h a t choice covers quite a wide band of frequencies, due to Doppler shifts caused by motions of both the t r a n s m i t t e r and receiver. These include p l a n e t a r y rotation, p l a n e t a r y revolution around its star, peculiar motions of the star, and revolution of the star around the galactic center. I t is common practice with hydrogen-line observations to remove effects due, to our own first three motions, b u t not the last. We have no way of knowing the doppler shift of the t r a n s m i t t i n g
TABLE I SKY ~OISE FROM VAI~IOUS SOIJffCCES
:Noise source Continuum (Fig. 3) Hydrogen relative to LSR (Fig. 4) Hydrogen relative to GC (Fig. 6)
% Sky > 50° % Sky -> 10° 0.3 4 0.2
8 37 3
beacon, and the beacon operators have no way of knowing our doppler shift, so we c a n n o t compensate for each othcr's shifts. On the other hand, we (lo know our own doppler shift, and the beacon operators can be assumed to know theirs, so it would seem possible t h a t if we each corrected for our own shifts, we wouht arrive at a unique c o m m o n frequency, t h e r e b y eliminating the unknown of frequency entirely. The nearest common reference point t h a t we sharc is the galactic center, so all doppler shifts should be, corrected to be at rest relative to t h a t point, including corrections for galactic rotation. This rotational correction is larger than the others an(1 is shown in Fig. 5 {br our own location. A n o t h e r reference frame t h a t might be considered in the future, particularly fbr extragalactic communications, is the 3°K background frame discussed by Conklin (1969), H e n r y (1971), and yon H o e r n e r
(1973). The frequency arrived at by making the galactic rotation corrections is a function of direction, amt thus there is the practical t)roblcm of making them. In the receiving case, it is trivial because it simply means tuning the receiver to a specific frequency determined b y where the a n t e n n a is pointing. In the t r a n s m i t t i n g ease it is somewhat more involved, because an omnidirectional dol)t)ler-removing beacon must transmit simultaneously on a (tiffercnt frequency in e v e r y different direction. There seems to bc no f u n d a m e n t a l reason why this cannot be done because it simply means t h a t the t r a n s m i t t e r power is distributed not only in direction, b u t in fre(luency as well. This does not necessarily mean t h a t the t r a n s m i t t e d signal in a n y given direction is weaker as a result of being distributed in frequency, because the frequency distribution is related to the direction distribution on a one-to-one basis. In a n y specific direction, only the frequency needs to be changed. A few ways in which this might be accomplished are given here, as a d e m o n s t r a t i o n t h a t it is not impossible, b u t it should be borne in mind t h a t the beacon operators could have much b e t t e r ways t h a n we can as y e t conceive.
BEACON SEARCH STRATEGY
I
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Fro. 5. Approximate velo(:ity (kin/see) of the local standard of rest relative to the galactic center, due to galactic rotation. METHODS OF OMNIDIRECTIONAL ])OI'PLER REMOVAL 1. Divide the sky into N solid angles. Construct N transmitters, each connected to an a n t e n n a having solid angle beamwidth 4rr/N, and cach operating at the appropriate frequency to remove average doppler shift in its particular direction. This quantization of direction will cause small fre(tueney errors in some directions, b u t the errors decrease for larger N and m a y be acceptable. 2. L a u n c h an interstellar communications satellite b a c k w a r d along the galactic orbit to achieve a s t a t i o n a r y position relative to the galactic center. Then t r a n s m i t omnidirectionally at the hydrogen rest frequency. Crudc calculations indicate t h a t thc satellite position will remain stable against stellar encountcrs or falling t o w a r d the galactic centcr for times which are long even for a beacon civilization. In addition, the satellite will remain within reasonable range of its host planet for a v e r y long time, so it could be r e m o t e l y controlled from thc host planet. This also insures t h a t its angular separation from the host planet would not lead us to incorrect conclusions as to thc source of the beacon. The satellite could also receive a n y replies
we might direct toward it and relay t h e m to the host planet, so angular separation might not be a problem a n y w a y (and might even be all insurance policy for timid beacon operators not wishing to reveal their e x a c t location!). In the c v e n t the satellitc ultimately got too far away from the host planet to be useful, it could simply be t u r n e d off and a new satellite launched. Power for such a satellite might be derived from fusion generators and any m a n u e v e r ing necessary could bc done b y low t h r u s t deviccs such as ion engines. The use of such a beacon satellite would remove the problem of astronomical selfinterference, because the t r a n s m i t t e d signal would e m a n a t e from only one point in their sky (their galactic latitude 0 °, their galactic longitude 270°), and be highly dopplershifted relative to c o m m o n l y used hydrogcn line obscrvational frequencies. I f circular polarization were used on the beacon, hy(lrogcn line receivers couhl be built to discriminate against t h a t polarization. 3. Construct an a n t e n n a and t r a n s m i t t e r system which removes all doppler shifts directly. We have t o d a y phased a r r a y r a d a r systems which e m p l o y large n u m b e r s of a n t e n n a elements interconnected b y electrical networks whose phase and delay
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characteristics are predetermined functions of frequency. As a result of this, when frequency is changed, the effective direction in which the a n t e n n a array points also changes in a predetermined way. These antennas are called frequency scanned arr;~ys and ]rove the p r o p e r t y t h a t the a n t e n n a t)ointing direction is tmi(tucly rclated to the opem~ting fl'equcncy ( H ansen, 1966). This prol)erty is exactly what is needed to remove doppler-shift simultaneously in all directions. An a r r a y of this t y p e could tie constructed to have the appropriate variation of direction with frequency to remove all doppler shifts. Then one could t r a n s m i t band-limited white noise into the a r r a y to achieve the desired simultaneous coverage of all directions. T r a n s m i t t e r modulation could be easily achieved by, ibr example, pulsing the entire t r a n s m i t t e r on and off or by changing the polarization at each element. F u r t h e r (tctails of such an a r r a y will be published elsewhere (Dixon, 1973). I f this metlm(1 were used from the surface of a planet, several arrays located equidistantly a r o u n d the globe would be desirable, and the necessary doppler shifts would be time-varying. This woul(t necessit a t e some t y p e of c o m p u t e r control of the phasing networks or a r r a y orientation. These time variations could be minimized b y locating the arrays at the geographical
poles of the planet, or on a planet v e r y distant from its star, or on a distantly orbiting satellite. ~II)E
BENEFI.TS OF I)OPPLEI¢, CORRECTION
In addition to providing a unique frequency, the complete doppler-correcting m e t h o d removes most of the hydrogen emission noise. I f galactic rotation doppler shift is removed, the receiver is in general t u n e d to frc(lucncies outside the band where hydrogen radi~ltes. Figure 6 (compiled from the same references as Fig. 4) indicates the hydrogen emission brightness t e m p e r a t u r e for this case. The temi)erature exceeds 10°K for only a b o u t 3°/(, of the known sky and thus would not c~use interfercnce even to a future 10°K receiving system. In fact, the continuum radiation is then the domim~nt noise, so t h a t even a "i)crfect '' receiver of ()°K would not tie appreciably affected by the hydrogen noise. One reason for this is t h a t the hydrogen emitting regions are in general much more distant t h a n the 1000 L. Y. range considered here, and hence have quite different galactic motions. There are ,~ few high velocity hydrogen clouds known to exist which were not included in the source d a t a for Fig. 6, b u t t h e y do not appreciably alter this result. Due to the rotation of the Earth, the
1 (.10°I( leo-
,~oo
,20-
9o"
69"
~
"-
koJ / 2
330"
3oo °
27o*
240"
21o"
18o"
i I //// -I FiG. 6. Hydrogen line brightness temperature at zero velocity relative to the galactic center.
BEACON S E A R C H S T R A T E G Y
receiver f r e q u e n c y must be changed as a function of time to provide the necessary doppler shift. An interfering terrestrial signal would not exhibit this frequency variation and would hence be easily identified. The possibility of an interstellar communications network among a n u m b e r of civilizations using the doppler-corrected method is a t t r a c t i v e from a communications enginccring standpoint, because the frequelmy used b y each one is inherently and simply d e t e r m i n e d b y its direction from any given point, and m u t u a l interfcrcnce couhl occur only when two such civilizations lay along the same line of sight, at diflbrent distances. Such beacons would also be useful for interstellar navigation, inasmuch as their received frequency indicates direction, and the received signal strength from a beacon of known power indicates its distance. HYDROGEN ABSORPTION Our knowledge of hydrogen absorption is r a t h e r meager, since the effect can only bc mcasurc(t in the direction of known radio sources, and this has been done for only a small n u m b e r of them (Clark et al., 1961 ; Riegal and Crutcher, 1971; Radhadkrishnan et al., 1972). I t appears however, t h a t
I
197
most absorbing regions are more distant t h a n 1000L.Y., are highly c o n c e n t r a t e d along the galactic plane, and the absorption b a n d w i d t h is quite small. Thus there is no practical difficulty arising from this effect. In a n y casc, w h a t c v c r hydrogcn might be present within 1000L.Y. is essentially at rest relative to us, since differential galactic r o t a t i o n is small at t h a t distance (Fig. 7). Thus thc m e t h o d of dopplercorrection would remove most hydrogen absorption effects, analogously with the emission effects. FREQUENCY SEARCH F r o m a practical standpoint, it would still be necessary for us to conduct a search in the frequency dimension, even using the doppler-correction method, because at the present timc we do not know our own galactic rotation velocity to b e t t e r t h a n a b o u t 10%. At the hydrogen frequency, this corresponds to a frequency u n c e r t a i n t y ranging from 0 to a b o u t + 1 2 5 k H z , depending on direction, with an average value of a b o u t ±50 kHz. This will of course decrease with time. l)espite this f r e q u e n c y uncertainty, the doppler-correcting m e t h o d narrows the necessary search range considerably, relative to the zero velocity case. I t is interesting to note t h a t if such a signal
O"
Fro. 7. Approximate velocity (km/sec) of the local standard of rest relative to points at 1000 L.Y. distance, due to differential galactic rotation.
198
DIXOh"
were e v e r d i r e c t e d , its f r e q u e n c y w o u l d i m m e d i a t e l y give us a n a c c u r a t e m e a s u r e o f o u r r o t a t i o n a l v e l o c i t y , since t h e s e n d e r p r e s u m a b l y k n o w s his v e r y a c c u r a t e l y .
CONCLUSIONS A s t e p has b e e n t a k e n t o w a r d a search s t r a t e g y b a s e d o n c o n s i s t e n t a p p l i c a t i o n of b a s i c p r i n c i p l e s to all a s p e c t s o f t h e p r o b lem. T h e P r i n c i p l e o f A n t i - C r y p t o g r a p h y l e a d s o n e to b e l i e v e t h a t i n t e r s t e l l a r b e a c o n s will t r a n s m i t o m n i d i r c c t i o n a l l y , a t a d o p p l e r - c o r r e c t e d h y d r o g e n line frequency, using binary sense-switched circular polarization modulation, with s w i t c h i n g r a t e s of t h e o r d e r o f l sec. C e r t a i n of t h e s e c o n c l u s i o n s are n o t new, b u t have been reaffirmed by a different a p p r o a c h . T h e s t r a t e g y p r o p o s e d is b y n o m e a n s t h e o n l y o n e possible, a n d m a y well c h a n g e as n e w ideas or facts come to light. N e v e r t h e l e s s , it is a s t r a t e g y which could be i m p l e m e n t e d to(lay, u s i n g t o ( l a y ' s knowledge, today's equipment, and today's sense of v a l u e s .
ACKNOWLEDGMENTS Tile author wishes to thank the following persons, each of whom contributed to this work in his own way: James Cook, Jerry Ehman, Mirjana Gearhart, John Kraus, Bernard Oliver, the entire Cyclops team, an(l two an
REFERENCES ALTEI~-HOFF, 1,V. J., DOWNES, D., GOAD, L., MAXWELL, A., AND ]~I~,'EHAR'r, ]~. (1970). Surveys of the Galactic Plane at 1.414, 2.695, and 5.000 GHz. Astron. Astrophys. Suppl. 1, 319. BECKMANN, P. (1968). "The Depolarization of Electromagnetic "~Vaves." The G(>lem Press, Boulder, Coh>rado. CAmEROn-, A. G. W. (ed.) (1963). "Interstellar Communication." W. A. Benjamin, Inc., New York, New York. CLARK,
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RADI-I ADKRISH NA-N', V.,
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~VILsON, R. W. (1962). The Hydrogen Line in Absorption. Astropkys. J. 135, 151 174.
COCCONI, G., AND MORRISON, P. (1959). Searching for Intorstellar Communications. In "Interstellar Communications" (A. G. W. Cameron, ed.), pp. 160-164. W. A. Benjamin, Inc., New York, New York. CONKLL\', E. K. (1969). Velocity of tho Earth with Respect to the Cosmic Background Radiation. Nature (Londcm) 222, 971. DIETER, N. H. (1972). Berkeley Survey of HighVelocity Interstellar Neutral Hydrogen: I. The S e c t i o n / 7 + - 1 5 ° t o + 1 5 ° . l - 10° t o 250 °. Astron. Astrophys. Suppl. 5, 21. DIETER, N. H. (1972). Berkeley Survey of HighVcl¢)city Interstellar Neutral Hydrogen: II. The Seeti(m f -, 15°. Astron. Astrophya. Suppl. 5, 313. Dlx()_~, R. S. (1973). An a n t e n n a system f(>r simultane<)us omni-dircctional Doppler shift removed. In p,.eparation. DRA~:E, F. D. (1961). Project Ozma. l'kysics Today 14, 40. HANSE:~, R. C. (1966). "Microwave Scanning Antennas," p. 137. Academic Press, New York, New York. HE.~RY, P. S. (1971). Isotrol)y of the 3K Background. Nature (London) 231,516. HILL, E. ]~. (1968). A (]ontinuum Survey of the Southern Milky Way at 1410 MHz. Aunt. J. l'kysics 21, 735. Icarus (1972). First Soviet-American Conference on Communication with Extraterrestrial Intelligence. lcaru~ 16, 412. KAPLAN, S. A. e(t. (19691. Extraterrestrial Civilizations: Problems of Interstellar Communication. NASA Technical Translation. TTF-631. KERR, F. J. (1969). Parkes Hydr<)gen-Line Survey of the Milky Way. Aust. J. Phys. 9,1. KERR. F. J. AND H]ND~IA.~, J. V. (1970). A Synoptic View of the Galactic Equator, l II =: 185° to 63 ° . Aust. J. Phys. 18, 1. LX.~DBLAD, P. O. (1966). Dwingeloo Atlas of 21-era Profiles, Part I. B.A.N. Suppl. 1, 53 LINDnI.AD, P. O. (19661. Dwingeloo Athos of 21-era Proliles, Part II with a Representation in Gauss|an Components. B.A.N. Suppl. 1, 177. I~[ATtlE~.VSON, D. S., .11EALY, J . 11., A.-N'I) I¢OI~iE,
,I. M. (1962). A Radio Survey of tile S<)uthcrn Milky "~Vay at a Frequency of 1440 Mc/s. ,,lust. J. Pkys. 15, 354. MCGEE, R. X., MII.TON, J. A. AND I,VOLFE, 1,V. (1966). A Sky Survey of Neutral Hydrogen at h21 era. Aust. J. t'ky.~. Suppl. 1.3. Mc(,'nn, ]~. X., MI'RRAY. J. I)., MILTON, J. A. (1963). A Sky Survey of Neutral Hydrogen at A21cm. ,,lust.,|. Phy.s'. 16, 136.
BEACON S E A R C H STRATEGY
MITTen, S. (1972). N e w e s t P r o b e of t h e R a d i o U n i v e r s e . N e w Sci. 56, 138-140. N A S A ( 1971 ). P r o j e c t Cyclops : A Design S t u d y of a S y s t e m for D e t e c t i n g E x t r a t e r r e s t r i a l I n t e l l i g e n t Life. C R 114445. I)~AI)~rADKmSm~'AN, V., et al. ( 1971 ). T h e P a r k e s S u r v c y of 2 1 - c e n t i m e t e r A b s o r p t i o n in Discrete-Source Spectra. J . S u p p l . 203, e n t i r e issue. RAI~fOND, E. (1966). Dwingeh)o At.hLs of 21-cm Profiles, P a r t II. B . A . N . Suppl. 1, 33. IR1EGEI,, K. ~,'V., AND C~¢UTCHEtt, R. M. (1972). N e u t r a l H y d r o g e n S e l f - A b s o r p t i o n in a L a r g e R e g i o n t o w a r d t h e Galuctic Center. Astron. Astrophys. 18, 55. SCm~ItDT, M. (1956). A Model of t h e D i s t r i b u t i o n of M~ss m t h e Gabmtic S y s t e m . B . A . N . 13, p. 15. SHKLOVSK lI, [. S. A.XD SA(;A.'¢, C. (1966). " I n t e l l i g e n t Lifi~ m t h e U n i v e r s e . " H o l d e n - ] ) a y , Inc., San Francisco, Cal. S'rEI.~, S. AND JO.~'T,:s, J. J . (1967). " M o d e r n C o m m u n i c a t i o n Prin(;iples." McGraw-Hill Co., New York, N.Y. TOV~ASVaX, G. M. (1964). E x t r a t e r r e s t r i a l Civilizations. N A S A T r a n s l a t i o n N6730330342. T T F 4 3 8 . TROITSKII, V. S., STARDOBT.'4EV, A. M., G~mSHTEIN, L. I., AND RAKHL[N, V. L. (1971). Search f()r M o n o c h r o m a t i c 927-MHz R a d i o
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E m i s s i o n from N e a r b y Stars. Soviet Astron. 15, 508. VAN KUILE.','B(:RO, J . (1972). A S y s t e m a t i c Search for H i g h V e l o c i t y H y d r o g e n O u t s i d e t h e G a l a c t i c P l a n e : I. Astron. Astrophys. Suppl. 5, I. VELDEN, L. (1970). A t l a s of 21-era Line Profiles of tile O u t e r P a r t of t h e G a l a x y in tile Region of t h e Galacti(: A n t i c e n t e r . Beitrage zur RcMio-a~tronomie 1, 7. VENU(;OPAL, 1,Y.R. AND SHUTER, U . L. H. (1970). A S u r v e y of Galactic N e u t r a l H y d r o g e n . Mere. R. Ast. Soc. 74, I. VERS(:HUUR, G. L. (1973). A Searc, h for N a r r o w B a n d 21-era W a v e l e n g t h Signals f r o m T e n N e a r b y Stars. Icarus 19, 329. Vo.',- H OER~,'ER, S. (1973). T h e Definition of R e s t m E m p t y Universe. Astrophys. J . 181, 261. "VV.a,.NrNIER,P., WRIXON, G. T. AND WILSON, R. "~V. (1972). A S u r v e y of P o s i t i v e Velocity N e u t r a l Hydrogen Above the Galactic Plane between l = 252 ° a n d l == 322 °. Astron. Astrophys. 18, 224. ~,VESTERIIOUT, (~. (1958). A S u r v e y of t h e C'ontinuous R a d i a t i o n from t h e G a l a c t i c Syst e m a t a l ' r e q u e n c y of 1390 Mc/s. B . A . N . 14, 215. Zt:CKEtt.~IA~,', B. (1973). O z m a I I . P a p e r p r e s c n t c d at U.S. Commission V U R S I Meeting, Socorro, New Mexico, J a n u a r y 8, 1973.
187--199 (1973)
A Search Strategy for Finding Extraterrestrial Radio Beacons R O B E R T S. D I X O N The Ohio Ntate (Tniversity Radio Ob~.ercatory, 2015 Nell Avenue, Columbus, Ohio 43210 Received April 23, 1973; rovised May 29, 1973 Tim I)rinciph; of Anti-Cryptography, together with our knowlcdgo of the structure and properties of the universe, lead to tim suggestion that ex~ratt~rrestrial ra(lio beacons will transmit continuously and omni(tirectionally, using binary sense-~witch(;d circular polarizaiton modulation. The expected frequency of transmission is the rest frequtmcy of hydrogen, relative t:o the galactic center. Probable signaling rates are within two orders of magnitude of one second. Meth()ds of implementing tho strategy are suggested, from both the receiving and transmitting viewpoints. THE PROBLEM
Our level of technology has reached the point whcre it is now possible to realistically consider searching for signals sent out b y extraterrestrial civilizations, in fact, a n u m b e r of limited searches have already been carricd out (I)rakc, 1961; Troitskii, 1.()71 ; Verschuur, 1973 ; Zuckerman, 1973) ; others are in progrcss or being planned (Zuckerman, 1973; lcaru,s. 1972). Before a large-scale search is begun, it seems appropriate to (leveh) I) a logical and consistent search strategy which will hopethlly maximize the i>robability of success. This work is an o u t g r o w t h of t h a t bcgun in P r o j e c t Cyclops (NASA, 1971), a feasibility and dcsign study of the subjcct. For general background discussions s e e . C~meron (1963), K a p l a n (1971 ), and Shklovskii and Sagan (1966). E x t r a t e r r e s t r i a l signals of intelligent origin may be of two types: those which are intended tbr the internal use of the sender but are incidentally radiated into space (e.g., radio, television, radar, etc.), and those, which are sent out for the express purpose of" initiating interstellar contact (e.g., beacons). The incidental signals are probably more difficult to detect, because they will be much weaker (they represent wasted power from the sender's viewpoint) Copyright ~5 1973 by Academic Press, Inc. All rights o f reproduction in any form reserved. Printed in Great Britain
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and because we cannot make m a n y assumptions a b o u t their characteristics. For beacons, however, we van assume the Principle of Anti-Cryptography, which encompasses the idea t h a t a beacon will bc designed and operated in such a way as to maximize its probability of discovery, both bv intcntiona l searches and by accidental observation (perhaps by astronomers). Another ihcet of this idea is t h a t since all choices of beacon characteristics must be made b y the sender alone, he will choose those which minimize the n u m b e r of unknown dimensions to be searched b y the recil)ient. I f there is a continuous range of choices bounded by definite limits, the principle can be implemented b y choosing one extreme or the other, r a t h e r t h a n b y making an indefinite "mi(hlle" choice. I f we neglect the incidental signals for the present, we can t)roceed to construct a search strategy based on this principle. We can also use the Assumption of Mediocrity (Shklovskii and Sagan, 1`()66), wherein we assu me t h a t we are just average among the inhabitants of the universe. Since beacon operation l)robably is a characteristic only of civilizations more socially a(ivaneed than ours let us assume tbr the present t h a t their technological capabilities also surpass ours, but only to the e x t e n t t h a t we can tbresee our own
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future capabilities. We assume t h a t a more advanced civilization will in some philosophical sense " a i m " their beacon toward being detected b y " e m e r g i n g " civilizations. W e can probably also assume their knowledge of physical laws and properties of the universe to be at least equal to ours. What, then, are the u n k n o w n dimensions t h r o u g h which wc must search~ T h e y are
:
1, Distance 2, ])irection 3. Time 4. Polarization 5. Signaling rate, bandwidth, modulation, all interrelated 6. Frequency.
and
L e t us consider each of these dimensions individually, and a t t e m p t to minimize their e x t e n t b y applying the anti-cryptog r a p h y and mediocrity ideas, as well as our own knowledge of the universe.
DISTANCE
Of all the unknowns to be searched, distance most lends itself to a progressive approach. I t is our n a t u r e to begin projects on a small scale, and e x p a n d with time, using the knowledge and experience gained at each step to guidc the n e x t step. Our technology will always set the ultimate distance limit to a beacon search at any given time. As distance increases, the only factor which improves the probability of beacon detection is t h a t the n u m b e r of possible stars increases. The n u m b e r of stars is proportional to distance cubed out to a b o u t 1000 L.Y. (NASA, 1972), and b e y o n d t h a t it increases more slowly.
The factors which reduce tile probability of beacon detection with increasing distance include : 1. The signal intensity decreases with distance, t h e r e b y increasing the power requirements of the t r a n s m i t t e r and the sensitivity requirements of the receiver. 2. Our knowledge of detailed stellar characteristics falls off rapidly in the 25100L.Y. range, although we can still make reasonable estimates of the fraction of inhabitable stellar systems beyond t h a t distance. 3. The longer light travel time requires greater cultural longevity. 4. The longer light travel time decreases the chances for and perhaps the a d w m t a g e s of an ultimate two-way exchange. For these reasons and others mentioncd in the Project Cyclops report, let us somewhat arbitrarily set our distance limit at a b o u t 1000L.Y. There are ample stars within this range to occupy us for some time, and it is sufficiently large to provide challenges to our receiver sensitivity and the available beacon power. Figure I shows thc size of a 1000L.Y. sphere relative to our g a l a x y (adopted from Schmidt, 1956), and giw~s perspective to the fact t h a t we are still only searching in our own " b a c k y a r d . "
DI[{ECTIO_N I t is generally agreed (Cameron, 1963; Shklovskii and Sagan, 1966) t h a t the planets of the civilizations we seek orbit stars having certain observable characteristics. A search for beacons should therefore perhaps be made in the directions of
.10 0
I
I [0,000 L Y.
F[(;. 1. Sizo and position of a 1000 L.Y. radius sphere (arrow) eent(,red oil tile Sun, relative to our galaxy. The contours show total mass density, relative to that near tile Sun.
189
BEACON SEARCH STRATEGY
those stars. For relatively nearby stars (100 L.Y.), beacons are individually searchable (there is of the order of 1 candidate star per 25 square degrccs of sky), but beyond th at our knowledge of detailed stellar characteristics is poor, and the stellar density is such that a continuous survey might be profitable (at 1000L.Y. there are of the order of 40 candidate stars per square degree of sky). Which search mode to use is related directly to the beam size of the rccciving antenna. A continuous survey has the advantage of avoiding any errors or assumptions made in choosing the candidate stars. Such a survey should perhaps concentrate on those directions where the stellar density is highest. At 1000L.Y., the total number of candidate stars in the direction of thc galactic poles is about 75% of t hat on thc galactic e(tuator (calculated from Cyclops Fig. 6-1). Thus the stellar distribution is relatively isotropic and no directions can be ignored. TIME
A beacon operator could adopt one of two time strategies. He could transmit continuously and omnidirectionally, or he could transmit periodically in specific directions. The latter method provides for stronger signals in the desired directions, and these directions might be chosen to coincide with likely nearby stars. On the other hand, if thc receiving civilization is looking in a diflbrent direction at the time the sender is beaming toward them, the signal will be missed, regardless of the fact th at it may be a strong one. The situation is anah)gous to two needles trying to find one another in the proverbial haystack. I t is better for one nccdlc to stand still while the other nccdle systematically searches thc haystack, since this method will a l w a y s lead to ultimate success. The alternative approach, where they search tbr each other simultaneously, may never be successful. The advantages of omnidirectional beacon transmission havc previously been shown from an information-theoretical standpoint (Townasyan, 1964) and from a systems engincering standpoint (NASA,
1971, p. 61). The operator of a very longrange beacon would perhaps not transmit truly omnidirectionally but would weight his transmitted powcr more heavily toward the galactic plane, in proportion to the integrated stellar density in each direction, out to his assumed range limit.
BANDW1DTI-I
A continuous range of values exists, hence anti-cryptography leads us to zero or infinity, with zero being the easier and more practical. Infinite bandwidth cannot be really approached in practice because it implies either infinite power or zero powcr spectral density, whereas zero bandwidth can be easily approached. From an ease-of-searcil stan(tpoint, any bandwidth " n e a r " zero is equivalent to exactly zero. A bandwidth near zero (rather than infinity), although requiring a frequency search, allows the transmitted signal to be placed where it is most likely to be ibund, and less likely to be interfcrcd with. From our own communications experience, we know t hat when it is desired to attain the utmost range with a given power, the optimum system uses a near-zero bandwidth. SIGNALING RATE
A continuous range of rates exist, hence anti-cryptography leads us again to zero or infinity. The dispersion in the interstellar medium (Kaplan, 1971) places an upper limit on signaling rate of a few thousand [)its per second. Since the infinite limit is unattainable, anti-cryptography leads us to the lowcr limit of zcro. As with bandwidth, any value " n e a r " zero is equivalent in case-of-search to exactly zero, from a practical standpoint. How " n e a r " to zero should be used is open to conjccture but several considerations may enter. It should probably not require more than some small fraction (say 1%) of the recipients lifetime to receive at lcast a simple messagc. Taking our lifetime to be 100yr, we guess l y r as the longcst reasonable time from our standpoint. By Mediocrity we surmise then
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t h a t other beings might have lifetimes within an order of m a g n i t u d e of ours, lea(ling to a message rate estimate of one every~ 0.1-10yr. From the s t a n d p o i n t of an emerging communicative civilization such as ours, it might t)e difficult to receive complete messages if t h e y required more t h a n a b o u t half our planetary rotation period to coral)let e, since multiph" receiving stations might be require(l. Assuming they take this into account, we estinmte a greater message rate of one ever), 1.2-120 hr, by the same reasoning as before. Considerations of the " r a t e - o f - t h i n k i n g " may., give_• some insight, to the signaling rate, as opposcd to the message rate. We regar(l signaling rates of say 0.1 [)it per second to bc as slow as it is practical to go. Slower rates lead to little practical iml)rovement. Thus, wc might estimate their rate to be 0.01 to l bit per second, or having a period of l to 100sec. Coincidentally the periods of known pulsars lit; in the range 0.01-10 sec, which adds attractiveness to t h a t choice because it would increase the chances of accidental discovery by i)ulsar searchers. An a r g u m e n t against too slow signaling is the fact t h a t if the modulation m e t h o d is binary, and only one state happens to be detecte(l, the signal direction might be observed entirely during the undetecte(1 state and hence missed completely. An a r g u m e n t against a high signaling rate, with its a c c o m p a n y i n g high bandwidth, is t h a t it is more difficult to detect. The tra(le off between intbrmation content and detectability must be hcavily weighted toward the detectability end because it is far b e t t e r to send a little infi)rmat.ion and have it be received by someone, than to send lots of information and have it. be received b y no one. '.l'he signalin~ rate might easily be increased after the initial acquisition by sending information as to how and where a second, higher signaling rate c o m m m d c a t i o n channel mi,,ht be found. Once a signal is detecte(I, the receiving systcnl can be reoptimize(I from a search mode into a s t u d y mode. This poses no great problem. The central problem is t h a t of the initial signal acquisition.
POLARIZATION
A narrow band signal is inherently polarized, with the most general polarization being elliptical. Linear polarization (along with all other elliptical polarizations e x c e p t circular) contains an additional unknown, namely the position angh; of the field vectors. ()ne coul(t argue t h a t the vectors could be aligne(l p a r a l M or perpendicular to the galactic plane (a " n a t u r a l " Mignment for inhabitants of our galaxy), b u t f a r a d a y rotation of the vectors in the interstellar medium would tend to destroy a n y such alignment. Thus, linear polarization seems unlikely. The remaining polarization, circular, contains the least n u m b e r of unknowns an(t is therefore the most likely one. There are only two possible senses, left-hand and right-hand, whereas all other polarizations have an infinite n u m b e r of possible position angles. There seems to be no preferred characteristic of nature to in(licate which sense might be chosen. This will be discussed f u r t h e r tamer modulation. MODULATION
I t is reasonable to assume t h a t the beacon signal will be m o d u l a t e d in some way so as to carry intelligence. T h a t modulation ean be accomplished b y intro(tucing time variations into one of the four characteristics which completely describe a signal. a. b. c. (1.
Amplitude Phase Frequency Polarization.
From our own experience, we know t h a t the most effective modulation methods for m a x i m u m range arc two-state (binary) methods. It is also most effective if the t r a n s m i t t e r is on continuously, but switched from one state to the other (i.e., the states should be - l , + l r a t h e r t h a n o, +l). Since one cannot t r a n s m i t a negative amplitude, and since a (0, + l ) method is undeteetable (luring the zero state and thus might be missed, wc can probably rule out amt)litude modulation.
BEACON S E ~ R C g S T R A T E G Y
Binary frequency modulation is obtained by switching the transmitter frequency between two fixed frequencies at appropriate times. This modulation method introduces another unknown (the other frequency) into the reception problem. Since a frequency search of some sort seems inescapable at the receiving end, it seems undesirable to use a mo(tulation method which complicates the search by changing the transmitter frequency. Binary phase modulation may be achieved by introducing a 180 ° phase shift into the transmitted signal at appropriate times. From an information carrying standpoint, phase modulation is superior to any of the other three methods, since it requires only half the signal-to-noise ratio of the others to send the same inibrmation (Stein and Jones, 1967). The signal as a whole, however, is not any easier to detect. On the other hand, phase modulation is least likely to be detected accidentally, since astronomical observations rarely measure phase. Thus while phase modulation is likely to be used where information transfer is of greatest importance, it offers no particular advantage to detecting the initial signal. Binary polarization modulation may he achieved by changing between two orthogonal polarizations at appropriate times, such as between two perpendicular linear polarizations, or between left- and righthand circular polarization. In the discussion of polarization above, linear polarization was ruled out, and there was no good reason to choose between left- or right-hand circular i)olarization. If one chose to modulate, by reversing the sense of circular polarization, not only would the difficulties of other modulation methods be avoided, but the choicc of which sense of circular polarization to use wouhl bc
191
obviated, because both senses would then be used. Thus this choice seems preferable, from the modulation and polarization standpoints, as well as from the anticryptograt)hic standpoint of avoiding arbitrary choices between e(tuMly likely alternatives. Polarization modulation is quite uncommon to us, probably because most o four communications are terrestrial, where depolarization effects make it less useful. Nevertheless it is used in laser range-finders (Mitton, 1972, p. 140), it is discussed by Beckman (1968), and could find wide application in space communications. A serendipitous side benefit of senseswitched circular polarization is that it could bc searched for using a sense-switched l)icke radiometer system which wouhl strongly discriminate against any linearly polarized signal. Since most terrestrial transmitters use linear polarization, interfcrence due to these transmissions wouM be reduced. Similarly, it would discriminate against unpolarized signals and hence remove most natural radio sources. In addition, the Dicke system would take full adwmtage of the sense reversals, giving a receiver output as shown in Fig. 2. In a binary modulation system, there still remains the unknown duty factor, or what average percentage of the total time is spent in each state? Clearly one cannot choose the extreme vah|es of 0%, 100% in either sense bccause then there is no modulation at all. The only remaining nonarbitrary allocation is 50%, 50%. This would allow for maximum integration time in each state, and seems the most likely choice. It should be noted that the anti-cryptograi)hy principle was applied indepen(tentlv to the tmknowns of bandwidth, signai ing rate and modulation, which are
RIGHT CIRCULAR
ZERO SIGNAL
LEFT CIRCULAR
FT('. 2. Output of a circular polarization sense-switched Dicke radiometer, in response to a binary circular polarization modulated signM.
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of course not independent. Nevertheless the results obtained are consistent with one another. FREQUENCY
I t is generally agreed (Cameron, 1963; Shklovskii and Sagan, 1966 ; Kaplan, 1971 ; NASA, 1971) that the optimuln frequency range for interstellar communications lies between approximately 1 and 10 GH z. Within t h a t range there are several spectral lines, notably hydrogen at 142o MHz and hydroxyl at 1665MHz. Since neither end of the frequency range is well defined, anti-cryptography leads us to choose one of the spectral line frequencies. The hydroxyl line can be ruled out because it is in fact multiple, consisting of several lines closely spaced. Another unknown would be introduced (that of which component to use) ifa multiple line were chosen. The remaining line, hydrogen, has a number of positive features that make it the best choice (most in keeping with the antieryptographic idea of choosing extremes wherever possible). It is the lowest frequency line within the microwave "window." It is the strongest emitting line in the window and hence most likely to be known and studied by mstronomcrs everywhere. It is perhaps the most astrophysically interesting line bccause the emitting hydrogcn is more widely dispersed throughout our galaxy than othcr emitting substances, and thercfore provides more inibrmation about the galaxy itself than other substances. Hydrogen is the lightest anti most abundant element in the universe. The emitting hydrogen is the simplest molecule among the other emitting molecules. Thc hydrogen line was originally suggested by Cocconi and Morrison (1963) as being a likely choice. Since then, however, several arguments have been advance(t as to why it might n o t be a good choice. The hydrogen emission itself might tend to drown out intelligent transmissions. Absorption by interstellar hydrogen at that frequency might prevent intelligent signals from reaching here. An): intentional transmissions near the hydrogen line might cause
serious interference to astronomers on the same planet who were trying to receive hydrogen emissions. On Earth, we do not allow any transmissions at all near the hydrogen line, for just t h a t reason. While all of these arguments against using the hydrogen line seem valid at first glance, a deeper look show's t h a t none really are. The local astronomer interference problem could be avoided in a number of ways, for example by transmitting from the far side of a satellite, or by transmitting on a frequcncy somewhat different from astronomical reception frequencies (a desired feature to be expanded upon later). The astronomical community of the beacon society may consider hydrogen line observations "pass6" by that time. In the ultimate analysis, it is only a societal value judgment to decide which is more vahmble to them---hydrogen line observations or an active search for other intelligent beings. Our sense of values today is such that we would probably choose the former, but that may not always be the case. And regardless of o u r sense of values, others may have a different set. HYDROGEN EMISSION INTERFERENCF
The deleterious effect of a particular noise added to a communications channel must be evaluate(| in comparison with other noises already present in the channel, rather than by itself in absolute terms. Thus it is of interest to compare the relative total noise level of the hydrogen line frequency with that of a nearby hydrogenfree frequency. The best receiving systems we have today for this frequency range have system noise temperatures of about 50°K. We can perhaps predict future improvement to 10°K. The galactic continuum radiation in this frequency range is shown in Fig. 3, which was compiled from Westerhout (1958), Altenhoff et al. (1970), Mathewson et al. (1962), and Hill (1968). The dotted lines indicate an area not covered by the above authors, and that area is subt.raeted from the total sky area in all calculations to follow. There is no reason to believe the missing area is funda-
BEACON SEARCH STRATEGY
193
/I // I
FIG. 3. Continuum brightncss temperature near the hydrogen line frcquency. mentally different from the othcr areas so its omission should not aflhct these results. The hydrogen line emission radiation is shown in Fig. 4, which was compiled from ])ieter (1972), Wannicr et al. (1972), Venugopal and Shuter (1970), Kerr and Hindman (1972), Kerr (1969), Vcldcn (1970), Kuilenburg (1972), McGee et al. (1963, 1966), Lindblad (1966), an<] Raimend (1966). This figure shows the radiation at the hydrogen line rest frequency, relative to our local standard of rest. That
,50°K
/
frequency is where the hydrogen radiation is strongest, and thus provides the "worst casc" results. In order to combine system temperature with the sky brightness temperaturcs shown in Figs. 3 and 4, the lattcr should be multiplicd by the receiving antenna beam cfficiency (typically 0.7), assuming the antenna bcamwidth is smaller than the angular extent of the emitting region, which is in general true. However, for the purpose at hand this factor can be neglected, thereby making the sky emis-
?
180"
FI(}. 4. Hydrogen line brightness temperature at zero velocity relative to the local standard of rest,.
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DIXON
sion interference appear somewhat worse t h a n it actually is. The sky areas of interest are t a b u l a t e d in Table I. The systein noise and the continuum radiation noise are present at both frequencies under comparison, whereas the hydrogen radiation noise is present at only one. Qualitatively, let us say t h a t if the combined sky noise is less t h a n the system noise (thereby degrading the system b y less t h a n a factor of 2), then serious interference is not present. Conversely, if the combined sky noise exceeds the system noise, wc will say t h a t serious interference is present. F r o m Table I, the sky noise exceeds 50°K only in a very small fraction of the sky, even if hydrogen emission is included. Thus the hydrogen emission is no problem tbr the 50°K receiving systems of today. F o r a 10°K system, however, interference will be found over a b o u t 40% of the sky, principally from hydrogen emission. Thus hydrogen emission would ultimately be the limiting factor at some time in the future, if the local standard of rest hydrogen frequency were to bc used. E v e n t h o u g h one m a y decide to use the hydrogen-line fl'equcncy, t h a t choice covers quite a wide band of frequencies, due to Doppler shifts caused by motions of both the t r a n s m i t t e r and receiver. These include p l a n e t a r y rotation, p l a n e t a r y revolution around its star, peculiar motions of the star, and revolution of the star around the galactic center. I t is common practice with hydrogen-line observations to remove effects due, to our own first three motions, b u t not the last. We have no way of knowing the doppler shift of the t r a n s m i t t i n g
TABLE I SKY ~OISE FROM VAI~IOUS SOIJffCCES
:Noise source Continuum (Fig. 3) Hydrogen relative to LSR (Fig. 4) Hydrogen relative to GC (Fig. 6)
% Sky > 50° % Sky -> 10° 0.3 4 0.2
8 37 3
beacon, and the beacon operators have no way of knowing our doppler shift, so we c a n n o t compensate for each othcr's shifts. On the other hand, we (lo know our own doppler shift, and the beacon operators can be assumed to know theirs, so it would seem possible t h a t if we each corrected for our own shifts, we wouht arrive at a unique c o m m o n frequency, t h e r e b y eliminating the unknown of frequency entirely. The nearest common reference point t h a t we sharc is the galactic center, so all doppler shifts should be, corrected to be at rest relative to t h a t point, including corrections for galactic rotation. This rotational correction is larger than the others an(1 is shown in Fig. 5 {br our own location. A n o t h e r reference frame t h a t might be considered in the future, particularly fbr extragalactic communications, is the 3°K background frame discussed by Conklin (1969), H e n r y (1971), and yon H o e r n e r
(1973). The frequency arrived at by making the galactic rotation corrections is a function of direction, amt thus there is the practical t)roblcm of making them. In the receiving case, it is trivial because it simply means tuning the receiver to a specific frequency determined b y where the a n t e n n a is pointing. In the t r a n s m i t t i n g ease it is somewhat more involved, because an omnidirectional dol)t)ler-removing beacon must transmit simultaneously on a (tiffercnt frequency in e v e r y different direction. There seems to bc no f u n d a m e n t a l reason why this cannot be done because it simply means t h a t the t r a n s m i t t e r power is distributed not only in direction, b u t in fre(luency as well. This does not necessarily mean t h a t the t r a n s m i t t e d signal in a n y given direction is weaker as a result of being distributed in frequency, because the frequency distribution is related to the direction distribution on a one-to-one basis. In a n y specific direction, only the frequency needs to be changed. A few ways in which this might be accomplished are given here, as a d e m o n s t r a t i o n t h a t it is not impossible, b u t it should be borne in mind t h a t the beacon operators could have much b e t t e r ways t h a n we can as y e t conceive.
BEACON SEARCH STRATEGY
I
195
0*
Fro. 5. Approximate velo(:ity (kin/see) of the local standard of rest relative to the galactic center, due to galactic rotation. METHODS OF OMNIDIRECTIONAL ])OI'PLER REMOVAL 1. Divide the sky into N solid angles. Construct N transmitters, each connected to an a n t e n n a having solid angle beamwidth 4rr/N, and cach operating at the appropriate frequency to remove average doppler shift in its particular direction. This quantization of direction will cause small fre(tueney errors in some directions, b u t the errors decrease for larger N and m a y be acceptable. 2. L a u n c h an interstellar communications satellite b a c k w a r d along the galactic orbit to achieve a s t a t i o n a r y position relative to the galactic center. Then t r a n s m i t omnidirectionally at the hydrogen rest frequency. Crudc calculations indicate t h a t thc satellite position will remain stable against stellar encountcrs or falling t o w a r d the galactic centcr for times which are long even for a beacon civilization. In addition, the satellite will remain within reasonable range of its host planet for a v e r y long time, so it could be r e m o t e l y controlled from thc host planet. This also insures t h a t its angular separation from the host planet would not lead us to incorrect conclusions as to thc source of the beacon. The satellite could also receive a n y replies
we might direct toward it and relay t h e m to the host planet, so angular separation might not be a problem a n y w a y (and might even be all insurance policy for timid beacon operators not wishing to reveal their e x a c t location!). In the c v e n t the satellitc ultimately got too far away from the host planet to be useful, it could simply be t u r n e d off and a new satellite launched. Power for such a satellite might be derived from fusion generators and any m a n u e v e r ing necessary could bc done b y low t h r u s t deviccs such as ion engines. The use of such a beacon satellite would remove the problem of astronomical selfinterference, because the t r a n s m i t t e d signal would e m a n a t e from only one point in their sky (their galactic latitude 0 °, their galactic longitude 270°), and be highly dopplershifted relative to c o m m o n l y used hydrogcn line obscrvational frequencies. I f circular polarization were used on the beacon, hy(lrogcn line receivers couhl be built to discriminate against t h a t polarization. 3. Construct an a n t e n n a and t r a n s m i t t e r system which removes all doppler shifts directly. We have t o d a y phased a r r a y r a d a r systems which e m p l o y large n u m b e r s of a n t e n n a elements interconnected b y electrical networks whose phase and delay
196
DIXOI~
characteristics are predetermined functions of frequency. As a result of this, when frequency is changed, the effective direction in which the a n t e n n a array points also changes in a predetermined way. These antennas are called frequency scanned arr;~ys and ]rove the p r o p e r t y t h a t the a n t e n n a t)ointing direction is tmi(tucly rclated to the opem~ting fl'equcncy ( H ansen, 1966). This prol)erty is exactly what is needed to remove doppler-shift simultaneously in all directions. An a r r a y of this t y p e could tie constructed to have the appropriate variation of direction with frequency to remove all doppler shifts. Then one could t r a n s m i t band-limited white noise into the a r r a y to achieve the desired simultaneous coverage of all directions. T r a n s m i t t e r modulation could be easily achieved by, ibr example, pulsing the entire t r a n s m i t t e r on and off or by changing the polarization at each element. F u r t h e r (tctails of such an a r r a y will be published elsewhere (Dixon, 1973). I f this metlm(1 were used from the surface of a planet, several arrays located equidistantly a r o u n d the globe would be desirable, and the necessary doppler shifts would be time-varying. This woul(t necessit a t e some t y p e of c o m p u t e r control of the phasing networks or a r r a y orientation. These time variations could be minimized b y locating the arrays at the geographical
poles of the planet, or on a planet v e r y distant from its star, or on a distantly orbiting satellite. ~II)E
BENEFI.TS OF I)OPPLEI¢, CORRECTION
In addition to providing a unique frequency, the complete doppler-correcting m e t h o d removes most of the hydrogen emission noise. I f galactic rotation doppler shift is removed, the receiver is in general t u n e d to frc(lucncies outside the band where hydrogen radi~ltes. Figure 6 (compiled from the same references as Fig. 4) indicates the hydrogen emission brightness t e m p e r a t u r e for this case. The temi)erature exceeds 10°K for only a b o u t 3°/(, of the known sky and thus would not c~use interfercnce even to a future 10°K receiving system. In fact, the continuum radiation is then the domim~nt noise, so t h a t even a "i)crfect '' receiver of ()°K would not tie appreciably affected by the hydrogen noise. One reason for this is t h a t the hydrogen emitting regions are in general much more distant t h a n the 1000 L. Y. range considered here, and hence have quite different galactic motions. There are ,~ few high velocity hydrogen clouds known to exist which were not included in the source d a t a for Fig. 6, b u t t h e y do not appreciably alter this result. Due to the rotation of the Earth, the
1 (.10°I( leo-
,~oo
,20-
9o"
69"
~
"-
koJ / 2
330"
3oo °
27o*
240"
21o"
18o"
i I //// -I FiG. 6. Hydrogen line brightness temperature at zero velocity relative to the galactic center.
BEACON S E A R C H S T R A T E G Y
receiver f r e q u e n c y must be changed as a function of time to provide the necessary doppler shift. An interfering terrestrial signal would not exhibit this frequency variation and would hence be easily identified. The possibility of an interstellar communications network among a n u m b e r of civilizations using the doppler-corrected method is a t t r a c t i v e from a communications enginccring standpoint, because the frequelmy used b y each one is inherently and simply d e t e r m i n e d b y its direction from any given point, and m u t u a l interfcrcnce couhl occur only when two such civilizations lay along the same line of sight, at diflbrent distances. Such beacons would also be useful for interstellar navigation, inasmuch as their received frequency indicates direction, and the received signal strength from a beacon of known power indicates its distance. HYDROGEN ABSORPTION Our knowledge of hydrogen absorption is r a t h e r meager, since the effect can only bc mcasurc(t in the direction of known radio sources, and this has been done for only a small n u m b e r of them (Clark et al., 1961 ; Riegal and Crutcher, 1971; Radhadkrishnan et al., 1972). I t appears however, t h a t
I
197
most absorbing regions are more distant t h a n 1000L.Y., are highly c o n c e n t r a t e d along the galactic plane, and the absorption b a n d w i d t h is quite small. Thus there is no practical difficulty arising from this effect. In a n y casc, w h a t c v c r hydrogcn might be present within 1000L.Y. is essentially at rest relative to us, since differential galactic r o t a t i o n is small at t h a t distance (Fig. 7). Thus thc m e t h o d of dopplercorrection would remove most hydrogen absorption effects, analogously with the emission effects. FREQUENCY SEARCH F r o m a practical standpoint, it would still be necessary for us to conduct a search in the frequency dimension, even using the doppler-correction method, because at the present timc we do not know our own galactic rotation velocity to b e t t e r t h a n a b o u t 10%. At the hydrogen frequency, this corresponds to a frequency u n c e r t a i n t y ranging from 0 to a b o u t + 1 2 5 k H z , depending on direction, with an average value of a b o u t ±50 kHz. This will of course decrease with time. l)espite this f r e q u e n c y uncertainty, the doppler-correcting m e t h o d narrows the necessary search range considerably, relative to the zero velocity case. I t is interesting to note t h a t if such a signal
O"
Fro. 7. Approximate velocity (km/sec) of the local standard of rest relative to points at 1000 L.Y. distance, due to differential galactic rotation.
198
DIXOh"
were e v e r d i r e c t e d , its f r e q u e n c y w o u l d i m m e d i a t e l y give us a n a c c u r a t e m e a s u r e o f o u r r o t a t i o n a l v e l o c i t y , since t h e s e n d e r p r e s u m a b l y k n o w s his v e r y a c c u r a t e l y .
CONCLUSIONS A s t e p has b e e n t a k e n t o w a r d a search s t r a t e g y b a s e d o n c o n s i s t e n t a p p l i c a t i o n of b a s i c p r i n c i p l e s to all a s p e c t s o f t h e p r o b lem. T h e P r i n c i p l e o f A n t i - C r y p t o g r a p h y l e a d s o n e to b e l i e v e t h a t i n t e r s t e l l a r b e a c o n s will t r a n s m i t o m n i d i r c c t i o n a l l y , a t a d o p p l e r - c o r r e c t e d h y d r o g e n line frequency, using binary sense-switched circular polarization modulation, with s w i t c h i n g r a t e s of t h e o r d e r o f l sec. C e r t a i n of t h e s e c o n c l u s i o n s are n o t new, b u t have been reaffirmed by a different a p p r o a c h . T h e s t r a t e g y p r o p o s e d is b y n o m e a n s t h e o n l y o n e possible, a n d m a y well c h a n g e as n e w ideas or facts come to light. N e v e r t h e l e s s , it is a s t r a t e g y which could be i m p l e m e n t e d to(lay, u s i n g t o ( l a y ' s knowledge, today's equipment, and today's sense of v a l u e s .
ACKNOWLEDGMENTS Tile author wishes to thank the following persons, each of whom contributed to this work in his own way: James Cook, Jerry Ehman, Mirjana Gearhart, John Kraus, Bernard Oliver, the entire Cyclops team, an(l two an
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~VILsON, R. W. (1962). The Hydrogen Line in Absorption. Astropkys. J. 135, 151 174.
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BEACON S E A R C H STRATEGY
MITTen, S. (1972). N e w e s t P r o b e of t h e R a d i o U n i v e r s e . N e w Sci. 56, 138-140. N A S A ( 1971 ). P r o j e c t Cyclops : A Design S t u d y of a S y s t e m for D e t e c t i n g E x t r a t e r r e s t r i a l I n t e l l i g e n t Life. C R 114445. I)~AI)~rADKmSm~'AN, V., et al. ( 1971 ). T h e P a r k e s S u r v c y of 2 1 - c e n t i m e t e r A b s o r p t i o n in Discrete-Source Spectra. J . S u p p l . 203, e n t i r e issue. RAI~fOND, E. (1966). Dwingeh)o At.hLs of 21-cm Profiles, P a r t II. B . A . N . Suppl. 1, 33. IR1EGEI,, K. ~,'V., AND C~¢UTCHEtt, R. M. (1972). N e u t r a l H y d r o g e n S e l f - A b s o r p t i o n in a L a r g e R e g i o n t o w a r d t h e Galuctic Center. Astron. Astrophys. 18, 55. SCm~ItDT, M. (1956). A Model of t h e D i s t r i b u t i o n of M~ss m t h e Gabmtic S y s t e m . B . A . N . 13, p. 15. SHKLOVSK lI, [. S. A.XD SA(;A.'¢, C. (1966). " I n t e l l i g e n t Lifi~ m t h e U n i v e r s e . " H o l d e n - ] ) a y , Inc., San Francisco, Cal. S'rEI.~, S. AND JO.~'T,:s, J. J . (1967). " M o d e r n C o m m u n i c a t i o n Prin(;iples." McGraw-Hill Co., New York, N.Y. TOV~ASVaX, G. M. (1964). E x t r a t e r r e s t r i a l Civilizations. N A S A T r a n s l a t i o n N6730330342. T T F 4 3 8 . TROITSKII, V. S., STARDOBT.'4EV, A. M., G~mSHTEIN, L. I., AND RAKHL[N, V. L. (1971). Search f()r M o n o c h r o m a t i c 927-MHz R a d i o
199
E m i s s i o n from N e a r b y Stars. Soviet Astron. 15, 508. VAN KUILE.','B(:RO, J . (1972). A S y s t e m a t i c Search for H i g h V e l o c i t y H y d r o g e n O u t s i d e t h e G a l a c t i c P l a n e : I. Astron. Astrophys. Suppl. 5, I. VELDEN, L. (1970). A t l a s of 21-era Line Profiles of tile O u t e r P a r t of t h e G a l a x y in tile Region of t h e Galacti(: A n t i c e n t e r . Beitrage zur RcMio-a~tronomie 1, 7. VENU(;OPAL, 1,Y.R. AND SHUTER, U . L. H. (1970). A S u r v e y of Galactic N e u t r a l H y d r o g e n . Mere. R. Ast. Soc. 74, I. VERS(:HUUR, G. L. (1973). A Searc, h for N a r r o w B a n d 21-era W a v e l e n g t h Signals f r o m T e n N e a r b y Stars. Icarus 19, 329. Vo.',- H OER~,'ER, S. (1973). T h e Definition of R e s t m E m p t y Universe. Astrophys. J . 181, 261. "VV.a,.NrNIER,P., WRIXON, G. T. AND WILSON, R. "~V. (1972). A S u r v e y of P o s i t i v e Velocity N e u t r a l Hydrogen Above the Galactic Plane between l = 252 ° a n d l == 322 °. Astron. Astrophys. 18, 224. ~,VESTERIIOUT, (~. (1958). A S u r v e y of t h e C'ontinuous R a d i a t i o n from t h e G a l a c t i c Syst e m a t a l ' r e q u e n c y of 1390 Mc/s. B . A . N . 14, 215. Zt:CKEtt.~IA~,', B. (1973). O z m a I I . P a p e r p r e s c n t c d at U.S. Commission V U R S I Meeting, Socorro, New Mexico, J a n u a r y 8, 1973.