Kotelnikov Radio Commn

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RADIO COMMUNICATION

WITH EXTRATERRESTRIAL CIVLIZATIONS

V.

A. KOTEL NIKO V

Ins t i tute of Radio Engineering and Electronics

USSR Academy of Sciences

The aim of this pap er is to con sider the possibi litie s of comm unication

with extra terres tr ia l civilizations whose technology is an outgrowth of

scientific pr inciples that a re already established on the Ear th and which

lead the Ea rth civilization by a few decades.

The Ea rth technology

is

assumed on the present-day level.

Although som e civilization s a r e probably

m or e advanced than we ar e by thousands o r even millions of ye ar s, our

res tr ic ted approach is apparently not unreasonable.

This paper does not

pretend to completeness: it only discus ses some par ticu lar exam ples,

which ar e not meant a s illustrations of optimal case s.

Let us consid er the tra ns mi ssi on of s igna ls in the fo rm of long mono-

chrom atic train s of pulses.

This technique

is

no le s s noiseproof than

othe r methods of transm issio n, and yet it i s s imp ler to achieve; ther efor e

it will probably be adopted in the beginning by young civilizations for

purposes of in ter ste lla r communication. The signal frequency

is

of course

not known in advance, and it m ay change when information i s being tr an s-

mitted.

In this ca se, the optimal recei ver w ill have a circu itry like that

block-diagrammed in the figure.

Her e A is the amplif ier, which may include a frequency changer; fi lt er s

with a band

f

overlapping the ent ire frequency range; D detectors; I inte-

gr ato rs which reco ver the energy passing through the filt er during the

integration time

T

NE suitable nonlinear elemen ts whose outputs a r e

added. A sign al

is

registered when the output of this receiver exceeds a

cert ain value.

F o r simpli city , without sacrif icing much of the noiseproof p rop ert ies

of the r ec ei ve r, the nonlinear elem ents can be repla ced with threshold



device s which produce an output signal only if the os cilla tory e nerg y

passing through the f i l te r in the t ime

T

ha s exceeded a cer tain threshold

value. I t i s this par t icula r receiver scheme that is conside red in what

follows.

Let the transmitting and the receiving antennas with effective surfaces

S and

S

be pointed at one anothe r. The maximum re ception range

is

then given by

where

P

i s the t r ansm i t t e r power,

the w avelength,

k =

1.38

~ / d e ~ ,

T rec eiv er noise temp erature; i s a function of

Af

the number of filters n

and thresho ld settin g, which depends on the probability of false response p f r

and the permissib le probabil i ty of s ignal loss by the rec eiv er ,

psi

When

con side red a s a function of hf Y ha s a minimum for

17.

F o r

p r

and

psl

le ss than lo-' , i t i s given by

sl

In some case s ,

when

T

i s l a rge , A f cannot be made e qua l to

I/?

s ince

due to the intr insic frequency drif t the signal will m is s the narrow p ass-

band of the fi lt e r. With A f > l / r we have

It i s undesirable to have f

< I / r

since th is wil l com plicate the design

and increa se Y , i . e. , reduc e

R.

We now consider a particular example.

Let the t ra nsm it ter power

commanded by an ex t ra ter re s t r ial c ivi l izat ion be

P=

l o 9 watt (1 of the

ele ctr ic power req uire me nts of the

USA .

The effective surface of the

transmitt ing antenna is

and the effective surface of our receiving antenna is

Set noise

T 30 .

Receiving antenna s with the se pa ra m et er s can be built without much

difficulty.

Tra nsm issio n wavelength



Fo r a frequency d rift of lo- (which

is

readily attainable at present),

we obtain for the filt er passband

With this passband we may tak e T ~ f

utting 7 1800 sec, pf = p S

and

l o 9 ,

we find

Fro m relat ion A) we then have

o r 128,000 light ye ar s, which is m or e than the di am ete r of the Galaxy.

If information is sent through this line, and the signal frequency is

changed fro m tran smi ssio n to transmission, the information rat e

is

I%?

1/60 bi ts per sec.

r

1800

The information ra te rapidly in cr ea se s a s dec reas es. If we put

T = A f - I = 3 3 sec, and pf =psl =

n 10 a s befo re, we find Y 3

and

R

1oZ0m,o r 10,000 light yea rs.

The corresponding information

ra t e i s

an

@ 9 bifs per set

T 3 3

F o r distan ces of 100 light year s,

T

can be reduced by four or de rs of

magnitude, and the information rat e will approximately increa se to the

sa m e extent.

The receiver being considered is provided with fairly narrow-band

f i l t e r s .

The transmitter and the receiver both accelerate and decelerate

due to the motion of the home plan ets in spa ce , and thi s obviously changes

the frequ ency of the signal.

These frequency changes must be compensated

on location, since otherwise the signal may mi ss the fi l te r s narro w pass-

band.

The compensation can be read ily introduced sin ce the accel erati on

of planetary motion is known at each point.

The ver y larg e number of sep arat e channels in the rec eiv er ( se e figure)

can be apparently replaced with a sim pler device performing the s am e

function.

How a re we going to find a s ta r with a powerful tran sm itt er located

on one of its planets?

Suppose that the tran smitting civilization has built a tra ns mi tte r with

the param ete rs from the previous example,

i e . , P

l o 9

W, S = lo5m2,

0.1 m. The antenna

is

pointed alternatively at different st a rs o r is

allowed to sc an the entire ce lestial sphere, drift ing acr os s a single st ar

In, say,

T

3 sec . Fo r l a rge

7

the problem is even simp ler.

X

The antenna beam fills a solid angle -- and it therefore

scans the en t i re

SI

celestial sph ere in the t ime



Fo r the above numerical p ara me ter s, we find 2 = 3.8 -10' s e c o r

2

years .

A considerably sho rter t ime is obtained if the antenna is pointed at

a certain s ta r and then rapidly switched to another sta r, etc.

The scanning

tim e fo r all the 10' s t a r s within the radius of 1000 light ye ar s is

T

3.107sec , o r 1 year .

If we assu me that a pre lim ina ry selection of the most promising

s t a r s reduces the population to 1 of the total ten millions, the scanning

time drops to T = 3 l o 5 se c, o r some four days. The scanning time

dec rea ses proportionately fo r st el la r populations within sph ere s of sm alle r

radii .

The technique of rapid antenna switching fr om one s t a r to another

thus redu ces the scanning tim e to a reasonable level.

Let the receiving sy stem be an ar ra y of beamed antennas cov ering the

entire celestia l spher e. In this case, a tran sm itte r with the above par a-

m et er s can be detected at a distance of 1000 light yea rs if the receiving

antenna sur fa ce , accordin g to equation (A),

is

S = 100m2,

It is assumed

that the re ceiver functions a s indicated

in

the figu re and that f T - I ,

T = 30'.

Seeing that the be am of thi s antenna occup ies a so lid angle h z / S

we find that

ar e required to cover the entire celestia l sphe re.

In this arrangement, the antenna is not expected to trac k the st ar . Each

antenna may therefor e have a s many as ten beam s. The number of

individual antennas may th ere for e be substantially le s s than m .

The

numb er of receivin g channels, howe ver, mu st be exactly

m

and each

receivin g channel should be equipped with fi lt er s overlapping the ent ire

relevant frequency band.

If we assu me that the transm ittin g civilization is sufficiently advanced

and its astron ome rs can actually sele ct the

1

of s t a r s which in principle

may support Earth-type civilizations, the scanning of al l the selected

s t a r s within a sph ere of 1000 light ye ar s r adius will take about four days.

The en tire c ele stia l sph ere need not be scanned at one time: different

a re a s of the sky, say , those having differen t declinations, can be scanne d

at different t imes .

Thus, if the sky is divided into 10 ar ea s, the su rv ey of

each ar e a can be completed in, sa y, one month (the signal, if any, w ill be

detected times during this period), and the entire celestia l sphe re will

be scanned in approx imately one ye ar . The num ber of receiving channels

and antennas can be further reduced by one order of magnitude.

The above receivin g network, though by no mea ns cheap o r eas y to

build, can be erected on Ea rth ,

This system wil l detect ext raterre str ia l

civilizations which have transmitters with the above parameters and are

located within the radius of 1000 light ye ar s from the Earth. Since th er e

ar e near ly

lo

s ta r s within this sp here, the sea rch will be success ful if

at lea st one of the ten million st ar s has a t ran sm itte r of the required kind.

If the distance scale i s reduced, the sear ch becomes progressively

simpler .



The table below lists some data for spheres with radius of 2000, 1000,

500,

and

200

light years with l o 8 ,

lo7 , l o 6 ,

and l o 5 stars, respectively.

The data of the table have been derived along the lines indicated above.

Column gives the number of ar ea s to be scanned separately

i f

the survey

of the entire c eles tial sph ere is to be completed in one ye ar . The scanning

time fo r each a re a was assum ed approximately

10

t imes gre ater than the

figure given

in

column

4.

We s ee fr om the table that

if

th er e i s a single Earth-type civilization in

10

s ta rs , it s detection at the prese nt s tage of our technological development

s

nearly impossible;

i f

there

s

a sin gle civilization in

lo7

sta rs , i t can

be detected with some effort;

i f

there

s

one civilization in

lo6

s t a r s , i t s

detection by the available means s quite probable.

If the extraterrestrial civilization commands more powerful resources,

we can of course d etect it fro m considerably gre ate r distance s.

Once the existence of a civilization has been established, a larg e

antenna should be pointed in the corresp ondin g direction, since beside s the

powerful call signals intended fo r detection by other civilizations, it

probably transmits information, meaningful messages that can be picked

up with high-efficiency antennas only. To esta blish a bil ate ral communication,

we should send a radio m es sag e to the discovered civilization. Our signal

will be picked up without any difficulty, since our transmitter can be pointed

precisely in the direction of the ex tra ter re str ial civilization that we have

previous ly discove red. After th is prel imin ary exchange of m es sa ge s, the

antennas of the two civ ilizatio ns will be pointed a t one another and a m or e

effective exchange of in form ation will be es tabli shed .

In conclusion let us c on sid er the possibility of detecting a civilization

even though it does not tra ns m it special detection sign als. The power

of the radio tra ns m itte rs used for internal purposes by these civilizations

i s probably of the or der of ten s of kilowatts, and the antennas in common

4 S,

use have a directive gain g, -(the probab ility of picking up na rrow er

Z

antenna beams

s

too small) . The receive r passband, a s before, is

0.3

c / s .

A narrowe r passband

s

inadvisab le, sinc e the Doppler frequency shift will

not be compensated on the transm itting side. Fo r the sa me reaso n we

z ,

4

SI

take r=3 se c ,

P = 1 0 5

watt,--=lo,

S ,=105 , Y=70,Tn=30.

Fromequation

A ) ,

i

,

2

E

2

9 2

,

Q

P

2 2 5

s -

.Z

8

QJ

m

y:

2~

400m2

100m2

25 rn2

4 m 2

2000

1000

500

200

m

.

g

Z ' g S ,

~ 2 3

480,000

120,000

30.000

4,800

10'

10'

lo

10'

g

.g

Q

c QJ

m

2

E L

1

10

100

1000

10

ye rs

1

y e r

36

d ys

4

d ys

M g

.5

2

8 .Z

8

.g

480,000

12,000

300

5

*

2

36

d ys

4

d ys

9 hrs

1 hr

,

A

2 z

2

4 :



R 3.1015mm, r 0.3 l ight ye ar s.

We se e that even the civi lizat ions occupying the nea res t s ta r s cannot be

detected unless they send special s ignals o r radiate (for some ob scure

rea so n) exceptionally high power.

O N L U S I O N S

If a civilization does not send sp ec ial detection1' sign als, i t apparently

cannot be detected even by i t s nea re st s te l la r neighbors.

If a civilization somew hat mo re advanced than we (app rox imate ly by a

few decades) sends spec ial radio signa ls, we can detect the se signa ls fr om

distan ces of 500-1000 light ye ar s.

Once civilizations have detected one another, they can establish radio

communicat ion on the galact ic scal e.

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