7/30/2019 Optimum Frequencies for Outer Space Communication 1

1/5

JOURNAL OF RESEARCH of the National Bureau of Standards-D . Radio PropagationVol. 64D, No.2 . March-Apr il 1960

ptimum Frequencies for Outer Space Communication 1George W. Ha ydon 2

(N ovemb er 10, 19 59 )Frequency depend ence of radio propagation and other techni cal facto rs which influenc e

ou te r space communi cations are examined to provide a basis fot' the selec tion of frequ enciesfot' communication betw een eart h an d a space ve hicle or for commun ication between spa cevehicles.1. Introduction

he probable future rapid advances in the use ofllites and space vehicles will in tensify the l'eq uirets for adequate space communications. Sin cemodest transmitter power will be initially availin the space vehicle, careful enginee ring of thece circui ts will be necessary to assure adequatecations and parLicular attention to the selecf radiofrequencies will be required. Opt imumon the basis of the signalransmitter power, a minim distortion of phase and amplitude, the minilikelihood of in terferen ce from other equipment,report takes signal-to-noise ratio as thethatction and other distortions may cause problemstracking and loca tion.

2. Factors Affecting the Selection ofFrequenciesetween earth and outer space

st pass through the earth's atmosphere (includingand ionosphere). Communicationsatellites will primal'lly involve radio pathsthe influence of the earth's atmosphere.he atmosphere is frequency selective, allowinges to pass through readily whileattenuating others. A range of frequencieswaves readily penetrate the atmosphere iscalled a "window."principal ranges of frequencies pass readilythe atmosphere. They are: (1) The rangf'ween ionospheric critical frequencies and frequenabsorbed by rainfall and gases (about 10 to000 Me), and (2) the combined visual and infraranges (about 10 6 to 10 9 Me).he atmosphere is known to be part ially transent in a third range below about 300 kc. Wavesthrough the ionosphere in this rangewhat is sometimes called the whistler mode.gation in this mode is not yet well understood.e range 10 to 10 ,000 M c is the most practicalcommunication purposes considering the pres-

e bas ic material in this paper was unanimonsly adopted hy the In ternaul tative Committee at the IX Plenary Assem bly in Losles, April 1959, and is being issued as CCIR Report No . 115, Factorsthe selection of freqnencies for telecommunication with and betweeued Sta tes Army Radio Frequency Engineering Office, Office of ChiefOfficer, 'l'he Pentagon, W ashington, D .C.; now with Cent ral Radioagation Laboratory, National Bureau of Standard s, Boulder, 0010.

ent state of development in radiofrequency powergeneration . The upper limit of this range m ay be aslow as 5,000 to 6,000 Me during heavy rainstormsand th e lower limit may be as high as 80 to 100 Mcdepending upon the degree of solar activity , thelocation of the earth terminal, and the geometry ofthe signal path. 01 1 the other hand , the windowmay extend from as low as 2 Me for polar locationsduring night-time periods to as high as 50,000 Mcat high alt i tude rain-free locatIOns.In the midportioD of this window favorable propagation conditions exist, and circui t performan ce canbe estimated on the basis of free-space propagationconditions by the following formula:, (P;JW )Ptex GtG r

where: P ;= requiTed tra nsmitter power, P; = minimum permissible receiver input power,j = frequency,d= dis tance between transmitter and receiver, Gt=transmitting antenna gain power, Gr= receivingantenna gain power.Actual propagation condItions vary substantiallyfrom this free space assumption at frequencies nearthe edge of the radio window, and it is necessary toco rrect for ionospheric and tropospheric effects toobtain a true estimate of frequency dependence.This co rrection require::; an estimate of troposphericabsorption [1] 3 at the higher frequencies and anestimate of ionospheric absorption at the lower frequencies [2] . In addition to estimating ionosphericabsorption, an estimate of the probability of radiosignals penetrating the ionosphere must be made [3].To determine optimum frequencies, the variationof background radio noise within the radio windowmust also be considered:(1) Cosmic noise predominates at the lower edgeof the radio window and decreases with frequencyuntil noise within the receiving equipment predominates.(2) In most present-day faciliti es the receivingequipment noise tends to predominate above about100 to 200 Mc for antennas directed toward averagesky noise areas and above about 300 to 500 Mc foran tennas directed toward high cosmic noise areassuch as the Milky Way.(3) I f low noise receiving equipment such as theMASER amplifier is used, receiver noise may predominate above about 600 to 1,000 Mc.3 Figures ill brackets iudicate th e literature references at the end of this paper.

105

7/30/2019 Optimum Frequencies for Outer Space Communication 1

2/5

(4) Noise within conventional receivers normallyincreases slowly as the operating frequency is increased, but may tend to decrease at the higherfrequenci.es if MASER amplifiers are employed.For antennas of fixed physical size, high frequencieshave the advantage of greater gain but the disadvantage of narrow beamwidths and associated tracking problems.High speed vehicles traveling so that the distancebetween transmitter and receiver is rapidly changinghave apparent frequencies differing from the actualtransmitter frequencies by the Doppler frequencyshift component in the direction of reception.Within the solar system there is evidence of appreciable densities of electrons out to great distancesfrom the sun .Transmission time delay will become substantialin outer space communications, e.g., 2.6 sec arerequired for a round trip radio signal to the moon.This time delay is essentially independent of operat-ing frequency.

3. DiscussionAlthough great distances arc involved, thepropagation medium in space beyond the first 500miles of the earth's atmosphere is believed to beessentially transparent to radio waves. Thus wemay estimate performance on the basis of free-spacepropagation. Frequency dependence of receiver input power under free-space propagation conditionsdepends upon the type of antenna at the transmitterand receiver. This frequency dependence is shownby the following free-space propagation formula :

P ( PtGtGr)roe fd7-where: P r is receiver input power, P t is transmitterpower, and other symbols have the same meaning asbefore.Frequency dependence of receiver input power forfree space propagation conditions can be summarizedas follows:(1) I f both the transmitting and receiving terminals of a free-space communication link use nondirective antenna (e.g., two vehicles in space) or ifbeamwidths at both terminals are fixed, the receiverinput signal power increases as the frequency isdecreased:

(2) I f one terminal of a free-space communicationlink uses a directive antenna of fixed physical sizeand can operate with narrower and narrower beamwidths as frequency incr eases and the other terminaluses a nondirective antenna or a fixed beamwidthantenna, e.g., a directive antenna on the earth's surface (Gt O(.F ) and a nondirect ive antenna on a spacevehicle, the receiver input signal power is independen t of frequ ency [P r 0 ( (P t/d2)] .

106

(3) I f both terminals of a free-space communication link use directive antennas of fixed physical sizeand can operate with narrower and narrower beam-widths as frequency increases, e.g., a directive antenna on the earth's surface and a directive antennaon a more elaborate space vehicle (Gt 0(.F andGr oc P), the receiver input signal power increases asthe frequency is increased [P r 0 ( (Pd2/d2)J.FreqLleney dependence for practical space-communication circuits requu'es that atmospheric effectsbe included. Receiver input signal power andreceiver input noise power for a directive receivingantenna and nondu'ective transmitting antenna areshown in figure l . The receiver input power in-cludes ionospheric and tropospheric effects for a1,000-mile propagation path tangential to the earth 'ssurface for summer midday operation during periodsof high solar activity and for moderate rain conditionssuch as experienced 1 percent of the time in theWashington, D.C., area. This is typical of themost adverse propagation conditions normally encountered i? the a b s e n c ~ of sudden i o ~ o p h e r i dis-turbances, mstances of mtense sporadIc E, areas ofauroral activity, or rain conditions of cloudburstproportions. During more favorable propagationconditions, such as a propagation path normal to theearth's surface during the night at the lower frequencies or in a high altitude rain-free location forthe higher frequencies, the receiver input power canbe expected to be essentially independent of frequency over a wider range of frequencies. Th ereceiver input power as shown between 100 and 500Mc in figure 1 will be typical over a much widerfrequency range during these favorable propagationperiods.Figure 2 shows essentially the same informationas figure 1, except that the distance is increased to300,000 miles, the receiving antenna diameter isincreased to 120 It, the use of cooled amplifiers suchas the MASER is anticipated, quasi-maximum

' - - ' ~ ~ ~ ~ ~ = = ~ ~ ~ ~ ~ = = ~ ~ ~ P E ~ N E = T R ~ ~ I O ~ N ~ T = = = T s = ' O S P ~ H E ~ R E - r90 ' ; POLAR REGION VERTICAL PATH\. V I POLAR REGION OBliaUE PATH OR T.1M r AL R ~ G I O N VCqTlCAL PATH100 ~ I G H T : ) : TROPICAL REGION ( ' e uW l p:.n '

....'f,\ : (' ~ ,...--. REr!.rVING A N T i : . ~ NA BEAMWIO'fH110 DAY "\. : : 20" 10 SO 60 ' 2" ,0 .5 -- .. .. -120 \. " \\ ..3i:f I 30' .. .2

MEDIAN \ 1 40"130"20 10" \ 5 2" ." .50...J 130 ATMOSPHERIC \ RECEIVING ANTENN A

NOISE IN : \. \ DIAMETER"9_140 AREA \\" ,1 I

-... -: .... ../ Y P ~ ~ ~ ~ E C O S M I CI

150li'160170

iii laoII'190200

- - - - - ~ R E C E I V E R NOISE

FREQUENCY, Me

CURRENT DESIGNEFFECT OF RAINAND GASEOUSABSORPTION

Fl ciuRE 1. T echnical considemtions in selecting fr equencies f or. radio communication to ewth from a space vehicle based on1,000 miles (typical satellite distance).Omnidirectional vehicle antelll1 a- 30- and 6Oft diam p ar abolic receivingantenna. One watt transmitter- on e kilocycle ba ndwidth .

7/30/2019 Optimum Frequencies for Outer Space Communication 1

3/5

r - - ~ r - - ' I - - ' ~ - - - r - - . - - . - - - - . - - . - - . - - - ' r - - ' - - ' - - -MINIMUM FREOUENCY TO ASSURE PENETR ATION OF EARTH'S IONOSPHEREI POLAR REGION VERTICAL PATH~ I POLAR REGION OBLIaUE PATH OR TROPICAL REGION VERTICAL PAlM

\ I Y I TROPICAL REGION OaLlQUE PATHI :/OA\ / \ : '

(\\ATM:ENRIC \.

H ~ ~ S ! O : ~ E : \ I IAREAS I I RECEIVING ANTENNA VERTICAL PATH'\ ...t, ,RECEIVING ANTENNA ( DIA METER - FEET IN DRY

EFFECT Of ~ .............. I BEAMWIOTH .,_ RAIN FREE AREA20. 1 ............. 10 . 5- 2- I- r20.5,-

7/30/2019 Optimum Frequencies for Outer Space Communication 1

4/5

.:::~ I O O

80UJ:;:,

7/30/2019 Optimum Frequencies for Outer Space Communication 1

5/5

t power is constant up to abou t 6,000higher frequencies may be used with only sligh tra tio but at the expense of increasedulty .more elaborate spa ce vebicles capable ofaining at titude and employing directive anas itre developed, the receiver inpu t power willquency and the background noisermine the . optimum frecy . The optimum fr equency will be govern edompromise between m ax imum practicalicitl antenna size and the minimum antennaon sistent with acquisition and tr ackinges. I f atti tude control of th e space vehid ea.cquisition and trach:in g limi tat ions of thesta tions establish minimum antenn a beamat both terminals, th e fL, ing of the maximumnna size at either terminal wi ll establishimum frequency and antenna siz e for therminal. As at ti tud e control and tr ackinges improve the optimum frequency in. As larger an tennas becom e pracLical t hefrequency decreases. Th e optimum freth erefore closely associitted wi th par ticularca tions and can be selected once the physical.nn a size and minimum beamwidths are es tabFor a I-deg minimum beamwidth for theantenna and a 20-deg minimum beamwidthntenna, op timum combinationsequencies and antenna s izes arc shown in table 1.

Earthantennadiam

It 3060120

SpaceOpt imum vehiclefrequency opt illlumantennadiam;l ie 24001200600

II

he optimum frequency for co mmunication beI t willnd upon radio noise in ou ter space and upon th ey of the vehicles to maintain at ti tudes and

109

thereby use directional an tennas. For omnidirectional an tenn as or for any fix ed an tenna beamwidththe optimum frequency will be the lowest frequencyco nsistent with the practical antenna size and ou terspace radio noise levels. Since opera tion at frequencies outside the radio window will tend tominimize the radio in te rference problem between thespace vehicles and ear Lh, space vehicles wiLh omnidirectional or broad beamwidLh an Lenna should beassigned trial frequencies below 10 Mc if an tennasat these frequcncies arc practical. For more clabora te space vehicles with the ability to properl y orientantennas with very narrow beamwidths, opera tion atfrequencies above or ncar the upper edge of the radiowindow is recommended (above 10,000 M c). I fphysical an tenna size limits the usc of frequenciesbelow 10 Mc and if the inability to orient antennaslimits the use of frequencies above 10 ,000 M c,an tenna siz e and an tenna beamwid th compromiseswill determine optimum frequencies. FrequenciC'smay then be selected by the u sE' of fi gure 5 in thesarne mann er as for communication between spacevehicles and earth, when both termin als usc direct iveantenn as except that the ear th's iono pheric andtroposph eric limit a tion will no longer apply.

5. References[II B. R. Bean and R. Abbott, Oxygen and wat er vaporabsorption of radio waves in the atmosph ere. Geo fi sicaP ura E App l. Milano 37, pp . 127- 144 (1957).[2] P. O. Lait in en and G. W. H ay don, Analysis a nd predictions of sky wave fi elds intensity , Radio PropagationAgency T ech. Rep t. No. 9. U.S. Arm y Signal RadioPropagation Agency, Fo rt Monm outh, N.J. (Aug. 1950).[31 D . H. Zacha risen, World maps of F2 cri tical frequenciesan d max imum usab le frequency facto rs, NBS Tech.

Not e No . 2 (PB151361) Apr . 19 59.Th e following paper ha s just been brought to the author'sa tt en tion:S. Perlman L. C. Ke lley, W. T . Russell, Jr ., and W. D.Stuart , Concerning optimum fr equenc ies for space communication, IRE T rans. on Communs. Systems, CS-7,167 (Sept . 19 59).

B OU LDER, C OL O. (Paper 64D2- 44)