JOURNAL OF RESEARCH of the National Bureau of Standards-D. Radio Propagation Vol. 64D, No.2. March- April 1960

Optimum Frequencies for Outer Space Communication 1

George W . Ha ydon 2

(N ovember 10, 1959)

Frequency dependence of radio propagation a nd other techni cal facto rs which in flu ence outer space communications are examin ed to provide a basis fot' the selection of frequencies fot' communication between earth and a space vehicle or fo r communication between space vehicles.

1. Introduction

The probable future rapid advances in the use of satellites and space vehicles will intensify the l'eq uire­m en ts for adequate space communications. Since only modest transmitter power will be initially avail­able in the space vehicle, careful engineering of the space circuits will be necessary to assure adequate communications and parLicular attention to the selec­tion of radiofrequencies will be required. Optimum frequencies can be selected on the basis of the signal­to-noise ratio for a g'ven transmitter power, a mini­mum distortion of phase and amplitude, t he mini­mum likelihood of interference from other equipment, etc. This report takes signal-to-noise ratio as the sole criteria of frequency selection recognizing that diffraction and other distortions may cause problems in tracking and location.

2. Factors Affecting the Selection of Frequencies

All communication between earth and outer space must pass through the earth's atmosphere (including the troposphere and ionosphere). Communication b etween satellites will primal'lly involve radio paths outside the influence of the earth's atmosphere.

The atmosphere is frequency selective, allowing some frequ encies to pass through readily while severely attenuating others. A range of frequencies in which waves readily penetrate the atmosphere is often called a "window."

Two principal ranges of frequencies pass readily through the atmosphere. They are: (1) The rangf' b etween ionospheric critical frequencies and frequen­cies absorbed by rainfall and gases (about 10 to 10,000 Me), and (2) the combin ed visual and infra­red ranges (about 106 to 109 Me).

The atmosphere is known to be partially trans­parent in a third range below about 300 kc. Waves are propagated through the ionosphere in this range

I by what is sometimes called the whistler mode. Propagation in this mode is not yet well understood.

The range 10 to 10,000 M c is the most practical for communication purposes considering the pres-

I The basic m aterial in this paper was unanimonsly adopted hy the Interna­t ional R adio Consultative Committee at the IX Plenary Assem bly in Los Angeles, April 1959, and is being issued as CCIR Report No. 115, Factors affecting the selection of freqnencies for telecommunication with and betweeu space vehicles.

' United States Army Radio Frequency Engineering Office, Office of Chief Signal Officer, 'l'he Pentagon, Washington, D .C.; now with Central Radio Propagation Laboratory, National Bureau of Standards, Boulder, 0010.

ent state of development in radiofrequency power generation . The upper limit of this range may be as low as 5,000 to 6,000 Me during heavy rainstorms and the lower limit may be as high as 80 to 100 Mc depending upon the degree of solar activity, the location of the earth terminal, and the geometry of the signal path. 011 the other hand, the window may extend from as low as 2 Me for polar locations during night-time periods to as high as 50,000 Mc at high alti tude rain-free locatIOns.

In the midportioD of this window favorable propa­gation conditions exist, and circuit performan ce can be estimated on the basis of free-space propagation condit ions by the following formula:

, (P;JW) Ptex GtGr

where: P ;= requiTed transmitter power, P; = mini­mum permissible receiver input power, j = frequency, d= distance between transmitter and receiver, Gt = transmitting antenna gain power, Gr=receiving antenna gain power.

Actual propagation condItions vary substantially from this free space assumption at frequencies near the edge of the radio window, and it is necessary to correct for ionospheri c and tropospheric effects to obtain a true estimate of frequency dependence. This correction r equire::; an estimate of tropospheric absorption [1] 3 at the higher frequencies and an estimate of ionospheric absorption at the lower fre­quencies [2] . In addition to estimating ionospheric absorption, an estimate of t he probability of radio signals penetrating the ionosphere must be made [3].

To determine optimum frequencies, the variation of background radio noise within the radio window must also be consider ed:

(1) Cosmic noise predominates at the lower edge of the radio window and decreases with frequency until noise within the r eceiving equipment pre­dominates.

(2) In most present-day facilities t he r eceiving equipment noise tends to predominate above about 100 to 200 Mc for antennas directed toward average sky noise areas and above about 300 to 500 Mc for antennas directed toward high cosmic noise areas such as the Milky Way.

(3) If low noise receiving equipment such as the MASER amplifier is used, r eceiver noise may pre­dominate above about 600 to 1,000 Mc.

3 Figures ill brackets iudicate the literature references at the end of this paper.

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(4) Noise within conventional receivers normally increases slowly as the operating frequency is in­creased, but may tend to decrease at the higher frequenci.es if MASER amplifiers are employed.

For antennas of fixed physical size, high frequencies have the advantage of greater gain but the disad­vantage of narrow beamwidths and associated track­ing problems.

High speed vehicles traveling so that the distance between transmitter and receiver is rapidly changing have apparent frequencies differing from the actual transmitter frequencies by the Doppler frequency shift component in the direction of reception.

Within the solar system there is evidence of ap­preciable densities of electrons out to great distances from the sun.

Transmission t ime delay will become substantial in outer space communications, e.g., 2.6 sec are required for a round trip radio signal to the moon. This time delay is essentially independent of operat­ing frequency.

3. Discussion

Although great distances arc involved, the propagation medium in space beyond the first 500 miles of the earth's atmosphere is believed to b e essentially transparent to radio waves. Thus we may estimate performance on the basis of free-space propagation. Frequency dependence of receiver in­put power under free-space propagation conditions depends upon the type of antenna at the transmitter and receiver. This frequency dependence is shown by the following free-space propagation formula :

P ( PtGtGr) roe fd7-

where: P r is receiver input power, P t is transmitter power, and other symbols have the same meaning as before.

Frequency dependence of receiver input power for free space propagation conditions can be summarized as follows:

(1) If both the transmitting and receiving termi­nals of a free-space communication link use non­directive antenna (e.g., two vehicles in space) or if beamwidths at both terminals are fixed , the receiver input signal power increases as the frequency is decreased:

(2) If one terminal of a free-space communication link uses a directive antenna of fixed physical size and can operate with narrower and narrower beam­widths as frequency increases and the other terminal uses a nondirective antenna or a fixed beamwidth antenna, e.g., a directive antenna on the earth's sur­face (G t O(.F) and a nondirective antenna on a space vehicle, the receiver input signal power is independ­ent of frequency [P r 0( (P t/d2)].

106

(3) If both terminals of a free-space communica- !

tion link use directive antennas of fixed physical size and can operate with narrower and narrower beam­widths as frequency increases, e.g., a directive an­tenna on the earth's surface and a directive antenna on a more elaborate space vehicle (Gt 0(.F and Gr oc P), the receiver input signal power increases as the frequency is increased [Pr 0( (Pd2/d2)J.

Freq Lleney dependence for practical space-com­munication circuits requu'es that atmospheric effects be included. Receiver input signal power and receiver input noise power for a directive receiving antenna and nondu'ective transmitting antenna are shown in figure l. The receiver input power in­cludes ionospheric and tropospheric effects for a 1,000-mile propagation path tangential to the earth's surface for summer midday operation during periods of hig. ·h solar activity and for moderate rain conditions I such as experienced 1 percent of the time in the Washington, D.C. , area. This is typical of the · most adverse propagation conditions normally en­countered i? the absenc~ of sudden io~ospheric dis- I turbances, mstances of mtense sporadIc E, areas of auroral activity, or rain conditions of cloudburst proportions. During more favorable propagation conditions, such as a propagation path normal to the earth's surface during the night at the lower fre­quencies or in a high altitude rain-free location for the higher frequencies, the receiver input power can be expected to be essentially independent of fre­quency over a wider range of frequencies. The receiver input power as shown between 100 and 500 Mc in figure 1 will be typical over a much wider frequency range during these favorable propagation periods.

Figure 2 shows essentially the same information as figure 1, except that the distance is increased to 300,000 miles, the receiving antenna diameter is increased to 120 It, the use of cooled amplifiers such as the MASER is anticipated, quasi-maximum

.0 '--'~M~'N'~MU~M~F=RE=aU~E~NC~Y~T~O =AS=SU~~~PE~NE=TR=~~IO~N~oTF =EA=RT="·Ts='OOO~SP~HE~RE~-r 90 ' ; POLAR REGION VERTICAL PATH \. V I POLAR REGION OBliaUE PATH OR T.1Mr AL R~GION VCqTlCAL PATH

100 ~IGHT : ) : TROPICAL REGION ('euWl p:.n '

~ .... 'f,\ : (' ~ ,...--. REr!.rVING ANTi:.~ NA BEAMWIO'fH

~ 110 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 ANTENNA ~ NOISE IN : \. \ DIAMETER

"9_140 AREA \\ :

" \,1 I

-... -: ..... ../ ~YP~~~~ECOSMIC I ', ~

~ 150

li' ~ 160

~ 170

iii lao II'

190

200

- - - - - ~RECEIVER NOISE

FREQUENCY, Me

CURRENT DESIGN

EFFECT OF RAIN AND GASEOUS

ABSORPTION

FlciuRE 1. T echnical considemtions in selecting frequencies f or . radio communication to ew·th from a space vehicle based on 1,000 miles (typical satellite dis tance).

Omnidi rect ional vehicle antelll1 a- 30- and 6O·ft di am p arabolic receiving antenn a. One watt transmitter- one kilocycle bandwidth .

eo r--~r--'I--'I~---r--.--.----.--.--.---'r--'--'---' MINIMUM FREOUENCY TO ASSURE PENETRATION OF EARTH'S IONOSPHERE

9 0

.... 100

Ii ~ 110 Z 0 120

g ~ 130

'Q~140

ffi ~ 150

~ 160

~ ~ 170

W IIKI

~ 190

200

I POLAR REGION VERTICAL PATH

~ I POLAR REGION OBLIaUE PATH OR TROPICAL REGION VERTICAL PAlM

\ I Y I TROPICAL REGION OaLlQUE PATH

I :/ OA\ /\ : ' (\\

ATM:ENRIC \. \

H~~S!O:~E : \ \ I I AREAS \~ I I RECEIVING ANTENNA VERTICAL PATH

'\ ...t, ,RECEIVING ANTENNA ( DIAMETER - FEET IN DRY

EFFECT Of ~ .............. ~ I BEAMWIOTH .,_ RAIN FREE AREA

20.1 ............. 10 ....... I~ 5- 2- I- r20.5,-·<::""-_' .. ' ·""""~,,, __ -+~ , ........ , .......

FIGURE 2. T echnical considemtions in selecting j1-equencies for mdio communication to earth j1-om a space vehicle based on 300,000 miles (distance to circle moon).

Omnidirectional velliele anlenna-60- and 12O·ft diam paraboljc receiving antCllJla. One watt transloittcr- onc kilocycle bandw idtll.

values of cosmic noise for high gain anLennas arc given, and receiver input power is shown for a vertical path in a dry rain-free location.

Figme 3 shows the theoreLical improvement at the higher Ireq uencies if fL directive antenna is used on the space vehicle. Although receiver input power is shown only for a 15-ft diam parabolic antenna on the space vehicle, this improvement wiLh increfLsed frequency applies for all directive antennas of flxed physical size. Since the increase in antenna gains at the higher frequencies more Lhan offsets the slightl~T increased power requirement clue to increased receiver noi se fLL these frequencies, the fiJ·st impres­sion is that the higher the frequenc~T the better the expected circuit performance as long as the frequency is below Lhe upper limit of the radio window. (About 6,000 M e for oblique paths during moderate rainfall and up to 50,000 Me for high altitude rain-free location .) The physical problem of antenna design and tmcking, however, establishes minimum per­missible antenna beamwidths fLnd is believed to place practical limits on this upper frequency at much lower values.

TheoreLical effective power requirements for greater distances are shown in figure 4 as a function of frequency. Power r equirements shown are the minimum detectable radiated power (6 db SIN ratio) from a space vehicle to a 60 ft cliam earth-based antenna under the most adverse propagation con­ditions normally encountered. Allowances for fading and antenna beamwidth limitations are not shown by the chart. Approximate distances from earth to the moon, the sun, certain planets, and to typical manmade satellites al'e s11O'wn.

For systems with minimum permissible beam­widths, any increase in antenna size reduces this pract ical upper frequency since antenna beamwidth decreases with increase in antenna size. The rela­tionship between antenna size, opemting frequency, and antenna befLmwidth for parabolic antennas is shown in figure 5. If either the ground terminal or

107

80 E 90 "-

f-IOO L ~ IGHT

1 MINI~UM FA~QUENCY TO ~~S~E PENETRATION OF EARTH S IONOSPHERE

POL AR REGION VER TICAL PATH

/ POL AR REGION OBUOUE PATH OR TROPICAL REGION VERTICAL PATH

t-/ I TROPICAL REGION OBLIOUE PATH

~ _/ ;; ~ " \ ~ 110 -- DAY,.A : .20 VERT ICAL PA~TH 6120 MEDIAN /'. \ IN DRY 3; ATMOSPHERtC\ I \ RAIN FREE AREA

~ 130 NOISE IN \ RECEIVE A ANTENN A 1° SO 10

:~140 HIG:~~ISf. I \ \ BEAMW1DTH 20 4° euauE PATH

<r \\ " 5' 3° SO VEHICLE ANTENNA IN HUMIO W 1, ° MODERATE i \ 1 BEAMWI DTH ~ 150 ..... \:i 10· 20" RAIN AREA

2160 " , 20 .. :15° 40" RECEIVER NOISE

~ 170 10N~:~~R~; : 4~.30' ~ ; \60:~ -----LC~R':'N~ ~Sl~ - - --3 ~ ABSORPTION I I AVERAGE COSMIC NOISE EFFEC T a: W IBO \,: RAIN ANO GASEOUS

~ ~ ABSORPTION -

'9 0

200

.J j 500 1000 2000 5000 I000O 20000 50000

FREQUENCY, Me

FIGURE 3. 'Technical considemtions in selecting fre quencies for radio communication to earth from a space vehicle based on 300,000 miles (distance to circle moon) .

l?iftecn fool diameler paraboli c vehicle antenna- 60-ft di am parabolic receiving anlenna. One watt transmitter- one kilocycle bandwjd th .

l e-li " 50

T r '+ JUPITER

100,000,000 MI LES

o SUN

~ ~::~~RY d MARS

~ 10,000,000

1.000,000

) MOO N

100,000

10,000

1958 BETA (VANGUARO)

1,000

FREQUENCY, Me

r

50000

FIGURE 4. Theol'etical effective mdiated power required from space vehicle to permit detection on earth .

One kilocycle bandwidth-6 db sjgnal-to-noise ratio- receiving antenna-60-!t parabola-daytjme operation- bigh solar activity-rain and gaseous absorption based on 1 percent of time (Wasbington, D,C.) . Radio path approximately horizontal to earth- receivers of current design.

space vehicle maximum antenna physical size and minimum antenna beamwidth is fixed, the optimum frequency can be estimated from figure 5.

Figure 6 is a nomogram to estimate the bandwidth allowance required to accommodate the Doppler frequency shift for radio transmissions from high speed vehicles.

For communication between vehicles in outer space, free-space propagation conditions apply over a wide frequency range. Frequencies above or below the earth's radio window can be expected to minimize interference problems with operations on earth. These frequencies will be below about 10 Mc or above about 10,000 Mc. Selection of an optimum frequency can be based on free-space

.::: ~IOO ~ 80 UJ :;:, <t 15 <t Z Z UJ

t;: 20 <t () 15 :J g 10 <t

~ 7 5 4

3

2

40 70 100 200 500 1000 2000 5000 10000

FREQUENCY, Me

FIGURE 5. Chart to estimate optim1lm frequencies for outer space communication when directive antennas are used at both terminals, and physical antenna size and minimum per· missible antenna beamwidth are fixed factors.

Example: Based on a 1 deg minimum beam width for the earth terminal, the optimum frequency for a 6O·ft diam parabolic is about ),200 Me and the optimum vehicle antenna size is a 3·ft diam parabolic based on a 20·deg minimnm beam· width criteria for the space vehicle antenn a.

1000 100,000

r : 80 50

60 45 800

70,000 (fJ 4 40 40 600 UJ .c u .J "-

50,000 :;; 0 35 E >-0 () u 0 0 <t

400 o co 2 20 :;; z

8~ <t

0 30 Cf) 0 ::>

(fJO 0 300 30,000 ~ UJ- I

df? >-250 0 I 10 25

0 bQ Il: Q UJ

700 7 0 2: 200 20,000 >- .J>- UJ

() ~~ ()

z UJ UJ 500 5 UJ 20 Il:

u 150 15,000 ~ ::> ::> :;; 0 0 0

,: UJ ~- - - -- >-,: 0::

0 "- _ 300 3 - - "- EXAMPlE: UJ u z ._ > Z 0:: A 10.000 MEGACYCLE SIGNAL UJ 100 10,000" ~ ~ UJ fi ::> o >- 200 2 >- WILL SHIFT 300 KILOCYCLES 0 UJ != >- IF THE TRANSMITTER IS

15 .J w 80 ~ UJ 0:: 0:: ::. 0:: "- 7,000 "- (fJ

150 1.5 (fJ TRAVELING 20,000 mph

a:: Z Z a:: <[ (fJ <[ RELATIVE TO THE RECEIVER . w 60 w a:: a:: >- >- >- ~IOO >->- >- . ()

~ 5,000 ~ >- >- 80 >-' (fJ (fJ "- 0 ~ z z _ 0

I 40 I =' 60 <[ <[ (fJ (fJ 10

a:: a:: '" 500 >-,>- f- >-0 0 9

30 3,000 z 40 Z w Cf)UJ ::> UJ::> 0 30 .JO 8 UJ o UJ a:: >- a::

20 2POO "- 0"-

a:: 20

=1 i 7

UJ .J

15 1,500 Cl. Cl. 6 0 0

10 100

10 1,000 8 80 5

.FIGURE 6. Chart to estimate Doppler frequency shifts.

propagation but r equires an estimate of nOIse po:vers in outer space, particularly as to the radio nOISe at frequencies between 2 and 10 Mc. Fre­quencies below 2 Mc are considered impractical because of antenna sizes required and the substantial plasma frequencies probably occW'ring in outer space dW'ing periods of severe magnetic storms. If radio noise is excessive below 10 Mc and if antenna orienta­tion problems limit the use of high gain antennas, the optimum frequency for communication between space vehicles may fall within the 10 to 10,000 Mc radio window.

4 . Conclusions

Communications between earth and outer space are theoretically possible within two broad frequ ency bands; about 10 to 10,000 Mc and in the infrared and optical r egions . At the cW'l'ent state of the equipment art the lower band is definitely favored .

The upper limit of the lower band is dependent upon tropospheric conditions, and the lower limit depends upon ionospheric conditions. The band, therefore, is not sharply defined but is dependent upon geographic location and time of operation. For reliable communication to any earth terminal location, this lower band narrows to about 70 to 6,000 Mc.

Within the 70- to 6,000-Mc band the optimum frequency will depend upon the specific communi­cation service required and will be a compromise between the maximum practical antenna size, the minimum b eamwidth which will permit acquisition or tracking, and the radio noise levels.

For space vehicles using essentially omnidirectional antennas communicating with earth terminals using directive antennas, the receiver input power will be constant with frequency over much of the 70- to 6,000-Mc band, and the background noise level and beamwidth requirements to assure tracking deter­mines the optimum frequency. Background noise from soW'ces within the antenna beam (cosmic noise) predominates at the lower edge of the band and noise generated within the first stages of the receiver predominates at the upper edge of the band. The crossover point between these noise soW'ces determines the frequency with the maximum signal­to-noise ratio and, therefore, the optimum frequency for communication if antenna beamwidths are satisfactory at these frequencies. These optimum frequencies are about as follows:

(1) 100 to 200 Me for conventional receivers with antennas directed toward average cosmic noise sources ;

(2) 300 to 500 Mc for conventional receivers with high gain antennas directed toward high cosmic noise sources such as the Milky vVay;

(3) 1,000 to 3,000 Mc if the r eceiver is equipped with cooled amplifiers such as the MASER.

Antenna beamwidths must always be consid­ered and compromises made between beamwidth and optimum signal-to-noise ratios. Since receiver noise increases only slowly with frequency and

108

I receiver inpu t power is constan t up to abou t 6,000 M c, higher frequencies may be used with only sligh t decrease in SIN ra tio bu t a t the expense of increased tracking difficulty .

As more elabora te space vebicles capable of maintaining attitude and employing direc tive an­tennas itre developed, t he receiver inpu t power will increase wi th frequen cy and the background noise level will no longer determine the . op timum fre­quency. The op timum frequency will be governed by a compromise between m aximum practical physicitl an tenna size and the minimum an tenna beamwid th consistent with acquisi tion and tracking techniqu es. If a t ti tude con trol of the space vehide and a.cquisition and trach:ing limi tations of the g!'oLmd stations establish minimum an tenna beam­widths at both terminals, the fL, ing of the maximum practical an tenna size at either terminal will establish the optimum frequen cy and an tenna size for the other terminal. As at ti tude control and tracking techniques improve the op timum frequency in­creases. As larger an tennas become pracLical t he optimum frequency decreases. The op timum fre­quency is therefore closely associitted wi th par ticular applications and can be selec ted once the physical

I ante.nna size and minimum beamwid ths are es tab­lished. For a I-deg minimum beamwidth for the

, earth an tenna and a 20-deg minimum beamwidth

I

for the space vehicle antenna, op timum combinations of frequencies and an tenna sizes arc shown in table 1.

E arth antenna

di am

It 30 60

120

Space Optimum vehicle frequency optilll um

antenna di am

;lie 2400 1200 600

II

The op timum frequency for communication be­tween outer space vehicles is unknown. It will depend upon radio noise in outer space and upon th e ability of the vehicles to maintain atti tudes and

109

thereby use directional antenn as. For omnidirec­tional an tennas or for any fixed antenna beamwidth the op timum frequ en cy will be the lowest frequency consisten t wi th the practical an tenna size and outer space radio noise levels. Since opera tion at fre­qu encies ou tside t he radio window will tend to minimize the radio in terference problem between the space vehicles and earLh, space vehicles wiLh omni­direction al or broad beamwidLh an Lenna should be assigned trial frequencies below 10 M c if an tenn as a t these frequcncies arc practical. F or more clabo­rate space vehicles wi th the ability to properl y orient an tennas with very narrow beamwidths, opera tion at frequencies above or ncar the upper edge of the radio window is recommended (above 10,000 M c). If physical antenna size limits the usc of frequencies below 10 M c and if t he inability to orien t an tenn as limi ts the use of frequencies above 10,000 M c, antenna size and an tenna beam wid th compromises will determine optimum frequencies. FrequenciC's may then be selected by the usE' of fi g ure 5 in the sarne manner as for communica tion between space vehicles and ear th , when both termin als usc direct ive an tennas excep t tha t the earth's iono pheric and tropospheric limita t ion will no longer apply.

5. References

[II B . R. Bean and R. Abbott, Oxygen and water vapor absorption of radio waves in t he atmosphere. Geofi sica P ura E Appl. Milano 37, pp. 127- 144 (1957).

[2] P . O. Lait inen and G. W. H aydon , An alysis a nd predi c­tions of sky wave fi elds intensity, R adio Propagation Agency T ech . R ep t. No. 9. U .S. Army Signal R adi o Propagation Agency, Fort Monmou th, N .J . (Aug. 1950).

[31 D . H . Zacharisen, World maps of F2 cri tical frequencies an d maximum usable frequency factors, NBS T ech. Note No . 2 (PB15136 1) Apr. 1959.

The following paper has j us t been brough t to t he author's a tten tion:

S. Perlman L . C. Kelley, W. T . R ussell, Jr., and W . D . Stuart, Concerning op t imum frequencies for space com­mu nication, IRE T rans. on Communs. Systems, CS-7, 167 (Sept. 1959).

B OU L DER, C OLO. (Paper 64D2- 44)