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Optimum Frequencies for Outer Space Communication 1 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, 19 59) Frequency dependence of radio propagation a nd other technical factors which in flu ence outer space communications are examined to provide a basis fot' the selection of frequencies fot' communication between earth and a space vehicle or for communication between space vehicles. 1. Introduction ent state of development in radiofrequency power generation. The upper limit of this range may be as The probable future rapid advances in the use of low as 5,000 to 6,000 Me during heavy rainstorms satellites and space vehicles will intensify the l'eq uire­ and the lower limit may be as high as 80 to 100 Mc m en ts for adequate space communications. Since depending upon the degree of solar activity, the only modest transmitter power will be initially avail­ location of the earth terminal, and the geometry of able in the space vehicle, careful engineering of the the signal path. 011 the other hand, the window space circuits will be necessary to assure adequate may extend from as low as 2 Me for polar locations communications and parLicular attention to the selec­ during night-time periods to as high as 50,000 Mc tion of radiofrequencies will be required. Optimum at high altitude rain-free locatIOns. frequencies can be selected on the basis of the signal­ In the midportioD of this window favorable propa­ to-noise ratio for a g'ven transmitter power, a mini­ gation conditions exist, and circuit performance can mum distortion of phase and amplitude, the mini­ be estimated on the basis of free-space propagation mum likelihood of interference from other equipment, conditions by the following formula: etc. This report takes signal-to-noise ratio as the sole criteria of frequency selection recognizing that P,tex (PGtG;JW) diffraction and other distortions may cause problems r in tracking and location. where: P ;= requiTed transmitter power, P;= mini­ 2. Factors Affecting the Selection of mum permissible receiver input power,j = frequency, d= distance between transmitter and receiver, Gt = Frequencies transmitting antenna gain power, Gr=receiving antenna gain power. All communication between earth and outer space Actual propagation condItions vary substantially must pass through the earth's atmosphere (including from this free space assumption at frequencies near the troposphere and ionosphere). Communication the edge of the radio window, and it is necessary to b etween satellites will primal'lly involve radio paths correct for ionospheri c and tropospheric effects to outside the influence of the earth's atmosphere. obtain a true estimate of frequency dependence. The atmosphere is frequency selective, allowing This correction require::; an estimate of tropospheric some frequ encies to pass through readily while absorption [1] 3 at the higher frequencies and an severely attenuating others. A range of frequencies estimate of ionospheric absorption at the lower fre­ in which waves readily penetrate the atmosphere is quencies [2] . In addition to estimating ionospheric often called a "window." absorption, an estimate of t he probability of radio Two principal ranges of frequencies pass readily signals penetrating the ionosphere must be made [3]. through the atmosphere. They are: (1) The rangf' To determine optimum frequencies, the variation b etween ionospheric critical frequencies and frequen­ of background radio noise within the radio window cies absorbed by rainfall and gases (about 10 to must also be considered: 10,000 Me), and (2) the combined visual and infra­ (1) Cosmic noise predominates at the lower edge red ranges (about 10 6 to 109 Me). of the radio window and decreases with frequency The atmosphere is known to be partially trans­ until noise within the r eceiving equipment pre­ parent in a third range below about 300 kc. Waves dominates. are propagated through the ionosphere in this range (2) In most present-day facilities the receiving I by what is sometimes called the whistler mode. equipment noise tends to predominate above about Propagation in this mode is not yet well understood. 100 to 200 Mc for antennas directed toward average The range 10 to 10 ,000 M c is the most practical sky noise areas and above about 300 to 500 Mc for for communication purposes considering the pres- antennas directed toward high cosmic noise areas I The basic m aterial in this paper was unanimonsly adopted hy the Interna­ such as the Milky Way. t ional Radio Consultative Committee at the IX Plenary Assem bly in Los Angeles, April 1959, and is being issued as CCIR Report No. 115, Factors (3) If low noise receiving equipment such as the affecting the selection of freqnencies for telecommunication with and betweeu MASER amplifier is used, receiver noise may pre­ space vehicles. ' United States Army Radio Frequency Engineering Office, Office of Chief dominate above about 600 to 1,000 Mc. Signal Officer, 'l'he Pentagon, Washington, D .C.; now with Central Radio Propagation Laboratory, National Bureau of Standards, Boulder, 0010. 3 Figures ill brackets iudicate the literature references at the end of this paper. 105 (4) Noise within conventional receivers normally (3) If both terminals of a free-space communica- ! increases slowly as the operating frequency is in­ tion link use directive antennas of fixed physical size creased, but may tend to decrease at the higher and can operate with narrower and narrower beam­ frequenci.es if MASER amplifiers are employed. widths as frequency increases, e.g., a directive an­ For antennas of fixed physical size, high frequencies tenna on the earth's surface and a directive antenna have the advantage of greater gain but the disad­ on a more elaborate space vehicle (Gt 0(.F and vantage of narrow beamwidths and associated track­ Gr oc P), the receiver input signal power increases as ing problems. the frequency is increased [Pr 0( (Pd2/d2)J. High speed vehicles traveling so that the distance Freq Lleney dependence for practical space-com­ between transmitter and receiver is rapidly changing munication circuits requu'es that atmospheric effects have apparent frequencies differing from the actual be included. Receiver input signal power and transmitter frequencies by the Doppler frequency receiver input noise power for a directive receiving shift component in the direction of reception. antenna and nondu'ective transmitting antenna are Within the solar system there is evidence of ap­ shown in figure l. The receiver input power in­ preciable densities of electrons out to great distances cludes ionospheric and tropospheric effects for a from the sun. 1,000-mile propagation path tangential to the earth's Transmission time delay will become substantial surface for summer midday operation during periods in outer space communications, e.g., 2.6 sec are of hi.g ·h solar activity and for moderate rain conditions I required for a round trip radio signal to the moon. such as experienced 1 percent of the time in the This time delay is essentially independent of operat­ Washington, D.C., area. This is typical of the · ing frequency. most adverse propagation conditions normally en­ countered i? the absenc~ of sudden io~o s pheri c dis- I 3. Discussion turbances, mstances of mtense sporadIc E, areas of auroral activity, or rain conditions of cloudburst proportions. During more favorable propagation Although great distances arc involved, the conditions, such as a propagation path normal to the propagation medium in space beyond the first 500 earth's surface during the night at the lower fre­ miles of the earth's atmosphere is believed to be quencies or in a high altitude rain-free location for essentially transparent to radio waves. Thus we the higher frequencies, the receiver input power can may estimate performance on the basis of free-space be expected to be essentially independent of fre­ propagation. Frequency dependence of receiver in­ quency over a wider range of frequencies. The put power under free-space propagation conditions receiver input power as shown between 100 and 500 depends upon the type of antenna at the transmitter Mc in figure 1 will be typical over a much wider and receiver. This frequency dependence is shown frequency range during these favorable propagation by the following free-space propagation formula : periods. Figure 2 shows essentially the same information P ( PtGtGr) as figure 1, except that the distance is increased to roe fd7- 300,000 miles, the receiving antenna diameter is increased to 120 It, the use of cooled amplifiers such where: P r is receiver input power, P t is transmitter as the MASER is anticipated, quasi-maximum power, and other symbols have the same meaning as before. Frequency dependence of receiver input power for .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 free space propagation conditions can be summarized \. V I POLAR REGION OBliaUE PATH OR T.1Mr AL R~GION VCqTlCAL PATH as follows: 100 ~IGHT : ) : TROPICAL REGION ('euWl p:.n ' (1) If both the transmitting and receiving termi­ ~ .... 'f,\ : (' ~ ,...--. REr!.rVING ANTi:.~ NA BEAMWIO'fH ~ 110 DAY "\. : : 20" 10° SO 60' 2" ,0 .5 -- .. ..- nals of a free-space communication link use non­ ~ 120 \." \\ .. 3i:f I 30' .. .2 directive antenna (e.g., two vehicles in space) or if ~ MEDIAN \ 1\ 40"130"20· 10" \ 5 2" ." .50 ...J 130 ATMOSPHERIC \ RECEIVING ANTENNA beamwidths at both terminals are fixed , the receiver ~ NOISE IN : \. \ DIAMETER input signal power increases as the frequency is "9_140 AREA \\ : \,1 I decreased: ~ 150 " li' -..
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