Page 1

Expl*r*fialr and Expl*it*tion

of the 3 cm fa 3 rnrn Wauelengtfi



*y Doc Ewen tHar*ld l. Ewen, Ph.D.) Originally published 1970 as part of Advances in Microwaves, Voiume 5 Edited by Leo Yaung Published by Academic Press, Inc.

Reprinted and published by Ewen Prime Company

David K. Ewen, M.Ed.

Witfl permission by Doc Ewen {Harold L Ewen, Ph.D.}

Ccpyright g 19?9, 2$13, Hareld l" Ewen, Ph.D.

ISBH-l3: 978-1491OL7296 ISBI{-1$: 1491f117295

Preface The lifth volume of Advances in Microwuves contains three chlpters that range in their coverage from low microwave frcquencics used to accelerate elementary particles, through cm and mm wavcs for exploring atntosphcric phenomena, and on to the microwave demodulation of light.

The chapter on high-speed photodetectors for recovering


signals modulated on a laser carrier is our first successful collahoration be-

tween authors working

for diflerent


L. K. Andcrson, iVI.

DiDomenico, Jr., and M. B. Fishcr are familiar with both microwaves ancl lasers. When large bandwidths Are to be transmitted on light beanrs' the signal must be ( I ) modulated onto the lascr carricr frequenc\' . (1) transmitted, and (3) demodulated. The chapter deals with the third topic. (The second topic was considered in G. Goubau's chapter in Volume 3 nnd A. E. Karbowiak's chaptcr in Volume I ; the first topic may bc the suhject of a future volume.) It is a pleasure to include a chapter b.,r H. I. Ewen. He has becn a pioneer in microwave radiometric measurements through and of thc atnrosphere. In the past, the frequency decade 10 to 100 GHz has heen used to probc the atmosphere and has yielded much meteorological information. This frequency band has long held out promise for microrvave conrmunications. a promise that seems to be on the point of being fulfilled via sltellites in splcc. Wr have included another contribution from abroad. French author Y. Garault writes on microwave hybrid modes. which ilre used to dcllect and separate high-energy particles in the linear accelcrators at CtrRN in Europe, and at Brookhaven and Stanford in America. We wish to acknorvledge the help and advice received from G. A. Loew in preparing this chapter. The reader is also referred to the chapter on the Stanford lincar:lccclcrator in

Volume l. This volume could not have been assembled without use of the facilities at Stanford Research Institute. We are also grateful to Miss Dianna Bremer

for her unfailing help in manv ways.

Lro Yourc;

Exploration and Exploitation of the 3cm to 3mm wavelength Region Harold I. Ewen EWEN KNIGHT CORPORATION EAST NATICK, MASSACHUSETTS



II. Microwave Radiometry

A. B.


Temperature calibration of the output Indicator Receiver Functions and Techniques

Microwave Radiometer Applications A. Radio Astronomy B. Microwave Meteorology

IV. A Look into the Future References.



Historically, the use of the millimeter portion of the spectrum


undergone cyclic periods of interest. Now increased attention has been directed to this wavelength region, spurred in large part by the worldwide explosion in communication needs. There are many inducements ro consider this portion of th: spectrum from a communication standpoint. Relatively high antenna gain is achieved with modest aperture dramerer; broad channel capability permits high information capacity, and the total available bandwidth, even within the restricted atmospheric windows, far exceeds the entire radio spectrum below 10GHz (3cm wavelength) . Exploitation of this wavelength region for communication has, in l-"rg" p".,, been paced by the need for reliable millimeter power generating devices and low-noise receiving systems. The required technological advancements


these areas appear imminent.

Though the prime interest, as in the past, has been the need to alleviate in the microwave communication bands, the latest resurgence of interest in millimeter waves has been aided by a passive, but not silent, congestion

partner. Exploration of this portion of the spectrum has been forging

ahead at an accelerating pace through the application of passive radiometric measurement techniques. Some of these investigations are concen-

trated in the available *atmospheric windows'



their future




potential for earth-space communication links. Several significant investigations, hou'et'cr, arc being directed to those portions of the spectrum u'here the level of atmospheric opacity is too great to be useful for comnltlnication. Radiometric sensing of thc electromagnetic em ission of the a tm osphere in these portions of the spectrum is providing a new and po\l'crful tool fclr the investigation of atmospheric structure and the associated physical processes. Today, \,4,e are at the dau'n of the new science of microwave mcteorolog,v. We can expect many startling discoveries bcginning in the decade of the 70s, ns microwave and millimeter radiometric sensors contribute to the challenge of global weather prediction. The new field of microwave meteorology \vas spawned by the young science of radio astronomy w'hich has produced so many startling discovcries conccrning our galaxy and the universe. From the earliest experiments performed in tlre HF and VHF bands, the radio astronomer's spectrum of interest has progressed tou'ard the millimeter wavelength rcgior , paralle ling the m ove of com m unication systems to higher freq uencies, rvith the uprvard step for each paced by advancements in instrument technology. Exploration and exploitation of the higher frequencies has historically favored the radio astronomer since the passive receiving devices needed for radio telescopes frequently become available bef-ore the po\e'er generating devices needed for communication systems are developed. Each upu'ard step in the spectrum has led to unanticipated discoveries. The significance of these discoveries has, ofl occasioD, suggested the exclusion of communication systems from certain portions of the microw'ave and millimeter spectrum. Radio astronomers and communicators share those portions of the spectrum frequently referred to as ths "atmospheric rvindo\t's" where electromagnetic radiation passes through the atmosphere r'vith least attenuation. These windou's, u'hich are centered near \{'avelengths of I and 3 mm, are open during clear \veather conditions, partially closed by heavy rvater laden clouds, and are essentially closed during occasional periods of heavy rain. The attenuation and noise characteristics of the atrnosphcrc in these rvindor.r's are of prime concern to both the radio astronomer and communicator. The astronomer must understand the propagation characteristics of the atmospheric medium in order to delete its contaminating effects from the analysis of the very faint signals received from space. The communicator must knou' how the atmospheric rnedium effects signal fading, angle modulatior, and correlation bandwidth in ordcr to determine the optimum systcrn design. Several significant communication research efforts in this area, today, are based on techniques devcloped in the field of radio astronom,v-. An obvious reciprocal benefit will be knowledge gained by the young science of micro\\'ave meteorology. The microwave radiometer is the common denominator in the explo-


ration of the 3 cm to 3 mm \\'a\relength region. Invented b1' Dickc Ilj less than three decades ago, â‚Źlrlbcllished anrJ exploited b1'lrdio asrronoml,, its use is rapidly' spreadint to a clivcrsitl' of scientiflc rescarch ancl cnginee ring disciplines and applications in the e.xplosive pioneering erplorarign sf thc

millimeter rt'avelength region. It u'ilt be helpl-ul in Oiscussing rhese applications if we first revier,r' certain radionlctric l'undamentals associateci with this portion of the freq uenc), specrru nl. II.


A microu'ave radiometric sensor is a device for thc dctection of elcctromagnetic energy which is noise-like in character. The spatial as u-ell as spectral characteristics of observed energ-v sr)urces determine the perforrnance requirements imposed on the functional subsystems pf the sensor. These subsystems includc an antenna,, and output indicator. Natural or non-man-made sourccs of radiation may be either spariall-n* discrete or extended. In the frecluency clomain, t6ese sources rlil\,be either broadband or of the resonant line r),pe. Scnsor design and performance characteristics are primarily deterrnined by the exrent to ryhich spatial and frequencl- parameters characte rize the radio noise soLlrce of interest to the observer. A micro\\'ave radiometric sensor is frequently referred tg 3s a temperature measuring device, since the output indicator is calibratcd in dcgrees Kelvin. The reason rvhy microu'ave radiomcters arc calibrated in temperature units and the modes of operation that are most frequently usecl arc described in the sections imrnediately follorving.


TrltPER,,\Tunr CALIBRAT-IoN oF Tt-tr

Oljrpur Irr:tcAToR

The ph-'-sical reasoning in support of calibrating the ourpur indiceror of a microw'ave radiometer in degrees Kelvin can bc deriyed from thcrmodynamic considerations and certain rvell-kno\tr,n properties of an antenna. The amount of energy absorbed b1'an antenna and presented at the input terminals of the receiver depends upon the orientation of the anrenna, the polarization of the \\'ave, and the im pedance match of the rece i'ing system ' Since al I an te nnas e re pola rized regard less of , design , the ma.ximum amount of cnergy accepted by an antenna, frorn a randomlv polarized wave, is one-half of the total energy content ol the waye. If \{,e assumc that an antenna is perfectl,v matched and that the incoming \\,il\.e is randoml)'polarized w'ith a po\r'er llux density S, then the absr-rrbed po\\.er PA is given by the expression








rvhere .4 is the efrectil'e antenna aperture area. In Eq' {l), the flu.x densit,l'S- of the radiation is assumed to be from a source of small angular size and is measured by the flow, of energy from the source through unit area in the \rrave front at the observing point. If


dE in the frequency range dv flows through area dA in tim e dt is long comparcd to the period ol one cycle of rhe radiation),

(rvhere r/r

then the flux densiry S is given by the expression


dE dAdyclt

/'l\, {t

which has the dimensions of po\r'er per unir area per unit bandrvidth. Norv con'sider a transmission line, one end of rvhich is terminated rvith a matched load and the other end of u'hich feeds an antenna in an absorbing medium. If *'e were to replace the antenna by' its .quivalent two-terminal crr lrsLlt netrvork ur K ilno and assume that tt it ls is a purery purely resistive r.ri, impedance and eqLlivalent to thc Ioad impedance, Ihen a transmission line terminated terminared in a matched antenna may be treatcd in a manner similar to a transmission line terminated n'ith a resistive load, as shor',,n in Fig. l. If the extent of THERMAL RESERVOIR








Ftc' I ' EquiValent circuit of an antenna

immersed in an absorbing medium at temperature I. In equilibriurn, the temperature of the load resistance is the same as the temperature of the absorbing medium.

the absorbing medium is suflrcient to completel),absorb all radiation from the antenna, the medium and the matched termination must then be at the same tem perature T, From Johnson noise po\r'er considerations, the termination u,ill radiate a po\\'er kTdv to the antenna. If the antenna, in turn, did not accept k Trtv

of radiation from the medium and transfer this power to the load, there u'ould be a net transfer of thermal energi- from one region to another at the same temperature rvithout application of rvork, in violation of the



second 'law o{' thermodynamics. This u'ould indicate that in the microwave and millimeter portion of the spcctrum, thc po\\'er deliverecl to the

receiving system input by an antcnna immersed in an absorbing medium at temperature I is independent of the frequency of observation. This conclusion can also be reached (see Fig . ?j by noting that the medium appears AS a blackbody to the radiation resistance of the anrenna, i-e., it absorbs all incident radiation and its radiation brightness P in rhe frequency inten'al dv in accord u'ith planck's larv is n , 1iO'-:


"hv3 .7 c-




n'here h


Bol Lzmann constant


and the brightness F unit bandwidth.


the power per unit area pcr unit solid angle, per



lF I







Flc. 2. Simplified block diagram of an antenna and receiver. When the anrenna is immersed in a blackbody at temperature T, the receiver input is equivalent to a resistive load R immersecl in a thermal bath at temDer-

ature T.

From the definition of flux density's,

it is evident that

.s- [,rro dO is the solid angle increment. This definition holds for any source of radiation over all solid angles. where


of flux


Harold I.


(3) indicares rhat the power received from a blackbody, the input terminals of the receiving system , is frequency measu red at Hence, the equivalence of blackbody radiation and Johnson n depcncle [. noise po\r,er appears inconsistent. The ensrver to this paradox lies in the Eqr.ration

characteristic frequency response of any antenna system and the frequency charitcreristic of blackbocll' brightness in the millimeter and microwave porrions of the spectrum. This can be most easily seen by recalling that the po\\,er absorbed by an antenna, from a randomly polarized source vo'ilen operating in the frequency'range dr, is



re 0 and p clescribe thc direction of the incoming ',va','e and A(0,Q) is thc:.rntenna aperrure receiving cross section in that direction. Hence, for an extended source w,lre


a') dQ F d, \,nto If the extendecl sourcc radiation is a blackbody, the antenna rvould then be erpressed in the form rR




2hv3 Ir fllf Ai.g,O)dQ+

f JJ"t",Yt*r4


the po\\'er receil'ed bY


[e^p (hvlk





of the photon hv is m uch less than the random thermal energy per degree of freedom kT at (3i] reduces rem perarure T , the e xpression for blackbody brightness [Eq. to the simplified expression In that porrion of frequency spectrum


w'here the energy

, : :?kT dv (RaYleigh-Jeans) A-

p au


is the rvavelength of observation. Hence, spcct rum Eq. {1) reduces to ,*,hcrc

in this portion of

(8) the


cross section for an)'antenna Rccalling that the ":'.rage'.u..,1t. may be expressed in the form brightness ol'uniform in a source immersed

o) r - +lu,o, 1;r


. : cl.Q,









.l_ 'tru



we arrive at the conclusion that the po\\'er received bf iln antcnna immersed in a blackbodl'at temperature T is frcquenc)'indcpcndent and equivalent to the Johnson noise po\\'er kT dv. As a consequence, the po\\'er received b,v a microwave or millimeter radiometer is conventionalil' described in terms of equivalent temperature units. The transition region in the frequency spectrum at u'hich t hc cncrgv of the photon is comparable to the random thermal energ)'per degree of freedom is, ol course, temperature dcpendent. Approximate values are shou'n in Table I. It is apparent from Table I that microu'ave nnd niillimeter \f,:evelength measurements of the earth terrain and atnlosphere (ambien t 290"K) at frequencies below' 300 GHz ii : I mnr i f'rll we ll rvithin that region of the spectrum w'here lu is less than kT. fable I Trvpr,nATUREs AND Connr,sPoNDINc WnvEl-r,hrcrlls THE ENr,ncv oF THE Ptrorox ft:"' Is EQunl ro k T

nr WstcH Temperature {'K) 300 11

20 4


Wavelength '. pm' '?o


900 6000 10,000

Wavelength {mm;

Frequenc) CHzi

0 .07


0.2 0.9 6.0

I 500






In summary, the power receivcd by an antenna immersed in a blsckbody at a temperature f is frequency independent and equivalent to the Johnson noise power that u'ould be radiated by an antenna if terminated in a matched resistive load at the same temperature T. These t\\'o fundamental sources of noise power are equivalent at m icrou'ave l'req uencies due to the inverse u'avelength squared dependence of blackbodl' brightncss, which is offset by the wavelength squared dependence of the antenna cross section. Hence, the noise po\{'er per unit cycle received b1'an antenna and presented at its output terminals is directly proportional to the cffective blackbod-y temperature lvhich characterizes t he source or source s in r*,'hich the antenna pattern is immersed. The proportionality factor is Boltzn-lann's constant ft. Since most natural sources are not blackbodies, their "signlil temlerature , " measured by a radiometric sensor, refers to the po\\'er leve I that would be received from a blackbody et a temperature which rr'ould provide an equivalent po\\,er level at the output terminals of the an[enne. This temperature concept is useful in describing the functians of the antenna and receiver in a microwave radiometric sensor. The entenna




extracts noise polver from thc radiation incident on its aperture and presentS i! noi)c pg\\'er at its output terminals rvhich can be described in terms of an efective blackbod-v- temperaturc. This noise po$'er represents a comFosire of the desired signal pog'er and undesired noise power from other sourges, since a practical.antenna alvv'ays looks to some degree in undesired directions: or the signal source may be immersed in-a background noise fie



mperature of the composite noise power presented ilr rhe output terminals of the antenna is I, and that portion associated r..ith useful signal power is Ir, then Tt:Ts* II', r,r'here f T, represents a summation of effective noise temperatures from the undesired sources of noise pou'er observed by the antenna. The siSnal-to-noise ratio at the ourpur rerminals of the antcnna is then TriLT,. The prime lunction o[ the recciver is to amplify and detect the input signal rvhich is characterized by the composite temperature ]n,{. All processcs of receiver amplification add noise to the received signal. This added noise is frequently referred to as the internal receiver noise q,"hich can be described by an effective temperature ?"n referred to the input terminals of the receiver. The ratio of antenna temperature to receiver noise temperature, at the interfaCe bctween the antenna and receiver, is then (fr * IT,)lT^. Note that the un\\'antcd noise power lf,, received by the antenna and presented at the input terminals o[ the rcceiver, cannot be differentiated from the desired signal temperature fs through amplification alone. Spatial differentiation betrveen ?'., and I ?i may be obtained by scanning of the antenna beam, if the source ol signal temperature ?t5 is spatially discrete and the sources contributing ro I7, are spatially extended. Similarly, the separation of I, from I I may be obtaincd in the lrequency domain by the receiver, if either the sourcc of signal temperature or background noise temperature exhibit markedly different frequency characteristics, such as a resonlnt line superimposed on a broadband continuum. In this case, the receiver can be scanned in the frequency domain to separate the signal temperature from the temperatures contribured by broadband background sources. The determination of the equivalent noise tem'perature of a receiving system is rclatctl to the method of noise figure measurement. The noise figure of a receiving system or net'*'ork is defined as the signal-to-noise rarioar the input, divided by the signal-to-noise ratio at the output, when the receiver or network is terminated in a matched load at a temperature ?'o of 190"K. A simplified equivalent receiver network, shown in Fig. 3, consists ol a net\l'ork rvith input terminals shunted by a resistor R and output terminals connected to a meter indicator. From the definition of noise figure F, the noise figure of the network shown in Fig' 3 is given by the effective

the expression




GkTodv G

+ GN

il ll

kTo dv


F-1+ If




kTo dv

now define the svstem noise temperature TR by the expression i13)

tf - kT*dv

we see that the relationship betrveen s-Ystem noise temperature and noise figure is i





=$ = dv








NOISE = k To dv NOISE =N




To dv

+ GN



noise relationshiPs of a four-terminal is immersed network with the input terminated in a resistive load which in a thermal bath at temPerature T'

Frc. 3. Input and output signal and

B. Recrtvtn FuNcrloNs

nNo TecuNlQurs

Theprimefunctionofthereceiverinamicrowaveradiometricsensor is to provide a measure of the antenna temperature' the As previously noted, the antenna temperature I' is' by definition' raised be ,r-p.r"rur" to *iri.h the-radiation resistance of the antenna must

inordertoproducethesamenoisepowerasthatcontributedbythevariouso[ brightness temperature sources observed by the antenna. It is also the antenna' would proa UtactUoay which, if it completely surrounded the describe the method To .vide the same noise p*". at the rlceiver input' we. will replace temperature' by which the receiver measures tbe 'antenna input' If the receiver the at load the antenna with an equivalent risistive power lnput nolse same the antenna temperature were I;, we would obtain a temperat bath in thermal a load to the receiver by placing the resistive zture TA.




The nced for signal amprification becomes readiryapparent u,hen one notes that the average noise pou'er per unit bandwidth produced by a resistor at an ambient rcmperature (290'K) is of the ordei of l0-20 rvatt. Typical detectors require a dri'e power of at least I0-e watt. 'The required input signal amplification must, of course, be increased if temperature changes less than 290"K are to be detected and recorded. The receiver must therelore be able to sense a low level change in noise power at its input and provide sufficient stable amplification io drive the output indicator s)'stem. Amplification stability is a prime requisite since the receiver must provide a consistent output responsc for the same input power change. The.rclatively poor gain stabirity of present receiving systems is overcome b1' the use of an input switch or modulator, to be discussed later.



The noise power output of a resistive termination is associated with the tlrermal agitation of electrons rvithin the resistive conductor which produce electronic collisions. As the thermal temperature of the resistor is increased, the thermal agitation increascs: and the number of collisions per unit time increases. The resultant noise pou'er output per unit cycle is direc'.ly proportional to the absolute temperature of the resistor. As indicated previously, the proportionality factor is Boltzmann's constant k. In this sense, a radio measurement of the thermal temperature of the input resistor may be described as a measurement of the electron collision frequency within the resistor. since the collisions are random, the number per second will vary; however, the mean of an infinite number of onesecond umples will lead to an exact value for the collision frequency. F-rorn statistical theory, the probable error in the measurement of a quantity of this type is inversely proportional to the square root of the number of measurements u'hich are made. If the number is infinite, the exact value is determined. lf we now measure the electronic collisions within a resistor, using an amplificr of finite bandwidth Au, the number of independent collisions per second which can be counted is equivalent to rhe receiver bandrvidth. Hence, the error in determining the mean value of the noise temperature (which is proportional ro the collision fniquency) will be inversely proportional ro the square root of the receiver bandwidth. If the

averaging process is extended over r seconds rather than one second, there will be, on the average, r Au independent collisions in each intervar of seconds, therefore




(1 5)

In most radiometric applications, the magnitude of the signal temper,a




ature is negligible when compared rvith the " receiver noise temperature T^' which describes the noise power added to the received signal by the various circuits rvithin the receiver.

Simplified functional block diagrams of the most commonll'used microwave and millimeter radiometric receiving systems arc shou'n in Fig . 4. The figure depicts the genealogical gro*'th of each receiver from Cryriol Video

Tunod Rodio




With Input SigrEl Anplifirr

Flc. 4. Simplified block diagrams of commonly used radio receivers. the one preceding. The crystal video receil'er is usually the first to be used

in a new portion of the frequency spectrum rvhere input signal frequency components required for the other modes of operation are not available.

The superheterodyne is the "workhorse" among receivers. The input circuit of the superheterodyne is a "mixer" in which the signal frequenc-y- is hetero-

dyned with tlfe local oscillator frequency. The difference or intermediate frequency between the signal and local oscillator is amplified by a tuned intermediate frequency amplifier, re ferred to as the IF amplifier. The addition of a low noise amplifier forward of the mixer in a superheterodyne mode will establish the receiving system noise temperature by,' providing adequate gain to overcome the conversion loss of the mixer. The most sensitive broadband radiometers operate in the TRF mode, where amplification is provided by the cascading of broadband low noise amplifiers, rypically, traveling wavetubes and, more recently, tunnel diode amplifiers. Today, completely solid-state TRF receivers, using tunnel diode amplifiers, provide nominal system noise temperatures of 1000"K and instantaneous




prcdcrection bandu'idths of l0 to | 5.,i of their operating frequency, at frequencies up to 20 GHz. The scnsitivity of a radiometric system, i.e., the minimum detectable signal. is determined by the amplitude nf the lluctuations present at the outflut inclicator in the absence of a signal. These fluctuations are attributable to two sources: il t The statistical fluctuations in a noise \\'aveform as described bv

Eq. (l 5) .

i:) Spurious gain fluctuations associated rvith the receiving netrvork. The amplitude of output fluctuations due to the first source cafl, in principle ,be reduced to any desired degree by reducing the postdetecrion band$'idth (increasing the integration time) . In practice,, the longest us:tble integration time is linrited b1,'the time available for observarion of the "signal." 2.

Gain llariatiotrs and the Dicke .Vode The second source of fluctuation s u,'hich occur at the receiver output are attributable to receiver gain instabilities. Their significance can be readili" grasped by the following example. If \\,e introduce values of TR - l000oK, Au: 2 x l0eHz, and r- I sec in Eq. (15) , we obtain an rms value for the amplitude of statistical noise fluctuations at the receiver output of the order of 0.03oK. This w'ould be the case if the receivcr \r'ere absolutely gain stable. Unfortunately, the best receivers, regard iess of type or frequency of operation, exhibit gain instabilities of the order of l,'/" during a time period comparable to that required for a noise measurement. As a consequence, a receiver with the performance characteristics describcd above u'ould provide an output fluctuation of 10"K if the gain changed by liv,. The noise measurement sensitivitv of

Irrc, 5. Simplified block diagram of the Dicke radiometer. A switch or modulator is introduced between the antenna output and receiver input.


the system would then be determined by the effect of gain variations rathe r than by the level of statistical noise fluctuations' The answer to this dilemma \r'as provided by Dicke Il] in the form of a single pole, double throw srvitch placed at the input of the receiver, as shown in Fig. 5. One of the input ports of the switch is connected to the antenna output terminal, the other to a resistive load held at a constant temperature ?.. The switch is driven sequentially in a square rvave fashion ar a frequency considerably higher (typically 30 to 1000 Hz) than that at

which a substantial receiver gain variation occurs. With the srvitch in operation, a signal at the switching or modulation frequency is presented

at the input terminals of the receiver rvith an amplitude proportional to the temperature difference TA- Tc, Becausc of the rapid su'itching rate, any receiver gain variation rvill operate equally on T^ { 7^ during onehalf of the srvitching cycle and on T.* T^ during the other half. rvith the result that it operates only on the difference T^ - Tr. If for e xample, the difference Te - 7. were loK, the effect of a 196 rcce ivcr guin variatiqn referred to the output indicator system would be 0'01'K' In the example given above, rhe introduction of the switch providcd a marked improvement in the noise measurement capability o[ a typical rersiver by eliminating the effect of receiver gain variations operating on the receiver noise temperature. The gain variations, horvevcr. contin:e to operate on the temperature differcnce presented at the two input ports of the switch. Tl'ris was not an important consideration in early' radiometers which had relatively high noise temperatures and narrorv bandwidths, leading to sensitivities of the order of a few degrees Kehin. Pre sent-day broadband radiometric receiving svstems' howcyer, have potential sensitivities of the order of 0.0-5'K rms for postdetection time constants of I second or less at frequencies up to and including 20 CHz. In rhis case, the effect of receiver gain variations, operating on an RF input unbalance (large temperature dilference betrveen input signal and comparison ports), is o[ far greater concern. Several techniques for reducing the RF input temperature unbalance are in comnton use. Thr-'se include addition of noise to the signal port of the radiometer, use of a lor1' temperature comparison source, and introduction of gain modulation' Addition of noise in the signal transmission line is frequentlv reserved for applications in rvhich the system noisc tcmperature is relatirely high, i.e., such that the added noise reprcsents e small perccntagc increlse in the overall system noise levcl. Radiometers r.r'ith maser or Iorv noise parametric input ampliliers normally. use a low te mpcrature comparison source such as a resistive load immersed in a liquid helium bath. The technique of "gain modulation," introduccd approximately one decade ago, involves the adjustment of receiver gain in synchronism rvith




the su'"itch or modulation frequency to provide an equivalent level of noise at the input to the envelope detector during both portions of the sg,itch c1'clc. This technique provides a convenient adjustment of the effective temperature level at either input port u'ithout adding noise to the receiver or changing the temperature of the comparison noise source. The gain modulator technique, horvever, is sensitive to changes in system noise figure and must be used rvith caution.


Temperature Calibration

The detection of a signal noise source and its measurement in absolute a prime objective in several radiometric receiving system applications. In addition ro sensitivity esrablished by the noisc characteristics of the dctector and, in large part, influenced by the gain stability of the overall receiving system, measurements of this type require: tem psrature units represents

{l; Knowledge of the output indicator

zero level

in absolute tem-

perature units. i'1


Calibration of the output indicator deflection in absolute temperature units.

The output indicator zero level corresponds

to the condition of


tem perature balance bet'uveen the input signal and comparison ports (switch ports) of the receiver. I-Jnder this condition, the comparison source tem-

perature referenced to the comparison input port provides the indicator " 7'ero let'el " for the indicator scale. Calibration of the output indicator deflection requires knou'ledge of the detector law and system gain. Of these tu'o requirements, the indicator zero level is by far the more difficult to achieve. Kno\\'ledgc of the detector law can easily be obtained through laboratory measurement. Sy'stem gain can be established at any time during a rneasurcment program b;- introducing a constant and fixed level of noise at the radiometer input. The noise level of this gain calibration noisc source need not be know'n precisel-v-. It is far more important that it remain constant and that it be used to establish the level of receiver gain during laboratory calibration of the radiometer response in equivalent tem perature units. This noise source is usually included as an integral pert of'a radiometer and is referred to as the "calibration or internal noise sOu

rce. "

Calibration of the output indicator - requires the introduction of


preciscly'knorvn temperature change at the input signal port of the radiomstrr. This mea.surement is usually' performed under carefully controlled labor;1torv conditions. This temperature change is lrequently generated b1'the scquential introduction of two vert'preci-].ly knor.r,n noise temper:


ature sources at the signal input port of the radiometer. Calibration of the internAl noise source in equivalent temperature units is automatically obtained as a by-product of this laboratory calibration procedure. From the foregoing, it is apparent that one intcrnal fixed calibration noise source, combined with knorvledge of the detector law characteristic and predetection attenuator values, provides all of the information required for the precise calibration of the output indicator reading in equivnlent temperature units. The internal calibration noise source level should be approximatelv two orders of magnitude greater than the amplitude of the peak-to-peak fluctuation level at the output indicator for the nominal value of postdetection integration time constant which will be used during the measurement program. This allows opportunity to establish the fullscale output indicator deflection level to an accuracy of at least \:ib. When measuring the amplitude of Iorv level signal temperatures, the introduction of the indicator calibration signal will normally require an output indicator scale change. This is usually achieved through ganged s',,r'itching of

the two functions in calibration noise source ignition and ind icator



In 1967 , Haroules et al . 12) described a passive circuit u'hich, u,hen introduced at the input of a relative po\r'er measuring radiometer, provides an absolute power measurement capability. The input circuit pcrformance is such that the zero position of the output indicator corresponds to zero degrees Kelvin. The effective blackbody temperature of a noise source coupled to the input of the radiometric receiver is read directll. in degrees


plif icr





nt:l qFab"._T.-p.: ty. _= _ T L

Flc. 6. Functional block diagram of absolute temperature







KelVin at the output indicator. A simplified functional block diagram sho$'ing the interconnection of RF components with the rypical Dicke modulator to achicve this absolute radiometric mode is shorvn in Fig.6. The concept of operation is predicated on the lact that any noise signal presented at the input port (1) of switch S will be attenuared by the RF losses a ssociated with the passive circuitry in the signal path from the input port of the srvitch to the inpu, por, of the Dicke modularor. In addition, these sarne passive RF circuit components will radiate noise as a conseq uence of the lact that they Are at a thermometric temperature above Absolute zero. This radiated noise will be combined u'ith the attenuated si.-enal and presented to the signal input port (l of the Dicke modulator' Since the attenuation of the input signaf .un be) characteri zed as a change in the gain of the total system, and the radiated noise of these components as a noise bias term, the desired calibration of the ourput indicator directly in clegrees Kelvin can be accomplished by a single adjustment of attenuator .4. Though the laboratory uOlustmeniofattenuator A requires a known source of noise power (cold load), a standard refercnce source of this type is not required for field operation of the




The number and diversity oI micro\\'ave radiometer applications today trul)'amazing rvhen one recalls that this instrument technique is just 27 )'ears old in 1970- Exploitccl first by the young science of radio astronomy, the power of this instrument has demonstrated its abilit,v to explore the unkno\\'n and provide many historic discoveries. Only a small fraction of the knorvledge gained has bcen anticipated. It was at first frustrating to learn that the brightness temperature of the sun, measured at low freo q uencies, was more than I ,000,000 K rather than the anticipated 6000" K , that the radio noise from our own galaxy was markedly diflerent from thc anticipated blackbocly radiatior, rhar spatially discrete sources of intense radio energy were present in space and could not be idcntificd w'ith optically observed sources. The impact of these and several othcr discoveries dispelled the frustration and replaced it with a humble ad m ission of the depth of our ignorance. Today, the unanticipated in galactic radio astronom)'is considered routine. We now knorv we are at the daw'n of a new era in astronom,v, Accumulating new knorvledge on which \\'e u ill build a nerv ancl deeper understanding in the years ahead. Spaw'ned by' the pioneering and explosive enthusiasm of the young sciencc clf radio astronomy, the improvement in micrclwave radiometric is


sensor capabilitics has bccn cqualll' startling. Tirc ternpcrittLlrc ntcllsurement sensitivity achieved tod;.r)- at a \\'avele ngth of 3 m nr mltche s tirc best that could be done 20 years ago ar a \\'avclength erf l0cm. In thcsc tvv'o decades, the measurcmcnt capabilitl, at l0 cnr has bccn inr pror cd b1' nrorc than t$'o orders of magnitude. The instrument of 30 )'ears ago flllerJ an entire room, consumine nearll,one kilorr,att of po\\'er. Its c()Llnterpert today is the size of a matchbor and requires less tlrln llve \\,ittts of irrput po\1'e


Paralleling the need for improved sensor capabilitl' has bcen the need for larger antennas of improved surlace tole rrnce to provide grcsrcr lngular resolution at short rvavelengths. with the combincd improrerncnts in angular resolution and tcmperature measurcment capability, the latest telescopes are nou'able to probe the farnili:lr neighbors of our o$'n solar svstem, mapping the surface of the sun and the moon and mcasuring rhe thermal radiation characteristics of the planets. Signals received from space b;- radio tclescopes et wavelentrhs shorter

than 3 cm are contaminatcd by atmospheric attenuation and noise. This interferring source of noise to thc radioastronomers has beco,nc thc signal for the micro$'ave meteorologist. The \\'ater vapor resonance at ll.l35 GHz and the complex resonant line structure of molecular arrnosphcric ox)'gen near a rvavelcngth of 5 mm bccame sources

of intense invcstigurion beginning in the early 1960s. The pattern of historical devclopment in rhe application of rhe radio telescope u'as just the reverse of u'hat one might have anticipated. fjrom the c.,r'ly observation of galactic radio noisc, follorvcd by rhe obscrrarion of our neighbors in the solar svsrem, the radio telescopc in .iust the past decade has been pointed u'ith greatcr intcrcst at the planet eiirth. The first step in this direction has been to obtain an improved undcrsranding ol the physical processes in our earth's atmosnherc. The microulvc rtdiometer offers the possibility to obrain a global picrure of rcmpcrarurc and $'ater vapor distributions, To sce, in clear air, large conccntralions of water vapor and associated temperature gradients, to detect air mass motion by listening to the radio signal from ozone as it moves like a tracer element through the je t streams, to predict n'here the cloudS u.ilt forrn and u.hether there rvill be a major storm. are the exploration frontiers of thc microu'avc meteorologist. A significant amount of fundamental research has already been accomplished. The initial resrs of rhis neu' potential capiibilitl,have been scheduled for satellite experim-ents to be performed in the decade of

the I 970s. In much the same u'ay that the science of radio astronomv spaw.ned the nerv science of micro*'ave mctcorology. thc kno*.lcdgc gaincd and the associated techniques of micro*'are meteorolosv have lcd to a hosl 9f 6srt'




I nci t'\citins areas of tpplication. Our limitcd objective in the discussion r,r'iricir i"rtllo\\'s u'ill be tc provide a bri':l sr-rmmar), of present-da). nricro\\'ii \ e rlrdiollle tcr applications in selccted areas of research. The erploratorv Ilittui'c of prcsent-dit1'rescitrch in each area emphasizes the ne\\,ness of the tllcli\urcmcnt irrstrtrmcnt. Exploitation of the knolvledge gained u'ill be paced b1' ollr ability' tct understand u'hat rr,'e learn and our abilitv to devclop u:cful s:-.stems bascd on that undcrstandins.


It .\nrr) AsrRolirlt\t y

lJv thc l.rte 195{}s, antenna and rcceiver technologies harJ extended

rlidio telcscopc operation to \\'avelengths shortcr than 3 cm. The milli-

nte tcr rcgittn beca me the pioneerirtg cha llenge of the I 960s. Earl), research in tlic 3 cnr to 3 tnnt rcgion \\'as concentrated primarill' on the cletermination of thc spectral indiccs ol the discrete radio sources u.hich had been previousl)'detected rtt the longcr \\'avelengths. Cosmic radio maps \\,ere extcrrdcd to tltc shorter vv'avelcngths and an intensir,'e search for a tenuous isotropic radiation \r,'as initiated b1' several research groups. The existence of-tlris rediaticln \\'as prcdicatcd on the theor,v of cosmic evolution u.hich postulittcs an initial primordiaI erplosion t3]. Verification requires an llh:;,:lute te mperitturc mcasurcment at scveral u'al'elengths, one of the most

difticr-rlt of-all radiometric nlc:lslrrentents. Partial success has been achieved bl' thc effnrts of severel investigators, placing the present brightness tempcr;tttlrc of this tcnuous radiation at a t'Alue appro.ximatel,v* 3" above ilb:rrlrrtc zcro. Expcrime nts of this t)'pe and others in thc millimeter \\'avelength rilnge ltre complicatcd by the far fainter cosmic signal levels received frttnt spacc in conlparison u'ith the energy received in tfie UHF and loq,er rnicro\\"ilvc rcsion. ()f equal signiflcance is the marked increase in atmospher-ic ltttcnuation and cmission at millimeter wavelengrhs [-l]. ,-\s \\'c itpproached the last ]'eAr of this decade, one might have said tlrat thc radio astronomy discoveries in thc millimeter region \\:ere insignifrcant irr conlparison *'ith the cxcitement produced at longer \\'avelengths b1' tlic discover-v of cluusi-stellar objects, pulsars, interstellar OH, helium, and several rcsonltnt lines of hvdrosen. As is so frequently the case in racl io itslrononl-v, thc picture suddcnlv changed w'hen in December 1968, :l rt\tlirch group et the Ljniversitl' of California [5lannouncecl the detection crl" rltdi<i e rrrission from antrltonia molecules in the interstcllar medium at a \"\;t\ clcngth of I .15 cm. 'f hc linc enrission \t'as observed at a frequency

cori-c)p(lnding to the inl'ersion transitions of the -I - l, K - I rotational levcls in the vibrational ground state of the NH3 rnolecule. The emission rcgiCrn \\'lls of srnall angular e.rtent, clisplaced to the south from the direction of the galactic centcr by approximatelv 3 arc minutes. The lg-foot



diarleter millimeter wa\/elength antenna at the I Ilt Crcek Stati*n of the Unir''ersity'of California's Radio Astronom)- Laborator\,\\,es,_:ed l'ith a Dicke t)'pe radiometer in n'hich the reference source at the input comparison port of the switch was provided b1' an off-axis anrcnne bcarn, displaced approximately 20 arc minutes l'ronr the boresighr of t5e lnrenna main beam' This technique is used e.xtensivell'in millimeter \r"-a'e rudio astronom-v for the observation of spatially' discrcte sources ol- rarjiation. The continuLlm radiation of the earth's atmosphere is intcrccpted b' borh antenna beaffiS, and hence its contribution results in a nulloutput through the po\\'er subtracting action of the Dicke su.'itch at thc radiomcter input. As astroph)'sicists \\'ere just beginning ro ponder the significance of this startling discovery, the same research group at Berkele' iinnounced the discovery of microu'ave emission from water !'apor in the interstellar medium' This discoverv \vas announccd ,nvithin less than 30 da_i,s follor'ing publication of their detection of interstellar ammonia. Thc rnicrorvave emission from water vapor \\,as associated *.ith the 6,0 * 5:., rotutional transition ' It \\'as observed in scveral dire ctions in space, one trl*,ard Sgr 82, the Orion Nebula, and also in the direction ol- the solrrcc w49. The emission of H1o in sgr 82 \\'as in trre same direction iceiestiar c.rLr;rdinatcs) in r'vhich emission from interstellar amnronia hacl prcviousll.bcen discovered ' The HtO radiation was impressivel,v intense producing , iin antcnna temperature of l4oK rvhen observed f'rom the Orion Nebula and an ilnrcnna temperature of at least 55oK from the dircction of thc s()urce. As a consequence of the small angular size of thc sourcc rcgions, the Berkeley group suggested that the brightness temperaturc ol'thc source in W.l9 mig6t be as great as l000oK. Measurements performed at the Nar,al Research Laboratorl', j"tlarl'land Point Observatory, a Icrv u.ccks lollo\r.ipg the announcement by the Berkelei- group, provided confirrnation br. ni.rsuring an antenna temperature of approximately 1000 " K from the \\/-lg source , indicating that the actual brightness temperiltu rc nlay bc even h igher. The antenna at the Naval Research Laboratorv, VIar:yland point Obssrvatori,is 85 feet in diameter. The measured increase in antenna tcmperaturc over tirat obtained by the :0-foot diameter te lescope ar rhe Universir),sf (-llifornia Hat Creek obsert'atory closely follo*'ed the ratio o'f the t\vo lnrenna aperture areas- Water vapor emission in the direction of' W'l9 is no\\. the most intense emission line detected in the interstellar medium. Onc cen onlr. imagine w'hat future applications u'ilI be macle of these exo-atmospheric point sources ol" coherent radiation, in addition to thc astroph)'sicul kno\o.ledge they will provide. with the advent of the space agc beginning in 1957. thc solar s'srern received increased attention as powerful rad io 1*lcscopcs in the 3 cm tc) 3 mnr wAvelength region \\rerc pointed tou'Ard our nearby celestial neish,




bors, The moon, of course, has been a prime target, Its surface, shape, and general form are wellknou'n from optical observations. From infrared obscrvations we know it's surface temperature and how it changes so drastically from lunar night to lunar day. with a radio telescope, *'e look belorv the surface. The heat u'ave from the sun propagates below the surface of the moon producing a temperature distribution at the lou'er levels n'hich is determined by the f'lux intcnsity of the heat rvave and the thermal properties of the subsurface material. The electromagnetic radiati<-rn originates below the surface and propagates uprvard and out through the surfacc. The longer the rva'clength, the deeper the source of radiation that is observcd. The thermal inertia of the subsurface material is the prime parameter of interest. The significant data is the brightness temperature of a selccted region observed during one complete lunation. The technique most frequently employcd is to obtain a daily map of the brightness distribution over the entire lunar disk and from these maps construct graphical plots of the brightncss temperature as a function of lunation phase. The important characteristics are the observed variations in thc amplitude of the brightness temperature and the phase lag in the heating and cooling portions oI the lunation cycle throughout a lunar month. As one might anticipate, the amplitude of lunar brightness temperature variations is barely dctectable at a u'avelength of 3 cm and is undetectable at meter wavelengths. Radiation at these wavelengths originates several mcters belorv the surface of the moon rvhere the heating and cooling of llre surface during lunar day and night (approximately 27 days) have a negligible effect. As u'e approach millimeter wavelengths, hou'ever, amplitude variations in the brightncss temperature are easily detected. Variations as -sreat at 200oK are typical at a wavelength of 3mm, and 100oK at 8 mm. 'I'he shorter the u'avelength, the greater the similarity ol the lunation brightness tcmperature u.ith the surface temperature observed at

infrared u,avclengths. Although the physical rcasoning associated with this area of lunar rescarch may appear relatively simple, the associated experimental, as well as analytical, problems represent a significant challenge. Among the several factors to be considered are the ability of the antenna to resolve a selected of the moon, the effects of surface roughness on the insolation phase, the and contaminating effects of the earth's atmosphere on the received signal. At a u'avelength of 8 mm, for example, a 30-foot diameter antenna provides a main beam angle response of approximately 4 arc ninutes, corresponding to an area approximatel-v 2"10 miles in diameter on the subterrestrial point of the moon. The main beam an-sle of the llO-foot diameter Havstack antenna at the M.l.T. Lincoln Laboratorv observes a circular area area



60 rniles in diametcr at this u'avclcngth. Since the moon subtends an angle 30 arc nrinutes in diameter, as r-lbserved from the earth, scr eral of the forvu'ard sidelobes of either a 30-loot or I l0-foot dirimeter arltrnn& ',r,ill intercept the lunar surlace. Although the sidelobrc lcvels lna) tru lovn'in comparison to the nrain beam response, the thermal energ)'radiated by'the moon and rcceived b1,the sidelobe structure is determined bi'thc intcgrel

of the brightness temperature of the nroon and the glin cif'thc


pattern over the solid angle subtcnded b,r' thc nloon. The lunar surlace is, perhaps, the most troublesorne in thc anall'sis of the observed data since one of the prime objectives is to relate the phasr of the observed thermal radiation Lo the phasc of the insolation or heat \^'ave penetrating into thc surface. The cornplcrit-"- of the problcm can be seen readill' b1,' considering the u,ell-kno\\'n Tl'cho crater rvhich has a small central prominence projecting from the cratcr floor. At lunar sunri::, one rvall of thc crate r is exposed to the solar heat u'ave, \\'hilc the othcr remirins in a shadc)\\'. The central prominence is psrtially heated bl'thc radient energl'from the u'all expcsed to the ra!'s of thc sun. thc othcr siclc cf tiie prominence remains cool in its ou'n shado\\'. As the sun riscs, ttre crater lloor and, ultimatell'. the prominence {lre erpos':d to the direct r3}'s of' the sun: and finally' the u,all on the sunrisc sidc of the cratcr is hclttcd dircctll' by the sun. shortll'after midday. As u"e proceed tou'ard sunsct, thc revcrse situation L)ccurs. It is readily'apparent that thc centrill pronrincncc, iIS *'ell as the floor of the crater, undergo il rnore cr)tnplcr cy'cle of heuting and cooling than a flat area of exposed lunar surlace ntatcrial. Trlnslating this situation into other areas u'hich are pock-markcd u'itlr manv small crevices, rills, and craters provides an appreciation l'or the challengc in this erea of exploration. Perhaps the grcatest challenge is the dcr e lopment of a unitred theorv cApable of erpltining \\'hat u'c sec in thc optical. infrared, :rnd micro\r'ave portions of thc specIrunt. both activelt' as r.'ell lrs passively, and the relationship of this underst{lnding to u'hat uc n'illsoon knou'about the ph1'sical and chcnrical charilctcristics of thc actu;.rl surfacc material from in situ measurenrcnt.- From tlris point of undcr:1.:.lnding, we ffra),'hope to remotelf'probe thc chtmcteristics of other setcliitcs

in our so[ar


Radio emission from all of the planets. rvith the exccptir-rn of Ncptune and Pluto, has no\r' been detected. Antenna resolving po\\'er is the tnost significanl. instrument limitation. Vcnus, nt closc approach to thc earth,

is onl,v I arc minute in dianretcr. - Larger collecting apcrturc:, pcrhaps in the form of multielement interferometers, ma)'he required beforc *'e u'ill be ahle tcl obtain detailed rnaps of the thermal radiation charactcristics ril' these nearby' celestial ncighbors rr hich rernain elusir"c bccau.;t of their :mall anslllar size.

flarold I.


.'\tltlosphcric conterninution ol the sign:rls received fronr the planets in the rniilirncte r regiott 5uggcsts that scnsor svstcms at these short \\'avelengths tlla!' brr morc r:se ful ii placed in carth orbit or, possibll', on the moon. ;\n llicr-nlitive, L')f cout-sc, is to send space vehiclcs to the planets. equipped u'ith scnsors covering thc cntire elcctronlagnetic spectrum. The {rrst historic vcn t ure ol' this t)'pc \\'ils thc llariner R l'tl)'-b,l' iV{ission to Venus in 1962. Simillrr s\'stents are no\tr'planned for the "Grand Tour" of the outer planets in l9?8. -l"he obvious advantage of planetary space probe is anguler r.:solving po\\'er. The gigantic 150-foot diameter radicl telescope at Jodrcll Rank in England, lor e.\ample, providcs a resolving po\\'er on the lunltl stlrface equivalcnt to an antenna the size of a quarter in a 100 mile

lunur rlrbit.

r\lthough our tnoon and t.hc planet Venus have been the subiects of intcnst inv'estigation cluring the past dccade. exploration of solar radio charilclcri.\tics has, pcrhaps, receiv'ed the greatest enrphasis. The need to undcl:tltnd and to hc able to predict the occurrence of solar activity', in piirticulai'thc timc and intensitv of a solar proton flare, has been emphasized b1' tlte cra ol- nranned space tlight. Be1'ond the protective sheath of thc ca rt h 's masnetosphere , thcse corpuscu lar streams of high cnerg-y particlcs c;in hrt,c ditnraging elfccts on lifc. -l-hc tempo of erploration of solar



?. Sun rnap at lr w'avelcngth of te

8 mm

taken by the Prospecr Hill radio Laboratories. Temper-

lescope crf r hc Ai r Force Cam bridge Research

ature contours are in 'K/10.




radio characteristics has increascd steadill' ovcr thc past I l -1*ur sol*r cycle' Today, gl0bal conlnlunication gricl ne't*'orks tr{lnsmit cilta t'ronr

earth based, optical, and radio telescopes anrJ solar orbiting pignce r sate llites to the solar actit'itv forecasting cienter located in Ccrloraci* Springs.

Ftc- 8. Air Force cambridge Research Lab'rarories, prosprect Hill millimerer radio telescope.





pilcd, red uced, and retransmitted to solar rese arch rs th roughout the u'orld. The ntajor efTort at several of these research ccntcrs is to scarch {'or tirc existence of sonle characteristic radio signature tlutt can bc usecl ils a rcliable indicator of a pending major solar evellt. The :cu rclt l-or such e precursor \\'ils extcnded into thc m illimeter \\'avele nrt it rcsion in thc elrrly I 960s. From prior research, it \\'as knor.r'n that the liigh encrs!' proton flares \\'erc associated u'ith regions of densely ionized clouds iplagc rcgionsj in the l'icinity of the solur chromosphere above the cipticallv observcd plrotosphere. It is in this region of the solar atmosphere thitt the bright flash of the optically obsen'cd solar llare occurs at the onset The rc thc data is conl ce n te

ii major solar event. It

\'!'us rcasoned that at millimeter wavelengths rr cd brightness temperature w'ould originate in this same general ;rlt itudc luver ol' the solar iltrnclspherc since radio penetration torvard the phr-rtosphcrc increascs its the \\'avelength of observation decreases.




T*'o t)'pcs of nrillime'ter solar radio telcscopes arc in use today. One sclrls a pcncil bcanl across the solur disk in a raster t)'pc manner to provide il de tailcd ntap of thc brightness tem perature distribution. The second cmirloys u llrge antenne beam encompassing the entire stllar disk to provide ir tneilsurc ol-thc intcgrated brightness temperaturc AS a function of time.


r:. 9 ,

r\cruspace Corponrtion 3.3 mm sun map for May 25, 1967. The contour plot is in l:i. intervals u'ith the reference contour denoted b-v 0.


The't'ierv of the integrated disk assLrres that all nrajor liarcs *'ill bc detected and rccorded. The mapping instrument runs thc risk of missing an cvcnt, or at least the onset of an evcnt, unlcss b1'sheer coincidence, the itntenna beam crosses the area of the llarc at the tirnc of occurrcncc. The ntapping












Flc. 10.

Aerospace Corporatirrn millimeter radio tclescope.





instrulnent is. of course, far more sophisticated than the instruments r.r,hich providc an integrated vien'ol'the total solar flux intensity. The antcnna motion required to achieve the raster scan is accomplished b-v computer pr{)Eramrning of thc drive s}'stem. Thc output data is computer-reduced to a n'lap d isplay ol hrightness tcm peri.lture contours distributed over the sur{ecc of the solar disk. C-omparison of these brightness temperature maps. in tinrc sequenct:, clcarlv indicates the growth and decay of active rcgions just abot'e thc photosphere. An I mm brightness temperature map of this t)'pe, obtained by P. Kalaghan of the Air Force Cambridge Research

Laboratories (AFCRL) Prospect Hill Observatory, is shown in Fig. 7 . A photograph of the {AFCRL) solar radio telescope is shown in Fig. 8. The antenna system is an elevation over azimuth mounted 30-foot diameter casscgrain. Solar brightness temperature maps obtained at a wavelength of 3 mm are shorvn for comparison in ltig. 9. This 3 mm merp \\'as recorded b-v the Aerospace Corporation radio telescope located in EI Segundo, California. A photograph of this ecluatoriallv mounted l5-foot diameter cassegrain ilntenna is shown in Fig. 10.

B. Il rcnow;\vr


\'licro\\'ave meteorolclgy is in part an outgror.r'th of radio astronomy and more rccently has become closely allied rvith atmospheric propagation studies necded to de termine system rcquirements for earth-space communication links at millimcter \\'avelengths. One area of millimeter communication svstems research is the measurement of atmospheric attenuation and noise chnracteristics in the "windows" betr,veen the rvater and oxygen resonanccs. These resonant lines occur at nominal frequencies of 22 and 60 CI{z respectively. ,{ second area of research in microwave meteorology is the measurement of the characteristics of at mospheric resonant lines themselves to capitalizc on their unique properties rvhich can be exploited for the purposes of determining the vertical temperature profile. water vapor distributiorl, and oz()ne distribution in the earth's atmosphere. Prcscnt-day i nvestigations have produced an i n terest ing cross-breeding betw'een research groups in communication, radio astronomy, and microwave meteorology. Celestial radio sources, for example, offer excellent exo-atmospheric tragets of opportunity for the communicator to investigate atmospheric propagation characteristics b,l' satisfying the requirement for

a transmitter of knorl'n flux intcnsity and position located beyond the carth's atmospherc. The validity of such measurements is, of course, predicrtcd on ilvaila ble know'lcdge concerning the characteristics of these celestial transmitting sources. As a consequence, el substantial portion of


our present-day knowledge conccrning the millimeter charactcristics of celestial radio sources is being provided bl' research scientists primarily concerned *'ith the communication aspects of the atmospheric medium, The common denorninator in each discipline is the micro\\'ar.c radiometric sensor. The xtmosphcric medium is,'of course, another common denominator. I-lou'ever, depending on the'rcsearch discipline, thc electrclmagnetic characteristics of the atmosphere ma.v be cCInsidered as eithcr signal or noise. In the discussion r,vhich follow's. \\'e u'illconsider first thc mcasuremcnt of atmospheric at.tenuation and noise in the "u'indo$'s." We *'ill then revie*' present-day exploration of atmosphcric gas lesonant line structure u'hich is located betw'een the " \\'indow's" and forms their boundaries. Tu'o areas of application in r.r'hich present knou'ledge concerning atrnospheric resonant, line structure is being exploited n'ill be revieu'ed in the third section




of .4tmosplrcric Attenuation and


At wavelengths shorter than 3 cm, electromagnetic \\'Aves can be severely atte n uated by rain. A possible solu tion for earth-to-satcllite communication systems would be the use of a diversity ground statian at each earth terminal. Under conditions ol' severe atten uation cnuscd b-v excessive rain at a ground terminal, the communication link could be srr'itched to the diversity ground station. The minimum separation betu'een the nvo earth based terminals requires a quantitative determinartion of attenuation statistics. The accumulation and analysis of the required statistical data is currently be ing pursued at the Air F orce Cambridge Reseilrch LaLroratory, the Betl Telephone Laborator\', and the NASA Electronics Research Center. [n lieu of the availability of a man-made satellitc to pror,'ide a calibrated exo-atmospheric transmitting sourcc, the sun is uscd as a target of opportunity since it is an exo-atmosptreric source of known llur intensity. Because of the relatively low'signal level received from the sun even u'hen unattenuated, a radio telescope is required for these measurements. The noise level received from thc sun as a function 'of \veather conditions determines the attenuation caused b-v the intervening atmospheric gases and associated condensation and precipitation products. During nighttime *'hen the sun is not available for absorption t)'pe nleasuremcnts, the radiometric sensor is pointed at elevation angles of interest, and atmospheric cffects arc derived from-obscrvcd variAtions in thc sky' noise temperature.

When the antenna is pointe cl at the slrn, the xntenna is given by










(- ') + (r

I exp (-i\r T))Irxr'ldQ



re Gi,0,P) is the gain pattern of the antenna in spherical coordinates, r is the atmospheric opacity, rr., is the exo-atmospheric brightness temper;tture of the sun at the frequency of obser'atinn, and 71., is the rvlre

mean thcrmometric temperature of the atmosphere along the path of observation. when the antenna beam is pointed a\\'ay from tlre sun toward some

cold poin t in the sky, the obserred antenna temperature TA

TA is

gi'en by



Ncrri' if the obscrvations of, Ts ancl 7.1 are made in sequence at the same ele'ation angle by pointing the antenna beanr first on then off the sun, the antcnna lempcrature differencc betu'een thc t\r'o measurements will be



*'here dO, is the solid angle of the sun. Introducing thc reasonable assumption that the atmospheric absorption cocfiicient $'ill be essentiall,v constant over the small angle subtended by the sun. the observed antenna tem peratures u,hen the ante nna beam is

pointe I



ard the sun and



n ton'ard the sky can be w,ritten in the


T'A--: phe








For clear \f,'eather condilions, one can obtain a measurc of the atmosric gas attenuation by assuming a uniform hori zogally stratified model

atmosphere' Under these conditions, rhe atmospheric opacity r can rec'xpressed in terms of the opacity observed in the zenith direction



r.r,'h c



0,is thc angle of obs*rr*l; Ht.l.a ro the zenith direction. Expressing r as a function of ro in Eqs. (1g) and (20) we can rear,

(2 l)

rangc terms and obtain the e.xpression

It - f,*, Tslv


exp {-rosec



from rvhich \\'c obtain the value of the verrical opacity ro in the form To

tT7--1 L


lskv -,

cos d .t



Alternatively, the value of ;o can be determined from the slope of the plot




7"r*, versus sec d..

For nighttime observations, a similar method can be applied t.o determine the atmospheric opacity from the measured sky antenna temperature. The value of ro is obtained from the derivative of E,q. (20) in the form t0

I T _T

t ,4

d(T 'sky






sec 0,


For large values of r, the assumed value of the mean thermometric temperature of the atmosphere T,ry becomes critically important. Onc can make the general observation that large values of attenuation are more precisely measured in absorption b:* using the sun as an exo-atmospheric source,

while small changes in the value of atmospheric et tenuation are nlore effectively sensed by sky temperature measurements obtained under conditions of relatively lorv attenuation values. The determination of atmospheric atten uation characteristics from sky temperature measuremen ts has bcen extcnsively use d and developed to a high degree of sophistication by the research group at the Air Force Cambridge Research Laboratory, under the direction of E. Altshuler. In addition to equipment located at the Prospect Hill Observutory in Waltham, Massachusetts, the AFCRL group operates a dual frequenc-y nleasurement


ll. Air Force Cambridge

Research Laboratories d'ual frequenc-v (15 GHz and 35 CHz) radiometric measurement s)'stem located at luount Hilo in the Hawaiian Islands,




instrumcnt at I-Iilo in the Hau'aiian islands. A photograph of this instrumcnt, ri,hich is opcratcd under the direction of K. Wulfsberg of AFCRL, is shou'n in Fig. 11. Thc entire equipment enclosure rotates in azimuth, and the cornucopia antenna attached to the side of the enclosure is adjusted in

eievation from thc operator control console located u'ithin the enclosure. A nricrowave radiometric sensor sy'stem assem bled by' the Bell Telephonc Laboratories for the accumulation of atmospheric attenuation stntistics at u'avelensths of 8 mm and 2 cm is shou,n in Fie . 12. The






The Bell Laboratories sun tracker in Holmdel, Neu' Jerse;- is used to tune in on sun signals at tr'o radio frequencies. A 5 x 9-foot metal mirror automaticalll'follow's the sun in its daill' path across the skl'. Other electronic equipment processes the signals and records the results. The apparatus is gathering data on the effect of rain on the signals receivecl. :3,'68.


5 X 9-foot plane rcllector is attached to an equatorial mount. The declinution angle of the reflecting plane is adjusted so that the sun's ravs are rellccted into the,l-foot aperture conical horn reflector antenna. The reflcctor is driven in the hour angle coordinatc b,v a clock mechanism w'hich itssures that the sun's ravs are continuously reflected into thc apertllre of the conical horn reflector. Throughout the observing period, the antenna beam is scanned on and oll thc sun at a I FIz rate, r'u'ith an angular excursion of 2.6*, b1' nrechanicalll' tilting the re flecting plane in the decli.t


nation angle coordinate. Automaiic opsration is another uniquc


of the instrument. It is preprogrem rned severat da1's in rtdvrtnce and provides continuous accumulation of data w'ilh Lrnattended rrperation. Nighttime observations of sk1, noisc are included in the c;bservins program seq uence


The Propsgation Studies Branclt at the NASA Electronic Rcscarch Center, under the direction of L. Roberts, undertook a similur scries of measurements, beginning in 196i. The ERC atmosphcric propagation measurernent system is shou,n in Fig. t3. Simultaneous ob:serv':ttions are obtained at wavelengths of 3 cm, I crn, 1.5 ctn, and [J nrm. The micro\\'ave radiometric sensors at cach \\'Avelcngttr are instrtllcd on individual equatorially mounted 5-foot diame ter searchlights.


\:crAl modcs of oper-

f'-i{ t lr ,4




iff' ;?ts:+



Flc. 13. NASA Electronics Research Center atmospheric research initrumentation. These sun tracking instruments are used to obrtain atmospheric attenuation data at four r,r'avelengths betu'een 3 cm and I mrn. The antenna s)'stems are cquatorialll' mounted S-foot searchlights.




ation arc incrudcd in thesc radiometric s'stems: antcnna bcam s*,itching, absolute tempcrature., and cxprod.a beam comparison. The explodcd bcam comparison modc prouiae, "ni.nn" oifferential temperature measuremcnt betu'een t\^.'o on-axir antenna " one beams, narro\r, beam boresi_ehted on the sun and a much rarger beam o'hich obtains a negrigible conrribution from thc sun.

Although the prinre objecti'c


or this type is to determine the statistics of atmospheric attcnuation and noise needed lor thc design of future earth-space communication rinks. ttre knor'teoge gained is userur in un.erstanding the physical processes of the armosphere . Sirrrultaneous observations at sJucrar'ruan.r.ngtt s selected to exploit the *'a'elengrh dependent a.tmospheric absorption coefficienf on rr-iir. poten_ tial for remorcry mapping clouds and ,ulather fronts. penetrarion to lhe rain cores rvithin clouds is accompristrea at the longer rval.erengths. Detection of high-artitude variations in u,ater vapor, prior to condensation, can be sensed at the shorter u'avelengths. The insirunr.nt"tion required t'or these measurements is markedr;- slmirar to that described above for communication systems research. From the routine dairy accumuration of attenuation statistics may evolve simirar instruments used b1. '' nricrorvave "' mcteor.r.-eists to dcterminc the "uh-v" or thcse ,ruti-rti.r-



Ab.srtrption and Rodiation h1, ,4rnrospheric Gases

The atmospheric gases rvhich pro'ide a signi'icant intcraction rvith

'microwavcs are water vapor' o.rygen. and ozone. watcr vapor rras strong absorption lincs at 1.3-5cm anC iO-t mnr. as u.ell as several stron-q linesat

submilrimeter rvaverengrhs. van Vreck[or ir,. n.'"gniruo" or thc l'35cm rine and the contriburion fromcarcutatca a'other lines. comparison

of his resulrs rvirh laborarory measurements by Bccker and Autrer [7] and Iro et al ' Iltl and *'ith atmospheric obscr'arions by Straiton and rorbert [9] shor'ed a substantiar discrepancl-. Bl adjusting the contribution from all other lines' the u'ater vapor absorption formulas summarized by Barrerr ard Chung I l0] represent the best arailable approximation for the l .35 cm line at temperature near 300"K. oxygen has a comprex spectrum, consisting of a band of resonant lines in rhe -5 mm u'a'erength range and ai isorated line at r.5 mm. Line lrequencies and bandrvidths hav'i been measured in the laboratory at prcssures up to l armosphere by'Artman and Gordontrr] ani Anderson et al.1121. Direct measurement of atmos-pheric absorption il, o*lrg.n i,", been made by scve ral investi-sators. The il.urur"n,.nt results and compu-

tations rvere reviewed by Mecks and Lillc_v f t 3t;";:';;;.;;;:;;i;."\r,arer and Strand [14j.


The interaction of ozonc *'ith micror'a'es is weak in conrparison rvith either oxygen or water Several resonant lines, ho* arc present throughout the entire 'apor. 3 cm to 3 mm u.avelengrh re-eion. Gora

has calculated the frequencies and intensitics

Il,s] for all iignificant lines of the

rotational spectrum ol ozone at lrequencies belorv 2700 GHz. The application of micror.r'ave radiomctric sensing of the u'eter 'apor resonant line is presently being exploited because it offiers the unique abilit;to yield a measurement of tropospheric water vapor in thc prescnce of clouds. Although a satellite borne *'atcr vapor scnsor *.ourd be cffective

only over oceans, oceans cover more than half thc earth's surfacc and are the spawning grounds of ma-ior storms. Micr.u,a'e radiometric sensing of the u'ater vapor resonance under clclr rvcather conditions pcrmits the measurement of integrated water vapor abundances and sprtial siz-e distri-

butions. Interest in the oxygen resonant line characteristics ncar a rvavelength of 5 mm has been stimulated by the fact that microwavc radiometric scnsing may provide the only remote se nsing technique capable of measuring armospheric temperature profiles in the prcsence of clouds. This ir.ould bc of considerable importance to global data collection for numerical u,eather prediction. This technique offers the potential capability of mcasuring the temperature profile from thc lou'er troposphere rvcll into the nresosphere. Microwa'e radiometric measure mcnt of the atmospheric ozone distribution has progressed at a slo*'er pace than studies of either oxy-gen or water vapor. Instrument technology. rather than meteorological intcrest, has set the pace in this area of rcscarch. ozone plays an important role in the organic and inorganic chemistry of the surface of thc earrh. Through a filtering action, it absorbs a lethal part of thc ultra'iolct radiation from the sun, thereby making life possibre on the surface of the earrh. ozone is also an important factor in our clirnatolog.r-, cstabrishing rhe balance betrveen exo-atmospheric radiation incident on the earrh and thc outgoing radiation from the earth, as a consequencc of its particular lhsorprion characteristics in the ultraviolct and infrarcd regions of thc spcclrum. Knou'ledge of the atmospheric ozone distribution in thc alrirude range

from l5 to 60km, obtained on a global scale. offers rhe ptrs:ibilirl


measuring air mass circulation as a conscquence of the fact thur ozonc in the lor.'"'er portions of the atmosphcre ma1'be consiclered an inert gas and its global distribution rvith time is. in large. dctermineel b1.rhe horizonral

motion and interaction of major air .masscs. As a preface to a rel'ie*'of thc currcnt status of crplorrtion .f atmospheric gas resonant characterisrics. it u'ill bc recall rhe rclaiion-

ship betu"een antenna tempcrature ancl thc elrective brightness tcrnpcrature

of an observed source of radiation. Since thc atmosphcre throughout most i

Harold r-rf



the 3 cm to 3 mm \,\'avelength region is semitransparent, the equation

of radiative transfer can be used to relate the brightness temperature Tu0) at the frequency v to the atmospheric composition and the temperature T'(si along the line of sight and to the brightness temperature TE of the background me diu m beyond the atmosphere. The equation of radiative transfer is expresscd in the form










r{s) exp




ln t q. {15) rtu'} is the total opacity of the atmosphere and a(r,, s) is the absorption cocfficient. Inspection of Eq. {25) shou's that the observed brightness temperature in any given direction is the sum of the background radiation and the radiation emitted at each point along the path of obsert'atioil, cach component attenuated by' the intervening atmospherc. The antenna temperature observed by a microrvave radiometric sensor looking into the atmosphere is, therefore, primarily determined by'the atmospheric absorption coelficient and temperature along tl're path of observation.

Sincc the integral of the product of the exponent and the absorption coefficient in Eq. (25) dctcrmines the contribution of the thermometric temperature along the path of obserl'ation, to the observed antenna temperature. it has become customary to refer to the value of this integral as a " weigh ting fu nction . " a. Oxygen. The micro\\'ave spectrum of the oxygen molecule results from llne structure transitions in rvhich the magnetic moment assumes various directions with respect to thc rotational angular momentum of the molecule . The un paired spin s of tw'o electrons produce the magnetic dipole moment of oxygen. Van Vleck [l6] was the first to develop the expression for the frequency, pressure, and temperature dependence of the oxygen absorption coefficient. This early work was reviewed by Meeks and Lilley [13]


Ilf{l in


1963, and later by Gautier and Robert

[7] in 1964, and


The complex of oxygen lines, in particular the atmospheric absorption coefl-icient as a function of frequenct- and altitude of observation, is shown in frig. l1 {Mceks and Lilley) . The general form of the rveighting functions


sclected freq ucncics, as computed by Meeks and Lille;-, is shown in





It is important to note that the expression for the oxygen absorption coeflicie nt has been derived from quAntum mechanical considerations in ri'hich the value of certain constants has been empirically selected to provide the best agreement rvith experimental data. If it \r'ere possible to directly' measure the absorption coefficient for all possible meteorological conditions of interest, the quantum mechanical approach could be dis;





g c I


f c(u



60 Frequency

Flc. 14.

65 (GC



The computed attenuation coefficient f ir: for air at three represe nrative heights. This figure sho*'s that the individual oxygen lines completely overlap at sea level, partly overlap at I km, and are resolved at 30 km. (Meeks and Lilll' Il3J.i

pensed rvith. Much of the prescnt-da-v research has been conccntrated oR the direct measrtrement of oxygen line profile characteristics ro provide an improvement in knowledge concerning the absorption coefTlcient values. Laboratory measurements have been performed by Stafford and Tolbert t l gl at the University of Texas, and balloon measuremenrs bf' Lenoir I l B] ar the Massachusetts Institute of Technology. Reber et al. [20] of the Aerospacc Corporation recentlv reported a '

very detailed analytical stud-v of this problem, supported b1, exrensivr3 measurements performed in a high-altitudc aircraft. Their published values are, perhaps, the most comprehensive and complete a\,ailable today. Measurements, utilizing the sun as a source, were rnade at sir discrete

altitudes ranging from sea level to 13.7 km. These measureme nts coyered the frequency range from 52 to 68 GHz (see Fig. 16). The more than 1500 independent attenuation measurements \r.ere used to calculate new










.37 gc/s


T (601



0 .00


u = 59.30

0 .08


T(oo) = zwoK T(ooo)= zztoK

0.04 0 .00



u =







= ztfrc


0 .04

57 .80 gc/s




4 F

0 .00







0 .04

56.60 gc/s

T(oo) =


t(eoo) =


0 .00 c .08

u = 55.40 gc/s TA(0") = zz3aK





0 .00

t/ =


TA(oo) = 0 .04

To( 6oo )

0 .00

54.30 gc/s 23oot<

= ?zf



HEIGHT (krn)


I 5.

\\'eighting f unctions for clctsrmination of the brightness temperature as a \^'e ighted a\erage of the kinetic tempcrature distribution. Weighting functiorrs arc show'n for six repre5entatil'e frequencies. iMeeks and Lille."- i l3l





I ti''l l ii ri i ':l.i I | r i i-i.ij i ' - io" s'it rll -. t

3 n't

6\., ..'"'. = ' .,t9 t!6
















l--. , lt1 sc






T' g -Tr


.*ilr' .-r,- -,' -

Ftc. 16. [.y features of the investigation of molecular atmospheric o,\ygen

characteristics performed by the Aerospace Corporation . tai The cornputed resonant profile characteristics of atmospheric ox!'gen as a function of the observers altitude over the frequenc.v rangeTr."tm 52 to 68 GHz' (b) Five-millimeter wavelength radiometric l.rsoi antenna s1'stem assembled for the aircraft measurement program. {cl RF portions of the 5 mm radiometric receiver used in the aircraft measurement program. This unit was ptrysically located directly behind the anrenna. (dJ



mm radiometric sensor instatled in ihe aircraft used in the

measurement program. The antenna can be seen behind the qlrartz window installed in the skin of the aircraft, just forwarrJ of rhe u ing.




values for rhe van vleck line-broadening coefficients. Zenith attenuations were computed utitizing these new coefficients over the frequency range 25 km. In addition, through the attenuations tangential and rates attenuation horizontal both altitudes' sevcral for u'ere computed atmosphere

4g to 72 GHz and for several altitudes from 0 to

|l,ater vapor. There is only one water vapor resonance in the 3 cm to 3 mm wavelength region. This occurs at a $'avelength of 1.35 cm. A second line occurs at a slightly shorter u'avelength of l'6 mm' There are a great number of strong lines at wavelengths shorter than I mm' The atmospheric opacity expressions for the 1.35 cm line were first developed by van vleck [6]. These were further refined by Barrett and Chung [10]. Tiey obtained relatively good agreement between theory and experiment by combining rhe van vleck and weisskopf [21] line shape with a nonresonant term which corresponds to contributions from the far wings of all of rhe lines at other frequencies. The most recent analytical and experimental work has been by Staelin [22] and Gaut [23]. Their experiat mental measurements were performed using a 28-foot apâ&#x201A;Źrture antenna microa S-channel with Lincoln Laboratory in Lexington, Massachusetts u.ave radiometer. The five selected frequencies were observed simultaneously to obtain an absorption profile using the sun as the- background source. The accuracy of thew profile measurements was of the order of 0.02 dB, or less than 5% of the total opacity' The measurement of the water vapor concentration in the atmosphere . by microwave radiometric techniques is complicated by the fact that the water vapor resonance at 1.35 cm is semitransparent- It is also relatively broad since most of the atmospheric water vapor is located at altitudes below l0 km, where pressure broadening effects dominate the line shape. Consequently, vertical sounding of the atmospheric water vapor distribution from satellite orbit will be contaminated by radiation emitted from the earth's terrain or ocean surface, as well as by clouds. For these reasons, the cxploration of the potential of this particular remote sensing capability is b"ing p.rrsued in several areas. These include a more precise determination of the characteristics of the absorption coemcient, the effect o[ clouds on received signal characteristics, and the radio emission characteristics of the ocean's surface. Observations obtained over the oceans will be the most usefuI since the emissivity of the ocean is approximatety one-half that of the land, thereby providing an adequate differential temperature contrast. Observations over land areas rvill prbvide little. if any, distinguishable signal since the temperature of the lower atmosphere, whâ&#x201A;Źre water vaPor is most adundant, is close to the earth ambient at the surface.



Ozone. The principal quantity which determines the transmission


coemcient of the atmosphere and the emission or effectivc brightness temperature of the atmosphere due to ozone is the absorption coefficient as a function of frequency and altitude. Those parameters g'hich determine the variation of the absorption coefljcient u'ith altitude are temperature, pressure, and ozone concentration. Gora [5] calculated the frequcncies

and intensities for all significant lines of the rotational spectrum of ozone at frequencies belou' 2700 GHz. The values of molecular constants were determined from the frequencies of ozone lines in the microu'ave spectrum' measured by Trambarulo el al . l}a) and by Hughes [25]. The averaSe half-width of the ozone lines in the 9.6-1t band, as determined by Walshau' [26], was used by Gora to calculate rvhat he termed the maximum absorption coefficient. He also used the Lorentz line shape lactor in this calculation since this function is a valid appro.rimation at the lou' pressures typical of those regions of the atmosphere where ozone is concentrated. Atmospheric measurement of the 36 GHz line in absorption by Mouu' and Silver [2?], the 37.8 GHz line absorption and the 30'l CHz line in emission by Caton et al .l?81, and 13.8 GHz line in emission by Barrett et al. l29l provided direct verification of the existence of these lines and their relative intensities as predicted by Gora. The experimental measurement o[a far more intense line at 101.7 GHz reported by Caton et al .130] offered the first opportunity to directly measure line prolile characteristics to an accuracy sufficient for inversion. The measured line u'idth ol this 3 mm transition is in excellent agreement with the predicted v'idth as inferred from the infrared measurements by Walshaw and the laboratory measurement of the 37.8 GHz line by Caton et al .1281. It will be helpful at this point to briefly review the methods used in the analysis of the observed ozone data to determine the percentage of ozone concentration relative to the total air content at various altitudes [3 l]. Certain disciplinary relationships are significant since there is a common use of terminology in the concept of " weighting functions. " The approach to ozone data analysis provides a simplification in measurement instrument requirements since an absolute temperature measurement is not required at a single frequency in order to deduce the ozone concentration. The difference temperature between measurements niade at two frequencies is used to infer the concentration in an atmospheric layer. A precise measure of the temperature difference between the two frequencies is required; however, the absolute value of either is not required. The relationship between the ozone absorption coefficient a and significant variable parameters may be expressed in the form

att) cc T-\ '

tt' I Av l- (v ;

-: (Au)2 vo)' * (Av)' *





r^"hcre ;\n, is the ozone conccntration. Ay is the line half-u,idth, y is thc t'recluency of observation, ancJ Ds the line frequencv. The line rvidth is proportional to pressure, n'hich is in turn proportionaI to the product of the

total number ol air molecules Nr and a temperature ternr ffexp (512 * ,:) l, u"here p is in the range 0.5 to 1.0. The absorption coeflcienr can, theref'orc, be rewritten in the form a







The constant of proportionality D includes molecular constants and geome trical factors associatcd rr'ith the path of'observation through the atmosphcre' -l he term in brackets is defined as a "single firequency r,r'eighting function" Itr'"' A "dilfercnce n'cighting function" $/,r w., is de{lned in the f'orm (Ay) '

\\t-,'-: : B

(2 8)

p is a constant r.r'hich normalizes the difTerencc f-requenc!,'u'eighting function to unit,t-. Inspcction ol Eq. (lSi indicates that the maxirnum valuc of trL,',,*". occurs r,r,,hen rvhcre




yil :- {Avll

(t e)

Since the observed brightness temperature at an]* single frequency of obser'"'ation is proport ional to t he in tegration of the absorption cclefficient

aiong t he path through the atmosphere, the ozone concentration can be dcrived from a dilTerence temperature measuremen t at trvo frequencies rvhich define thc attitude limits of thc observed lzry'er. A graphical plot of sir single frequencv u'eighting functions is sho\\rn in 1tig. l7a. The corresponding diffcrence w'cighting functions arc sholl'n in Fig. l7b. The three <lifl'e rence frequency *'eighting functions sho\\'n in solid line are obtained from the differences of the paired sets of the four single frequency u,eighting functions, also shou'n in solid line in Fig. l6a. The single frequency rveighting functions at 2 and 20 MHz (shown dotted), which combine to form the difference frequenc!'weighting function at 6.3 MHz (dotted) show the degree to u'hich difference frequency weighting functions at 2, 6.3, and l() I{FIz tend to overlap and thereby provide interdependent samples of the atmosphere. The development of the rveighting functions in Fig. 17 demonstrates the dual use ol data obtained at intermediate observing frequencies, i.e ., obseru'ational data obtained at 6.3 MHz can be used for the upper and middle difference freq uency u'eighting functions, 3s u,ell as for both the middle and lorver difference frequency \\'eighling functions.




: r


u u-l




: o t LI',

o lll


-6.3 MHz 20 t*rtz






-200 MHz

0.4 0.6 0.8 t.0 (o)




- \l'eighting

functions of atmospheric ozonc. iaJ Norm;rlizetJ singic frefunctions lV, \,ersus freque ncy displacement from the line frequency. (bt Normalized difference frequenc-y- r+.eighting func_ tions W'ur*:,2 Versus freque nc]' and altitude. quenc-v weighting

The natural limit of the diffcrence weighting funcrion hall'-rvidrh is not immediately' apparent from the example of atmosphe ric ozone weighting functions shown in Fig. 17. This natural limit is shorvn graphi.rtty in Fig. 18. Recalling that the observed temperaturc at anln frequency cif observation is proportional to the inregral of the absorpt.ion coefficient over the ra)" path of observation, a factor C(trj, represenrrng all terms in the integrand other than those in the weighting function, takes the typical

form shown in Fig. 18a for standard ARDC model atmospheric tempera-

ture and pressures and for values of atmospheric ozone cCIncentration as calculated by Hunt [321. The form of C(h) is primarily determined by the factors outside the bracket in Eq. (26). From Eq. (28) , it is apparenr thar the rvidth of the weighting function IV,r-,, is derermined b:r the rario

(v, - vo) lQt - Lt) . The width of 9{/,r-,, is'constant for values of the ratio. A plot of this ratio versus the u'idth of the difference frequency weighting function decreases to a minimum oi about 14.5 km as the clifference {r, - u,) decreases to zero (see Fig. l BbJ . This indicates that as rhe tu'o frcquencies of observation required to define a difference freque nc!, u,eighring function approach the corresponding " frequâ&#x201A;Źncy of the u,eighring func-





d UJ tl,



J v d40 o ;)



c (ro-1


lf-w idth




- uz




l/t-uo = 2MHz Uz-Uo = 4MHz 0.5 Alrirude Width



,r- r,


(kr) (d)

Frc. 18. Half-width limits of ozone difference frequency weighting functions. iai C{hi vsheight(h) in km for standard ARDC model atmosphere and 03 concentration. (Afte r Hunt [32).) (b) Width of difference frequency ozone weighting function vs (u1 - vo) l\rz - vd . (c) Normalizing factor B vs width of the difference frequency weighting function Wyr"2. (d) Normalized weighting function 1l:iFr;2 VS altitude (where 2 vs: width determined by the atmospheric pressure at the altitude of the peak





tion," the minimum half-u'idth of the rveighting function becomes 14.5 km. The minimum width of the difference rveighting function for useful data can also be seen directly if one plots the value of the factor;g in Eq. (2S) as a function of the lr'idth of the difference frequency weighting function, since this normalizing factor is directly indicative of the energv conrained under the area of the weighting function u'hen plotted as a function of altitude and amplitude. The graph of i9 vcrsus the',r'idth o f H''-r*,., is shorvn in Fig. l8c. It is evident from this graph that the minimum usable u'idrh of a difference frequency weighting function is approrimately 15 km. A graph of a difference frequency '''veighting function near this optimum width o[ 15 km is shown in Fig. l8d. The corresponding frequencies of observation for the single-frequency weighting functions are displaced from the line frequency b"v 2 and 4 MHz respectively'. Comparison of the shape and width of this weighting function (Fig. 18d) rvith the rveighting function shown graphically by the dotted line in Fig. l7b indicares thar a weighting function developed frorn observing frequencics displaced 2 l!{FIz and 20 MHz from the center frequency of the line prc)vides nearl,v the same definition of the atmospheric layer as that provided by' the combination of frequencies displaced 2 MHz and 4 MFIz from the line center. This tends to suggest the value of using intermediate frequencies ol observation to perform a dual role in the derivation of ozone concentration for adjacent weighting functions.

It is apparent from the foregoing that, at most, four independent samples of atmospheric ozone concentration can be obtained. The minimum half-u'idth of each sample layer rvill be of the order ol' l5 km. A measure of the ozone concentration in any selccted layer is computed from measured temperature differences at two frequencies. The microwave radiometer used by' Caton et al. [30] rvas a double

conversion superheterodyne. First intermediate frcquency' arnplification was provided by three travelling \,'ave tubes in cascade \\rith an instantaneous bandwidth of 2 GHz centered at 3 GHz. The input signal to the second converter was coupled from an intcrstage transmission line betw'een the second and third traveling \r'ave tubes. Six second interrnediate frequency amplifiers were provided in the form of five contiguous filters, eAch 10 MHz wide, and one filter covering the entire 50 MHzband. A seventh broadband 2 GHz response was derived from the output of the third traveling wave tube. Absorption measurements were performed using the sun as a background source. The gomparison load in the Dicke mode of operation was provided by a gas discharge noise source, fed through a servo controlled attenuator to the comparison port of the ferrite modulator. The control signal for the servo loop \\'as derived from the broadband (2 GHt) channel. The servo control loop performed the function of stabi-


rold I .

En'e n



4.p ;Jl

L..G *:4


,? ','4+

,{ 'rX | 2i



f -.1 .-l; :ri j i*-'-*--*:t^,


, rlrl ;|} t\ i "'r,. : I rl+: ll :


*.r ir-..,



h' .*


-,j* .i>., * '+r



tt **-


,. ."

.. *

.L.,".' 'la'


fi*. ., il *. rt [i.'. t;-{tt :ii-';--.-.]. a


I .*



Flc' 19. NASA Electronics

Research Center atmospheric ozone raOiometric senior used in the initial detection of the resonant line at 101.7 CHz. {a) Equatorially mounted five-foot diameter searchlight antenna. 'b} Antenna and radiometric signal processing control console.



lizing the output from the various channels by discrirninating against small in the observed sun antenna temperature produced b.v clouds drifting through the antenna bcam during the pcriod ol'cbseryation. Even under clear \\leather conditions, \'il riations in the sun antenna remperature as great as 100'K \\'ere freque ntl,v clbscrvcd at this w'avelength. These broadband variations appeared in all ch:lnnels r1'hen t5c noise feedback loop was inoperative. -fhe antenna \\,as an equatoriallv molinted 5-foot searchlight. Tracking of'the sun \\'as provirlcd bl' e s)'nchronous clock mechanism. Photographs of the antenna ancl control console are shown in lrigs. 19a and b respcctivcly. variations


Related Areas

0f Appticutiott

The absorption charactcristics of molccular atmospheric,,;qr,'gefi oll"er great opportunity for exploitation . The a t te n Lurt ion expericncr.tJ f'rorn se:.r

level along a r.ertical path through the atmosphcre is nearll' l0{; dB


resonant Iine frequenc-ies near 60 GY'lz. SatclIitc-to-satclIitc communicatirin at thcse frequencies would be free of nliln-ntader noise originuting at the

carth's surface. A colnmunication link of this t)'pc ri oulrJ alsg bc undetectable at the earth's surlace. Cclnlrnunicatitln links bctriecn high-altitudc aircraft, operating at frequencies in the ri'clls bct\r'ucn resontrnt line.\, u'0uld

joy the sa tne bcne {i ts. The ability to vieu'tlre earth lrorri srrtcllitc urbit w,irh a ladiorllutric sensor and see a uniformly bright matttlc in this \\'avelcngth regic',n luts suggested the possibility of an earth vcrtical sensor more accurirLc rhan an IR horizon scanner. Another possible application is thc abilitl'r6 rclnrttcll,rcqse r"cgiilps of clear air turbulence (CA'fi in the foru'ard llight parh ot' supcrs6nic high altitude aircraft. A millinretcr \\'ave racliornctcr, tunccl Io the ox],gen \\/avelength band, rnay pror,'ide this capabilitr-'. Ternfcr1rrurr anonralies appsar to be associatcd u'ith C'A-f rcgions. -l-he rangc ar ri iricl: ir "niillimeter \\'aVe thermometer" is projectcd l'or*'ard of rhc lrircnil't altrng irs flight path can be adjustcd b,"- sclecting the \\'avclengrI sf t-,b>urvarisp, capi ralizing again on the \t'avelcngth de pende ncc of the or] gcn absoi'piiol en

coefllcient. The satellite r:artlt-r'ertical scnsor and the CAl" cletcctcr et;nccflI ure

discussed in greater dctail in thc section.s u'hicii follLr\\,. 'l-hesc;rpplicarigns are typical exarnples of the cxploitation ol'knorr,,lc'clgc cLlr-rt'irtl).h,eing gained and applied to inrprove prescnt ci-lpabilitics rlirrlugh nr\\'conccprs

and techniques.

o. '4tt Earth I'erlical Sensor. Thc mLlst cont nlun nte tht,d i'urr passir,'e remote scnsing of thc c{lrth ve rtical ;,''rbit is prcrJicarcC on ,silisllire

Harold I.


the s1'mmetr-v and stability of the earth's infrared horizon about the local satcllite vertical. The performance of an IR horizon scanner is cletermined b1' natural linritittions' [t has been suggested t33] that the molecular atm0spheric oxygen mantle might offer a superior reference for earth vertical sensing' Although the system concept of rim cutting used in IR horizon definition could be applied to sensing of the molecular atmospheric oxygen hori zan mantle, the antenna aperture size to obtain an equivalent pencil



Ftc' ?0. Typical atmospheric temperature at latitudes of


vs height profiles observed in Januarv

30, 40, 60 and 75o north.


beam would be unreasonably large for satellite application. Fortuitously,, the thermal radiation characteristics of molecular atmospheric oxygen

negate the need for a rim cutting technique. The relationship between the oxygen emission spectrum and the te mperature as a function of altitude above sea level shows that the observed emission is frequency selective and represents the average temperature in

an atmospheric layer of air approximarely 7





to l0 km deep. The mean

RE ('K)

Typical atmospheric temperature vs height profiles obserr,:tJ in July at latitudes of 15, 30, 45, 60 and 75o north.




height of the observed layer is determincd by rhe frequency of observation anrl the observation angle relativc to the nadir. The basic concept of a molccular atmospheric oxygen vertical sensor is predicated on selection of an observing frequcncy u'hich provides thermal sensing of the atmospheric temlerature at an altitudc {determined also by the obse rving angle relative to thc nadir) at rvhich a near uniform global temperature distribution is an

tici pated.

Typical temperature hcight profiles for the month of January as a function of latitude are shou'n in Fig. 20. Corresponding temperature hcight profiles for the month olJuly are shou'n in Fig.2l. With the exccption of the rcported high temperatures during rvinter in northern latitucles. one rvould anticipate temperature variations of approximately :l0'K from the poles to the equator in the altitude range from 25 to 35 knr, indepcndent ol season. Rcferring to the tempe rature-height profiles (F-igs. 20 and 2l), it is of interest to note that the temperature difference bcrrveen widely separated latitudes is less at higher altitudes. It should be notefl thatatmgspheric tcmperature data above 25 km is quite sparse; and in particular, above 35 km (balloon altitude) is obtaincd by isolated rocket probcs. The general form, horvever, of the temperature-height profiles suggests the eflicacy of observing a near uniform global temperature mantle

at altirudes above 25 km. As previously dcscribed, the absorption characteristics

of molecular observation of atntospheric oxygen are such that one can sclect a frequency sea level to from altitude to obtain a temperature sounding at any desired 'approximately 75 km. The "rveighting function" for a particular frequency of observation describes the altitude interval-temperature contribution for that frequency. Asshown in Fig. 15, thedepth of the weighting functions tends to increase with altitude. All weighting functions shown in Fig. I 5 are associated rvith frequencies located between oxygen line rcsonant frequencies; i.e., they are located in line "wells" as opposed to

line "cores." It is of intcrcst to note, in reference to Fig. 15, that the mcan altitude of the u'eighting function increases rvith the angle of obser!'ation off the nadir direction. The increase for a 60 zenith angle is of the order ol 5 km as shorvn in Fig. l-5. The mean altitude of the rr,cighting function incrcases sharply as one approaches a zenith angle of the horizon as viewed from orbit. At 60.8 GHz, for example, the altitude of the rvei-qhting function increases l1 km over the altitude that u'ould be probcd at the s,Ime lrecluency in the nadir direction. This feature is o[ considerable advantage in system design since the selection ofan operating frequency such as 60.8 GHz in thc "ivell" betu'een line "cores" eases restrlints on frequency stability, while at the same time providing a u'eighting function altitude in the l5 to 30 km range, $'here temperature variations


of -r 10"K. The radiometric mode of opcratiofl f'or a moleculer atnlospircric ox]"gen earth vertical sensor is frequentl,v refcrred to its thermal ceittroid sensing. This pitssive micro\\'ave technique \\'as clcveloped :hortl)'af'ter World War I I and has undergone scveral gcneric advancenle n ts since that time. A major area of application has bccn in the design ol- r'adiomcrric sextants used for the navigation of mobile vehicles {primarily,'ships and submarines) under foul \\'eathcr conditions. Cclcstial raclio sources sucli as the sun, moon, and radio stars are used in thcse applicatir;ns. In thc radiometric' sextant mode. the projection of the antennr bcarn on the celestial sphere is invariabll'much lergcr than tire solid anglc subtenclcd by t.he celestial source. The position of the source in crrrhogonal coclrdinates about the radio boresight of the antennt is dctt:rmined b1'cornparing the power received b1, dual antcnna bcams in either coordinat.e. The t\r,o beams are usually'displaced to provide a 3 dB responsc on thc anrenna boresight axis. The general form of thc anglc-tracking error funcrion ar. the output o{' the radiometric receiver in either coord inatc is independent of the method of angle sensing; holvevcr, the achier':lble signal-to-noise ratio is critically dependent on thc modc of operation and, hcnce, deterrvith latitude are of rhe order

mincs the rnts angle tracking accurac!'. The developmcnt clf the expression


for the anglu-tracking accuracy in a radiometric scxtant confrguration u'ill bc he lpful in the

analysis of the earth vertical sensor concept,since the lattcr is a degenerate case of radiclmetric sextant thermal centroid tracking. In the certh vertical sensing application the antenna beam anglc is srlri"lllcr than thc solid angle of the celestiul source iearth) . Thc form of the anele error function in either coordinate in the radiometric sextant mode is sho\\:n in fig. 22. Note rhat rhe solid angle cf tlie source is smaller than cither antcnne beam projection on the ce lsstiai sphcre . The tr'vo antenna bearns are identicul, each prol'iding a half-po\\'er response on the boresight axis. A characteristic "s-curve" is devcloped as tire source passes through the line of ccntcrs of the two bcams and thc boresight aris. The S-curve is derivcd b1'subtracting the po\r'er received b)' antenna

beam B from that received bi" antenna bcam A'. Noise lluc'tuations associated n'ith the thermal noise characteristics of the targert and inhcrent receiver noise are superimposed on the S-curve. The rms angle tracking accuracy can be derivcd from the follou'ing geometricl I consideratiotls. The slope of the st raight linc connccting thc pcuks of thc S-curvc n-ith thc targer source on boresight is

Straight linc slopc







(P* to P*)




Ftc. 12. Ceneral form of a radiometric sextant angle error curve iS-curve) de-

rived b.v tu'0 antenna beams r','hose frc\\.er patterns intersect at their half-pnwer points on'the boresight axis of the antenna system. The anglc error curve is obtained by subtracting the power received b-v antenna beam B from that received by antenna beam A as a celestial sourcc isun. moon, or radio star; passcs along the line of ccnters of the t*'o antenna bcams. The "figure-of-merit" of the curve is defined as the ratio of the peak-to-pe ak amplitucle ipx to px) s to the peak-to-peak level of receiver noisc fiuctuations iPt: to Pxl FL, superimposedon the


rvlrere tPx to P")^t is the pcitk-to-peak amplitude of the S-curye and 0.4 is thc ;rnsular scparation ol the t\\'o peaks of the S-curve. f1rr a cos2 antenna aprrtLlre illumination, the slope is ril times greatcr than the sr.raight line slope. or: S-curvc sloPe -:- T,/2 {P^ tcl P")S 0,1



Tlrc rms angle rracking e rror at borcsigh L 60 multiplied by the slope of thc S-curve At boresight, is eq ual to thc rms ^, value of the fluctuat ing noise collrp()ncnt sup(]rimposed on the S-curve, or


; 'i2 {Pr


(F,, to P"\F[-

)'s of\ -



r11 1J


\\'here tPx to Pr') FL is the peak-to-peak noise fluctuation level supcrirnpcsed

on the S-curve. Therelore

.:0 00,


3r lI{,


(P^ to P")S


{P^ to




Ms is defined as the frgure-of-mcrit of the S-cur\:e and reprcscnts a measure of the signal-to-noise ratio associated rr'ith the anglc-tracking error function. The angle sensing mode of opcration must bc sclected to maximize fr|.r. The equation for the tigure-of-merit M, ma]- bc re-expresscd in radiometric system parameters b1'recalling that the pcak-to-peak amplitude of the S-curve is twice the observed antenna temperature when the source is centered in either antenna bearn and that the peak-to-peak noise lluctuation level is 6 tintes the rms radiomctric sensitivitv {tw'ice 3o) , i.e .,

(Pr to P";S



'1=, : f r( _\.-0: i,




(P* ro PxIFL

= 6Af, =

{;(E - r)l, \/



r.r'here TB

0r 0^


f" pFptTa -

rms sensitivitv of thc rarjiometcr, oK the antcn na apcrturc eflicicncy

the receiver noisc figure the receiver predetection bendu'idth

thc receiver postdetection integration time constant 290"K

The foregoing anall'sis can be extended to the carth verricul thcrrnal centroid sensor mode by noting that for this casc the antenne bcanr is smaller than the titrget, &S show'n in Fig. 13. If thc squint anglc betrveen






General form of the angle error curve developed by a satellite earth rertical sensor when operating in the thermal centroict sensing mode. The earth vertical direction is coinciclent w'ith the null crossover pnint of the angle error curve. This is a degenerate case of the radiometric sextant mode shou'n in Fig . 22.

antenna bcam rl and B is adjusted to provide simultaneous interception of opposing earth hori zan lines on lhe corresponding antenna boresight axis, the n an S-curl'e w'ill be developed as the tn'o antenna beams are scanned across the earth, as sho\\''n Fig . 23. In this case, the antenna tem perature Tu $'ill be the brightness temperature of the earth oxygen mantle at the frcquency' ol observation (for an antenna aperture efficiency of 100 To1 . Inspection of this special case leads to the conclusion ihut Eq. (33) is directly'applicable to the determination of the rms angle tiacking u..uracy associated u'ith the crossover or null point of the S-curve. As a typical example oi anticipated performance, \\'e ma-y assume that the mantle temperature of or)'gen at an altitude of 30 km u'ill be in the order of 230oK. I'le nce, for a 3 oK rms se nsitivity achieved rvith a postdetection time const;tnt of 0.1 sec, the figure-of-merit of the S-curve g,ill be


&_230 64r.-




For a l0o antenna bcam angle, the anticipetcd rms angle tracking is therefore 1.5 arc minutes (rms).


A photograph of the first radiometric sensor ass!'mble d l'or satcllite -f flight to evaluate the efficac-v of this concept is sho\\'n in ftig. :4, his




,. . : .i':!i


r t


s li r 5 T




sensor developed hrl'the Air Forcc Carnbndgc Research Laboratories. This unit uas launched into earth orhit in Jul-r'

Frc.24. A 5-mm radiometric t967


unit \\'as dcveloped by the Air Force C'ambridge Rcsearch Laboratorles under the direction of J. Aarons and D. Cuidicc. A rnodihcd absolutc temperature mode of operation \\'us used, u,'ilh thc output indicator zero adjusted to correspond to an input signal tcmperaturc of l5()'K. The ob.iective of the experiment was to dctermine if the atmospheric temperature observed from satellite orbit-at a frcqLlcncy ol 60.|i GFlz \\'as consistent with the predicted almospheric model. Thc observe d tenlperature as a function latitude \vas a prime rneasurcment objcctive. Thc radiometcr \^,as launched into a near-circular polar orbit on July )'i , lqrr7. Prior to stabilization, the satcllite tumble rate of I rpm \\'i-ls casill'dctcctt-'d as thc

I{a rolcl I . Ev'e rt

singlc antcnna brearn scanncd across the earth and sky. Diflcull'\\,as encountercd in achicving vehicle stabilization w,ith the desired alritude. The :iatcllite ultimatcly stabilized in an upside-down atrirucle, pointing the scnsor an{cnna into -\pacc. For a period of t}rree months, the unit faithfullr" recordcd that the observed temperaturc \\,,as nca r zero. This \\-as an inrportant pionecring expcrinrent, hou'ever, since it demonstrated that rlrdionleter instrument technology at 5 mm was rcad-v* for t5e challenge of e

xploration from space at this \\,AVelength.

h' Detect ion of Clear Air Turbulence. The abilit y ro det ect tem perItltre anonlalies in the forrvard flight path of a high-altitude aircrAft at a \"'ilVClength of 5 m m is predicated on the large dynamic rangc of atmosph cric absorption coelllcicn ts ivhich are available ove r a relativelv small \\'ilvclength ranse near 5 mm as a consequence of thc resonant tine profile cltaractcristics of molecular atmospheric ox),gen. The intensity,and ran_se

to an ntmospheric tempcraturc Anomali* along the foru,'arcl flight path of an ;tit'cral't is senscd b1'operating at tu,o or more frequencies selected ro prot''idc lltmosphcric absorption coelllcients ar the flieht altitude in the ranse t"rorn 0.1


1.0 dBikrn.

Assuming iIn idcal pencil bcam antenna pattern {the antenna pattern unitv c)\-er an angle corresponding to 3 dB antenna beamu'idth and zera e lscu herci , thc expression for the ohsenn'ed antenna temperature fsee Eq. 'l-i) | takes the lornr (3 8)

ri lrcrc rt i.>. ,t



* thcrmometric


of the atmtlsphere et range



integrand is thc product of Iw'c lactors. The first is the temperature distribution along thc ray path, tlte second is a space-dependent function of thc attenuation cocflicient along the ra)'path. The secoJrd factor is largest fnr those regions ncaresl the antenna and exponentially' decrease as the distilnce from the clenrent of atmosphere Iocated in range inte6,,a I ds bccomes progressivell' llrther from the ante nna. Thus, this factor enrpSasizcs splttial elentcn ts clf the tcmperature distribution at ranses near the antcnna and providcs a decreasing contritrution to antenna lemperarure for tltose elemcnts u'ell rcmovecl from the antenna. Because of this spatial sclection propertY, the sccond factor in the integrand is frequently referred t{, ils thc " horizontal wcighting function " of the temperature distribution alcng the rav plrth ,




It is apparent from Eq. i3S) that the contribution to the obscrt.ed

re)'path is delermined bl' the value of the u'eighting function n'hich is in turn determined b1'the value of the absorption coefficient at the frequencv of obscrvation. Thc temperature contribution from a region rr,'cll forn'ard of tlte ltntenne can be made to contribute a significant portion to the ovcrall antenna temperature by obscrving at a frequenc\r u'ith a relatively large value for the absorption coefTicient. Selection of the obscrving frequenc)' is based on knou'ledge ol' the frequcnc)' dependence of n,., at the flight altitude. Probing atmosphcric temperatures along the forw'ard llight path of an aircraft can be accomplished b)'a multichannel (multifrequencl') radiometer in rvhich the channel frequencies arc selected to prol'ide the dcsired combination o f a,,, value s rcquircd for detection of te mperature anoirlalies ahead of the aircraft, A ntinimum o{'trr'o freclucncies of ohservtrtiofl itre

antenna temperature from an)'region along the


To illustrate the rangc capabilit,v of thc tempcraturc sensing systcm of this t)'pe, \\,e u'ill rcrvrite Eq. (3 S I for the case in rvhich thc absorption coefficienr is constant along the ra-v path, l-his is a reasonable assuntption for a horizontal path. The antenna tcmpemture for this condition is (


de pendent terrn r.r'hcn the range u'eighting function. lts maximum value occtlrs steadill'decreases, Itor all other values of s, the n'eighting functiott

We now note that the exponential factor is the onll'renge

in the s:0.

thereby rcducing contributions l'rom regions at large range r"e lucs. If rt'e define the disrance ar r.r'hich the exponential factor is l?L ol'its lrlllximum value as S,o. i.e.,



ln r00



then regions at distance S greater than Srr contribute less tha n l'o' to the total antenna tempcrature, and S,r defincs the rangc intervitl r.r'hich contributcs 99r; to the observed antennr tempsrature . T1'pical velues of S.rn

andcorreSpondingvaluesofatindBikm)are:a-0.l,5rl a - 0.5, Sr, - 40 km; and a - 1.0, S.,,

To obtain a qLrantitative picture of anticipatcd pcrformence , let us consider a 5 mm radiomctric s;-stcm nlountcd in an aircraft *'ith its antsnnii beam pointing alon-r the horizontal flight path. lf the ambient te mperaturc along the flight path is T r and a tem pereture anomaly described b.,v e function A f(S) is present in the foru'ard range inter\'al S, to 5,, the anttnntl temperat.ure sensed b1'the radiometer r,r'ill be

Ilarold I. T


:-: T, +


j,; a {y) A f iS) [e xp

e (y'i.t ] d.S



Notice that in the itbscnce of a temperature anomall, A r(,.t), the antenna temperaturc is simpl,v thc ambient tcmperarure at the flight altitude Tr. \vhcn a te mperaturc anomaly is includecl, rhe limits of the second integral ilre 0nlY ovcr thc regiott rt'here the anomal-"- i, present since the integrand c5



^lol |t-l


ItI l.< l'{ ,<l F-^l





4 '-tt







+ '1


50 40 302C


, o


Ftc. l-{. The normalized multifrequenc,v response of a radiometric sensor to a srep temlxrature anomal.v of A Io, l0 km in e.\tent, at a horizontal from the sensor. The desired a-tmospheric abscrption coefficienr 1.5 dB,'km determines the corresponding frequencies of observation. For a practicaI sy'stem, the observing frequencies can be confined to a relatively narrow bandwidth by opeiating range


u in the range from 0.1 to

in the Vicinitl'of a resonant Iine of an atmospheric gas, such as moie-

cular o.\)gen.




\ D


PI-{]I'I'A I



is zero elsc$'hcre. As an e.\anrple, lct A 7'l.I) eclual a cL)nstunt r llur: l Z; over the rangc intcrval from ^Sr to ^S,, lrncl zero clse u'hcrc. In thi: cilse the

antenna temperature



i t1r

Thus, thc presencc of thc temperaturc anonralv appears i-ls a cSlingc in antennu tenrperature about tlre ambicnL 7,. As an illLlstrative cxample o{'tlre magnirucJc of rhe anriciplred change in antenna tcmperaturc as il l-unction clf range and clbserving f'rcqLlency

(a value) ' conside r thc cilse of' a l0o(-- tempcruture anonlalr,. 1() km in eltent, in thc interval range Sr to.5,. Figurc l5 shorvs il grapliicel plot of the change in antcnnl1 tenlperature relativc to thc antbient tempcrature

at the flight altitude' as a function ol'the rangc to thc Ie mpe raturc unonrai,v, for selected I'alues ol'a from 0.I to I.5 dl]'km. Il \\'e lssumc tlut thc sensor has a temperature scnsing capabilit,v of r).5'K {ipdicatiye 3l' prescnt capabilit)') , it is appare nt from I'tig. 25 that thc renrpcrarurc anL)rnlly would be observed at a distant'e of'59 km at ult obscrring l-rcqucnc!, for which a: 0" I dB,1krn. .tl knr for a 0.1dB,krn, and ll kln I-t)r rr 0.5 dBi km. Refcrring to lrig. 15, it is of interest to rtotc the rcsponsc churlr:r1,ri:rics of the various channels to thc ilssurncd tentpcraturc irnonrall'. ,,\lthouglr initiallf insensitive to the clisturbance, the ft * 0.: dlJ'knr chrnncl subsequentl)' responds ver)' quickly and, at a ritnge of' l5 km, prrrr,i,Jes itn outptlt signat which exceeds the signal lcvct in thc a * 0.1 dB,'knt channcl.

A sirnilar "crossover point" for thc channel pair (r'- 0.5cJIJkrn and a - 0.1 dBikm occttrs at a range of I knr. The prcscnce clf t6c:e "cr()ssover points" betw'een indiviclual channels oll'crs an;rtlditional ranqc indicator for the anomalous temperatLrre rcgion. To demonstrilte that the results are not criricalll'dcpcncicnr upon rhe temperature profile of the cssunred discontinuit-\', a similar' ;lnill1,:is can be applied to two other forms of tempcraturc anomal)'. In onc ci.rsc, \\,e ri'ill assume a linear transition fronr tcm pcraturc T-, to T, + I 0'- C' and in r l:e other an expclnential transition frorn T/ to T, + l0'C'. Fpr cach c1sc. \r'e lvill assume a half amplitudc *'iclth of l0 knt, corrbsponcling ro the first case considered. The enticipated antcnna tcmperature changc \.ersus range to the anomalous tempcritturc rcgion rcsulting from thcse distributions are shown in Figs. l6 and 17, respectivcl\,, for comparison r,,,ith thc case descrihcd by Fig. 25. It is of intcrest to note that {}\,erali signature characteristics are the seme for all three cascs. This indicatcs th;rr the basic range sensing capabilitf is not criticlilll'dcpcndent ()n the tcnrperature profile of the anomalv but rather on thc fundamenral ratliative prL)perties of molecular atmospheric oxvgcn as a func.tion of frcqLrcncv anci altitudc







n? V, I

"^l F-l, o r lr 'la Fl \J





4\,/ -/"t

/ (,/

/r /2 /



-/ 40 30 2A tO too 9c Bo 70 60 50 Ronge



S rn



The normalized multifrequency response of a racJionrctric sensor to a ramp tcmperature anomaly of A Io, 10 km in e.xtent, at a horizontal range S from the sensor. isee Fig. 25 for comparison of the response characte ristics fr-rr rarious values of the atmospheric absorption coeflicicn t


tli- obicr\';rticln.

,,\ dultl channcl 5 mnt radiomcter scnsor \\,as cleveloped in 1968 b1'the of the NASA Electronics Research Center to investigatc Cl,{T dctection capabilitv. A photogreph of this millimeter racliometric scnsor is show'n in ftig. 2,!. The antenna is a single conical lcns J'cd horn. "fhc antenna output is fed via an orthogonal mode transducer Pi-r.rpilgations Studies [Jrancir

thc inputs of tu't'l radiomctric recei\:ers. Onc receiver is tunable over the lrcquenc)' range l'rom 5l to 53 GI-lz and rhe other from 57 to 59 GHz. t"o




r--oi , o ll-

'i< fi







Ronge S in km




The normalized multifrequency response of a railiometric sensor ro an exponential temperature anomal,v u'hich has an average value of J f0 over a range interval l0 km in e.xtent, at a horizontal range 5 frgrn the sensor. {See Fig- 25 for comparison of the response charactcristics for various values of the atmospheric absorption coefficient rr. r

The individual tuning ranges and the frequency separation beru'een channels allorv selection of corresponding atmr)spheric atrsorption coefircient yAlues

for observation 60,000 feet

of 0.1 and I dBlknr for flight alti-tudes from 3i),000 ttl

The first aircraft flight test has been scheduled for the fall of 1969. As a part of this flight program, i[ is planned to poinr the antenna verti-

cally down from high altitude. This should provide an interesting er,aluation of vertical sounding of the at mosphgric tenrperature prglile in rhe




Flc. 18. 5-mm wavelength dual channel radiometric

sensor developed by the

i{ASA Electronics Research Center to experimentally'verify the ability to detect clear air turbulence regions along the forw'ard flight path of high alritudc aircraf t.

ioli'er troposphere. Ttre diversity of the tw'o disciplines, CAT detection and mcteorological meesurcments. $'ill be joined by one sensor as a common

dcnominator in this dual experiment to explore and exploit.



I-listoric;rll)', our communication needs and associated technological l-crluirentents havc provided thc stintulus for expanding our radio capahilitics t'rom long to shorter \\'avelengths. There has been no slackening qlf thc pace as this need now focuses attcntion on the 3 cm to 3 mm wavelcngth region. This time, horvever, the communicator has a silent and pcrsistcnt pertner alread_v- activel,v exploring this region of the spectrum u'ith clcfinite plans for exploitation. The ability to obtain a global picture of


atmospheric rvater vapor and tempcraturc distributions, combined u'ith air mass circulation under cleer air conditions, offers the potential to predict in advance the formation of storm clouds and thcir motions. Several significant applications can be accornplished only in this portion of the spectrum as a consequence of the nature of the ph,vsical processes involvcd. In rhe future, &s communicators look through the atmospheric *'indows, microu'ave meteorologists tvill be measuring the globei structure of the atmosphere in the spectrum benveen the u'indo*'s. The y' ma)' botir join together, horvever, in earth orbit at opcrating wavelengths in the vicinity of 5 mm, since here the cornnlunicator is assured an envircnment free of man-made electromagnetic interference, protected bt' the several

hundred dB attenuation of the ox)'gen blanket surrounding tire earth. Here, the needs of the meteorologists *'ill parallel those of the prcscnt day radio astronomcr in rcaching agreentents on "quiet bands'' to bc used exclusivel.v for passive remote sensing. This is not an insigni{icant problem. Without carel-ulconsideration, harmful interfcrence to passive studies and applications may result. F-or example, a microu'ave mapping radiometer has been proposeci for an progranl on thc liimbus series of satellites. A frequenc-v of 19.35 MFIz wAS chosen b$ceusc it is in the region of the spectrum u.'here the brightness tcmperature of smooth sea water is practicalif independenI of the \\'ater tcmperature. This frequency is, coincidentl)'. in a radio astronom]- band presenti''' prcltccted from man-made electromagnetitc transmission. It tt'as rccrntl-'* suggested that this particular radio astronom-Y band be rclocated to:3.55 GHz in order to make n'at l'or space-to-earth coltl nt un icatiuns. The advent of space-to-earth comnlunication s)'stcnts at selcctcd frcq ucRcit":s of this type ma-v seriously elTcct rernotc sensing applications u'hich cltnnot change freq




The erplosir,e explclration of the 3 cm tcl 3 mm lr'at'eleltgth region u''ill continue at an accelerating pace since instrunlent capllbilit;" is no longer the limiting l'actor. Within the next half dccade a radiomctric temper&ture sensing capability of better than l"K rr'il1 be achievable throughout this entire \\:avelength region, r'u'ith postdetection integration tinre constants no greater than l sec. The cl'rallenge r+'il1 be to exl.ract kno*'ledge and understanding from the centimeter to millimeter \vavelength signals that are naturall,n- emitted by thc atmosphere , the oceAns, and all surfacc terrain materials. It is never an easy task, [1s\\'ever, u'hen thc unkno\l'n is so close to home. As J. P. Wild said at the Fourth Parvsel'Memorial Lecture at the Llniversit,v of Queensland, Australia, in April of 196 |i, speaking about our knou'ledge of the sun: 'o You see the sun is rather an enigrna in astrophl'sics. We appeer to knorv so much about astrophysics-about the galaxy and the un iversc and so on ; thcre might be

Hurold I.


a !'r:u'controversial alternatives w'hcn astronomers talk of cosmology or rlLursurs ()r pulsars, but on the u'hole the."" sit back and ret'ie$' their acliieven"lcnts *'ith rcmarkable satisfaction. . . When you know' t\\'o or thrcr'things about something thcre is no difficulty in producing a theory knou'a thousand things, the theory becomes to sxplain it. But "r'hen )'ou more diflicult, and u'hen )'ou know that another thousand things are ri'rriting to be discovercd the theorists get lrightened off. And so, apart frrrrn a fcr,'' of thc braver theorists, the onus is left in the hands of experime ntal ph1-sicists: cspecially those prepared to persevere gradually, step bv ste p, n'ith the scientific mcthod; and especialli" those prepared to fashion ne\1" lines of attack." ,4s \\'e enter this ne\\'era n:i1|1 the c'apAbility to passivell"and remotely' scnse thc location, identit,v, and condition of our earth resources from slitcllite orbit, our succcss rvill be measured by our perseverance and intcgritl' to exploit thc micro\\'ave spectrum in the best interest of all mil


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Exploration and Exploitation of the 3 cm to 3 mm Wavelength Region  

Exploration and Exploitation of the 3 cm to 3 mm Wavelength Region. Originally published in 1970 by Academic Press and republished in 2013 b...

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