Expl*r*fialr and Expl*it*tion

of the 3 cm fa 3 rnrn Wauelengtfi Seg isn

*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 thatrange in their coverage from low microwave frcquencics used to accelerateelementary particles, through cm and mm wavcs for exploring atntosphcricphenomena, and on to the microwave demodulation of light.

The chapter on high-speed photodetectors for recovering nricrou,avesignals modulated on a laser carrier is our first successful collahoration be-tween authors working for diflerent companics. L. K. Andcrson, iVI.

DiDomenico, Jr., and M. B. Fishcr are familiar with both microwaves ancllasers. When large bandwidths Are to be transmitted on light beanrs' thesignal must be ( I ) modulated onto the lascr carricr frequenc\' . (1) trans-mitted, and (3) demodulated. The chapter deals with the third topic. (Thesecond 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. Inthe past, the frequency decade 10 to 100 GHz has heen used to probc theatmosphere and has yielded much meteorological information. This fre-quency band has long held out promise for microrvave conrmunications. apromise 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 andseparate high-energy particles in the linear accelcrators at CtrRN in Europe,and at Brookhaven and Stanford in America. We wish to acknorvledge thehelp and advice received from G. A. Loew in preparing this chapter. Thereader is also referred to the chapter on the Stanford lincar:lccclcrator inVolume l.

This volume could not have been assembled without use of the facilitiesat Stanford Research Institute. We are also grateful to Miss Dianna Bremerfor her unfailing help in manv ways.

Lro Yourc;

Exploration and Exploitation ofthe 3cm to 3mm wavelength Region

Harold I. Ewen

EWEN KNIGHT CORPORATIONEAST NATICK, MASSACHUSETTS

Introduction

Microwave RadiometryA. Temperature calibration of the output IndicatorB. Receiver Functions and Techniques

Microwave Radiometer ApplicationsA. Radio AstronomyB. Microwave Meteorology

IV. A Look into the Future

References.

I. INTRODUCTION

Historically, the use of the millimeter portion of the spectrum hasundergone cyclic periods of interest. Now increased attention has beendirected to this wavelength region, spurred in large part by the worldwideexplosion in communication needs. There are many inducements ro con-sider this portion of th: spectrum from a communication standpoint. Re-latively high antenna gain is achieved with modest aperture dramerer;broad channel capability permits high information capacity, and the totalavailable bandwidth, even within the restricted atmospheric windows, farexceeds the entire radio spectrum below 10GHz (3cm wavelength) . Ex-ploitation of this wavelength region for communication has, in l-"rg" p".,,been paced by the need for reliable millimeter power generating devicesand low-noise receiving systems. The required technological advancementsin these areas appear imminent.

Though the prime interest, as in the past, has been the need to alleviatecongestion in the microwave communication bands, the latest resurgenceof interest in millimeter waves has been aided by a passive, but not silent,partner. Exploration of this portion of the spectrum has been forgingahead at an accelerating pace through the application of passive radio-metric measurement techniques. Some of these investigations are concen-trated in the available *atmospheric windows' to

"tt"blirh their future

I.

II.

III.

Harold I. En'en

potential for earth-space communication links. Several significant inves-tigations, hou'et'cr, arc being directed to those portions of the spectrumu'here the level of atmospheric opacity is too great to be useful for com-nltlnication. Radiometric sensing of thc electromagnetic em ission of thea tm osphere in these portions of the spectrum is providing a new andpo\l'crful tool fclr the investigation of atmospheric structure and the as-sociated physical processes. Today, \,4,e are at the dau'n of the new scienceof microwave mcteorolog,v. We can expect many startling discoveriesbcginning in the decade of the 70s, ns microwave and millimeter radio-metric sensors contribute to the challenge of global weather prediction.

The new field of microwave meteorology \vas spawned by the youngscience of radio astronomy w'hich has produced so many startling dis-covcries conccrning our galaxy and the universe. From the earliest ex-periments performed in tlre HF and VHF bands, the radio astronomer'sspectrum of interest has progressed tou'ard the millimeter wavelengthrcgior , paralle ling the m ove of com m unication systems to higher fre-q uencies, rvith the uprvard step for each paced by advancements ininstrument technology. Exploration and exploitation of the higher fre-quencies has historically favored the radio astronomer since the passivereceiving devices needed for radio telescopes frequently become availablebef-ore the po\e'er generating devices needed for communication systemsare developed. Each upu'ard step in the spectrum has led to unanticipateddiscoveries. The significance of these discoveries has, ofl occasioD, sug-gested the exclusion of communication systems from certain portions ofthe microw'ave and millimeter spectrum. Radio astronomers and com-municators share those portions of the spectrum frequently referred to asths "atmospheric rvindo\t's" where electromagnetic radiation passes throughthe atmosphere r'vith least attenuation. These windou's, u'hich are centerednear \{'avelengths of I and 3 mm, are open during clear \veather conditions,partially closed by heavy rvater laden clouds, and are essentially closedduring occasional periods of heavy rain. The attenuation and noise charac-teristics of the atrnosphcrc in these rvindor.r's are of prime concern to boththe radio astronomer and communicator. The astronomer must understandthe propagation characteristics of the atmospheric medium in order todelete its contaminating effects from the analysis of the very faint signalsreceived from space. The communicator must knou' how the atmosphericrnedium effects signal fading, angle modulatior, and correlation bandwidthin ordcr to determine the optimum systcrn design. Several significant com-munication research efforts in this area, today, are based on techniques de-vcloped in the field of radio astronom,v-. An obvious reciprocal benefit willbe knowledge gained by the young science of micro\\'ave meteorology.

The microwave radiometer is the common denominator in the explo-

EXPLORATIO\ A\ D EXPLOITATIO\;

ration of the 3 cm to 3 mm \\'a\relength region. Invented b1' Dickc Ilj lessthan three decades ago, €lrlbcllished anrJ exploited b1'lrdio asrronoml,, itsuse is rapidly' spreadint to a clivcrsitl' of scientiflc rescarch ancl cnginee ringdisciplines and applications in the e.xplosive pioneering erplorarign sf thcmillimeter rt'avelength region. It u'ilt be helpl-ul in Oiscussing rhese ap-plications if we first revier,r' certain radionlctric l'undamentals associateciwith this portion of the freq uenc), specrru nl.

II. MICROWAVE RADIOMETRY

A microu'ave radiometric sensor is a device for thc dctection of elcctro-magnetic energy which is noise-like in character. The spatial as u-ell asspectral characteristics of observed energ-v sr)urces determine the perforrn-ance requirements imposed on the functional subsystems pf the sensor.These subsystems includc an antenna, reccir.er, 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\,beeither broadband or of the resonant line r),pe. Scnsor design and per-formance characteristics are primarily deterrnined by the exrent to ryhichspatial and frequencl- parameters characte rize the radio noise soLlrce ofinterest to the observer.

A micro\\'ave radiometric sensor is frequently referred tg 3s a temper-ature measuring device, since the output indicator is calibratcd in dcgreesKelvin. The reason rvhy microu'ave radiomcters arc calibrated in temper-ature units and the modes of operation that are most frequently usecl arcdescribed in the sections imrnediately follorving.

A. TrltPER,,\Tunr CALIBRAT-IoN oF Tt-tr Oljrpur Irr:tcAToR

The ph-'-sical reasoning in support of calibrating the ourpur indicerorof a microw'ave radiometer in degrees Kelvin can bc deriyed from thcrmo-dynamic considerations and certain rvell-kno\tr,n properties of an antenna.

The amount of energy absorbed b1'an antenna and presented at theinput 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'ingsystem ' Since al I an te nnas e re pola rized , regard less of design , the ma.xi-mum amount of cnergy accepted by an antenna, frorn a randomlv polarizedwave, is one-half of the total energy content ol the waye. If \{,e assumcthat an antenna is perfectl,v matched and that the incoming \\,il\.e israndoml)'polarized w'ith a po\r'er llux density S, then the absr-rrbed po\\.erPA is given by the expression

Harold I. En'en

p^A (li

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 froma source of small angular size and is measured by the flow, of energy fromthe source through unit area in the \rrave front at the observing point. Ifenergy dE in the frequency range dv flows through area dA in tim e dt(rvhere r/r is long comparcd to the period ol one cycle of rhe radiation),

then the flux densiry S is given by the expression

s- dEdAdyclt

/'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 terminatedrvith a matched load and the other end of u'hich feeds an antenna in anabsorbing medium. If *'e were to replace the antenna by' its .quivalenttwo-terminal netrvork and assume that it is a purery r.ri,crr lrsLlt ur K ilno assume that tt ls a purely resistive impedance

and eqLlivalent to thc Ioad impedance, Ihen a transmission line terminaredline terminatedin a matched antenna may be treatcd in a manner similar to a transmissionline terminated n'ith a resistive load, as shor',,n in Fig. l. If the extent of

Ftc' I ' EquiValent circuit of an antenna immersed in an absorbing medium attemperature I. In equilibriurn, the temperature of the load resistanceis the same as the temperature of the absorbing medium.

the absorbing medium is suflrcient to completel),absorb all radiation fromthe antenna, the medium and the matched termination must then be atthe same tem perature T,

From Johnson noise po\r'er considerations, the termination u,ill radiatea po\\'er kTdv to the antenna. If the antenna, in turn, did not accept k Trtvof radiation from the medium and transfer this power to the load, thereu'ould be a net transfer of thermal energi- from one region to another atthe same temperature rvithout application of rvork, in violation of the

*se

THERMAL RESERVOIR AI TEMPERATURE T

RADIATTON RESISTANCE R

LOAD RESISTANCE R

EXPLOR,\TIO).J AI{D EXPLOITATIO\

second 'law o{' thermodynamics. This u'ould indicate that in the micro-wave and millimeter portion of the spcctrum, thc po\\'er deliverecl to thereceiving system input by an antcnna immersed in an absorbing mediumat temperature I is independent of the frequency of observation.

This conclusion can also be reached (see Fig . ?j by noting that themedium appears AS a blackbody to the radiation resistance of the anrenna,i-e., it absorbs all incident radiation and its radiation brightness P in rhefrequency inten'al dv in accord u'ith planck's larv is

n , "hv31iO'-: .7c-

dv i3)lexp (-hvlkr)-t]

n'here

h

k : Bol Lzmann constant

c

and the brightness F it the power per unit area pcr unit solid angle, perunit bandwidth.

l-

ANTENNA

lFI

where dO is the solid angle increment.holds for any source of radiation over all

RECEIVERII

{,4)

This definition of flux densitvsolid angles.

ANTENNA RADIATION RESISTANCE R

Flc. 2. Simplified block diagram of an antenna and receiver. When the anrennais immersed in a blackbody at temperature T, the receiver input isequivalent 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

A^^PLIFIER DFTECTOR INIEG.

Harold I. Ewen

Eqr.ration (3) indicares rhat the power received from a blackbody,

measu red at the input terminals of the receiving system , is frequency

depcncle n [. Hence, the equivalence of blackbody radiation and Johnson

noise po\r,er appears inconsistent. The ensrver to this paradox lies in the

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

PR

w,lre 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

prR a') dQ F d, (6)

po\\'er receil'ed bYIf the extendecl sourcc radiation is a blackbody, the

the antenna rvould then be erpressed in the form

(5)

+l \,nto

r f f 2hv3D I ll Ai.g,O)dQ+r p f JJ"t",Yt*r4 C_

dv 7)

(8)

(10)

[e^p (hvlk r) - I ]

In that porrion of frequency spectrum w'here the energy of the photon hv

is m uch less than the random thermal energy per degree of freedom kT at

rem perarure T , the e xpression for blackbody brightness [Eq. (3i] reduces

to the simplified expression

,n , ?kTp au : : dv (RaYleigh-Jeans)

A-

,*,hcrc I is the rvavelength of observation. Hence, in this portion of the

spcct rum Eq. {1) reduces to

PR

Rccalling that the ":'.rage'.u..,1t. cross section for an)'antenna

immersed in a source ol'uniform brightness may be expressed in the form

r - +lu,o,1;r J

o) cl.Q, - )2

. : .l_'tru

I

\ero, ?) dQ : A7

EXPLORATION AND EXPLOIT,\TIO}i

we arrive at the conclusion that the po\\'er received bf iln antcnna im-mersed 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' de-

scribed 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 offreedom is, ol course, temperature dcpendent. Approximate values are

shou'n in Table I. It is apparent from Table I that microu'ave nnd niilli-meter \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 llrvithin that region of the spectrum w'here lu is less than kT.

fable I

Trvpr,nATUREs AND Connr,sPoNDINc WnvEl-r,hrcrllsnr WstcH THE ENr,ncv oF THE Ptrorox ft:"' Is EQunl ro k T

Temperature {'K) Wavelength '. pm' Wavelength {mm; Frequenc) CHzi

30011

20

4

1.5

'?o

200

9006000

10,000

0 .070.20.96.0

r0.0

4300

I 500

J J.-1

50

3il

In summary, the power receivcd by an antenna immersed in a blsck-body at a temperature f is frequency independent and equivalent to the

Johnson noise power that u'ould be radiated by an antenna if terminatedin a matched resistive load at the same temperature T. These t\\'o funda-

mental 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 cffectiveblackbod-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 temler-

ature , " measured by a radiometric sensor, refers to the po\\'er leve I thatwould be received from a blackbody et a temperature which rr'ould providean 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

Harold L Even

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 ofan efective blackbod-v- temperaturc. This noise po$'er represents a com-

Fosire of the desired signal pog'er and undesired noise power from othersourges, 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 ld.Il the effective te 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 ofnoise 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 amplificationadd 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 differentiationbetrveen ?'., 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 ofI, 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 resonlntline 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-noiserarioar 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 ofnoise figure F, the noise figure of the network shown in Fig' 3 is given by

the expression

EXPLORATIO]V AND EXPLOITATION

or

tf - kT*dv

we see that the relationship betrveen s-Ystem noise

figure is

TR

F_ GkTodv + GN

!xRlT

stGNAL - s

RESISTIVE LOAD NOISE = k To dv

INTERNAL NOISE = N

Frc. 3. Input and output signal and noise

network with the input terminated in

in a thermal bath at temPerature T'

G kTo dv

l/F-1+kTo dv

If r.ve now define the svstem noise temperature

il ll

{ 1l)

TR by the expression

i13)

temperature and noise

i 11)

lxj rruT

SIGNAL = G5

TOTAL NOISE = G t'. To dv + GN

relationshiPs of a four-terminala resistive load which is immersed

B. Recrtvtn FuNcrloNs nNo TecuNlQurs

Theprimefunctionofthereceiverinamicrowaveradiometricsensoris to provide a measure of the antenna temperature'

As previously noted, the antenna temperature I' is' by definition' the

,r-p.r"rur" to *iri.h the-radiation resistance of the antenna must be raised

inordertoproducethesamenoisepowerasthatcontributedbythevarioussources observed by the antenna. It is also the brightness temperature o[

a UtactUoay which, if it completely surrounded the antenna' would pro-

.vide the same noise p*". at the rlceiver input' To describe the method

by which the receiver measures tbe 'antenna temperature' we. will replace

the antenna with an equivalent risistive load at the receiver input' If the

antenna temperature were I;, we would obtain the same nolse power lnput

to the receiver by placing the resistive load in a thermal bath at a temper-

zture TA.

NETWORI(

GAIN = $BANDWIDTH = dv

INTERNAL NOIST N

Harold L Ew,en

The nced for signal amprification becomes readiryapparent u,hen onenotes that the average noise pou'er per unit bandwidth produced by aresistor 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

requiredinput signal amplification must, of course, be increased if temperaturechanges less than 290"K are to be detected and recorded. The receivermust therelore be able to sense a low level change in noise power at itsinput and provide sufficient stable amplification io drive the output indi-cator s)'stem. Amplification stability is a prime requisite since the receivermust provide a consistent output responsc for the same input power change.The.rclatively poor gain stabirity of present receiving systems is overcomeb1' the use of an input switch or modulator, to be discussed later.

I. Sensitivitl'

The noise power output of a resistive termination is associated withthe tlrermal agitation of electrons rvithin the resistive conductor whichproduce electronic collisions. As the thermal temperature of the resistoris increased, the thermal agitation increascs: and the number of collisionsper unit time increases. The resultant noise pou'er output per unit cycleis direc'.ly proportional to the absolute temperature of the resistor. As indi-cated previously, the proportionality factor is Boltzmann's constant k. Inthis sense, a radio measurement of the thermal temperature of the inputresistor may be described as a measurement of the electron collision fre-quency within the resistor. since the collisions are random, the numberper second will vary; however, the mean of an infinite number of one-second umples will lead to an exact value for the collision frequency.F-rorn statistical theory, the probable error in the measurement of a quantityof this type is inversely proportional to the square root of the number ofmeasurements u'hich are made. If the number is infinite, the exact valueis determined. lf we now measure the electronic collisions within a resistor,using an amplificr of finite bandwidth Au, the number of independentcollisions per second which can be counted is equivalent to rhe receiverbandrvidth. Hence, the error in determining the mean value of the noisetemperature (which is proportional ro the collision fniquency) will beinversely proportional ro the square root of the receiver bandwidth. If theaveraging process is extended over r seconds rather than one second, therewill be, on the average, r Au independent collisions in each intervar ofseconds, therefore

AI* I=- ,-{c Au

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

(1 5)TR

EXPLORATIOIV A}iD EXPLOITATION

ature is negligible when compared rvith the " receiver noise temperatureT^' which describes the noise power added to the received signal by thevarious circuits rvithin the receiver.

Simplified functional block diagrams of the most commonll'usedmicrowave and millimeter radiometric receiving systems arc shou'n inFig . 4. The figure depicts the genealogical gro*'th of each receiver from

Cryriol Video

Tunod Rodio Frcqrrncy

fuprrhotrrodync

SuperhetorodynoWith 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 usedin a new portion of the frequency spectrum rvhere input signal frequencycomponents required for the other modes of operation are not available.The superheterodyne is the "workhorse" among receivers. The input circuitof the superheterodyne is a "mixer" in which the signal frequenc-y- is hetero-dyned with tlfe local oscillator frequency. The difference or intermediatefrequency between the signal and local oscillator is amplified by a tunedintermediate frequency amplifier, re ferred to as the IF amplifier. The ad-dition of a low noise amplifier forward of the mixer in a superheterodynemode will establish the receiving system noise temperature by,' providingadequate gain to overcome the conversion loss of the mixer. The mostsensitive broadband radiometers operate in the TRF mode, where amplifi-cation is provided by the cascading of broadband low noise amplifiers, rypi-cally, 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

Harold L Ev,en

prcdcrection bandu'idths of l0 to | 5.,i of their operating frequency, atfrequencies up to 20 GHz.

The scnsitivity of a radiometric system, i.e., the minimum detectablesignal. is determined by the amplitude nf the lluctuations present at theoutflut inclicator in the absence of a signal. These fluctuations are at-tributable to two sources:

il t The statistical fluctuations in a noise \\'aveform as described bvEq. (l 5) .

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

2. Gain llariatiotrs and the Dicke .Vode

The second source of fluctuation s u,'hich occur at the receiver outputare attributable to receiver gain instabilities. Their significance can bereadili" grasped by the following example. If \\,e introduce values ofTR - l000oK, Au: 2 x l0eHz, and r- I sec in Eq. (15) , we obtainan rms value for the amplitude of statistical noise fluctuations at thereceiver output of the order of 0.03oK. This w'ould be the case if thereceivcr \r'ere absolutely gain stable. Unfortunately, the best receivers,regard iess of type or frequency of operation, exhibit gain instabilities ofthe order of l,'/" during a time period comparable to that required for anoise measurement. As a consequence, a receiver with the performancecharacteristics describcd above u'ould provide an output fluctuation of10"K if the gain changed by liv,. The noise measurement sensitivitv of

Irrc, 5. Simplified block diagram oflator is introduced between

the Dicke radiometer. A switch or modu-the antenna output and receiver input.

EXPLORATION AND EXPLOITATION

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 inoperation, a signal at the switching or modulation frequency is presented

at the input terminals of the receiver rvith an amplitude proportional tothe temperature difference TA- Tc, Becausc of the rapid su'itching rate,

any receiver gain variation rvill operate equally on T^ { 7^ during one-

half 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' radio-

meters which had relatively high noise temperatures and narrorv band-

widths, 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 com-

parison 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' temper-

ature 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

Harold I. Ew,en

the su'"itch or modulation frequency to provide an equivalent level of noiseat the input to the envelope detector during both portions of the sg,itchc1'clc. This technique provides a convenient adjustment of the effectivetemperature level at either input port u'ithout adding noise to the receiveror changing the temperature of the comparison noise source. The gainmodulator technique, horvever, is sensitive to changes in system noisefigure and must be used rvith caution.

3. Temperature Calibration

The detection of a signal noise source and its measurement in absolutetem psrature units represents a prime objective in several radiometric re-ceiving system applications. In addition ro sensitivity esrablished by thenoisc characteristics of the dctector and, in large part, influenced by thegain stability of the overall receiving system, measurements of this typerequire:

Knowledge of the output indicator zero level in absolute tem-perature units.Calibration of the output indicator deflection in absolute tem-perature units.

The output indicator zero level corresponds to the condition of RFtem perature balance bet'uveen the input signal and comparison ports (switchports) 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 indicatordeflection requires knou'ledge of the detector law and system gain. Ofthese tu'o requirements, the indicator zero level is by far the more difficultto achieve. Kno\\'ledgc of the detector law can easily be obtained throughlaboratory measurement. Sy'stem gain can be established at any timeduring a rneasurcment program b;- introducing a constant and fixed levelof noise at the radiometer input. The noise level of this gain calibrationnoisc source need not be know'n precisel-v-. It is far more important thatit remain constant and that it be used to establish the level of receiver gainduring laboratory calibration of the radiometer response in equivalenttem perature units. This noise source is usually included as an integralpert of'a radiometer and is referred to as the "calibration or internal noisesOu rce. "

Calibration of the output indicator - requires the introduction of apreciscly'knorvn temperature change at the input signal port of the radio-mstrr. This mea.surement is usually' performed under carefully controlledlabor;1torv conditions. This temperature change is lrequently generatedb1'the scquential introduction of two vert'preci-].ly knor.r,n noise temper-

:

{l;

i'1 \

EXPLORATION AND EXPLOITATION

ature sources at the signal input port of the radiometer. Calibration ofthe internAl noise source in equivalent temperature units is automaticallyobtained as a by-product of this laboratory calibration procedure.

From the foregoing, it is apparent that one intcrnal fixed calibrationnoise source, combined with knorvledge of the detector law characteristicand predetection attenuator values, provides all of the information requiredfor the precise calibration of the output indicator reading in equivnlenttemperature units. The internal calibration noise source level should be

approximatelv two orders of magnitude greater than the amplitude of thepeak-to-peak fluctuation level at the output indicator for the nominalvalue of postdetection integration time constant which will be used duringthe measurement program. This allows opportunity to establish the full-scale output indicator deflection level to an accuracy of at least \:ib. Whenmeasuring the amplitude of Iorv level signal temperatures, the introductionof the indicator calibration signal will normally require an output indi-cator scale change. This is usually achieved through ganged s',,r'itching ofthe two functions in calibration noise source ignition and ind icator scale

change.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, providesan absolute power measurement capability. The input circuit pcrformanceis such that the zero position of the output indicator corresponds to zerodegrees Kelvin. The effective blackbody temperature of a noise source

coupled to the input of the radiometric receiver is read directll. in degrees

r-

1-En v i rorme nt:l qFab"._T.-p.: ty. _= _ T L

plif icr

Flc. 6. Functional block diagram of absolute temperature measureme nt moCe.

Harold I. Ewen

KelVin at the output indicator. A simplified functional block diagramsho$'ing the interconnection of RF components with the rypical Dickemodulator to achicve this absolute radiometric mode is shorvn in Fig.6.The concept of operation is predicated on the lact that any noise signalpresented at the input port (1) of switch S will be attenuared by the RFlosses a ssociated with the passive circuitry in the signal path from theinput port of the srvitch to the inpu, por, of the Dicke modularor. Inaddition, these sarne passive RF circuit components will radiate noise asa conseq uence of the lact that they Are at a thermometric temperatureabove Absolute zero. This radiated noise will be combined u'ith the at-tenuated si.-enal and presented to the signal input port (l ) of the Dickemodulator' Since the attenuation of the input signaf .un be characteri zedas a change in the gain of the total system, and the radiated noise of thesecomponents as a noise bias term, the desired calibration of the ourputindicator directly in clegrees Kelvin can be accomplished by a singleadjustment of attenuator .4. Though the laboratory uOlustmeniofattenu-ator A requires a known source of noise power (cold load), a standardrefercnce source of this type is not required for field operation of thedcvice.

III. MICROWAVE RADIOMETER APPLICATIOI{S

The number and diversity oI micro\\'ave radiometer applications todayis 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 theunkno\\'n and provide many historic discoveries. Only a small fraction ofthe knorvledge gained has bcen anticipated. It was at first frustrating tolearn that the brightness temperature of the sun, measured at low fre-q uencies, was more than I ,000,000 o K rather than the anticipated6000" K , that the radio noise from our own galaxy was markedlydiflerent from thc anticipated blackbocly radiatior, rhar spatially discretesources of intense radio energy were present in space and could not beidcntificd w'ith optically observed sources. The impact of these and severalothcr discoveries dispelled the frustration and replaced it with a humblead m ission of the depth of our ignorance. Today, the unanticipated ingalactic radio astronom)'is considered routine. We now knorv we are atthe daw'n of a new era in astronom,v, Accumulating new knorvledge onwhich \\'e u ill build a nerv ancl deeper understanding in the years ahead.

Spaw'ned by' the pioneering and explosive enthusiasm of the youngsciencc clf radio astronomy, the improvement in micrclwave radiometric

EXPLORATI(]\ AND EXPLOI'I'AI-IO\

sensor capabilitics has bccn cqualll'ment sensitivity achieved tod;.r)- at a

that could be done 20 years ago ar adecades, the measurcmcnt capabilitl,than t$'o orders of magnitude. Theentire room, consumine nearll,onetoday is the size of a matchbor andpo\1'e r.

startling. Tirc ternpcrittLlrc ntcllsure-\\'avele ngth of 3 m nr mltche s tirc best\\'avclength erf l0cm. In thcsc tvv'o

at l0 cnr has bccn inr pror cd b1' nrorcinstrument of 30 )'ears ago flllerJ ankilorr,att of po\\'er. Its c()Llnterpertrequires less tlrln llve \\,ittts of irrput

Paralleling the need for improved sensor capabilitl' has bcen the needfor larger antennas of improved surlace tole rrnce to provide grcsrcr lngularresolution at short rvavelengths. with the combincd improrerncnts inangular resolution and tcmperature measurcment capability, the latest tele-scopes 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 thermalradiation characteristics of the planets.

Signals received from space b;- radio tclescopes et wavelentrhs shorterthan 3 cm are contaminatcd by atmospheric attenuation and noise. Thisinterferring source of noise to thc radioastronomers has beco,nc thc signalfor the micro$'ave meteorologist. The \\'ater vapor resonance at ll.l35GHz and the complex resonant line structure of molecular arrnosphcricox)'gen near a rvavelcngth of 5 mm bccame sources of intense invcstigurionbeginning in the early 1960s.

The pattern of historical devclopment in rhe application of rhe radiotelescope u'as just the reverse of u'hat one might have anticipated. fjromthe c.,r'ly observation of galactic radio noisc, follorvcd by rhe obscrrarionof our neighbors in the solar svsrem, the radio telescopc in .iust the pastdecade has been pointed u'ith greatcr intcrcst at the planet eiirth. Thefirst step in this direction has been to obtain an improved undcrsrandingol the physical processes in our earth's atmosnherc. The microulvc rtdio-meter offers the possibility to obrain a global picrure of rcmpcrarurc and$'ater vapor distributions, To sce, in clear air, large conccntralions ofwater vapor and associated temperature gradients, to detect air mass motionby listening to the radio signal from ozone as it moves like a tracer elementthrough the je t streams, to predict n'here the cloudS u.ilt forrn and u.hetherthere rvill be a major storm. are the exploration frontiers of thc microu'avcmeteorologist. A significant amount of fundamental research has alreadybeen accomplished. The initial resrs of rhis neu' potential capiibilitl,havebeen scheduled for satellite experim-ents to be performed in the decade ofthe I 970s.

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

Harold I. En,en

I nci t'\citins areas of tpplication. Our limitcd objective in the discussionr,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 erploratorvIlittui'c of prcsent-dit1'rescitrch in each area emphasizes the ne\\,ness of thetllcli\urcmcnt irrstrtrmcnt. Exploitation of the knolvledge gained u'ill bepaced b1' ollr ability' tct understand u'hat rr,'e learn and our abilitv to de-vclop u:cful s:-.stems bascd on that undcrstandins.

,{. It .\nrr) AsrRolirlt\t y

lJv thc l.rte 195{}s, antenna and rcceiver technologies harJ extendedrlidio telcscopc operation to \\'avelengths shortcr than 3 cm. The milli-nte tcr rcgittn beca me the pioneerirtg cha llenge of the I 960s. Earl), researchin tlic 3 cnr to 3 tnnt rcgion \\'as concentrated primarill' on the cletermi-nation of thc spectral indiccs ol the discrete radio sources u.hich had beenpreviousl)'detected rtt the longcr \\'avelengths. Cosmic radio maps \\,ereextcrrdcd to tltc shorter vv'avelcngths and an intensir,'e search for a tenuousisotropic radiation \r,'as initiated b1' several research groups. The existenceof-tlris rediaticln \\'as prcdicatcd on the theor,v of cosmic evolution u.hichpostulittcs an initial primordiaI erplosion t3]. Verification requires anllh:;,:lute te mperitturc mcasurcment at scveral u'al'elengths, one of the mostdifticr-rlt of-all radiometric nlc:lslrrentents. Partial success has been achievedbl' thc effnrts of severel investigators, placing the present brightness tem-pcr;tttlrc of this tcnuous radiation at a t'Alue appro.ximatel,v* 3" aboveilb:rrlrrtc zcro. Expcrime nts of this t)'pe and others in thc millimeter \\'ave-length rilnge ltre complicatcd by the far fainter cosmic signal levels receivedfrttnt spacc in conlparison u'ith the energy received in tfie UHF and loq,errnicro\\"ilvc rcsion. ()f equal signiflcance is the marked increase in atmos-pher-ic ltttcnuation and cmission at millimeter wavelengrhs [-l].

,-\s \\'c itpproached the last ]'eAr of this decade, one might have saidtlrat thc radio astronomy discoveries in thc millimeter region \\:ere insigni-frcant irr conlparison *'ith the cxcitement produced at longer \\'avelengthsb1' tlic discover-v of cluusi-stellar objects, pulsars, interstellar OH, helium,and several rcsonltnt lines of hvdrosen. As is so frequently the case inracl io itslrononl-v, thc picture suddcnlv changed w'hen in December 1968,:l rt\tlirch group et the Ljniversitl' of California [5lannouncecl the detectioncrl" rltdi<i e rrrission from antrltonia molecules in the interstcllar medium ata \"\;t\ clcngth of I .15 cm. 'f hc linc enrission \t'as observed at a frequencycori-c)p(lnding to the inl'ersion transitions of the -I - l, K - I rotationallevcls in the vibrational ground state of the NH3 rnolecule. The emissionrcgiCrn \\'lls of srnall angular e.rtent, clisplaced to the south from the di-rection of the galactic centcr by approximatelv 3 arc minutes. The lg-foot.,

:XPLORATIO\,{\D EXpLOtT,\TION

diarleter millimeter wa\/elength antenna at the I Ilt Crcek Stati*n of theUnir''ersity'of California's Radio Astronom)- Laborator\,\\,es,_:ed l'ith aDicke t)'pe radiometer in n'hich the reference source at the input com-parison port of the switch was provided b1' an off-axis anrcnne bcarn,displaced approximately 20 arc minutes l'ronr the boresighr of t5e lnrennamain beam' This technique is used e.xtensivell'in millimeter \r"-a'e rudioastronom-v for the observation of spatially' discrcte sources ol- rarjiation.The continuLlm radiation of the earth's atmosphere is intcrccpted b' borhantenna beaffiS, and hence its contribution results in a nulloutput throughthe po\\'er subtracting action of the Dicke su.'itch at thc radiomcter input.As astroph)'sicists \\'ere just beginning ro ponder the significance ofthis startling discovery, the same research group at Berkele' iinnouncedthe discovery of microu'ave emission from water !'apor in the interstellarmedium' This discoverv \vas announccd ,nvithin less than 30 da_i,s follor'ingpublication of their detection of interstellar ammonia. Thc rnicrorvaveemission from water vapor \\,as associated *.ith the 6,0 * 5:., rotutionaltransition ' It \\'as observed in scveral dire ctions in space, one trl*,ard Sgr82, the Orion Nebula, and also in the direction ol- the solrrcc w49. Theemission of H1o in sgr 82 \\'as in trre same direction iceiestiar c.rLr;rdinatcs)in r'vhich emission from interstellar amnronia hacl prcviousll.bcen dis-covered ' The HtO radiation was impressivel,v intense , producing iin antcnnatemperature of l4oK rvhen observed f'rom the Orion Nebula and an ilnrcnnatemperature of at least 55oK from the dircction of thc w.tg s()urce. As aconsequence of the small angular size of thc sourcc rcgions, the Berkeleygroup suggested that the brightness temperaturc ol'thc source in W.l9 mig6tbe as great as l000oK. Measurements performed at the Nar,al ResearchLaboratorl', j"tlarl'land Point Observatory, a Icrv u.ccks lollo\r.ipg the an-nouncement by the Berkelei- group, provided confirrnation br. ni.rsuringan antenna temperature of approximately 1000 " K from the \\/-lg source ,indicating that the actual brightness temperiltu rc nlay bc even h igher. Theantenna at the Naval Research Laboratorv, VIar:yland point Obssrvatori,is85 feet in diameter. The measured increase in antenna tcmperaturc over tiratobtained by the :0-foot diameter te lescope ar rhe Universir),sf (-lliforniaHat Creek obsert'atory closely follo*'ed the ratio o'f the t\vo lnrenna aper-ture areas- Water vapor emission in the direction of' W'l9 is no\\. the mostintense emission line detected in the interstellar medium. Onc cen onlr.imagine w'hat future applications u'ilI be macle of these exo-atmosphericpoint 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'srernreceived 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,

Harold I. Etren

bors, The moon, of course, has been a prime target, Its surface, shape,and general form are wellknou'n from optical observations. From infraredobscrvations we know it's surface temperature and how it changes sodrastically from lunar night to lunar day. with a radio telescope, *'e lookbelorv the surface. The heat u'ave from the sun propagates below thesurface of the moon producing a temperature distribution at the lou'erlevels n'hich is determined by the f'lux intcnsity of the heat rvave and thethermal properties of the subsurface material. The electromagnetic radi-ati<-rn originates below the surface and propagates uprvard and out throughthe surfacc. The longer the rva'clength, the deeper the source of radiationthat is observcd. The thermal inertia of the subsurface material is theprime parameter of interest. The significant data is the brightness temper-ature of a selccted region observed during one complete lunation. Thetechnique most frequently employcd is to obtain a daily map of thebrightness distribution over the entire lunar disk and from these mapsconstruct graphical plots of the brightncss temperature as a function oflunation phase. The important characteristics are the observed variationsin thc amplitude of the brightness temperature and the phase lag in theheating and cooling portions oI the lunation cycle throughout a lunarmonth.

As one might anticipate, the amplitude of lunar brightness temper-ature variations is barely dctectable at a u'avelength of 3 cm and is unde-tectable at meter wavelengths. Radiation at these wavelengths originatesseveral mcters belorv the surface of the moon rvhere the heating and coolingof llre surface during lunar day and night (approximately 27 days) have anegligible effect. As u'e approach millimeter wavelengths, hou'ever, ampli-tude variations in the brightncss temperature are easily detected. Variationsas -sreat at 200oK are typical at a wavelength of 3mm, and 100oK at8 mm. 'I'he shorter the u'avelength, the greater the similarity ol the lu-nation brightness tcmperature u.ith the surface temperature observed atinfrared u,avclengths.

Although the physical rcasoning associated with this area of lunarrescarch may appear relatively simple, the associated experimental, as wellas analytical, problems represent a significant challenge. Among the severalfactors to be considered are the ability of the antenna to resolve a selectedarea of the moon, the effects of surface roughness on the insolation phase,

and the contaminating effects of the earth's atmosphere on the receivedsignal. At a u'avelength of 8 mm, for example, a 30-foot diameter antennaprovides a main beam angle response of approximately 4 arc ninutes, cor-responding to an area approximatel-v 2"10 miles in diameter on the subter-restrial point of the moon. The main beam an-sle of the llO-foot diameterHavstack antenna at the M.l.T. Lincoln Laboratorv observes a circular area

EXPLOR A.I-IO\ ,.\ND EXPLOITA] IO\

60 rniles in diametcr at this u'avclcngth. Since the moon subtends an angle30 arc nrinutes in diameter, as r-lbserved from the earth, scr eral of theforvu'ard sidelobes of either a 30-loot or I l0-foot dirimeter arltrnn& ',r,illintercept the lunar surlace. Although the sidelobrc lcvels lna) tru lovn'incomparison to the nrain beam response, the thermal energ)'radiated by'themoon and rcceived b1,the sidelobe structure is determined bi'thc intcgrelof the brightness temperature of the nroon and the glin cif'thc antennapattern over the solid angle subtcnded b,r' thc nloon.

The lunar surlace is, perhaps, the most troublesorne in thc anall'sisof the observed data since one of the prime objectives is to relate the phasrof 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 smallcentral prominence projecting from the cratcr floor. At lunar sunri::, onervall of thc crate r is exposed to the solar heat u'ave, \\'hilc the othcr remirinsin a shadc)\\'. The central prominence is psrtially heated bl'thc radientenergl'from the u'all expcsed to the ra!'s of thc sun. thc othcr siclc cf tiieprominence remains cool in its ou'n shado\\'. As the sun riscs, ttre craterlloor 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 *'ellas 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. Trlnslatingthis situation into other areas u'hich are pock-markcd u'itlr manv small

crevices, rills, and craters provides an appreciation l'or the challengc inthis erea of exploration. Perhaps the grcatest challenge is the dcr e lopmentof a unitred theorv cApable of erpltining \\'hat u'c sec in thc optical. infra-red, :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'illsoonknou'about the ph1'sical and chcnrical charilctcristics of thc actu;.rl surfaccmaterial from in situ measurenrcnt.- From tlris point of undcr:1.:.lnding,we ffra),'hope to remotelf'probe thc chtmcteristics of other natur.il setcliitcsin our so[ar svstem.

Radio emission from all of the planets. rvith the exccptir-rn of Ncptuneand 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 *'eu'ill be ahle tcl obtain detailed rnaps of the thermal radiation charactcristicsril' these nearby' celestial ncighbors rr hich rernain elusir"c bccau.;t of their:mall anslllar size.

flarold I. Ey,'en

.'\tltlosphcric conterninution ol the sign:rls received fronr the planets inthe rniilirncte r regiott 5uggcsts that scnsor svstcms at these short \\'avelengthstlla!' 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. equippedu'ith scnsors covering thc cntire elcctronlagnetic spectrum. The {rrst historicvcn 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 outerplanets in l9?8. -l"he obvious advantage of planetary space probe is angu-ler r.:solving po\\'er. The gigantic 150-foot diameter radicl telescope atJodrcll Rank in England, lor e.\ample, providcs a resolving po\\'er on thelunltl stlrface equivalcnt to an antenna the size of a quarter in a 100 milelunur rlrbit.

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

680

Frc;. ?. Sun rnap at lr w'avelcngth of 8 mmte lescope crf r hc Ai r Force Cam bridgeature contours are in 'K/10.

taken by the Prospecr Hill radioResearch Laboratories. Temper-

EXPLORATIO\ A \ D E,\PI.OI'TATIO\

radio characteristics has increascd steadill' ovcr thc past I l -1*ur sol*rcycle' Today, gl0bal conlnlunication gricl ne't*'orks tr{lnsmit cilta t'ronrearth based, optical, and radio telescopes anrJ solar orbiting pignce r sate l-lites to the solar actit'itv forecasting cienter located in Ccrloraci* Springs.

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

.t

Harold I. Ev'en

The rc thc data is conl pilcd, red uced, and retransmitted to solar rese archce n te rs th roughout the u'orld. The ntajor efTort at several of these researchccntcrs is to scarch {'or tirc existence of sonle characteristic radio signaturetlutt 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 \\'ave-le nrt it rcsion in thc elrrly I 960s. From prior research, it \\'as knor.r'n thatthe liigh encrs!' proton flares \\'erc associated u'ith regions of densely ionizedclouds iplagc rcgionsj in the l'icinity of the solur chromosphere above thecipticallv observcd plrotosphere. It is in this region of the solar atmospherethitt the bright flash of the optically obsen'cd solar llare occurs at the onseto{" ii major solar event. It \'!'us rcasoned that at millimeter wavelengthsthc obse rr cd brightness temperature w'ould originate in this same general;rlt itudc luver ol' the solar iltrnclspherc since radio penetration torvard thephr-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 provideil de tailcd ntap of thc brightness tem perature distribution. The second

cmirloys u llrge antenne beam encompassing the entire stllar disk to provideir tneilsurc ol-thc intcgrated brightness temperaturc AS a function of time.

f: r:. 9 , r\cruspace Corponrtionplot is in l:i. intervals

3.3 mm sun map for May 25, 1967. The contouru'ith the reference contour denoted b-v 0.

EXPLORAI'IO\ AND EXPI-OITATIO\

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

.t"**titrtl*..*-on

-t- '+^

Flc. 10. Aerospace Corporatirrn millimeter radio tclescope..,

L

v,.*)}t

zt

I

I{arold I. Ev'en

instrulnent is. of course, far more sophisticated than the instruments r.r,hichprovidc an integrated vien'ol'the total solar flux intensity. The antcnnamotion required to achieve the raster scan is accomplished b-v computerpr{)Eramrning of thc drive s}'stem. Thc output data is computer-reducedto a n'lap d isplay ol hrightness tcm peri.lture contours distributed over thesur{ecc of the solar disk. C-omparison of these brightness temperaturemaps. in tinrc sequenct:, clcarlv indicates the growth and decay of activercgions just abot'e thc photosphere. An I mm brightness temperature mapof this t)'pe, obtained by P. Kalaghan of the Air Force Cambridge ResearchLaboratories (AFCRL) Prospect Hill Observatory, is shown in Fig. 7 . Aphotograph of the {AFCRL) solar radio telescope is shown in Fig. 8. Theantenna system is an elevation over azimuth mounted 30-foot diametercasscgrain. Solar brightness temperature maps obtained at a wavelength of3 mm are shorvn for comparison in ltig. 9. This 3 mm merp \\'as recordedb-v the Aerospace Corporation radio telescope located in EI Segundo, Cali-fornia. A photograph of this ecluatoriallv mounted l5-foot diameter casse-grain ilntenna is shown in Fig. 10.

B. Il rcnow;\vr METEoRoLoGY

\'licro\\'ave meteorolclgy is in part an outgror.r'th of radio astronomyand more rccently has become closely allied rvith atmospheric propagationstudies necded to de termine system rcquirements for earth-space communi-cation links at millimcter \\'avelengths. One area of millimeter communi-cation svstems research is the measurement of atmospheric attenuation andnoise chnracteristics in the "windows" betr,veen the rvater and oxygen res-onanccs. These resonant lines occur at nominal frequencies of 22 and60 CI{z respectively.

,{ second area of research in microwave meteorology is the measure-ment of the characteristics of at mospheric resonant lines themselves tocapitalizc on their unique properties rvhich can be exploited for the purposesof 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-breedingbetw'een research groups in communication, radio astronomy, and micro-wave meteorology. Celestial radio sources, for example, offer excellentexo-atmospheric tragets of opportunity for the communicator to investigateatmospheric propagation characteristics b,l' satisfying the requirement fora transmitter of knorl'n flux intcnsity and position located beyond thecarth's atmospherc. The validity of such measurements is, of course,predicrtcd on ilvaila ble know'lcdge concerning the characteristics of thesecelestial transmitting sources. As a consequence, el substantial portion of

EXPLORATIO}J AND EXPLOITATION

our present-day knowledge conccrning the millimeter charactcristics ofcelestial radio sources is being provided bl' research scientists primarilyconcerned *'ith the communication aspects of the atmospheric medium,

The common denorninator in each discipline is the micro\\'ar.c radio-metric sensor. The xtmosphcric medium is,'of course, another commondenominator. I-lou'ever, depending on the'rcsearch discipline, thc electrcl-magnetic characteristics of the atmosphere ma.v be cCInsidered as eithcrsignal or noise.

In the discussion r,vhich follow's. \\'e u'illconsider first thc mcasuremcntof atmospheric at.tenuation and noise in the "u'indo$'s." We *'ill thenrevie*' present-day exploration of atmosphcric gas lesonant line structureu'hich is located betw'een the " \\'indow's" and forms their boundaries. Tu'oareas of application in r.r'hich present knou'ledge concerning atrnosphericresonant, line structure is being exploited n'ill be revieu'ed in the thirdsection .

1. Measurentent of .4tmosplrcric Attenuation and iVoise

At wavelengths shorter than 3 cm, electromagnetic \\'Aves can be

severely atte n uated by rain. A possible solu tion for earth-to-satcllite com-munication 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 tothe diversity ground station. The minimum separation betu'een the nvoearth based terminals requires a quantitative determinartion of attenuationstatistics. The accumulation and analysis of the required statistical data iscurrently be ing pursued at the Air F orce Cambridge Reseilrch LaLroratory,the Betl Telephone Laborator\', and the NASA Electronics Research Cen-ter. [n lieu of the availability of a man-made satellitc to pror,'ide a cali-brated exo-atmospheric transmitting sourcc, the sun is uscd as a target ofopportunity since it is an exo-atmosptreric source of known llur intensity.Because of the relatively low'signal level received from the sun even u'henunattenuated, a radio telescope is required for these measurements. Thenoise level received from thc sun as a function 'of \veather conditionsdetermines 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, theradiometric sensor is pointed at elevation angles of interest, and at-mospheric cffects arc derived from-obscrvcd variAtions in thc sky' noisetemperature.

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

Harold I. Ew,en

rs: Iryir', exp (- ') + (r exp I -i\r(- T))Irxr'ldQI

(16)

rvlre 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 meanthcrmometric temperature of the atmosphere along the path of observation.when the antenna beam is pointed a\\'ay from tlre sun toward somecold poin t in the sky, the obserred antenna temperature TA is gi'en by

TAJ4; (r7)

Ncrri' if the obscrvations of, Ts ancl 7.1 are made in sequence at the sameele'ation angle by pointing the antenna beanr first on then off the sun, theantcnna 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 $'ill be essentiall,v constant over the small angle subtended bythe sun. the observed antenna tem peratures u,hen the ante nna beam ispointe d to$ ard the sun and the n ton'ard the sky can be w,ritten in theI rlrm

TS

T'A--: [1 exp (- r) llo"For clear \f,'eather condilions, one can obtain a measurc of the atmos-

phe ric gas attenuation by assuming a uniform hori zogally stratified modelatmosphere' Under these conditions, rhe atmospheric opacity r can berec'xpressed in terms of the opacity observed in the zenith direction ra,r.r,'h c re

(2 l)and 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-rangc terms and obtain the e.xpression

It - f,*, - exp {-rosec d.) tzz)T,t Tslv

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

(l a;

(1e)

(20)

tT7--1To

L I S lskv -,

cos d

.t

(23)

EXPLORATION AND EXPLOITATIO}.i

Alternatively, the value of ;o can be determined from the slope of the plotof 7's - 7"r*, versus sec d..

For nighttime observations, a similar method can be applied t.o deter-mine 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

I d(T 4 r,*" )

T _Tt ,4 'sky

For large values of r, the assumed value of the mean thermometric temper-ature of the atmosphere T,ry becomes critically important. Onc can makethe general observation that large values of attenuation are more preciselymeasured in absorption b:* using the sun as an exo-atmospheric source,while small changes in the value of atmospheric et tenuation are nloreeffectively sensed by sky temperature measurements obtained under con-ditions of relatively lorv attenuation values.

The determination of atmospheric atten uation characteristics fromsky temperature measuremen ts has bcen extcnsively use d and developedto a high degree of sophistication by the research group at the Air Force

Cambridge Research Laboratory, under the direction of E. Altshuler. Inaddition to equipment located at the Prospect Hill Observutory in Waltham,Massachusetts, the AFCRL group operates a dual frequenc-y nleasurement

Ftc. ll. Air Force Cambridge Research Laboratories d'ual frequenc-v (15 GHzand 35 CHz) radiometric measurement s)'stem located at luount Hiloin the Hawaiian Islands,

t0 f rl)d sec 0,

Harold I. Ev,en

instrumcnt at I-Iilo in the Hau'aiian islands. A photograph of this instru-mcnt, 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, andthe cornucopia antenna attached to the side of the enclosure is adjusted ineievation from thc operator control console located u'ithin the enclosure.

A nricrowave radiometric sensor sy'stem assem bled by' the Bell Tele-phonc Laboratories for the accumulation of atmospheric attenuationstntistics at u'avelensths of 8 mm and 2 cm is shou,n in Fie . 12. The

Frc. The Bell Laboratories sun tracker in Holmdel, Neu' Jerse;- is used totune in on sun signals at tr'o radio frequencies. A 5 x 9-foot metalmirror 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 re-ceivecl. :3,'68. r

5 X 9-foot plane rcllector is attached to an equatorial mount. The decli-nution angle of the reflecting plane is adjusted so that the sun's ravs arerellccted into the,l-foot aperture conical horn reflector antenna. The re-flcctor is driven in the hour angle coordinatc b,v a clock mechanism w'hichitssures that the sun's ravs are continuously reflected into thc apertllre ofthe conical horn reflector. Throughout the observing period, the antennabeam is scanned on and oll thc sun at a I FIz rate, r'u'ith an angular ex-

cursion of 2.6*, b1' nrechanicalll' tilting the re flecting plane in the decli-.t

i4iplK

II

EXPLORATION A\D EXPLOI TATIO\

nation angle coordinate. Automaiic opsration is another uniquc f-eature

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

The Propsgation Studies Branclt at the NASA Electronic Rcscarch

Center, under the direction of L. Roberts, undertook a similur scries ofmeasurements, beginning in 196i. The ERC atmosphcric propagationmeasurernent system is shou,n in Fig. t3. Simultaneous ob:serv':ttions areobtained at wavelengths of 3 cm, I crn, 1.5 ctn, and [J nrm. The micro-\\'ave radiometric sensors at cach \\'Avelcngttr are instrtllcd on individualequatorially mounted 5-foot diame ter searchlights. Se \:crAl modcs of oper-

f'-i{t

rllr

,4

ffi::'::"trrrr* T iff'

;?ts:+

4

ar"

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

Harold I. Ew.en

ation arc incrudcd in thesc radiometric s'stems: antcnna bcam s*,itching,absolute tempcrature., and cxprod.a "ni.nn" beam comparison. The ex-plodcd bcam comparison modc prouiae,

" oifferential temperature measure-mcnt betu'een t\^.'o on-axir antenna beams, one narro\r, beam boresi_ehtedon the sun and a much rarger beam o'hich obtains a negrigible conrributionfrom thc sun.

Although the prinre objecti'c .f nreasurements or this type is todetermine the statistics of atmospheric attcnuation and noise needed lorthc design of future earth-space communication rinks. ttre knor'teogegained 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 lherain cores rvithin clouds is accompristrea at the longer rval.erengths. De-tection of high-artitude variations in u,ater vapor, prior to condensation,can be sensed at the shorter u'avelengths. The insirunr.nt"tion requiredt'or these measurements is markedr;- slmirar to that described above forcommunication systems research. From the routine dairy accumuration ofattenuation statistics may evolve simirar instruments used b1. nricrorvavemcteor.r.-eists to dcterminc the "uh-v" or thcse ,ruti-rti.r- '' "'

2. 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 strongabsorption lincs at 1.3-5cm anC iO-t mnr. as u.ell as several stron-q linesatsubmilrimeter rvaverengrhs. van Vreck[or carcutatca ir,. n.'"gniruo" orthc l'35cm rine and the contriburion from a'other lines. comparisonof his resulrs rvirh laborarory measurements by Bccker and Autrer [7] andIro et al ' Iltl and *'ith atmospheric obscr'arions by Straiton and rorbert [9]shor'ed a substantiar discrepancl-. Bl adjusting the contribution from allother lines' the u'ater vapor absorption formulas summarized by Barrerrard Chung I l0] represent the best arailable approximation for the l .35 cmline at temperature near 300"K.

oxygen has a comprex spectrum, consisting of a band of resonantlines in rhe -5 mm u'a'erength range and ai isorated line at r.5 mm. Linelrequencies and bandrvidths hav'i been measured in the laboratory atprcssures up to l armosphere by'Artman and Gordontrr] ani Andersonet 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,arerand Strand [14j.

EXPLORATION A],JD EXPLOITATION

The interaction of ozonc *'ith micror'a'es is weak in conrparisonrvith either oxygen or water

'apor. Several resonant lines, ho*.er.er. arcpresent throughout the entire 3 cm to 3 mm u.avelengrh re-eion. Gora Il,s]has calculated the frequencies and intensitics for all iignificant lines of therotational spectrum ol ozone at lrequencies belorv 2700 GHz.

The application of micror.r'ave radiomctric sensing of the u'eter 'aporresonant line is presently being exploited because it offiers the unique abilit;-

to yield a measurement of tropospheric water vapor in thc prescnce ofclouds. Although a satellite borne *'atcr vapor scnsor *.ourd be cffectiveonly over oceans, oceans cover more than half thc earth's surfacc and arethe spawning grounds of ma-ior storms. Micr.u,a'e radiometric sensing ofthe u'ater vapor resonance under clclr rvcather conditions pcrmits themeasurement of integrated water vapor abundances and sprtial siz-e distri-butions.

Interest in the oxygen resonant line characteristics ncar a rvavelengthof 5 mm has been stimulated by the fact that microwavc radiometric scnsingmay provide the only remote se nsing technique capable of measuring armos-pheric temperature profiles in the prcsence of clouds. This ir.ould bc ofconsiderable importance to global data collection for numerical u,eatherprediction. This technique offers the potential capability of mcasuring thetemperature profile from thc lou'er troposphere rvcll into the nresosphere.

Microwa'e radiometric measure mcnt of the atmospheric ozone distri-bution has progressed at a slo*'er pace than studies of either oxy-gen orwater vapor. Instrument technology. rather than meteorological intcrest,has set the pace in this area of rcscarch. ozone plays an important rolein the organic and inorganic chemistry of the surface of thc earrh. Througha filtering action, it absorbs a lethal part of thc ultra'iolct radiation fromthe sun, thereby making life possibre on the surface of the earrh. ozoneis also an important factor in our clirnatolog.r-, cstabrishing rhe balancebetrveen exo-atmospheric radiation incident on the earrh and thc outgoingradiation from the earth, as a consequencc of its particular lhsorprioncharacteristics in the ultraviolct and infrarcd regions of thc spcclrum.Knou'ledge of the atmospheric ozone distribution in thc alrirude rangefrom l5 to 60km, obtained on a global scale. offers rhe ptrs:ibilirl .fmeasuring air mass circulation as a conscquence of the fact thur ozonc inthe lor.'"'er portions of the atmosphcre ma1'be consiclered an inert gas andits global distribution rvith time is. in large. dctermineel b1.rhe horizonralmotion and interaction of major air .masscs.

As a preface to a rel'ie*'of thc currcnt status of crplorrtion .f atmos-pheric gas resonant characterisrics. it u'ill bc helpful.ro recall rhe rclaiion-ship betu"een antenna tempcrature ancl thc elrective brightness tcrnpcratureof an observed source of radiation. Since thc atmosphcre throughout most

i

Harold I. Ex'en

r-rf the 3 cm to 3 mm \,\'avelength region is semitransparent, the equationof radiative transfer can be used to relate the brightness temperature Tu0)at the frequency v to the atmospheric composition and the temperatureT'(si along the line of sight and to the brightness temperature TE of thebackground me diu m beyond the atmosphere. The equation of radiativetransfer is expresscd in the form

T6iv) * Tnexp [- r{y) ] + sld{s) (25)

ln t q. {15) rtu'} is the total opacity of the atmosphere and a(r,, s) is theabsorption cocfficient. Inspection of Eq. {25) shou's that the observedbrightness temperature in any given direction is the sum of the backgroundradiation and the radiation emitted at each point along the path of obser-t'atioil, cach component attenuated by' the intervening atmospherc. Theantenna temperature observed by a microrvave radiometric sensor lookinginto the atmosphere is, therefore, primarily determined by'the atmos-pheric absorption coelficient and temperature along tl're path of observation.Sincc the integral of the product of the exponent and the absorptioncoefficient in Eq. (25) dctcrmines the contribution of the thermometrictemperature along the path of obserl'ation, to the observed antenna tem-perature. 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 resultsfrom llne structure transitions in rvhich the magnetic moment assumesvarious directions with respect to thc rotational angular momentum of themolecule . The un paired spin s of tw'o electrons produce the magneticdipole moment of oxygen. Van Vleck [l6] was the first to develop theexpression for the frequency, pressure, and temperature dependence of theoxygen absorption coefficient. This early work was reviewed by Meeksand Lilley [13] in 1963, and later by Gautier and Robert [7] in 1964, andLenoir Ilf{l in 1968.

The complex of oxygen lines, in particular the atmospheric absorptioncoefl-icient as a function of frequenct- and altitude of observation, is shownin frig. l1 {Mceks and Lilley) . The general form of the rveighting functionsfor sclected freq ucncics, as computed by Meeks and Lille;-, is shown inI=ig. I 5 .

It is important to note that the expression for the oxygen absorptioncoeflicie nt has been derived from quAntum mechanical considerations inri'hich the value of certain constants has been empirically selected to pro-vide the best agreement rvith experimental data. If it \r'ere possible todirectly' measure the absorption coefficient for all possible meteorologicalconditions of interest, the quantum mechanical approach could be dis-

;

ft*ur , fs

-I, r{s) exp

E{-ogcIofc(u

EXPLORATIO\I AND EXPLOITATION

DU 55 60 65 70Frequency (GC /S)

Flc. 14. The computed attenuation coefficient f ir: for air at three represe nrativeheights. This figure sho*'s that the individual oxygen lines completelyoverlap 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 oRthe direct measrtrement of oxygen line profile characteristics ro provide animprovement in knowledge concerning the absorption coefTlcient values.Laboratory measurements have been performed by Stafford and Tolbert t l glat the University of Texas, and balloon measuremenrs bf' Lenoir I l B] arthe Massachusetts Institute of Technology. '

Reber et al. [20] of the Aerospacc Corporation recentlv reported avery detailed analytical stud-v of this problem, supported b1, exrensivr3measurements performed in a high-altitudc aircraft. Their published valuesare, perhaps, the most comprehensive and complete a\,ailable today.Measurements, utilizing the sun as a source, were rnade at sir discretealtitudes ranging from sea level to 13.7 km. These measureme nts coyeredthe frequency range from 52 to 68 GHz (see Fig. 16). The more than1500 independent attenuation measurements \r.ere used to calculate new

ZotrU7:u-()4FIIL1 J

0.08

0.04

0 .00

0 .08

0.04

0 .00

c.08

0 .04

0 .00

0.08

0 .04

0 .00

c .08

0.04

0 .00

0.08

0 .04

0 .00

l:tc:. I 5.

Harold L Ev'en

80

HEIGHT (krn)

\\'eighting f unctions for clctsrmination of the brightness temperature as

a \^'e ighted a\erage of the kinetic tempcrature distribution. Weightingfunctiorrs arc show'n for six repre5entatil'e frequencies. iMeeks andLille."- i l3l ,

U

T (0")

T (601

& .37 gc/s

22f K

2330K

9=6o0 u = 59.30 gc/s

T(oo) = zwoK

T(ooo)= zztoK

u = 57 .80 gc/s

T(oo) = ztfrc

T(60o)= 2lBoK

v = 56.60 gc/s

T(oo) = 2lgoK

t(eoo) = ztfr

u = 55.40 gc/s

TA(0") = zz3aK

To(ooo) = 2lBoK

t/ = 54.30 gc/s

TA(oo) = 23oot<

To( 6oo ) = ?zf K

I'Ci --'

rd;3 n't

= 6\., ..'"'.

' .,t9 t!6

o)

EXPLORATION AND EXPLOITATION

I ti''l l ii ri i' xo - io" s'it ' rll uc

t{*rft. ir.l

-. t ;

r i i-i.ij i!r$ lto lfn lE

;llt.':l.i I

lla 1.9 $i| -",, l--. ^

sc lt1 ilj

'-r*,.-t{'

T' :g-Tr

.*ilr'

.-r,- -,' -

Ftc. 16. [.y features of the investigation of molecular atmospheric o,\ygencharacteristics performed by the Aerospace Corporation . tai The corn-puted resonant profile characteristics of atmospheric ox!'gen as a func-tion of the observers altitude over the frequenc.v rangeTr."tm 52 to 68GHz' (b) Five-millimeter wavelength radiometric l.rsoi antenna s1'stemassembled for the aircraft measurement program. {cl RF portions ofthe 5 mm radiometric receiver used in the aircraft measurement pro-gram. This unit was ptrysically located directly behind the anrenna.(dJ The 5 mm radiometric sensor instatled in ihe aircraft used in themeasurement program. The antenna can be seen behind the qlrartzwindow installed in the skin of the aircraft, just forwarrJ of rhe u ing.

Harold I. Ewen

values for rhe van vleck line-broadening coefficients. Zenith attenuations

were computed utitizing these new coefficients over the frequency range

4g to 72 GHz and for several altitudes from 0 to 25 km. In addition,

both horizontal attenuation rates and tangential attenuations through the

atmosphere u'ere computed for sevcral altitudes'

b. |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 non-

resonant term which corresponds to contributions from the far wings of

all of rhe lines at other frequencies. The most recent analytical and ex-

perimental work has been by Staelin [22] and Gaut [23]. Their experi-

mental measurements were performed using a 28-foot ap€rture antenna at

Lincoln Laboratory in Lexington, Massachusetts with a S-channel micro-

u.ave radiometer. The five selected frequencies were observed simultane-

ously 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 distribu-

tion 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 ofthe 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, wh€re water vaPor

is most adundant, is close to the earth ambient at the surface.

c. Ozone. The principal quantity which determines the transmission

EXPLORATION AND EXPLOITATION

coemcient of the atmosphere and the emission or effectivc brightness tem-perature 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 ab-

sorption 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 inemission 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 measure-

ment 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 in-ferred from the infrared measurements by Walshaw and the laboratorymeasurement 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 ofozone concentration relative to the total air content at various altitudes [3 l].Certain disciplinary relationships are significant since there is a commonuse of terminology in the concept of " weighting functions. " The approachto ozone data analysis provides a simplification in measurement instrumentrequirements since an absolute temperature measurement is not requiredat a single frequency in order to deduce the ozone concentration. The

difference temperature between measurements niade at two frequencies isused 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 sig-nificant variable parameters may be expressed in the form

(Au)2 -:

(v vo)' * (Av)' *;

att) cc T-\ ' tt' IAv l-|'16t

Harold I. E*,en

r^"hcre ;\n, is the ozone conccntration. Ay is the line half-u,idth, y is thct'recluency of observation, ancJ Ds the line frequencv. The line rvidth isproportional to pressure, n'hich is in turn proportionaI to the product of thetotal 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 tv),\tr -*

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

\\t-,'-: : B(Ay) '

{Ayl: -('t -7\

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

\vt vo) b: yil :- {Avll

(2 8)

(t e)

Since the observed brightness temperature at an]* single frequency ofobser'"'ation is proport ional to t he in tegration of the absorption cclefficientaiong t he path through the atmosphere, the ozone concentration can bedcrived from a dilTerence temperature measuremen t at trvo frequenciesrvhich define thc attitude limits of thc observed lzry'er. A graphical plot ofsir single frequencv u'eighting functions is sho\\rn in 1tig. l7a. The corre-sponding diffcrence w'cighting functions arc sholl'n in Fig. l7b. The three<lifl'e rence frequency *'eighting functions sho\\'n in solid line are obtainedfrom the differences of the paired sets of the four single frequency u,eight-ing functions, also shou'n in solid line in Fig. l6a. The single frequencyrveighting functions at 2 and 20 MHz (shown dotted), which combine toform the difference frequenc!'weighting function at 6.3 MHz (dotted) showthe degree to u'hich difference frequency weighting functions at 2, 6.3, andl() I{FIz tend to overlap and thereby provide interdependent samples ofthe atmosphere.

The development of the rveighting functions in Fig. 17 demonstratesthe 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 andmiddle difference freq uency u'eighting functions, 3s u,ell as for both themiddle and lorver difference frequency \\'eighling functions.

rt1uu-lt-LI',:otlJ..t

o:)

u,rrj

rlJ:orlll

o3

EXPLORAI-ION AND EXPI-OI"TATIO\

-2 l"lHz

-6.3 MHz

20 t*rtz

-200 MHz

0.4 0.6 0.8 t.0

Ftc- l7 - \l'eighting functions of atmospheric ozonc. iaJ Norm;rlizetJ singic fre-quenc-v weighting functions lV, \,ersus freque ncy displacement from theline frequency. (bt Normalized difference frequenc-y- r+.eighting func_tions W'ur*:,2 Versus freque nc]' and altitude.

The natural limit of the diffcrence weighting funcrion hall'-rvidrh isnot immediately' apparent from the example of atmosphe ric ozone weight-ing functions shown in Fig. 17. This natural limit is shorvn graphi.rttyin Fig. 18. Recalling that the observed temperaturc at anln frequency cifobservation is proportional to the inregral of the absorpt.ion coefficientover the ra)" path of observation, a factor C(trj, represenrrng all terms inthe integrand other than those in the weighting function, takes the typicalform shown in Fig. 18a for standard ARDC model atmospheric tempera-ture and pressures and for values of atmospheric ozone cCIncentration ascalculated by Hunt [321. The form of C(h) is primarily determined by thefactors outside the bracket in Eq. (26). From Eq. (28) , it is apparenr tharthe 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 weightingfunction 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 frc-quencies of observation required to define a difference freque nc!, u,eighringfunction approach the corresponding " frequ€ncy of the u,eighring func-

(o) (b)

Harold I. Ev+'en

vt&dUJ

tl,

*soUJvd40o;)

5go

c (ro-1 Ho lf-w idth

(b)

II

of W v, - uz (km)

(o)

0.5Alrirude Width of W ,r- r, (kr)

(c)

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 frequencyozone weighting function vs (u1 - vo) l\rz - vd . (c) Normalizing factorB 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

rcsponse). ; ,

(d)

l/t-uo = 2MHz

Uz-Uo = 4MHz

EXPLORATION AND EXPLOIT,.\TIO}{

tion," the minimum half-u'idth of the rveighting function becomes 14.5 km.The minimum width of the difference rveighting function for useful datacan 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 conrainedunder the area of the weighting function u'hen plotted as a function ofaltitude and amplitude. The graph of i9 vcrsus the',r'idth o f H''-r*,., is shorvnin Fig. l8c. It is evident from this graph that the minimum usable u'idrhof a difference frequency weighting function is approrimately 15 km. Agraph of a difference frequency '''veighting function near this optimumwidth o[ 15 km is shown in Fig. l8d. The corresponding frequencies ofobservation for the single-frequency weighting functions are displaced fromthe line frequency b"v 2 and 4 MHz respectively'. Comparison of the shapeand width of this weighting function (Fig. 18d) rvith the rveighting func-tion shown graphically by the dotted line in Fig. l7b indicares thar a

weighting function developed frorn observing frequencics displaced 2 l!{FIzand 20 MHz from the center frequency of the line prc)vides nearl,v thesame definition of the atmospheric layer as that provided by' the combi-nation of frequencies displaced 2 MHz and 4 MFIz from the line center.This tends to suggest the value of using intermediate frequencies ol obser-vation to perform a dual role in the derivation of ozone concentration foradjacent weighting functions.

It is apparent from the foregoing that, at most, four independentsamples of atmospheric ozone concentration can be obtained. The mini-mum half-u'idth of each sample layer rvill be of the order ol' l5 km. Ameasure of the ozone concentration in any selccted layer is computed frommeasured temperature differences at two frequencies.

The microwave radiometer used by' Caton et al. [30] rvas a doubleconversion superheterodyne. First intermediate frcquency' arnplificationwas provided by three travelling \,'ave tubes in cascade \\rith an instantane-ous bandwidth of 2 GHz centered at 3 GHz. The input signal to thesecond converter was coupled from an intcrstage transmission line betw'eenthe second and third traveling \r'ave tubes. Six second interrnediate fre-quency amplifiers were provided in the form of five contiguous filters, eAch

10 MHz wide, and one filter covering the entire 50 MHzband. A seventhbroadband 2 GHz response was derived from the output of the thirdtraveling wave tube. Absorption measurements were performed using thesun as a background source. The gomparison load in the Dicke mode ofoperation 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-

Ha rold I . En'e n

:;.94.p

;Jl

L..G*:4;g

, rlrl;|}t\:

i "'r,.I rl+:

*.r, ir-.., -"t.

h' .*

fi*.il *.

[i.'.t;--tt {-l?.,.

,?','4+

| 2i,{'i---'-' - 'rXf -.1

: .-l;:ri j

i*-'-*--*:t^,: f, -,j*ll .i>.,* '+r

tt ,. ..* _ir:- **- ." *rq: .L.,".'

'la'.,:ii-';--.-.].

Ia ". .*rt ..r..

Flc' 19. NASA Electronics Research Center atmospheric ozone raOiometricsenior 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.

;,

EXPLORATION AND EXPLOIT,\]-IO};

lizing the output from the various channels by discrirninating against smallvariations in the observed sun antenna temperature produced b.v cloudsdrifting through the antenna bcam during the pcriod ol'cbseryation.Even under clear \\leather conditions, \'il riations in the sun antenna rem-perature 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 feed-back loop was inoperative. -fhe antenna \\,as an equatoriallv molinted5-foot searchlight. Tracking of'the sun \\'as provirlcd bl' e s)'nchronousclock mechanism. Photographs of the antenna ancl control console areshown in lrigs. 19a and b respcctivcly.

3. Related Areas 0f Appticutiott

The absorption charactcristics of molccular atmospheric,,;qr,'gefi oll"ergreat 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 arresonant Iine frequenc-ies near 60 GY'lz. SatclIitc-to-satclIitc communicatirinat thcse frequencies would be free of nliln-ntader noise originuting at thecarth's surface. A colnmunication link of this t)'pc ri oulrJ alsg bc unde-tectable at the earth's surlace. Cclnlrnunicatitln links bctriecn high-altitudcaircraft, operating at frequencies in the ri'clls bct\r'ucn resontrnt line.\, u'0ulden joy the sa tne bcne {i ts.

The ability to vieu'tlre earth lrorri srrtcllitc urbit w,irh a ladiorllutricsensor and see a uniformly bright matttlc in this \\'avelcngth regic',n lutssuggested the possibility of an earth vcrtical sensor more accurirLc rhan anIR horizon scanner.

Another possible application is thc abilitl'r6 rclnrttcll,rcqse r"cgiilpsof clear air turbulence (CA'fi in the foru'ard llight parh ot' supcrs6nic highaltitude aircraft. A millinretcr \\'ave racliornctcr, tunccl Io the ox],gen\\/avelength band, rnay pror,'ide this capabilitr-'. Ternfcr1rrurr anonraliesappsar to be associatcd u'ith C'A-f rcgions. -l-he rangc ar ri iricl: ir "niilli-meter \\'aVe thermometer" is projectcd l'or*'ard of rhc lrircnil't altrng irsflight 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'piiolcoefllcient.

The satellite r:artlt-r'ertical scnsor and the CAl" cletcctcr et;nccflI urediscussed 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,einggained and applied to inrprove prescnt ci-lpabilitics rlirrlugh nr\\'conccprsand techniques.

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

Harold I. Ev+,en

the s1'mmetr-v and stability of the earth's infrared horizon about the localsatcllite vertical. The performance of an IR horizon scanner is cleterminedb1' natural linritittions' [t has been suggested t33] that the molecular at-m0spheric oxygen mantle might offer a superior reference for earth verticalsensing' Although the system concept of rim cutting used in IR horizondefinition could be applied to sensing of the molecular atmospheric oxygenhori zan mantle, the antenna aperture size to obtain an equivalent pencil

TE MPERATURE (OK)

Ftc' ?0. Typical atmospheric temperature vs height profiles observed in Januarvat latitudes of li, 30, 40, 60 and 75o north.

JA NUAR Y

\\i\l--

EXPLORATION AND EXPLOITATION

beam would be unreasonably large for satellite application. Fortuitously,,the thermal radiation characteristics of molecular atmospheric oxygennegate the need for a rim cutting technique.

The relationship between the oxygen emission spectrum and the te m-perature as a function of altitude above sea level shows that the observedemission is frequency selective and represents the average temperature inan atmospheric layer of air approximarely 7 to l0 km deep. The mean

T E M PERATU RE ('K)

Flc' 2l ' Typical atmospheric temperature vs height profiles obserr,:tJ in July atlatitudes of 15, 30, 45, 60 and 75o north.

Harold I. En'en

height of the observed layer is determincd by rhe frequency of observationanrl the observation angle relativc to the nadir. The basic concept of a

molccular atmospheric oxygen vertical sensor is predicated on selection ofan observing frequcncy u'hich provides thermal sensing of the atmospherictemlerature at an altitudc {determined also by the obse rving angle relativeto 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 temperaturehcight profiles for the month olJuly are shou'n in Fig.2l. With the ex-

ccption of the rcported high temperatures during rvinter in northernlatitucles. one rvould anticipate temperature variations of approximately:l0'K from the poles to the equator in the altitude range from 25 to35 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 rocketprobcs. 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

atntospheric oxygen are such that one can sclect a frequency of observation

to obtain a temperature sounding at any desired altitude from sea level to'approximately 75 km. The "rveighting function" for a particular fre-quency of observation describes the altitude interval-temperature contri-bution for that frequency. Asshown in Fig. 15, thedepth of the weightingfunctions tends to increase with altitude. All weighting functions shownin Fig. I 5 are associated rvith frequencies located between oxygen linercsonant frequencies; i.e., they are located in line "wells" as opposed toline "cores." It is of intcrcst to note, in reference to Fig. 15, that themcan altitude of the u'eighting function increases rvith the angle of obser-

!'ation off the nadir direction. The increase for a 60 arc.de-eree zenith angle

is of the order ol 5 km as shorvn in Fig. l-5. The mean altitude of therr,cighting function incrcases sharply as one approaches a zenith angle ofthe horizon as viewed from orbit. At 60.8 GHz, for example, the altitudeof 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 operatingfrequency such as 60.8 GHz in thc "ivell" betu'een line "cores" eases re-

strlints on frequency stability, while at the same time providing a u'eighting

function altitude in the l5 to 30 km range, $'here temperature variations

EXPLORATION,AND EXPLOITT\I IO\

rvith latitude are of rhe order 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 ceittroidsensing. This pitssive micro\\'ave technique \\'as clcveloped :hortl)'af'terWorld War I I and has undergone scveral gcneric advancenle n ts since thattime. A major area of application has bccn in the design ol- r'adiomcrricsextants used for the navigation of mobile vehicles {primarily,'ships andsubmarines) under foul \\'eathcr conditions. Cclcstial raclio sources suclias the sun, moon, and radio stars are used in thcse applicatir;ns. In thcradiometric' sextant mode. the projection of the antennr bcarn on thecelestial sphere is invariabll'much lergcr than tire solid anglc subtenclcdby t.he celestial source. The position of the source in crrrhogonal coclrdin-ates about the radio boresight of the antennt is dctt:rmined b1'cornparingthe power received b1, dual antcnna bcams in either coordinat.e. The t\r,obeams are usually'displaced to provide a 3 dB responsc on thc anrennaboresight axis. The general form of thc anglc-tracking error funcrion ar.

the output o{' the radiometric receiver in either coord inatc is independentof the method of angle sensing; holvevcr, the achier':lble signal-to-noiseratio is critically dependent on thc modc of operation and, hcnce, deter-mincs the rnts angle tracking accurac!'.

The developmcnt clf the expression for the anglu-tracking accuracyachieved in a radiometric scxtant confrguration u'ill bc he lpful in theanalysis of the earth vertical sensor concept,since the lattcr is a degeneratecase of radiclmetric sextant thermal centroid tracking. In the certh verticalsensing application the antenna beam anglc is srlri"lllcr than thc solid angleof the celestiul source iearth) .

Thc form of the anele error function in either coordinate in the radio-metric sextant mode is sho\\:n in fig. 22. Note rhat rhe solid angle cf tliesource 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 responseon the boresight axis. A characteristic "s-curve" is devcloped as tire sourcepasses through the line of ccntcrs of the two bcams and thc boresightaris. The S-curve is derivcd b1'subtracting the po\r'er received b)' antennabeam B from that received bi" antenna bcam A'. Noise lluc'tuations as-

sociated n'ith the thermal noise characteristics of the targert and inhcrentreceiver noise are superimposed on the S-curve. The rms angle trackingaccuracy can be derivcd from the follou'ing geometricl I consideratiotls.

The slope of the st raight linc connccting thc pcuks of thc S-curvcn-ith thc targer source on boresight is

Straight linc slopc(],

t 3t"ti

Harold I. En'err

(P* to P*) S

eA --fFtc. 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 theirhalf-pnwer points on'the boresight axis of the antenna system. Theanglc error curve is obtained by subtracting the power received b-vantenna beam B from that received by antenna beam A as a celestialsourcc isun. moon, or radio star; passcs along the line of ccnters of thet*'o antenna bcams. The "figure-of-merit" of the curve is defined as theratio of the peak-to-pe ak amplitucle ipx to px) s to the peak-to-peaklevel of receiver noisc fiuctuations iPt: to Pxl FL, superimposedon thecurve.

rvlrere tPx to P")^t is the pcitk-to-peak amplitude of the S-curye and 0.4 isthc ;rnsular scparation ol the t\\'o peaks of the S-curve. f1rr a cos2 antennaaprrtLlre illumination, the slope is ril times greatcr than the sr.raight lineslope. or:

S-curvc sloPe -:- T,/2 {P^ tcl P")S0,1

Tlrc rms angle rracking e rror at borcsigh L 60 ^, multipliedthc S-curve At boresight, is eq ual to thc rms value of thecollrp()ncnt sup(]rimposed on the S-curve, or

i3 t;

by the slope offluctuat ing noise

EXPLORATIO}i AND EXI'I-OIT.\TION

; i2 {Pr - P^ )'s of\ -' 0o

(F,, to P"\F[- r111J ,i

level supcrirnpcsed\\'here tPx to Pr') FL is the peak-to-peak noise fluctuationon the S-curve. Therelore

luvhefe

Ms- (P^ to P")Sil-1j

{P^ to Px )[.[-

Ms is defined as the frgure-of-mcrit of the S-cur\:e and reprcscnts a measureof 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 radio-metric system parameters b1'recalling that the pcak-to-peak amplitude ofthe S-curve is twice the observed antenna temperature when the source iscentered in either antenna bearn and that the peak-to-peak noise lluctuationlevel is 6 tintes the rms radiomctric sensitivitv {tw'ice 3o) , i.e .,

.:000,

3r lI{,

(Pr to P";S : 7Tn : f r( -0: '1=,

_\. 0,u i,

i3i

f 15)

iJc j(P* ro PxIFL = 6Af, ={;(E - r)l,

\/r.r'here

TB

0r

0^

A f" - rms sensitivitv of thc rarjiometcr, oK

p - the antcn na apcrturc eflicicncy

F - the receiver noisc figure

p- the receiver predetection bendu'idth

t - thc receiver postdetection integration time constant

Ta - 290"K

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

Harold I. En'en

Flc;. 13. General form of the angle error curve developed by a satellite earthrertical sensor when operating in the thermal centroict sensing mode.The earth vertical direction is coinciclent w'ith the null crossover pnintof the angle error curve. This is a degenerate case of the radiometricsextant mode shou'n in Fig . 22.

antenna bcam rl and B is adjusted to provide simultaneous interception ofopposing 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 scannedacross the earth, as sho\\''n Fig . 23. In this case, the antenna tem peratureTu $'ill be the brightness temperature of the earth oxygen mantle at thefrcquency' ol observation (for an antenna aperture efficiency of 100 To1 .

Inspection of this special case leads to the conclusion ihut Eq. (33) isdirectly'applicable to the determination of the rms angle tiacking u..uracyassociated u'ith the crossover or null point of the S-curve. As a typicalexample oi anticipated performance, \\'e ma-y assume that the mantle tem-perature 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 timeconst;tnt of 0.1 sec, the figure-of-merit of the S-curve g,ill be

&_23064r.- 9/t{r: \Jt)

EXPI-ORATIO\ ,\ND E,XPLOIT\I IO\

For a l0o antenna bcam angle, the anticipetcd rms angle tracking i.tccurac)'

is therefore 1.5 arc minutes (rms).

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

t

e-s

,. . : .i':!i#Ir s li

trT

t

5,T

Frc.24. A 5-mm radiometric sensor developed hrl'the Air Forcc CarnbndgcResearch Laboratories. This unit uas launched into earth orhit in Jul-r'

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 temper-

ature observed from satellite orbit-at a frcqLlcncy ol 60.|i GFlz \\'as con-

sistent with the predicted almospheric model. Thc observe d tenlperatureas a function latitude \vas a prime rneasurcment objcctive. Thc radiometcr\^,as launched into a near-circular polar orbit on July )'i , lqrr7. Prior tostabilization, 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 en-countercd in achicving vehicle stabilization w,ith the desired alritude. The:iatcllite ultimatcly stabilized in an upside-down atrirucle, pointing thescnsor an{cnna into -\pacc. For a period of t}rree months, the unit faith-fullr" recordcd that the observed temperaturc \\,,as nca r zero. This \\-as aninrportant pionecring expcrinrent, hou'ever, since it demonstrated thatrlrdionleter instrument technology at 5 mm was rcad-v* for t5e challenge ofe xploration from space at this \\,AVelength.

h ' Detect ion of Clear Air Turbulence. The abilit y ro det ect tem per-Itltre 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 atmos-ph cric absorption coelllcicn ts ivhich are available ove r a relativelv small\\'ilvclength ranse near 5 mm as a consequence of thc resonant tine profilecltaractcristics of molecular atmospheric ox),gen. The intensity,and ran_seto 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 pro-t''idc lltmosphcric absorption coelllcients ar the flieht altitude in the ranset"rorn 0.1 to 1.0 dBikrn.

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

(3 8)

ri lrcrc

rt i.>. ,t )

7-{.\'i * thcrmometric tcmperatLrre of the atmtlsphere et range .r.-[he integrand is thc product of Iw'c lactors. The first is the temperaturedistribution along thc ray path, tlte second is a space-dependent function ofthc attenuation cocflicient along the ra)'path. The secoJrd factor is largestfnr those regions ncaresl the antenna and exponentially' decrease as thedistilnce from the clenrent of atmosphere Iocated in range inte6,,a I dsbccomes progressivell' llrther from the ante nna. Thus, this factor enrpSa-sizcs splttial elentcn ts clf the tcmperature distribution at ranses near theantcnna and providcs a decreasing contritrution to antenna lemperarure fortltose elemcnts u'ell rcmovecl from the antenna. Because of this spatialsclection propertY, the sccond factor in the integrand is frequently referredt{, ils thc " horizontal wcighting function " of the temperature distributionalcng the rav plrth

,

EXPLORATIO).i A}.ID T,OITATION

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

antenna temperature from an)'region along the re)'path is delermined bl'the value of the u'eighting function n'hich is in turn determined b1'thevalue of the absorption coefficient at the frequencv of obscrvation. Thctemperature contribution from a region rr,'cll forn'ard of tlte ltntenne can

be made to contribute a significant portion to the ovcrall antenna temper-

ature 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') radio-meter in rvhich the channel frequencies arc selected to prol'ide the dcsired

combination o f a,,, value s rcquircd for detection of te mperature anoirlaliesahead of the aircraft, A ntinimum o{'trr'o freclucncies of ohservtrtiofl itrerequired.

To illustrate the rangc capabilit,v of thc tempcraturc sensing systcm ofthis 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

( 3e)

We now note that the exponential factor is the onll'renge de pendent terrn

in the u'eighting function. lts maximum value occtlrs r.r'hcn the range

s:0. Itor all other values of s, the n'eighting functiott steadill'decreases,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 lrlllximumvalue as S,o. i.e.,

r00 i.40i

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 con-

tributcs 99r; to the observed antennr tempsrature . T1'pical velues of S.rn

andcorreSpondingvaluesofatindBikm)are:a-0.l,5rla - 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 antsnniibeam pointing alon-r the horizontal flight path. lf the ambient te mperaturcalong 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 anttnntltemperat.ure sensed b1'the radiometer r,r'ill be

s,r- I lnfr,-.;

Ilarold I. Ex,en

T oi.>i :-: T, + a {y) A f iS) [e xp e (y'i.t ] d.Sj,; i4 1)

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

c5

L,i..+

o3

a2

too '-tt l BO

^lol|t-l o

It-I l.<l'{

,<lF-^l

ol

70 60

Ronge

50 40 302C lO o

Sinkm

Ftc. l-{. The normalized multifrequenc,v response of a radiometric sensor to asrep temlxrature anomal.v of A Io, l0 km in e.\tent, at a horizontalrange 'S from the sensor. The desired a-tmospheric abscrption coefficienru in the range from 0.1 to 1.5 dB,'km determines the correspondingfrequencies of observation. For a practicaI sy'stem, the observing fre-quencies can be confined to a relatively narrow bandwidth by opeiatingin the Vicinitl'of a resonant Iine of an atmospheric gas, such as moie-cular o.\)gen.

t,

/ +4

,'1

,

E.X PLOI{ ATION ,A \ D E.X PI-{]I'I'A I ILI \

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 theantenna temperature is

T,r(uj i t1r

Thus, thc presencc of thc temperaturc anonralv appears i-ls a cSlingc in an-tennu tenrperature about tlre ambicnL 7,.

As an illLlstrative cxample o{'tlre magnirucJc of rhe anriciplred changein 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 ineltent, in thc interval range Sr to.5,. Figurc l5 shorvs il grapliicel plotof the change in antcnnl1 tenlperature relativc to thc antbient tempcratureat 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 thcsensor has a temperature scnsing capabilit,v of r).5'K {ipdicatiye 3l' prescntcapabilit)') , it is appare nt from I'tig. 25 that thc renrpcrarurc anL)rnllywould be observed at a distant'e of'59 km at ult obscrring l-rcqucnc!, forwhich a: 0" I dB,1krn. .tl knr for a - 0.1dB,krn, and ll kln I-t)r rr - 0.5dBi km.

Refcrring to lrig. 15, it is of interest to rtotc the rcsponsc churlr:r1,ri:ricsof the various channels to thc ilssurncd tentpcraturc irnonrall'. ,,\lthouglrinitiallf insensitive to the clisturbance, the ft * 0.: dlJ'knr chrnncl sub-sequentl)' responds ver)' quickly and, at a ritnge of' l5 km, prrrr,i,Jes itnoutptlt 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 anda - 0.1 dBikm occttrs at a range of I knr. The prcscnce clf t6c:e "cr()ss-over points" betw'een indiviclual channels oll'crs an;rtlditional ranqc indi-cator for the anomalous temperatLrre rcgion.

To demonstrilte that the results are not criricalll'dcpcncicnr upon rhetemperature profile of the cssunred discontinuit-\', a similar' ;lnill1,:is can beapplied to two other forms of tempcraturc anomal)'. In onc ci.rsc, \\,e ri'illassume a linear transition fronr tcm pcraturc T-, to T, + I 0'- C' and in r l:eother an expclnential transition frorn T/ to T, + l0'C'. Fpr cach c1sc. \r'elvill assume a half amplitudc *'iclth of l0 knt, corrbsponcling ro the firstcase considered. The enticipated antcnna tcmperature changc \.ersusrange to the anomalous tempcritturc rcgion rcsulting from thcse distri-butions are shown in Figs. l6 and 17, respectivcl\,, for comparison r,,,ith thccase descrihcd by Fig. 25. It is of intcrest to note that {}\,erali signaturecharacteristics are the seme for all three cascs. This indicatcs th;rr the basicrange sensing capabilitf is not criticlilll'dcpcndent ()n the tcnrperatureprofile of the anomalv but rather on thc fundamenral ratliative prL)pertiesof molecular atmospheric oxvgcn as a func.tion of frcqLrcncv anci altitudc

.,

Harolcl I. En'en

nq

Aa

n?

o?

too 9c Bo 70 60 50 40 30 2A tO O

Ronge S rn km

[:lr-;- 15. The normalized multifrequency response of a racJionrctric sensor to aramp tcmperature anomaly of A Io, 10 km in e.xtent, at a horizontalrange S from the sensor. isee Fig. 25 for comparison of the responsecharacte ristics fr-rr rarious values of the atmospheric absorption coef-licicn t rr

tli- obicr\';rticln.,,\ dultl channcl 5 mnt radiomcter scnsor \\,as cleveloped in 1968 b1'the

Pi-r.rpilgations Studies [Jrancir of the NASA Electronics Research Center toinvestigatc Cl,{T dctection capabilitv. A photogreph of this millimeterracliometric scnsor is show'n in ftig. 2,!. The antenna is a single conicallcns J'cd horn. "fhc antenna output is fed via an orthogonal mode transducert"o thc inputs of tu't'l radiomctric recei\:ers. Onc receiver is tunable overthe lrcquenc)' range l'rom 5l to 53 GI-lz and rhe other from 57 to 59 GHz.

.t

I"^lF-l, or lr'la

,.{

\J

Flvt

V,

/

---€-)

4\,/-/"t

-/

(,/

/2/r/

c-r5

EXPLORATIO)J AND EXI'I-OI .\TIO\

t(X) Bo 60 40 2C 3Ronge S in km

Ftc. 27 - The normalized multifrequency response of a railiometric sensor ro anexponential temperature anomal,v u'hich has an average value of J f0over a range interval l0 km in e.xtent, at a horizontal range 5 frgrn thesensor. {See Fig- 25 for comparison of the response charactcristics forvarious values of the atmospheric absorption coefficient rr. r

The individual tuning ranges and the frequency separation beru'een channelsallorv selection of corresponding atmr)spheric atrsorption coefircient yAluesfor observation of 0.1 and I dBlknr for flight alti-tudes from 3i),000 ttl60,000 feet

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,alu-ation of vertical sounding of the at mosphgric tenrperature prglile in rhe

r--oi , oll-'i<fi

Harold I. Evten

Flc. 18. 5-mm wavelength dual channel radiometric sensor developed by thei{ASA Electronics Research Center to experimentally'verify the abilityto detect clear air turbulence regions along the forw'ard flight path ofhigh alritudc aircraf t.

ioli'er troposphere. Ttre diversity of the tw'o disciplines, CAT detectionand mcteorological meesurcments. $'ill be joined by one sensor as a commondcnominator in this dual experiment to explore and exploit.

IV. A LOOK INTO THE FUTURE

I-listoric;rll)', our communication needs and associated technologicall-crluirentents havc provided thc stintulus for expanding our radio capa-hilitics t'rom long to shorter \\'avelengths. There has been no slackeningqlf thc pace as this need now focuses attcntion on the 3 cm to 3 mm wave-lcngth region. This time, horvever, the communicator has a silent andpcrsistcnt pertner alread_v- activel,v exploring this region of the spectrumu'ith clcfinite plans for exploitation. The ability to obtain a global picture of

EXPLORATIO\ A)(D EXPLOITATIO};

atmospheric rvater vapor and tempcraturc distributions, combined u'ith airmass circulation under cleer air conditions, offers the potential to predict

in advance the formation of storm clouds and thcir motions. Several signi-ficant applications can be accornplished only in this portion of the spectrumas a consequence of the nature of the ph,vsical processes involvcd.

In rhe future, &s communicators look through the atmospheric *'in-dows, microu'ave meteorologists tvill be measuring the globei structure ofthe atmosphere in the spectrum benveen the u'indo*'s. The y' ma)' botirjoin together, horvever, in earth orbit at opcrating wavelengths in thevicinity of 5 mm, since here the cornnlunicator is assured an envircnmentfree 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 prob-

lem. Without carel-ulconsideration, harmful interfcrence to passive studies

and applications may result. F-or example, a microu'ave mapping radio-

meter has been proposeci for an experimcnl.al progranl on thc liimbusseries 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 fre-quency is, coincidentl)'. in a radio astronom]- band presenti''' prcltccted

from man-made electromagnetitc transmission. It tt'as rccrntl-'* sug-

gested that this particular radio astronom-Y band be rclocated to:3.55GHz 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 ofthis type ma-v seriously elTcct rernotc sensing applications u'hich cltnnot

change freq ue nc\' .

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 longerthe limiting l'actor. Within the next half dccade a radiomctric temper&-

ture sensing capability of better than l"K rr'il1 be achievable throughoutthis entire \\:avelength region, r'u'ith postdetection integration tinre con-

stants no greater than l sec. The cl'rallenge r+'il1 be to exl.ract kno*'ledgeand 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'nis so close to home. As J. P. Wild said at the Fourth Parvsel'MemorialLecture 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 ratheran enigrna in astrophl'sics. We appeer to knorv so much about astro-physics-about the galaxy and the un iversc and so on ; thcre might be

t.

Hurold I. En'en

a !'r:u'controversial alternatives w'hcn astronomers talk of cosmology orrlLursurs ()r pulsars, but on the u'hole the."" sit back and ret'ie$' theiracliieven"lcnts *'ith rcmarkable satisfaction. . . When you know' t\\'o orthrcr'things about something thcre is no difficulty in producing a theoryto sxplain it. But

"r'hen )'ou knou'a thousand things, the theory becomesmore diflicult, and u'hen )'ou know that another thousand things areri'rriting to be discovercd the theorists get lrightened off. And so, apartfrrrrn a fcr,'' of thc braver theorists, the onus is left in the hands of experi-me ntal ph1-sicists: cspecially those prepared to persevere gradually, stepbv ste p, n'ith the scientific mcthod; and especialli" those prepared tofashion ne\1" lines of attack."

,4s \\'e enter this ne\\'era n:i1|1 the c'apAbility to passivell"and remote-ly' scnse thc location, identit,v, and condition of our earth resources fromslitcllite orbit, our succcss rvill be measured by our perseverance andintcgritl' to exploit thc micro\\'ave spectrum in the best interest of allmil n k ind.

R EFER ENCES

Dicke. R. I{. The measurement of thcrmal radiation at micro*'al'e frequencies.

.rter,. Scf . lnstr. 17, 268-.215 (.1946,.

Haroulcs. G. C., Bror,r'n, W.E., IlI, and Erven, H. I. Method and Means for Pro-viding an Absolutc Pow'er l\'Ieasurement Capabilit"v. Patent application, February,le6i "

Dickc, R. I{.. Pee bles, P. J. E., Roll, P.G., and Wilkinson, D. T. Cosmic black-body

radiirtion . ,,lstroplr-r's. J, 112, 414-419 {t965;.Thonrpson, W. 1., IIl, and Haroules, G. G. A review of radiometric measurements

trf atmospheric atrenuation at wavclengths from 75 centimeters to 2 millimeters.N ASA TN-D-5087 lJanuarv 1969r.

5. Cheung. A, C., Rank, D. M., Tou'nes, C. H., Thornton, D. D., and Welch, W. J.

Dcrection of NHr molecules in thc interstellar medium bl'their microwave emission.Pht's" Rer'.21, l70l-1705 r1968i.

Van Vleck, J. H. Absorption of microvvaves bf' u'ater vapor.1941 :.

Ilcckcr, C.8., and Autler, S. H. Water vapor absorption of electromagnetic radia-

tion in the centimeter wal'e-length range. Pht,s. Re',,.70, 300 (1946i.

[-lo. W.. Kaufman, I. A., ancl Thaddeus. P. Laboratory measufement of microwave

irbsorption in rnotlels of the atmosphere of Venus. J. Geoph)'s. Research7l,509lI 966;.

Straiton, A. W.., and Tolbert, C. W. Anornalies in the absorption of radio u'aves by

atrnospheric gases . Proc. IEEE. -18, 898 -9$3 /1960r.

Barrcit, A. H.. and Chung, V.K. .,\ method for the determination of high altitudewate r-\'apor abunrlancc from ground-based microu'al'e obscrvations. J. Geophys.

Resea rch 67 ,4259 il96lr.I l. r\rtnran, J. O... and Cordon, J. P. Ahsorption of microwaves b.v or)-gen in the mil-

limeter uarelength region. Pht's. Rev'.96. l23i il954i.

8.

9.

l0

EXPLORATION,\ND EXPL()ITA'I'IO\

12. Anderson, R. S., Smith, \'. \'.. and Gordl', w. lv{icrcr*a\c spectrunr of o\}'-*cn.Ph1' .r. Rev. 87, 571 11952,.

13. lv{eeks' M. L., and Lille-r', A. E. The microware sFectrum of or}'gcn in tire eurth'satmosphere. J. Geop hy s. Re searcft (r8, l6g3 . l96J, .

14. Westu'atcr. E. R.., and Strand, O. N. Application cf statistical estinratisr: lechniqucsto ground-based passive probing of the troposphcric temperaturc structurc. U. S.Dep't. of Commerce, ESSA Technical Report I[:R 37-ITSA ]7, I]ouitJer. Ct-rlt-rrarjcri1967,.

15. Gora, E.K., The rotational spectrunr of ozone. J. .)Iol. Spectrosc()p.\ -1. lB lgSg;.16. Van Vleck, J. H. The absorption of microrvaves h1' ox)'gcn . I'h.t'.s. Rn'. 71. 413-41-t

{t917i.17. Gautier, D. and Robert, A. Calcul du coeflficie nt d'absorption des ondc-i rlillinretri-

ques dans I'o,xy"gene moleculaire en presence d'un champ magnctique faifric. applica-tion a l'atmosphere terrestre. Ann. Geoplr1's. 20,41i0 i1964',.

18. Lenoir, W. B. Microwave spectrum ol' molccular or1'gen in thc mesosphcre . J. Gco-phys. Researclt 73.361 il968i.

19. Stafford, L. F., and Tolbert, C. W. Shapes of ox)'gen absorption lincs ip rhe micro-wave frequenc-v region. J. Geoplr-l'.s. Researcr 6ti. 3431-3435 il9fr3 .

20. Reber, E. E., Mitchell, R. L., and Carter. C. J. Or1'gcn absorption in the earrh'sa'mosphere. Air Force Report No. SA MSO-TR-68-488, Acrospece Rcp'-rrt No. TR-0200 14230-46,-3 { 1968).

21 . Van Vleck, J. H., and Weisskopf, V. F. On the shape cf collision brorrdeneti lines.Reus. lr{odern Phv s. 17, 227 -236 (1945: .

22. Staelin, D. H. Measurements and interpretation of the microw'ave spucr runl of thcterrestrial atmosphere near 1-centimeter rr'avelength. J. Geopht's. Re.seurt:h 71. 28i5-2881 i.1966 .

23. Gaut, N. R. Studies of Atmc-rspheric 1*'ater Vapor by lr{cans of Passir c \{icrorraveTechniques. Ph. D. Thesis, Dept. of lv{eteorology'. lviassachusetts lnstitLrle trf "cch-nology' (1967

) .

24. Trambarulo, R., Ghosh, S. N., Burrus, C. A., Jr.. and Gorcll', \\'. Thr molecul,rrstructure, dipole moment, and a g factor of ozone from its microw'ave spcctrunt. J.Chem. Ph],s. ?1, 851-854 i1953r.

25. I{ughes, R. H. Structure of ozone from the microu'ave spcctrunt beluesn 9,000 ancl45,000 Mc. J. Chenr. Phv s. 2{, I 3 I - I 38 'l 956,.

26. Walshaw, C. D. Line widths in the 9.6 1e band of ozone. Proc. Phrs. Serc. Lontlon,,A6g' 530 ilg55j.

27. Mouu', R. 8., and Silver, S. Solar radiation and atmospheric absc-rrprion for theozone line at 8.3 mm . Inst. Eng.Res. Ser.6Oi2i7:., Universit)' of Caliiornir, Berkelc-r'i 1960i.

28. Caton, W. M.. Welch, W. J., and Silrer, S. Absorption'and cmission in the 8-mmregion bi ozone in the upper atmosphere. Space Sci. Lah., Ser. No. 8. Issus 12;1967 i .

29. Barrett, A. H., Neal, R. W.. Staelin, D. fI., and Weigand, R. lU. Racliornctric iJctcc-tion of atmospheric ozone. Quart. Prog. Rept., Res. Lab. o.f Elecrrur;ic"s. l\{. l. T.tJul)',196-it.

30. Caton, w. lvI., Mannella, G. C., Kalaghan, P. lv1.. Barrington, A. E., anti Euen, l-1. I.Radio measurement of the atmosphcric clzone transition ar l0l.7 CH,,. ,^lstroplr1,s.J.;Lerters, l5l, L 153 {1968,.

31. Caton,, W.h{., Private col'pmunication

Harold I. Ew,en

32' FIunt' B' c' Photochemistrl'of ozonc in a moist atmosphere. J. Geoph),s. Re.rearch71, l3tl5 {1966\.

33' Itadi'rnetric charactcristics of the atmosphere for referencc-clircction sensing inspecc r rhicle navigation. .r\ir Force Conrract AF lg i62g; 3239.