IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. GE-25, NO. 6, NOVEMBER 1987 647

Extrapolation of Earth-Based Solar IrradianceMeasurements to Exoatmospheric Levels for Broad-Band and Selected Absorption-Band ObservationsJOHN A. REAGAN, SENIOR MEMBER, IEEE, PETER A. PILEWSKIE, IAN C. SCOTT-FLEMING,

BENJAMIN M. HERMAN, AND AVISHAI BEN-DAVID

Abstract-This paper describes techniques and presents results for is negligible as is normally the case for narrow field-of-extrapolating earth-based spectral-band measurements of directly view instruments. The straight-line Langley plot fittingtransmitted solar irradiance to equivalent exoatmospheric signal lev- technique then may be used to extrapolate the surface sig-els. These techniques have been used to aid in determining system gain n ltsettings of the Halogen Occultation Experiment (HALOE) sunsensor nals to the equivalent exoatmospheric signal levels. How-system being developed for the NASA Upper Atmosphere Research ever, the Bouguer-Lambert law, when integrated over aSatellite (UARS) and for the Stratospheric Aerosol and Gas (SAGE) II broad nonabsorbing spectral region or even over a nar-instrument launched in October 1984, on the NASA Earth Radiation row, fairly weak spectral absorption region, does not yieldBudget Satellite (ERBS). A band transmittance approach was em-

a. .

p

ployed for the HALOE sunsensor, which has a broad-band channel a l L pdetermined by the spectral responsivity of a silicon detector. A modi- termined band transmittances and/or modified Langleyfied Langley plot approach, assuming a square-root law behavior for plot fitting techniques must be used to transform surfacethe water-vapor transmittance, was used for the SAGE II 940-nm signals to exoatmospheric levels.water-vapor channel. The band transmittance approach was employed in the

Index Terms-Exoatmospheric, solar spectral irradiance, solar ra- case of the HALOE sunsensor system, which, for thediometer, Langley plot. coarse azimuth and elevation position sensors, has broad-

band channels set by the spectral responsivity of the sili-con detectors used for the sensors. The SAGE II instru-

I. INTRODUCTION ment has only narrow-band channels, but one falls in the

DERFORMANCE testing/calibration of spaceborne so- 940-nm water-vapor absorption band. A modified Lang-1lar radiometer systems typically includes pre-launch ley plot approach, assuming a square-root law behaviorsurface solar observations and/or ground-reference mea- for the water-vapor transmittance, was employed to de-surements with auxiliary instruments once the system is termine the exoatmospheric signal level in this case. De-in orbit. In either case, the problem arises of how to tails of how these approaches were implemented are pre-transfer surface solar measurements to their equivalent sented in the following sections of this paper.top-of-the-atmosphere exoatmospheric levels. This paperdescribes techniques that have been employed to extrap- II. STATEMENT OF THE PROBLEMolate earth-based spectral-band measurements of directly On a purely monochromatic basis, the directly trans-transmitted solar irradiance to equivalent exoatmospheric -2 -transmltted~~ ~ ~ ~ ~ ~ ltedsoa spectral irradiance Fx(w *m *nm ) atsignal levels. These techniques have been used to aid in wavelength spiciden onteeat' s m ay b re-determining system gain settings for the Halogen Occul- watedetoth top of the eatmospric or zero-

lated to the top of the atmosphere e'xoatmosphen'c or zero-tation Experiment (HALOE) sunsensor system being de- airmass solar spectral irradiance F0 at X through the Bou-veloped for the NASA Upper Atmospheric Research Sat- guer-Lambert lawellite (UARS) and for the Stratospheric Aerosol and Gas(SAGE) II instrument launched in October 1984, on the Fx =FoxTx (1)NASA Earth Radiation Budget Satellite (ERBS). dFor spectral bands that are fairly narrow and outside of an

gaseous spectral absorption regions, the surface signals Tx exp [-m (TX + rxg)1 (2)may be accurately described in terms of the Bouguer-Lambert law, assuming the diffuse radiation contribution where

m is the atmospheric relative airmass for the solarManuscript received January 12, 1987; revised July 3, 1987. This work position (zenith angle) at the time of observa-

was supported in part by NASA under Purchase Orders L 83161 B and A tion of FAX32033 C and NOAA under Grant NA80RAD00065. Ti i h topei pcrlotcldpha x

The authors are with the University of Arizona, Tucson, AZ 85721. .X i h topei pcra pia et tXxIEEE Log Number 8716812. cluding any gaseous spectral absorption,

0196-2892/87/0600-0647$01.00 ©C 1987 IEEE

648 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. GE-25, NO. 6, NOVEMBER 1987

Txg is the atmospheric gaseous spectral absorption op- andtical depth at X,

TX is the atmospheric spectral transmittance at X for RX:Fox em`9adXairmass m and total optical depth TX + rXg, and (7)

Fo0 is the zero-airmass solar spectral irradiance at X Txg =for the earth-sun separation at the time of ob- RxFox dAservation.

where Txg is the band-weighted transmittance determinedTo obtain a measurable signal with a radiometer sys- bhe pag .i spectal-absorptionabe tergas

tem, Fx must be collected over some nonzero wavelength in question lan spectralfaturestofFehand Rf Eveninterval and receiver area. The collected flux is also con- ifFetoandRw eefectivl constantover aX, the strnverted to a voltage by the system's photodetector and spectralvaRiation ofectyicly exhibitoe byegseoua

elecroncs pckae. Asumng F isconsantoverthere-spectral variation of -rxg typically exhibited by gaseous ab-electronics package. Assuming Fx is constant over the re- sorption lines is sufficient to require the band-weightedceiver area AR, as should be the case for practical receiver t A

area sizes, the quasi-monochromatic output signal voltage Lambetawca Txg geeal cannot be te mod-Vx for a spectroradiometer with a small wavelength band- Lereby anexponentialwi cannetive acrguent mof -

pass AXcenteredon X is~~eled by an exponential with a negative argument of air-mass times a constant band-weighted optical depth. Vari-ous relations have been developed to model gaseous ab-

= AR RxFX dX (3) sorption band transmittance (e.g., [4]-[6]). The statistical(Goody [4]) band model yields a square-root behavior

where RX ( V wW-1) is the system spectral responsivity where Txg may be modeled by an exponential with a neg-including the spectral dependence of both the photodetec- ative argument proportional to the square-root of the totaltor and the wavelength/bandpass selection system of the path absorber amount for the case where individual ab-spectroradiometer. sorption lines are fairly narrow compared to their spectral

Equation (3) may be specialized to apply to one of three separation distances. This result appears to apply fairlysituations, depending on the size of AX and the location well for water-vapor bands in the near infrared [7], [8].of X. If AX is fairly small, say less than about 10 nm, and For wide-band applications where AX = X2 - X1 ex-X is in a region excluding any gaseous spectral absorption tends over a wide spectral range (i.e., 100 nm or more),(i.e., where Txg = 0), then (3) is well approximated by (3) takes the formtaking erI outside the integral V= (8)

Vx = Voxe-mm (4) and

and rX2VO = AR RxFox A (9)

= AX T = RxFox exp [-m(rx + Txg)] dX (10)

where Vox~ is the radiometer's exoatmospheric or zero-air- where T is the wide-band transmittance. As with the nar-mass signal level. In this case the radiometer output signal row-band spectral gaseous absorption case, (8) does notis of the same form and, thus, obeys the Bouguer-Lam- follow the Bouguer-Lambert law. This would be the casebert law. Given thatTd iS spatially/temporally constant even if AX were chosen where Txg = 0, because TX stillover the range of airmasses that measurements are made varies significantly over AX for the wide-band case.in, a plot ofln Vx. versus m, a Langley plot, will yield a Moreover, due to the combined spectral dependencies ofset of data points distributed along a straight line with RX, FOX, and TX + TX, it is generally impossible to accu-slope -t and intercept ln V0 . The Langley method for rately represent T by some simplified band model. Thusdetermining Ven and TX is simply that of applying a extrapolating V to the equivalent exoatmospheric level V0straight-line fit to a Langley plot to obtain the intercept ln requires evaluating T.Ve tand slope -T. It is an approach that has been usedextensively to monitor atmospheric turbidity and solar ir- vII EXOATMOSPHERIC SIGNAL LEVEL DETERMINATIONradiance and to calibrate radiometer systems (e.g., [1]- FOR HALOE SUNSENSOR[3]).

If AX is still fairly small but X is within a gaseous spec- The HALOE instrument employs the solar occultationtral absorption band such as the oxygen and water-vapor technique to scan the Earth's limb during satellite sunrisebands found in the visible and near-infrared regions, (3) and sunset events. Acquisition and tracking of the sun ismay be expressed in the form accomplished by coarse azimuth and elevation analog

sensors with broad-band channels defined by the spectralVAn= VOie ly toTmi (6) responsivity of the silicon detectors used as the sensors.

REAGAN et al.: EXTRAPOLATION OF SOLAR IRRADIANCE MEASUREMENTS 649

The elevation-axis control also has a digital fine sunsensor 1.2_for solar scanning and edge tracking. This sensor systemhas an interference filter defined narrow bandpass of AX 1.0,EG+G= 3 nm centered at X = 700 nm followed by a siliconarray detector (see [9] for added details on the HALOEinstrument and experiment). /0.8 | NASATo maintain solar tracking deep into the Earth's atmo- J

spheric limb, the instrument sunsensor systems must have .6- /wide dynamic ranges. Also, to make maximum use of 0

these ranges, the system gains must be set so that the exo- 0.4L Iatmospheric signal levels are near the top, but do not ex- | |ceed, the signal windows of the sunsensor systems. Asthe gains must be set prior to launch, this is why it was 0.2 l /necessary to establish the expected exoatmospheric signal / dlevels from ground-based measurements. 0 4 6 8 1 1200

200 400 600 800 1000 1200The sunsensor calibration tests were conducted on X (nm)

Mount Lemmon (elev. 2790 m) near Tucson, AZ, al- Fig. 1. R,xFo versus X computed for NASA and EG&G responsivities.

though some backup measurements were also made inTucson. A high-altitude dry clean site such as MountLemmon offers the advantage of minimizing atmospheric components, rx may be estimated fairly accurately by ex-attenuation, thereby yielding a surface signal closer to the trapolating between values of Tx determined at only a fewexoatmospheric level. Nevertheless, transmittance cor- wavelengths.rections still must be made to accurately determine the To obtain these few required values of Trx, concurrentexoatmospheric signal level. As can be seen from (8), the measurements were made with a narrow-band (AX =_ 10wide-band exoatmospheric signal V0 may be determined nm) solar radiometer with nine channels positioned inby dividing the signal observed at the surface V by the nongaseous spectral absorption regions over the wave-wide-band transmittance T. length range 370 to 1030 nm. The rx values were deter-To determine T requires knowing at least the relative mined by straight-line fitting the Langley plot data ob-

spectral behavior of the exoatmospheric spectral irradi- tained for each radiometer channel. The results obtainedance FOX' the relative spectral variation of the system re- on one of the Mount Lemmon observation days (4/19!sponsivity Rx, and the total atmospheric spectral optical 85), including the smooth curve extrapolated throughdepth Tx + TXg. The NASA standard developed by The- the experimentally determined TX values, are plotted inkaekara [10], which is sufficiently accurate for the prob- Fig. 2.lem addressed in this paper, was assumed for FoX, and Rx The only gaseous spectral absorption of any real con-was taken as the spectral responsivity of a silicon photo- sequence for the silicon photodiode bandpass occurs duediode. Two silicon photodiode responsivity relations were to water-vapor absorption bands around 720, 840, and 940considered. One was a generic silicon responsivity curve nm and oxygen absorption in a band around 760 nm. The(circa mid- 1970's) which the NASA Langley Research gaseous band transmittances Txg for each of the bands wereCenter Sunsensor Team had used for modeling calcula- computed using the Lowtran VI computer code [12].tions, while the other was taken from EG&G product lit- Pressure, temperature, and dew point temperature profileerature [11]. The products of Fox times Rx for each of information required for the Lowtran computations werethese responsivities are plotted versus wavelength in Fig. obtained from weather station radiosonde measurements1. It can be seen that the EG&G responsivity weights made at Tucson on the same day. For Txg determined forshorter wavelengths more strongly than the NASA curve. a given airmass m (i.e., for a given observation-time), theThe actual responsivities of the HALOE sunsensor pho- spectral transmittance Tx for an absorption band may betodiodes are assumed to yield F0 XRX products that fall accurately approximated bysomewhere between the two curves of Fig. 1. The areas T = -MTX A

under these two curves were summed numerically, in 10 Tx e Txg (11)nm steps, to yield the effective exoatmospheric signal lev- for small AX where TX is evaluated at the center of AX.els V0 required in (10). Using Tx determined in this manner, the products of RX,The next step in computing T is to determine T,. as F0 X' T, were determined at selected times (airmass val-

defined in (2), so that the product RX F0 X TXi can be eval- ues) for each observation day. Results are shown in Figs.uated for the numerator integrand of (10). This requires 3, 4, and 5 for two days of measurements on Mount Lem-determining TX, and, for regions where gaseous spectral mon and one day in Tucson. As with Fig. 1, the compu-absorption occurs, TXg. As TX iS due to components that tations were made for both the NASA and EG&G respon-vary rather smoothly with wavelength, namely, the mo- sivity relations. The areas under the curves in these figureslecular (Rayleigh) scattering component, the aerosol ex- were summed numerically, in 10-nm steps as with Fig. 1,tinction component, and the continuum gases absorption to yield the effective surface signal levels V. The wide-

650 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. GE-25, NO. 6, NOVEMBER 1987

1.0 - 1.24/23/85

A Data Points m-1.5031.0

JOt 0.8

| 01 8 | o D ~~~~~~~~~~EG+G A

0.6-NASA

0.01CC

0.4-360 560 760 960 1160

X (nm)

Fig. 2. rx versus X determined from spectral solar radiometer observations 02at Mount Lemmon, AZ, on April 19, 1985 from 7:02 to 10:01 MST. /

o d' I, i I ,t I1.2 200 400 600 800 1000 1200.2 )d~~~~~~~~~~~~~~~~~~~~~~~~~~~~knm)

4/19/85 Fig. 5. RxFoxTx versus X for Tx determined for April 23, 1985 16:53 MST1.0 _ m-1.585 Tucson, AZ solar observations.

0.8 EG+G (<} \sponding computed (by (8)) HALOE exoatmospheric sig-EG, / 8 a nal levels are given in Table I. The three-day averages of

106L0 I6 -NASAV0 for each sensor and assumed responsivity relation have

,/^0 ,, \ percent standard deviations of 1.4 percent or less, whilej<r11 / t the V0 averages for the two assumed responsivities differ

0.4 by less than 4 percent. Thus, it is estimated that the wide-l ; band transmittance approach permitted extrapolation of

/ the HALOE sunsensor surface measurements to their0.2 / equivalent exoatmospheric levels with an uncertainty

i \m within about 6 percent.200 400 600 800 1000 1200

X (nm) IV. EXOATMOSPHERIC SIGNAL LEVEL DETERMINATIONFig. 3. RxFox Tx versus X for Tx determined for April 19, 1985 9:00 MST FOR 940-nm WATER-VAPOR CHANNEL

Mount Lemmon, AZ solar observations.

Measurements were also made on Mount Lemmon, AZ,1.2- to provide information to aid in determining the system

gain settings of the science data channels of the SAGE IIm4/2.173 instrument [13], which, like the HALOE instrument, em-

1.0m = 2.173ploys the solar occultation technique to scan the Earth's

atmospheric limb. The SAGE II instrument has seven nar-08 row-band (AX < 20 nm) science data channels between

EG+G 380 to 1020 nm. Only one channel, the 940-nm water-vapor channel, falls in a gaseous spectral absorption re-

0v6 / NASA gion (other channels fall in nitrogen dioxide and ozone0.4 / / absorption bands, but these are continuum absorption re-

gions). Thus, it was possible to use the Langley plot tech--/ j ,; nique to accurately establish the expected exoatmospheric

02 signal levels for all but the 940-nm water-vapor channel.The SAGE II instrument was not actually taken into the

o ½ I', | , | , | ,\ ifield for gain setting measurements. Instead, a separate200 400 600 800 1000 1200 narrow-band radiometer with similar channels was takenA(nm)to Mount Lemmon and calibrated by the Langley plotFig. 4. R,iFo,2R, versus X for T, determined for April 20, 1985 8:02 M5T tehiutofrtsabshtsxamsprcsgnl1v

Mount Lemmon, AZ solar observations. ehqutofrtetbs lSxamsprisgnlevels. This radiometer was then operated beside the SAGEII system, in a ground-base, laboratory environment,

band transmittance T is obtained by dividing these areas while both instruments tracked the sun. In this manner, itby the areas determined for the curves in Fig. 1. The re- was possible to transfer the radiometer calibration to thesulting transmittances, the concurrent HALOE coarse SAGE II instrument. However, the field measurementssunsensor surface signal measurements, and the corre- also demonstrated that the Langley technique appeared to

REAGAN et al.: EXTRAPOLATION OF SOLAR IRRADIANCE MEASUREMENTS 651

TABLE ICOMPUTED WIDE-BAND TRANSMITTANCES AND HALOE COARSE SENSOR

SYSTEM EXOATMOSPHERIC SIGNAL LEVELS

Location, Date Computed Observed HALOE CSS Computed HALOE CSSTime and Wide-Band Sunsensor Surface Exoatmospheric Signal Level*Airmass for Transmittance Signal Level* VMeasurement 0

T V Azimuth Sensor Elevation Sensor

NASA EG&G Azimuth Elevation NASA EG&G NASA EG&GRespon. Respon. Sensor Sensor Respon. Respon. Respon. Respon.

Mount Lemmon 0.8728 0.8469 7585 7645 8690 8956 8759 90264/19/85-9:00 MSTm= 1.585

Mount Lermmon 0.8419 0.8060 7274 7333 8639 9024 8709 90974/20/85-8:02 MSTm = 2.173

Tucson 0.8076 0.7759 7131 7192 8831 9192 8905 92694/23/85-15:36 MSTm = 1.503

Avg. of Avg. of Avg. of Avg. of8720 9057 8791 9131±99 ±121 ±102 ±125

* Normalized to mean earth-sun separation; values are in relative counts (millivolts) of theinstrument output signal digitizer.

underestimate the exoatmospheric signal rather grossly for bythe 940-nm channel, and a conservative safety factor hadto be included in setting the gain for this channel. Further xg-exp - (12)investigation of the 940-nm channel problem led to the where m is airmass, u is the columnar absorber amount,results presented here. and k is a constant that depends on the absorption struc-For narrow-band radiometer measurements within a ture of the band in question and on the form of the radi-

gaseous spectral absorption band such as the 940-nm ometer responsivity/filter function Rx over this band.water-vapor band, the surface measurement may be re- Using this relation in (6) and rearranging terms yieldslated to the exoatmospheric signal by (6), which is re-peated below Vx ernx Voxexp[k-kVfm] (13)

Vx = Vox e-ma Txg which is in a convenient form to apply a modified Langleyplot fit. The term on the left is the effective surface signal

Given that TX has been determined, which could be done if the only atmospheric transmittance loss was due to Txg.by the same technique described earlier for the wide-band A plot of the natural logarithm of this effective signal ver-transmittance case, Vo0x still cannot be obtained from Vx sus sIm, a modified Langley plot, should yield a straightunless Txg either can be determined or somehow modeled line of slope - k v4 and intercept In V0X. Thus, a straight-to permit a Langley plot like extrapolation to V0,. Low- line fit to the modified Langley plot may be used to findtran could be used to compute Txg, but, due to the empir- Vox without having to know k or u, although u must re-ical and limited adjustable parameter format on which the main temporally/spatially constant over the range of air-model is based, the error in Tx,g computed by this ap- masses that measurements are made.proach probably would be at least 5 percent. This error A unique opportunity to test the modified Langley plotlevel was acceptable for the wide-band case where gas- approach occurred during a radiometer field calibrationeous spectral absorption contributed only a fairly small experiment at Mauna Loa Observatory (MLO), Hawaii,amount to the overall transmittance. However, it is un- in August 1985. Data were collected with the same 10-acceptable for the narrow-band case where the error in Txg channel narrow-band solar radiometer used in support oftranslates directly to the estimate of V0,. Lowtran or any the Mount Lemmon HALOE sunsensor and SAGE II ex-other band transmittance computation scheme also suffers periments. The purpose of the experiment was twofold.from the drawbacks of requiring as inputs 1) pressure, One was to update the Vox calibration of this instrument,temperature, and dew point temperature profiles for the and the other was to provide a calibration cross check forobservation site, and 2) the specific form of the respon- a NASA airborne sunphotometer also brought to MLO forsivity/filter function R, of the radiometer for the particular ground-based calibration tests.channel in question. Thus, it would be more desirable if The 10-channel solar radiometer has, as indicated ear-Txg could be modeled in such a way to permit a Langley lier with regard to Fig. 2, nine channels in nongaseousplot fit type of approach to extrapolate Vx to V0X-* spectral absorption regions, but the remaining channel isAs noted in Section II, a square-root law transmittance in the 940-nm water-vapor band. Prior to this experiment,

model appears to apply fairly well for near-infrared water- determination of V0 x for the 940-nm channel had beenvaporbands. Specifically, Txg is approximated in this case accomplished only by transferring V0X~from an adjacent

652 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. GE-25, NO. 6, NOVEMBER 1987

TABLE IIVox DETERMINATIONS FOR 940-nm CHANNEL

Date Modified Nornal1 Standardand Langley Langley LampLocation Plot Fit* Plot Fit* Transfer*

Aug. 9, 1985 1139.9 862.2MLO

Aug. 10, 1985 1138.2 983.6MLO

June 1985 1107 to 1124Tucson

* Normalized to mean earth-sun separation; values are in relative counts(millivolts) of the instrument output signal digitizer.

TABLE IIILOWTRAN AND MODIFIED LANGLEY PLOT DETERMINATIONS OF VOX AND Txg

FOR 940-nm CHANNEL MLO OBSERVATIONS

Date LOWTRAN Modified Langleyand Plot FitAi rmass T VO* T V *

Agox Xg ox

Aug. 9, 1985

m = 2.062 0.7141 1018.6 0.6380 1139.9

m = 2.537 0.6849 1046.2 0.6285 1140.0

Aug. 10, 1985

m = 2.056 0.7497 1189.2 0.7831 1138.3

m = 2.541 0.7226 1207.2 0.7664 1138.1

* Normalized to mean earth-sun separation; values are in relative counts(millivolts) of the instrument output signal digitizer.

absorption-free channel (870- or 1030-nm channels) to the proach is clearly superior and that it can provide a fairly940-nm channel through standard lamp measurements accurate estimate of the radiometer exoatmospheric signal(this approach also requires knowledge of the relative level for the 940-nm channel.spectral irradiance properties of the lamp and the sun).The V0o obtained by applying the modified Langley plot V. CONCLUSIONfit to two days of 940-nm channel measurements are listed Techniques have been presented for extrapolating earth-in Table II. The TX at 940 nm required in (13) were de- based spectral-band measurements to equivalent exoat-termined by interpolating from rx at adjacent channels mospheric signal levels. These techniques have been suc-analogous to the procedure described earlier with regard cessfully applied to measurements for both wide-bandto Fig. 2. Also included in the table are the V0x deter- visible/near infrared and 940-nm water-vapor band obser-mined by a normal Langley plot fit to the 940-nm channel vations. The wide-band transmittance approach permitteddata and the V0 x determined earlier by the standard lamp extrapolation of the HALOE sunsensor surface measure-calibration approach. It can be seen that the results ob- ments to their equivalent exoatmospheric levels with antained by the modified Langley plot fit and the standard apparent uncertainty within about 6 percent. Much of thislamp transfer agree quite well, while the results of the uncertainty was due to not knowing the exact spectral re-normal Langley plot fit are grossly different. Lowtran sponsivity of the sunsensors. The modified Langley plotcomputations also were run for these two days using the approach, which assumes a square-root law transmit-Hilo, HI weather station radiosonde data for the required tance, permitted determination of the 940-nm water-vaporinputs. The Txg and corresponding V0x obtained using channel exoatmospheric signal level within about 3 per-Lowtran are compared with the modified Langley plot fit cent of the value obtained using a standard lamp transferresults in Table III for two specific times (airmasses) dur- method. This was significantly better agreement than thating each of the two days. The Lowtran results differ from obtained by using a normal Langley plot fit or by usingthe modified Langley plot fit results by 4.5 to 10.5 per- Lowtran to compute the band transmittance.cent. It also can be seen that the water-vapor transmit-tance was lower (i.e., more water-vapor) on August 9, REFERENCES

which s proably he reaon fo the oorer lower v0 X [1] A. Angstrom, "On the atmospheric transmission of sun radiation andvalue obtained by the normal Langley plot fit for that day. dust in the air," Geogr. Ann., vol. 11, pp. 156-166, 1929.The results indicate that the modified Langley plot ap- [2] G. E. Shaw et al., "Investigations of atmospheric extinction using

REAGAN et aL.: EXTRAPOLATION OF SOLAR IRRADIANCE MEASUREMENTS 653

direct solar radiation measurements made with a multiple wavelength Ian C. Scott-Fleming was born in New York City

radiometer," J. Appl. Meteorol., vol. 12, pp. 374-380, 1973. on January 26, 1955. He received the B.A. degree13] J. A. Reagan et al., "Assessment of atmospheric limitations on the in economics from the College of William and

determination of the solar spectral constant from ground-based spec- Mary, Williamsburg, VA, in 1977, and the M.S.

troradiometer measurements," IEEE Trans. Geosci. Remote Sensing, degree in electrical engineering from the Univer-vol. GE-24, pp. 258-266, 1986. sity of Arizona, Tucson, in 1987. He is currently

141 R. M. Goody, Atmospheric Radiation L. Theoretical Basis. Oxford: working.toward the Ph.D. degree in electrical en-

Clarendon, 1964, pp. 125-170. gineering at the University of Arizona.

[5] W. M. Elsasser and M. F. Culbertson, "Atmospheric radiation ta- He has taught in the System Engineering De-bles," Meteorol. Monogr., vol. 4, pp. 1-43, 1960. - partment at the University of Arizona. His pre-

[6] K.-N. Liou, An Introduction toAtmosphericRadiation. New York: vious research includes infrared sensor perfor-Academic, 1980, pp. 87-121. mance modeling, radar control systems, and signal processing. His current

[7] F. E. Volz, "Economical multispectral sunphotometer for measure- areas of interest include atmospheric remote-sensing techniques and instru-ments of aerosol extinction from 0.44 to 1.6 am and precipitable mentation, microprocessor applications, and atmospheric modeling.water," Appl. Opt., vol. 13, pp. 1732-1733, 1974.

[81 C. Tomasi et al., "Multiwavelength sunphotometers for accuratemeasurements of atmospheric extinction in the visible and near-IRspectral range," Appl. Opt., vol. 22, pp. 622-630, 1983.

[9] A. S. Moore et al., "Calibration and performance of the HalogenOccultation Experiment sunsensor," SPIE Proc., vol. 685, paper 27,1986. *

[10] M. P. Thekaekara, "Extraterrestrial solar spectrum, 3000-6100 A atI A intervals," Appl. Opt., vol. 13, pp. 518-522, 1974.

[11] "Photoconductive silicon photodiode application notes," EG&GElectro Optics, Salem, MA, EG&G Appl. Notes D3000C-3, July1980.

[12] F. X. Kneizys et al., "Atmospheric transmittance/radiance: Com-puter code Lowtran 6," AFGL-TR-83-0187, 1983. Benjamin M. Herman received the B.S. and

[13] L. E. Mauldin et al., "Stratospheric aerosol and gas experiment 'I M.S. degrees in 1951 and 1954, respectively, frominstrument: A functional description," Opt. Engr., vol. 24, pp. 307- New York University, New York, and the Ph.D.

312, 1985. degree in 1963 from the University of Arizona,Tucson.

* l _ Since 1963, he has been on the faculty of theDepartment of Atmospheric Sciences and a Re-search Scientist at the associated Institute of At-

John A. Reagan (M'72-SM'83) was born in mospheric Physics at the University of Arizona.Grandview, MO, on May 2, 1941. He received His areas of research have been radiative transfer,the B.S. degree in physics and the M.S.E.E. de- multiple scatter phenomena, aerosol optics, andgree from the University of Missouri, Rolla, in atmospheric remote sensing.1963 and 1964, respectively, and the Ph.D. de- Dr. Herman is a fellow of the American Meteorological Society, and a

gree in electrical engineering from the University member of the American Geophysical Union and the American Associationof Wisconsin, in 1967. for the Advancement of Science.

Since 1967, he has been with the Departmentof Electrical and Computer Engineering at theUniversity of Arizona, Tucson, where he now isa Professor. His areas of interest and research in-

clude atmospheric optics, electromagnetic propagation, and atmosphericremote-sensing techniques and instrumentation.

Dr. Reagan has served on the GEOSCIENCE AND REMOTE SENSING SocI- *

ETY AdCom and is now Secretary-Treasurer of the society. He is a memberof the American Meteorological Society, where he has been actively in-volved in the AMS Laser Radar Conferences. He is also a member of TauBeta Pi, Sigma Pi Sigma, and Phi Kappa Phi. He is a registered Profes-sional Engineer in the State of Arizona.

* Avishai Ben-David was bom in Jerusalem, Is-rael, on August 2, 1956. He received the B.S. de-gree in atmospheric sciences in 1981 from the He-

Peter A. Pilewskie was born in Erie, PA, on Sep- brew University of Jerusalem, Israel, and the M.S.tember 30, 1961. He received the B.S. degree in and Ph.D. degrees in atmospheric sciences in 1983meteorology in 1983 from Pennsylvania State and 1986, respectively, from the University of

University, University Park, and the M.S. degree Arizona, Tucson.in atmospheric sciences in 1986 from the Univer- Since 1986 he has held a postdoctoral positionsity of Arizona, Tucson, where he is working to- in the Institute of Atmospheric Physics at the Uni-ward the Ph.D. degree. versity of Arizona. His areas of research have in-

His areas of interest include atmospheric re- cluded atmospheric remote sensing and methodsmote sensing, cloud physics, and atmospheric op- of solution for the inverse problem, radiative transfer, and multiple scat-

tical phenomena. tering in the atmosphere.