Optical Measurements from High Altitude Balloons

David G. Murcray, James N. Brooks, Norman J. Sible, and Herbert C. Westdal

A description is given of a system that has been constructed to make radiometric measurements fromhigh altitude balloons. The instrumentation consists of a 20-cm aperture, 30-cm focal length radiom-eter that has been equipped to scan in azimuth and elevation, a filter system so that spectral data canbe obtained, an onboard digital magnetic tape recording system for recording the data generated duringthe flight, auxiliary electronics and power supplies for operation of the equipment, and a gondola toserve as a suitable scanning platform and to protect the equipment when it is returned to the ground byparachute at the end of the flight. The results obtained on a balloon flight made with this equipmenton May 8, 1959 are also presented. For this flight the radiometer was equipped with a thermistorbolometer detector with a KRS-5 window and with filters that transmitted radiation from 1 to 2 ,2 ,u to 3 A, 3 u to 5 p, 5 p to 8 /, and 8 p to 35 a. These results are presented in the form of isoradiance plots.

Introduction

The present study has as its objective the measure-ment of the spectral and spatial distribution of theradiation received from the earth and its atmosphereat high altitudes. In order to accomplish this ob-jective a radiometer system capable of making thesemeasurements automatically and light enough to betransported to high altitudes by means of large bal-loons has been constructed. A description of theequipment and the results obtained when the equip-ment was flown on May 8, 1959 form the body of thisreport.

Instrumentation

The radiometer consists of a 20-cm aperture,30-cm focal length Cassegrainian telescope. A planemirror which makes an angle of 500 with respect tothe optical axis of the radiometer is placed at the frontaperture of the radiometer. Scanning is accomplishedby rotating this mirror about the optical axis of theradiometer, which is depressed 100 from the horizontalduring flight. This combination of mirror angle anddepression angle results in a conical scan that passesthrough the nadir but only within 200 of the zenith,thus missing the balloon. Azimuth scan is achievedby rotating the whole radiometer with respect to abase plate which is fixed with respect to the gondola

The authors are at the Denver Research Institute, Universityof Denver, Denver, Colorado.

Received 7 November 1961.Research reported in this document has been supported by

the Aeronautical Systems Division of Air Force Systems Com-mand.

that houses the equipment during flight. The planemirror is rotated through 3600 in 30 sec and the radi-ometer traverses 180° in azimuth in 5 min. The azimuthscan is limited to 180° by means of microswitches whichreverse the direction of rotation of the radiometerand are activated by the radiometer motion.

The incoming radiation is interrupted 80 times persecond by means of an eight-bladed chopper. Theradiation also passes through openings in a filter wheelwhich is mounted coaxially with the chopper. Thefilter wheel has positions for 12 filters and is rotatedthrough one step each time the direction of motion ofthe azimuth scan is changed. Thus every 5 min ascan is completed with a different filter in front of thedetector.

For the flight of May 8, 1959 the radiometer wasequipped with a 2.5 mm square thermistor bolometerdetector equipped with a KRS-5 window. With thisdetector the radiometer system has a /2 X /20 squarefield of view. The detector is housed in a blackbodycavity and as the chopper interrupts the incomingradiation the detector gives rise to an ac voltage thatis proportional to the difference between the incomingradiation and the radiation from the blackbody cavity.The blackbody is not maintained at a constant tem-perature but is allowed to assume ambient tempera-ture and the temperature is monitored. The ac signalfrom the detector is amplified, synchronously rectified,and filtered. The details of the radiometer are shownschematically in Fig. 1.

A knowledge of the position of the scanning mirror,the orientation of the radiometer with respect to thegondola, and orientation of the gondola with respect

March 1962 / Vol. 1, No. 2 / APPLIED OPTICS 121

Ftig. 1. Schematic diagram of balloon borne radiometer.

to the earth is required for the determination of thedirection from which the radiation is being received.The position of the scanning mirror and the orienta-tion of the radiometer with respect to the gondola aredetermined by monitoring the voltages across potenti-ometers that are coupled to the two drive systems. Theorientation of the gondola with respect to the earth isdetermined by means of two magnetometers whoseprobes are placed at right angles to each other. Byusing two probes in this fashion it is possible uniquely todetermine the position of the gondola with respect tothe earth's magnetic field.

The data are recorded in digital form on magnetictape by means of a digital tape recorder which iscarried aloft with the rest of the instrumentation.The recorder consists of an analog to digital converterthat converts the input voltages into a binary codeddecimal suitable for recording on magnetic tape and aseven channel 12-mm tape transport. The analog todigital converter samples 10 input channels whosevoltages range from 0.00 to 9.99 volts, ten times persecond, with an accuracy of 0.01 volt. The tapetransport takes the standard 27-cm magnetic tape reels.On this flight the data were recorded using a 25 mm/sectape speed and 0.025-mm magnetic tape. This com-bination of tape thickness and tape speed gave an over-all recording time of 12 hr.

All mechanical rotations are accomplished by meansof 400 cycle, 115 volt single phase synchronous motors.

The 400 cycle power is supplied by means of a sinewave inverter. The main source of power is a 28 voltlead acid battery. All other voltages except the detec-tor bias voltage are either supplied by the 28 voltsource or derived from the 400 cycle source.

The equipment is housed in a gondola which is usedto provide a means of suspending the equipment fromthe balloon without interfering with its scanningmotion and also to protect the equipment when it isreturned to the ground by parachute. The gondola isconstructed from electrical conduit. The radiometersystem is suspended in the gondola by springs. Thesprings are tied with light cord so that the radiometeris supported on a rigid base during flight. Upon impactthe cords break and the springs are extended beyondtheir elastic limit. This system reduces the forceson the radiometer during impact.

Calibration Procedures

The radiometer was calibrated using a Barnes En-gineering Company 30-cm aperture blackbody sourcewhich was placed as close to the scanning mirror aspossible to reduce atmospheric transmission losses.The source temperature was varied from 40 to 120'Cin 10° increments and the corresponding signal voltagesrecorded. This calibration was performed the daybefore the flight using the same bias batteries andpower supplies that were used during the flight.

Since the gondola is constructed of electrical conduit

122 APPLIED OPTICS / Vol. 1, No. 2 / March 1962

it was also necessary to calibrate the magnetometers inposition on the gondola. This was accomplished byorienting the gondola toward magnetic north andrecording the magnetometer outputs. The gondolawas then rotated through 100 and the outputs recorded.This process was continued until the gondola had beenrotated through a complete circle.

Description of the Flight

The results presented in this report were obtainedin a balloon flight that was made on May 8, 1959, fromHolloman Air Force Base, New Mexico. The equip-ment was launched at 0515 and rose with an averageascent rate of 400 cm/sec. It reached a floating altitudeof 26 km and was allowed to float at altitude until1715 when it was cut down by command from theground. The equipment impacted near Bingham, NewMexico. The sky was clear during the early morninghours but there was a slight cloud buildup by noon.

Data Reduction

The data obtained on this flight were recorded indigital form on magnetic tape. This digital tape wasplayed back directly into the Denver Universitydigital computer where the calculations necessary toconvert the voltages into radiance versus directionwere performed, and the results were printed out in theform of tables of radiance versus direction.

These tables were used to plot the results so thatthe isoradiance plots could be drawn. The drawingof the isoradiance lines after the values have beenplotted is, to a certain extent, subjective since the in-formation available is limited. In addition a greatdeal of fine structure has to be ignored in order to beable to draw the isoradiance lines at all.

No attempt is made to orient the gondola duringflight. The gondola motion consists mainly of a slowrotation. This rotational motion is superposed in theazimuthal scanning motion and as a result there areoccasionally gaps in the scan. Because of the place-ment of the filters, each hour three successive scansare made without any filter in front of the detector.The combining of the three scans makes it possible todraw a much more accurate isoradiance diagram thanfrom a single scan. Therefore plots are presented foreach hour since the background radiation was notchanging rapidly with time. This is not the case forthe readings taken during ascent, however; these willbe presented in a separate report.

The quantity that is measured when a filter is inposition in front of the detector is given by

KAV = AN = f NB()F(X)dX-f N,(T)F(x)dx

where NB(X) is the background radiance, Ni(XT) isthe radiance of a blackbody at temperature T F(X)

is the transmission function of the filter, and K isthe calibration constant for the system. Since Ni(XT)and F(X) are known functions it is possible to deter-mine the second integral by numerical integration.This integration was performed on a digital computerfor the various filters for the temperatures encounteredduring flight. The calibration constant K is also afunction of the temperature since the sensitivity of thedetector is a function of temperature. This variationof sensitivity was determined in a separate study andthe calibration constant adjusted accordingly. Withthese quantities known it is possible to determine thevalue for the quantity

f NB(X)F(N)dx.

The quantity of actual interest is NB(X); however, inorder to determine this quantity it is necessary to makesome assumption as to the wavelength distribution ofthe incoming radiation. Rather than making suchassumptions the isoradiance plots are given for thevalues of the integral and the estimate of the wave-length dependence of N is left to the discussion.

The filter functions for the various filters are givenin Fig. 2. The signals obtained when filters no. 2and 3 were in front of the detector were greater thanthe system noise only when the radiometer field of viewwas scanning close to the sun in the upper hemisphereand in the direction of maximum scattering in the lowerhemisphere, so the results were not plotted.

Results and Discussion

The equipment, with the exception of one magnetom-eter, operated as expected and data were obtainedfrom launch until 1100. The output of one magnetom-eter fluctuated rapidly about its proper value. Be-cause of this fluctuation the orientation of the gondolahad to be determined on the basis of the output of theother magnetometer using the faulty magnetometeronly to determine the proper quadrant. Since theoutput of a single magnetometer is a function of thecosine of the angle between the magnetic field and theaxis of the probe there are regions where the outputvoltage is insensitive to slight changes in the gondolaorientation. Therefore the error in determining thegondola orientation was increased, and the valuesgiven for the azimuth angle are only accurate to within

-5o.The data obtained without any filter in front of the

detector are given in Figs. 3 through 5. Although scanswere made every ten minutes without any filters infront of the detector, selected results only are presentedsince the radiance was changing only slowly with time.Examination of these isoradiance plots indicates thata large part of the radiation observed toward the east inthe early morning is due to scattered or reflected sun-

March 1962 / Vol. 1, No. 2 / APPLIED OPTICS 123

Z0

U)

.9 1.0 1.- 0 41 22 03

-40-

30

20

10

0.9 1.0 1.5 2 3 4 5 6 7 8 9 10 14 18 22 26 30 34 /

WAVE LENGTH IN MICRONS

Fig. 2. Plots of transmission versus wavelength for filters no. 1 through no. 5. Numbers above the curves refer to the correspondingfilter.

000

Fig. 3. Isoradiance plot open filter. 0635-0650 hr M.S.T.,8 May 1959. Isoradiance values are in mw/cm 2 -sterad. Azi-muth values are with respect to magnetic north. 0 elevation ishorizon, 900 is nadir. Solar azimuth from magnetic north 670,solar elevation 170.

Fig. 4. Isoradiance plot open filter. 0735-0750 hr M.S.T.,8 May 1959. Isoradiance values are in mw/cm 2-sterad. Azi-muth values are with respect to magnetic north. elevation ishorizon, 900 is nadir. Solar azimuth from magnetic north 760,solar elevation 310.

124 APPLIED OPTICS / Vol. 1, No. 2 / March 1962

Fig. 5. Isoradiance plot open filter. 0935-0950 hr M.S.T., Fig. 6. Isoradiance plot filter no. 1. 0750-0755 hr M.S.T.,8 May 1959. Isoradiance values are in mw/cm'-sterad. Azi- 8 May 1959. Isoradiance values are in mw/cm'-sterad. Azi-muth values are with respect to magnetic north. 0 elevation is muth values are with respect to magnetic north. 0 elevation ishorizon, 900 is nadir. Solar azimuth from magnetic north 960, horizon, 90° is nadir. Solar azimuth from magnetic north 770,solar elevation 550. solar elevation 320.

light. The strong dependence of the radiation on theangle between the sun and the viewing angle indicatesthat the process is probably one of scattering. Al-though the weather was clear at Alamogordo on theday of the flight the daily weather map for May 8,1959 indicates the presence of a large storm overwestern Texas at midnight. Photos taken with a wideangle camera which was carried aloft with the equip-ment indicate a long thin cloud bank to the east at adepression angle of 15° from the horizontal. Betweenthis cloud layer and the horizon there is a very brightregion not associated with any visible cloud layer butwhich appears to be due to scattered sunlight. Thesephotos were taken using color movie film which respondsto shorter wavelength radiation than the thermistorbolometer since the KRS-5 window does not transmitbelow 600 m/a. On the basis of the photographs it wouldappear that the radiation arises from sunlight that isbeing scattered by something other than the tops ofclouds associated with the storm.

The results obtained when filter no. 1 is in front ofthe detector are presented in Figs. 6 and 7. The radia-tion observed when this filter is in front of the detectordefinitely arises from scattered or reflected sunlightsince the amount of radiation emitted by a blackhodyat the temperature of the earth's surface in this spectralwavelength interval is considerably less than thethreshold sensitivity of the instrument.

90

Fig. 7. Isoradiance plot filter no. 1. 0950-0955 hr M.S.T.,8 May 1959. Isoradiance values are in mw/cm2-sterad. Azi-muth values are with respect to magnetic north. 0 elevation ishorizon, 900 is nadir. Solar azimuth from magnetic north 99°,solar elevation 57°.

Comparison of Fig. 6 with Fig. 4 (the correspondingscan taken without any filter in front of the detector)verifies the fact that the radiation toward the horizonin the east in the early morning is due to scatteredsunlight. Similar comparisons of the data taken later

March 1962 / Vol. 1, No. 2 / APPLIED OPTICS 125

Fig. 8. Isoradiance plot filter no. 4. 0720-0725 hr M.S.T., Fig. 9. Isoradiance plot filter no. 4. 0920-0925 hr M.S.T.,8 May 1959. Isoradiance values are in mw/cm 2 -sterad. Azi- 8 May 1959. Isoradiance values are in mw/cm2-sterad. Azi-

muth values are with respect to magnetic north. 0 elevation is muth values are with respect to magnetic north. 0 elevation ishorizon, 90° is nadir. Solar azimuth from magnetic north 740, horizon, 90° is nadir. Solar azimuth from magnetic north 920,solar elevation 270. solar elevation 51 0.

in the day fail to yield the high correlation present inthe earlier data. This is not surprising since there area number of factors such as altitude of reflecting surface,reflectivity of the surface, etc., that affect the radiationdifferently in different wavelength regions.

It is possible to calculate the albedo of the earth inthis wavelength interval by assuming that wavelengthdependence of the incoming solar radiation is similarto a 6000'K blackbody. If one makes this assumptiona simple numerical integration indicates that the filtertransmits 38% of the solar radiation in the 1.05 to1.95 u region. Thus all of the radiance values quotedon the plots for this filter should be multiplied by afactor 2.6 to convert them to the radiance values thatone would observe without the filter if one assumes thatthe observed radiation has the same wavelength de-pendence as the incoming solar radiation. Assumingfurther that the solar constant is 2 cal/cm 2-min thenthe radiance of a perfectly diffuse reflector in the wave-length interval passed by the filter would be 8.9 coso mw/cm 2 -sterad. Examination of the values obtainedduring the scan made at 1050(0 = 23°) gives albedovalues in this wavelength region ranging from 0.32to 0.96 with the majority of the values falling in therange between 0.32 and 0.64. These values are basedon the assumption of a diffuse surface and in some casesthis condition is not satisfied. It should be noted thatthese values represent the albedos as measured ataltitude and may be quite different from the values onewould obtain if the measurements were made close tothe reflecting surface, since the intervening atmosphere

[so

Fig. 10. Isoradiance plot filter no. 5. 0730-0735 hr M.S.T.,8 May 1959. Isoradiance values are in mw/cm2-sterad. Azi-muth values are with respect to magnetic north. 0 elevation ishorizon, 900 is nadir. Solar azimuth from magnetic north 740,solar elevation 280.

will modify the results by scattering and absorbingthe solar radiation.

The results obtained when filters no. 2 and 3 were infront of the detector were seldom greater than thesystem noise so the results were not plotted.

126 APPLIED OPTICS / Vol. 1, No. 2 / March 1962

180

Fig. 11. Isoradiance plot filter no. 5. 0830-0835 hr M.S.T.,8 May 1959. Isoradiance values are in mw/cm2-sterad. Azi-muth values are with respect to magnetic north. 0 elevation ishorizon, 90° is nadir. Solar azimuth from magnetic north 840,solar elevation 420.

Fig. 12. Isoradiance plot filter no. 5. 1030-1035 hr M.S.T.,8 May 1959. Isoradiance values are in mw/cm 2-sterad. Azi-muth values are with respect to magnetic north. 0 elevation ishorizon, 900 is nadir. Solar azimuth from magnetic north 1110,solar elevation 640.

The data obtained with filter no. 4 in front of thedetector are presented in Figs. 8 and 9. The wavelengthregion passed by this filter lies almost entirely in the6.3 ,u water vapor band. Most of the radiation reachingthe radiometer in this case comes from the emission of

radiation by the water vapor in the atmosphere. Theonly detail sufficiently gross to be drawn on the isoradi-ance plots is the so-called "limb-darkening" effect, thefall-off in the incoming radiation as the scan approachesthe horizon.

The data obtained when filter no. 5 is in place infront of the detector are given in Figs. 10 through 12.This filter and the KRS-5 window in front of the de-tector pass radiation between 8 yu and 35 Au. This wave-length interval contains the so-called "window"between 8 /u and 13 1u and the intense absorption bandsdue to CO2 at 15 ,u, and the water vapor pure rotationalband which starts at 20 /i and extends over the remain-der of the interval. Since the incoming radiation isagain a function of the absorption and emission of theatmospheric gases limb darkening is also observedin the isoradiance plots obtained with this filter. Somestructure is observed in addition to the limb darkening.Since the CO2 is probably uniformly distributedthroughout the atmosphere this structure has to be dueto variations in temperature or emissivity of the groundor else to variations in the water vapor distribution.The amount of radiation leaving the top of the at-mosphere in the wavelength interval beyond 5 u is ofconsiderable meteorological interest and has been thesubject of many theoretical studies. One of the morerecent of these is the study by Hales and Zdunkowski1

using the new Atmospheric Radiation Tables preparedby Elsasser and Culbertson.2 These calculations aremade for the total long wavelength flux leaving theearth under different meteorological conditions. Inorder to compare our results with these calculationsit is necessary to convert our results from radiancevalues to flux and also remove the effect of the filter.An additional term will have to be added to correctfor the radiation from 5 to 8 Au which lies outside thewavelength interval passed by the filter. Assumingthat the wavelength distribution of the incomingradiation can be approximated by a blackbody of 270'Kresults in a multiplication by a factor of 2.8 to convertthe radiance as observed through the filter into radianceat the aperture. The problem of converting the radi-ance values to flux values entails calculating 2rXfq/2 N(0) cos0 sinG dO. This could be done numericallyusing the data obtained during the flight. However, arigorous comparison of the observed results datatheoretical calculations would require certain with(temperature of the ground over the region observeddistribution and temperature of water vapor in theregion being scanned, etc.) that are not available; henceit was not considered worthwhile to perform theintegration. Instead the radiation was assumed to beisotropic and the observed radiances close to the nadirare merely multiplied by r to change them to fluxes.When these corrections are made one obtains fluxes offrom 242 watts/meter2 to 285 watts/meter2. The

March 1962 / Vol. 1, No. 2 / APPLIED OPTICS 127

theoretical flux quoted for Broken Arrow, Oklahoma,varies from 285 watts/meter 2 for a ground temperatureof 400C to 259 watts/meter 2 for a ground temperatureof 150C. The ground temperature in the vicinity ofAlamogordo changes very rapidly as the sun heats itand quite often runs 15'C higher than the air tempera-ture, and can run as high as 450C. The SacramentoMountains which lie to the east of Alamogordo areconsiderably higher and cooler than the Tularosa Basinand in addition are covered with trees, so the effectivesurface temperature of this region could be lower than150C. Thus the observed values appear reasonable inview of the many assumptions that had to be made in

order to compare the results.The authors wish to thank the personnel of the

Balloon Branch at the Air Force Missile DevelopmentCenter for the excellent job they did of launching andrecovering the equipment.

References1. J. V. Hales and W. G. Zdunkowski, "The long wavelength

flux leaving the earth under various meteorological condi-tions," Scientific Report No. 1, Contract AF 19(604)-2418.Univ. Utah (1960).

2. W. M. Elsasser and M. F. Culbertson, "Atmospheric radiationtables," Final Report Contract AF 19(604)-2413. ScrippsInst. of Oceanography, Univ. of Calif. (1960).

PhIoto D. J. Troy

Scene in London Airport on trip home from the IC() meetings-Neil Hochgraf Rochester, L. Mertz Block Associates, D. R.Herriott Bell Telephone, and R. Carpenter Geophysics Corpora-tion of America. See page A22 for application form for 1962 group

flight to Munich Meeting.

Photo D. L. MacAdam

Have you seen this one? S. Q. Duntley, who was awardedthe Ives Medal at the Los Angeles Meeting, and Vern E. Hamil-ton, Chairman of the Program Committee for the L. A. October

1961 OSA 46th Annual Meeting.

128 APPLIED OPTICS / Vol. 1, No. 2 / March 1962

STOP PRESS

OSA President D.L. MacAdam will lecture to the Local Sectionsas follows:

Detroit March 26 Subject to be announcedRochester April 17 Color in CourtSan Francisco April 30 Subject to be announcedLos Angeles May 2 Optical EngineeringSan Diego May 4 Optical Engineering

Sigma Xi ClubDates for other sections will be announced later