Infrared multidetector spectrometer for remote sensing of temperature profiles in the presence of clouds H. H. Aumann and M. T. Chahine An infrared multidetector spectrometer with channels in the 4. 3 -jumand 15-Mm CO 2 bands for the remote sensing of temperature profiles in the presence of clouds is described. Results obtained from aircraft flights in July 1975 over ocean sites under various conditions of cloudiness demonstrate the capability of the dual frequency technique to recover surface temperatures to an accuracy of O0.5 K in the presence of up to 90% cloud cover. Introduction There is considerable need for determining atmo- spheric vertical temperature profiles and surface tem- peratures from measurements of the outgoing radiance by instruments carried on satellites. The vertical temperature profiles, humidity profiles, and surface temperatures obtained from existing sounders on the various Nimbus and NOAA satellites indicate that good accuracies can be achieved under cloudless conditions. In general, however, clouds exist in the fields of view and do affect the values of the measured radiances. It is necessary, therefore, to take the effects of clouds into account if accurate and reliable temperature values are to be determined. The 4.3-gm CO 2 absorption band is the best spectral region for recovering accurate temperature profiles with high vertical resolution in the lower troposphere, near the earth's surface. But, the problem of multiple cloud layers has so far prevented the use of 4.3-gm observa- tions on a routine basis. However, by making addi- tional measurements in properly selected parts of the 15-gm CO 2 band, Chahinel has shown that the effects of clouds on the outgoing radiance can be eliminated and that the corresponding clear column radiance values can be accurately reconstructed, leading to the recovery of clear column temperature profiles with the same degree of vertical resolution and accuracy per- mitted under cloudless conditions. Once the temper- ature profile is known, the fractional cloud covers, heights of clouds, and the humidity profiles can be re- covered, provided that all the channels on the instru- ment observe the same field of view at the same time. The authors are with Jet Propulsion Laboratory, Space Sciences Division, Pasadena, California 91103. Received 25 February 1976. Thus, the multidetector approach is essential in order to insure the simultaneity of all measurements and to make certain that all channels register the effects of the same clouds. No assumptions are made about the spectral properties of clouds or their distribution. In the final data analysis, the terms containing spectral effects are eliminated by analyzing data from four ad- jacent fields of view, while terms containing the frac- tional cloud covers are determined from the 15-Am channels. The vertical temperature profile is recovered from the 4.3-gm channels. The surface temperature is obtained from the 3.7-gm window channels, and the water vapor mixing ratio is obtained from channels in the 6.3-gm H20 band. In order to verify experimentally this dual frequency concept we have made a number of aircraft flights at up to 7.6-km altitude over land and ocean sites under various conditions of cloudiness and sun angle, using a multidetector ir radiometer covering the required wavelength region. In this paper we describe the in- strument, its calibration, and its flight performance. A detailed analysis of the data obtained from these air- borne observations which demonstrates the accuracy and capability of this concept is presented elsewhere. 2 Instrument Description The instrument is a modified version of a balloon- borne grating spectrometer, described by Schaper and Shaw. 3 The original instrument was used to verify experimentally the concept of temperature sounding in a cloud-free field of view using the 4.3-gm CO 2 band covered only the 3.7-5 gm wavelength region. This wavelength region was expanded through the use of a coarser grating to incorporate four cloud sounding channels between 11 gm and 14 gm and three channels near 5.4 gm for the determination of the water vapor profile in the short wavelength wing of the 6.3 gm H20 band. In order to adapt the instrument to the aircraft September 1976 / Vol. 15, No. 9 / APPLIED OPTICS 2091
Transcript
Infrared multidetector spectrometer for remote sensing of
temperature profiles in the presence of clouds
H. H. Aumann and M. T. Chahine
An infrared multidetector spectrometer with channels in the 4.3-jum and 15-Mm CO2 bands for the remote
sensing of temperature profiles in the presence of clouds is described. Results obtained from aircraft flights
in July 1975 over ocean sites under various conditions of cloudiness demonstrate the capability of the dual
frequency technique to recover surface temperatures to an accuracy of O0.5 K in the presence of up to 90%
cloud cover.
Introduction
There is considerable need for determining atmo-spheric vertical temperature profiles and surface tem-peratures from measurements of the outgoing radianceby instruments carried on satellites. The verticaltemperature profiles, humidity profiles, and surfacetemperatures obtained from existing sounders on thevarious Nimbus and NOAA satellites indicate that goodaccuracies can be achieved under cloudless conditions.In general, however, clouds exist in the fields of view anddo affect the values of the measured radiances. It isnecessary, therefore, to take the effects of clouds intoaccount if accurate and reliable temperature values areto be determined.
The 4.3-gm CO2 absorption band is the best spectralregion for recovering accurate temperature profiles withhigh vertical resolution in the lower troposphere, nearthe earth's surface. But, the problem of multiple cloudlayers has so far prevented the use of 4.3-gm observa-tions on a routine basis. However, by making addi-tional measurements in properly selected parts of the15-gm CO2 band, Chahinel has shown that the effectsof clouds on the outgoing radiance can be eliminatedand that the corresponding clear column radiancevalues can be accurately reconstructed, leading to therecovery of clear column temperature profiles with thesame degree of vertical resolution and accuracy per-mitted under cloudless conditions. Once the temper-ature profile is known, the fractional cloud covers,heights of clouds, and the humidity profiles can be re-covered, provided that all the channels on the instru-ment observe the same field of view at the same time.
The authors are with Jet Propulsion Laboratory, Space SciencesDivision, Pasadena, California 91103.
Received 25 February 1976.
Thus, the multidetector approach is essential in orderto insure the simultaneity of all measurements and tomake certain that all channels register the effects of thesame clouds. No assumptions are made about thespectral properties of clouds or their distribution. Inthe final data analysis, the terms containing spectraleffects are eliminated by analyzing data from four ad-jacent fields of view, while terms containing the frac-tional cloud covers are determined from the 15-Amchannels. The vertical temperature profile is recoveredfrom the 4.3-gm channels. The surface temperatureis obtained from the 3.7-gm window channels, and thewater vapor mixing ratio is obtained from channels inthe 6.3-gm H20 band.
In order to verify experimentally this dual frequencyconcept we have made a number of aircraft flights at upto 7.6-km altitude over land and ocean sites undervarious conditions of cloudiness and sun angle, using amultidetector ir radiometer covering the requiredwavelength region. In this paper we describe the in-strument, its calibration, and its flight performance. Adetailed analysis of the data obtained from these air-borne observations which demonstrates the accuracyand capability of this concept is presented elsewhere.2
Instrument Description
The instrument is a modified version of a balloon-borne grating spectrometer, described by Schaper andShaw.3 The original instrument was used to verifyexperimentally the concept of temperature soundingin a cloud-free field of view using the 4.3-gm CO2 bandcovered only the 3.7-5 gm wavelength region. Thiswavelength region was expanded through the use of acoarser grating to incorporate four cloud soundingchannels between 11 gm and 14 gm and three channelsnear 5.4 gm for the determination of the water vaporprofile in the short wavelength wing of the 6.3 gm H20band. In order to adapt the instrument to the aircraft
Fig. 1. Optical and mechanical layout of the infrared multidetectorspectrometer.
environment the spectrometer was antivibrationshock-mounted inside a container maintained at apressure of 100 mm Hg with dry nitrogen and separatedfrom the outside atmosphere by a ZnSe pressure bulk-head window. ZnSe was chosen because of its strengthand moisture resistance, although its high reflectivitycomplicates the radiometric calibration. A liquid ni-trogen Dewar provides cooling for the detectors and achopper and purge gas for the instrument and the out-side of the pressure bulkhead window. The spec-trometer and a 35-mm camera are mounted downlooking together with the nitrogen Dewar on a pallet atflight station 863.00 in the cargo bay of a P3A aircraft.The camera is bore-sighted with the spectrometer andserves as a visual monitor of cloud conditions. Theelectronic control and recording equipment aremounted in the passenger compartment.
The optical and mechanical layout of the instrumentis shown in Fig. 1. Radiance from an extended sourcepasses through the 0.3-mm thick ZnSe pressure bulk-head window, enters the spectrometer at f13 through a1.5-cm X 0.3-cm entrance slit, and is collimated onto a80-lines/mm grating blazed for 11.35 gm. The gratingis used in first, second, and third order. The second andthird order dispersed image of the entrance slit is dis-played on an array of nineteen PbSe detectors, the firstorder is directed by a germanium beam splitter to fourpyroelectric channels. All detectors view the same fieldof view simultaneously, since the grating is the commonaperture stop. The proper wavelength positions of thedetectors were determined from a ray tracing and were
subsequently fine adjusted using a monochromator witha resolving power of 1000. The monochromator wascalibrated with a He-Ne laser in high orders. Thewavelength and bandwidth of the twenty-three chan-nels are given with their measurement objectives inTable I.
Immediately in front of the entrance slit, but behindthe ZnSe window, is a 296-Hz tuning fork chopperwhose blades are blackened on the side facing the slitand gold plated on the other side. The chopper bladesare cooled by cold nitrogen gas flow to the base of thechopper assembly. Temperature sensors and servo-controlled heaters, mounted to each chopper blade,maintain the mean temperature of the chopper bladesat 240 i 0.4 K. Because of the location of the chopper,the calibration has to account for radiance emitted bythe entrance slit cavity and reflected off the ZnSe win-dow. For this reason the entrance slit cavity, as well asall other surfaces requiring high emissivity without theaid of multiple reflections, was painted with 3M black,with emissivity E 0.92.4 Its temperature, monitoredas part of the engineering data, dropped during opera-tion to between 250 K and 260 K.
Each pyroelectric detector assembly consists of anorder isolation filter, a 0.6 X 1.3-cm antireflection coatedgermanium Fabry lens, which sets the nominal spectralresolution at 2%, and a 3-mm diam blackened LiTaO3detector.5 The pyroelectric array is thermally tied tothe spectrometer housing which cools during operationto slightly below 260 K. The nominal noise equivalentpower of the detectors ranged from 3 X 10-9 W/Hz1 /2to 5 X 10-9 W/Hz 1 /2 at 296 Hz.
The PbSe detectors6 are cooled to 210 I 0.1 K by aservocontrolled cold nitrogen gas flow. They are ar-ranged in three wavelength groups under order isolation
filters. The detectors are 1.5 mm in diameter, mountedon the backsides of hemispherical, antireflection coatedstrontium titanate immersion lenses. Individual 0.3 X
1.3-cm antireflection coated silicon Fabry lenses imagethe grating onto the detectors and set the nominalspectral resolution at 1%. The detectors achieve a noiseequivalent power between 10-11 and 4 X 10-12 W/Hz1 /2at 296 Hz. The outputs from the detector preamplifiersare filtered, demodulated at the chopper frequency, andfed into separate 4-sec integrators and sample-and-holdcircuits.
A two-point internal radiometric calibration of theinstrument is accomplished by periodically inserting acalibration plate and a zero shutter into the optical path.The optical zero of the instrument is measured byclosing the entrance slit with a shutter, thus preventingany chopped flux from reaching the detectors. Theresponsivities of the detectors are determined by in-serting a 3M blackened calibration plate in front of thechopper which fills the instrument field of view. Themean temperature of the calibration plate is monitoredby a themister and maintained at 282 I 0.1 K by a smallheater.
The observing and calibration sequence of the in-strument consists of repeated scans of nine frames, eachof 20-sec duration. The optical zero is determinedduring frames 1,2, and 3, during frames 4,6,7, and 9 theinstrument measures the target radiance, and frames5 and 8 are used to measure the signal from the cali-bration plate. At the start of each frame, t = to, all in-tegrators are set to zero, and a multiplexer starts sam-pling the temperature and voltage monitors. At t = to+ 6 sec signal integration is started. At t = to + 10 secall detector sample-and-hold circuits are clamped, and,in frames 4, 6, 7, and 9, the camera is triggered. Thedetector outputs are then sequentially sampled by themultiplexer. The output of the multiplexer is AIDconverted into twelve bits and stored on magnetic tapefor subsequent computer processing.
The voltage output of a detector channel at frequencyvi viewing a target of radiance Ni is then given by
where S(i) = voltage responsivity of detector (i);Ei = emissivity of 3M black;
Ti = window transmission at frequency vi;TcH = temperature of the chopper blades;
TE = temperature of the entrance slit cavity;B(vT) = Plank blackbody function at v and T;
Vo(i) = zero offset voltage of channel (i).
The quantity Vo(i) is measured directly during frames1, 2, and 3. The voltage responsivity-emissivity prod-uct is determined during frames 5 and 8 by measuring
VC(i) = S(i)ei[B(vj,Tc) - B(vjTcH)] + Vo(i),
where Tc is the temperature of the internal calibrationblackbody.
Because of the relatively low temperature of the en-trance slit cavity compared to ambient at flight altitude(-245 K), the calculation of Ni is much more sensitiveto Ti/Eq than to Ti. Based on the laboratory blackbodycalibration we adopted Ti/ei = 0.735 i 0.015, inde-pendent of wavelength. The difference between thisvalue and the expected calculated value of 0.76 is smalland probably due to temperature gradients and thecumulative effects of thermistor calibration uncer-tainties. This method was adequate to calculate ab-solute radiances to within 2% for effective temperaturesin the 240-300K range.
30'
290
t!280=1t:
27°1
Radiometric CalibrationIn order to determine vertical temperature profiles
and surface temperatures to within 1 K, absolute radi-ances have to be measured to better than 4% in allchannels. This requires a careful calibration of thedetector responsivities and of the effects of the pressurebulkhead window. The detector responsivities aredetermined during the internal calibration and zeroshutter frames as part of the observing sequence. Theeffect of the ZnSe window was determined in the labo-ratory by filling the instrument beam with a blackbodyat temperatures ranging from 77 K to 323 K. Thewindow transmits a fraction T - 0.71 of the source ra-diance and adds to the signal the reflected componentR of the radiance emitted by the entrance slit cavity, asseen by inspection of Fig. 1. The window is made by thechemical vapor deposition techniques and exhibits ac-cording to the manufacturer virtually no absorption inthe wavelength region of interest, i.e., the reflectedcomponent R = 1 - T.
26°
1.0_= 0.8_
5 o 0.6- - 0.4•1-0=
LONGITUDE (WEST)
1n,0. 2 Io L1 I InrjlLl
310
304
302 _
300 SHORE SEA lAND
298 , I i i0 2 4 6 8 10 12 14 16 18 20 22 24 26 28
STATIONSI , I I
48 50 55 60 65
SUN ZENITH ANGLE
*MEASURED BUCKET TEMPERATURE, K
Fig. 2. Flight plane and results of the 7-24-75 test flight.
The instrument was installed on a NASA P3A Earthresources aircraft (NASA 927) at Ellington AFB, Texas,during June 1975. An engineering flight over a cleararea of the Gulf of Mexico, 32 km (20 miles) offshorefrom Louisiana, on 16 July 1975 with 20-min runs at0.3-km, 4.8-km, and 7.6-km altitudes was used to checkout the instrument and verify its ability to measureaccurate surface temperatures under clear conditions.Three afternoon and one sunrise flight on 21 July 1975,24 July 1975, 25 July 1975, and 31 July 1975 from El-lington AFB, each at 7.6-km altitude and of approxi-mately 3.5-h duration, were timed to pass over radio-sonde launches from Lake Charles, Louisiana, orBrownsville, Texas, for truth tests. The aircraftheadings were adjusted in flight by one of the investi-gators (HHA) to cover conditions ranging from visuallyclear, hazy, general low overcast to overflights of smallforming or dissipating thunderstorm heads.
The instrument performed essentially as anticipated.The PbSe detector channels were twelve-bit digitizationnoise limited. The performance of the pyroelectricdetectors degraded due to their susceptibility to mi-crophonics, but this effect could be held to acceptablelevels by maintaining the indicated airspeed near 315km/h (ground speed 480 km/h), resulting in SNR near40:1 for effective temperatures near 270 K, as comparedto 100:1 under laboratory conditions.
Figure 2 shows the surface temperature recovered inthe presence of up to 90% cloud cover and the fractionalcloud distribution of the 24 July 1975 flight as a functionof scan number and sun zenith angle. Each scan pointrepresents the results obtained from the analysis of fourconsecutive fields of view, 2.5 km X 3 km, with centersseparated by 2.7 km. Between scan 4 (100 km SW of'Lake Charles) and scan 17 (120 km NE of Brownsville)the flight path was more than 50 km offshore. DuringJuly and August the Gulf water surface temperature atthis distance from shore is no longer effected by coastalwater runoff and is nearly constant -302 K. This is ingood agreement with the recovered surface tempera-tures of 302.1 0.5 K shown in Fig. 2. The pointsmarked at scans 1 and 15 correspond to bucket tem-peratures cataloged by the U.S. Navy Fleet NumericalWeather Central at Monterey, California. At scan 19no data were taken because of a major heading change
from SW to NNE. Scans 20 through 24 are over coastalwater range (Laguna Madre), where the measuredtemperature was somewhat below the open Gulf surfacewater temperature, possibly due to water runoff.Cloud-top height was estimated and recorded on theflight log by the onboard observer and agreed with therecovered cloud-top height to within the 300-m ac-curacy of the visual height estimate. However, thefractional cloud covers determined from the ir dataagreed, as should be expected, with estimates based onthe pictures from the onboard camera monitor only inthe case of thick clouds. A detailed analysis of the data,including temperature and humidity profiles, cloudheight, and cloud type classification and correlationwith radiosonde and visual observations will be pre-sented elsewhere.2
The data reported in this paper could not have beenobtained without a considerable cooperative effort.The ray trace and optical modification of the spec-trometer were performed under contract to BeckmanInstruments, Anaheim, California. In particular weacknowledge the effort of M. Kassen, who constructedthe analog and digital interface and operated the in-strument during the flights, and the support receivedfrom the Earth Resources Group and the P3 pilots atthe Johnson Space Flight Center, Houston, Texas.
This paper presents the results of one phase of re-search carried out at the Jet Propulsion Laboratory,California Institute of Technology, and was sponsoredin part by the National Science Foundation, Office ofInternational Decade of Ocean Exploration, and theOffice of Naval Research as part of the North Pacificexperiment (NORPAX) under National Science Foun-dation Research grant AG505 and by the NationalAeronautics and Space Administration under contractNAS7-100.
References1. M. T. Chahine, J. Atmos. Sci. 31, 233 (1974).2. M. T. Chahine, H. H. Aumann, and F. W. Taylor, submitted to J.
Atmos. Sci. (1976).3. P. W. Schaper and J. H. Shaw, Appl. Opt. 9,924 (1970).4. D. L. Stierwalt, Appl. Opt. 5,1911 (1966).5. Laser Precision, Inc., Yorkville, N.Y.6. Santa Barbara Research Company, Santa Barbara, Calif.7. CVD process, Raytheon, Waltham, Mass.
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