Observations From the Nimbus I Meteorological Satellite

8/7/2019 Observations From the Nimbus I Meteorological Satellite

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, .IACCESSION6 6 -UMBE RI -12 130N(-rTHRU)112

I.(CODE)

P2(PAGES) II7 0(NASA CR OR TMX OR AD NUMB ER) ICATEGORY)

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OBSERVATIONS F ROM TH E


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T ~ M B U SMETEOROLOGICALSAT ELL IT^Six papers presented bg Goddard Space Fl ightCenter personnel at the western annuaI meetingof the American Geophysical Union, Seatt le,Washington, December 29, 1964---.. .

Prepared by Goddard Space Flight Center- -l_l__._

Scientific and Technical lnformation Division 1 9 6 5NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONWashington, D.C.

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For sale by the Clearinghouse for Federal Scientific ond Technical InformationSpringfield, Virginia 22151 - Price $1.00

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I11 THEORETICAL RELATIONSHIP BETWEEN EQUIVALENT BLACKBODYTEMPERATURES AND SURFACE TEMPERATURES MEASURED BYTHE NIMBUS HIGH RESOLUTION INFRARED RADIOMETER

Virgil G. Kunde.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

CONTENTS

pageFOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiI THE NIMBUS I FLIGHT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ilbur B. Huston and Harry Press. 1 YI1 THE HIGH RESOLUTION INFRARED RADIOMETER (HRIR) EXPERIMENT

L. L. Foshee, I. L. Goldberg, and C. E. Catoe . . . . . . . . . . . . . . . . . . . . . . . 13 1

V POLAR EXPLORATION WITH NIMBUS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 /. Popham and R. E. Samuelson.VI EXAMPLES OF THE METEOROLOGICAL CAPABILITY OF THE ,-IMBUS SATELLITELewis J . Allison, J . S. Kennedy and G. W. Nicholas. . . . . . . . . . . . . . . . . . . . 61 I'

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THE NIMBUS I FLIGHTWilbur B. Huston* and Harry Press7

INTRODUCTIONNimbus I w a s successfully launched by a Thor/Agena launch vehicle fr om the West ern Te st

Range (WTR) t Vandenberg A ir Force Base on August 28 , 1964. The flight marked the first inNASA's advanced ser ie s of meteorological satellites. The purpose of th is paper is (1) o de-scr ibe the Nimbus spac ecraf t and the Nimbus sy stems, (2 ) to descr ibe the meteorological sen-sors carried on this flight, and (3) to define the scope, charac teris tics, and limitations of th emeteorological data collected. This paper wi l l serve as an introduction and background fo r themore deta iled and sp ecific pa per s which follow and which deal with the meteoro logical signifi-cance of the da ta obtained f ro m this flight.

NIMBUS PROGRAMTh e Nimbus prog ram is a meteoro logical rese ar ch and development prog ram of the National

Aeronautics and Space Adminis tration. The program involves the flight of a se ri es of large,amply powered, ea rt h stabilized, omnibus space craf t which a r e to be launched in relativ ely lowaltitude (500-750 nautical miles) near-pola r orbi ts. This orbital choice per mit s f u l l earth cover-age and dat a collection on a daily basis. The spacecr aft and supporting ground complex ar e con-figu red to facil itate re al- tim e data collection, processing, and application. Subsequent flights willutilize the sam e bas ic spacecraft, with an expanding set of advanced meteorological sensors,which a r e cur rent ly under development by NASA and by the scientific community.

THE SPACECRAFTFigure 1 shows the Nimbus I spacecraft in flight configuration. The spacecra ft is roughly

ten feet tall from the base of the se nso ry rin g to the top of the command antenna. The weight atlaunch was 832 pounds. The spacecraft consists of three major elements: the sol ar paddleswhich are 8 feet by 3 feet, and provide the basic elect ric power supply; the upper hexagonalpackage which contains the complete attitude stabilization and control s yste m fo r the spacecraft;and finally, the lower sens ory ring which co nsis ts of a 54-inch diameter toroidal structure.This tor us contains eighteen standard -size bays which house the major electronic elementsof the space craf t. Larg e and bulky elements such as camer as and tape r eco rde rs a r e mountedin the open center portion of the torus.*Deputy Nimbus Project Manager, GSFC.?Nimbus Project Manager , GSFC.

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WILBUR B. HUSTON AND HARRY PRESS

ATTITUDE CONTROL HOUSINGTHERMAL 1

INTERCONNECTI

HIGH -RESOLUTIONINFRARED RECORDERHIGH -RESOLUTION'

AUTOMATIC ' AUTO'MAT CPICTURE -TRA NSM ISSIO N PICTURE- TRANSMISSIONANTENNA CAMERA

Figure 1-Nimbus I spacecraft.

BEAlTELEMEls t

CO N'RY A

HORIZON SCANNER

YAW NOZZLESSOLAR PADDLE

9 DIRECTION IN ORBIT\

. c '

Solar paddles provide 450 watts underf u l l so lar illumination. The attitude controlsystem houses a solar ar ra y drive which canprovide rotation of the paddles about thepitch axis of the spacecraft. The attitudecontrol system is capable of keeping thespac ecra ft pointed towards the e ar th with apointing accuracy in all thre e axes of betterthan one degree on a lar ge par t of the mis-sion. In addition, the spacec raft rotationalra tes are extremely slow, l es s than 0.05 de-gr ee s per second, and well within the re-quiremen ts for picture taking. The basicspacecraft also contains a command systemand a housekeeping telemetry system (4000data points per minute) for monitoring theengineering perfor mance of the s pace craf tin orb it. These housekeeping da ta includespacecraft atti tude e rr or measurements

which ar e required f or the interpr etation and accurate ear th positioning of the sens ory data.To provide an optimum ther mal environment for the many electronic and solid sta te devices(4000 transistors, 6000 diodes), ther mal control shu tte rs wer e provided whicn maintain thetemperatu re of all components at 25C f 3 " at all times.

The spacecraft carri ed th ree meteorological sens ors located on the sensory ring as shownin Figure 1. The largest, the advanced vidicon camera system (AVCS)wa s designed to meetthe needs of the national meteorolog ical se rv ic es fo r global weathe r da ta, especially cloudcover. The data ar e stored on board in a 1200-foot magnetic tape r eco rd er and a r e playedback on command to either of two command and data acquisition s it es which are linked to theGoddard Space Flight Center at Greenbelt, Maryland and to the National Weather SatelliteCenter at Suitland, Maryland by wide-band communication l ink s.

The automatic picture transmission system (APTS) provided simi la r cloud mapping photo-graphs of somewhat lesser resolution to local weather users who employed a simple inexpen-sive ground station (Refere nces 1 and 2) developed as pa rt of the Nimbus pro gra m. Some 65of these stations were in operation throughout the world by th e U. S. Weather Bureau (USWB),the A i r Weather Servic e, the U. S. Navy, U. S. Army, national weather se rv ic es of manycountries , and s ome private u se rs who built thei r own stations using the design data in R efer -ence 1. Pictures a r e transmitted continuously during satellite day, the s pacec raft command atthe local A P T stations is not required.

Both camera sy ste ms provided pi ctur es taken in daylight. At nighttime the high resolutioninfrared radiometer (HRIR) sys tem provided a continuous scan of the e ar th to detect emittedtherm al radiation. These data a r e stored on tape in the space craft and read out on commandover the S-bmd data link at the Same time as the AVCS pictures are read out over this link atth e command and data acquisition (CDA) sites.


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THE NIMBUS I FUGHT 3The communication and data handling aspects of the Nimbus s ys tem a r e discussed in m ore

detail in Reference 3; the spa cec raf t development and qualification are described in Reference4 . Since sensor y syst em development and data chara cteri stics a r e intimately related to thechoice of the desi red Nimbus orbit and to the actua l orbit achieved, so me desc ript ion of both isindicated prior to a mor e detailed description of the sensory sy ste ms.

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THE NIMBUS ORBITThe design orb it of the Nimbus syst em -

circu lar, sun-synchronous high-noon (Figu re2) - w a s chosen to take advantage of t he pre-cess ional moments induced on the orbit planeof a satellite by the equatorial bulge of theearth. For an orbit of a given semimajorax is and eccen trici ty, an inclination anglewith respect to the equator can befound suchthat the precession rate of the orbit plane

I 99% RETROGRADEN -SYNCHRONOUS ORBITORBITAL MOTIONLATITUDINAL

RTH ROTATIONLONGITUDINAL COVERAGE5

just matches the averag e rotational ra te of Figure 2-The high-noon sun-synchronous or bi t formeteorological satelI tes.he ear th about the sun, 0.9856 degr ees perday (Reference 1).

.circular orbit at an altitude of 500 nautical mil es (576 statu te mi le s or 929 kilome ters ) w a schosen f or Nimbus I. A t that altitude, the required inclination angle to maintain synchronismbetween the precession of the orbit plane and t h e mean motion of the sun in right ascension, is99.05 degrees.

On the basis of the resolution and coverage constraints imposed by the optical sensor, a

Th e time of lifto ff and thus of injection into orbit, was chosen so that the angle between theorbit plane and the earth-sun line would be small, establishing a "high-noon" orbit . Th is choicecoupled with sun s ynch ron ism made all northbound equator cro ssi ngs occur near local noon,and southbound crossings near local midnight.

Because of an undetected leak in the Agena fuel transfer equipment at the WTR launch pad,approximately 40 pounds of fuel were not loaded as planned, and the desire d ci rcul ar o rbit wasnot obtained. A comparison of the desir ed and actual orb its is shown in Figure 3 . Apogeealtitude w a s within two nautical m ile s of the desired value, but a short second burn of theAgena resulted in a velocity deficiency which caused the elliptic orbit . Per ige e was locatedinitially 20 degr ees above the equator on the dar k or nighttime portion of the orbit. Northwardmotion of the agru men t of perigee at the rate of 3 .1 degrees/day resulted in a dr if t of the lowpor tion of th e or bi t toward the command and data acquisition (CDA) station ne ar Fairbanks,Alaska, with a consequent reduction in the duration of pas se s and of the time availablefor data readout. A s a result, not al l data recorded on the spacecraft could be read outand ther e were gaps in the sensory data coverage for this reason.


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4 WILBUR B. HUSTON AND HARRY PRESSSENSORY DATA

Advanced Vidicon Camera SystemEach of the th re e espec ially developed

television ca mera s in the AVCS has a squarefield of view of 37 degrees. The centralcamera poin t s str aig ht down toward the cen-t e r of the earth; the side cam era s areoriented at angles of 35 degrees from thenadir, thus having a two-degree overlap atthe sides of the central picture. The earthprojection of the th re e- ca me ra field of viewillustrated in the top portion of Figure 4would at the design altitude have providednearl y continental coverage. The pict uresrecorded by the three came ras ar e illus-trat ed in the lower portion of F igu re 4. Thegrid lines shown are written over the pic-tures by a computer tied into the AVCSground station. They are computed on theba si s of the spacecraft ephemer is, and thetime of shut ter opening as recorded on thepict ure timing channel. The grid lines a r eplotted at two degree intervals; the latitudeand longitude legend, al so wri tten on the pic-ture by the computer, identifies the inter-section shown by the arrow. North and southlatitude is distinguished by the let te r N; th elongitude given by the final th re e digi ts of thesix-digit field is expressed in degrees ea stof Greenwich.

3 .1 DEG/DAY/--f--.. -0:-'

' IDESIRED ACTUAL

576 579 - 263929 932 - 423103.5 98 .7f 8 -8.1'f 2 . 3 + 3 . 313.9 14.72 - 3 2 - 7

ORBIT (MI )P ERIOD (M I N )H I G H N O O NS U N S Y N C . ( D E G / M O )BLIND ORBlTS/DAY

(KM)

ORBITS/ DA Y

Figure 3-The Nimbus I orbit.

AVCS FIELDj35 125 11 5 IO5

Figure 4-The f ield of view of the advanced vidiconcamera system (AVCS).

An example of a s t ri p of AVCS pictures is shown in Figure 5. Each set of picture tripl etshas been mosaiced; the gaps between su cces sive tr ipl ets res ult f rom the lower than design alti-tude in the elliptic orbit. The se pictures, which we re re cord ed over the Nile valley and Eas ter nMediterranean on September 16, 1964, wer e tran smit ted fr om the sp acec raft tape rec or de rOver the S-band data link to the ground station in Alaska, where they were relayed simultane-ously over two 48-kilocycle extre mely high quality dat a links to the Nimbus Data Cen ter atGreenbe lt, Md. and the USWB National Weather Satellite C ente r. A s a consequence of the real-tim e da ta handling capability of the Nimbus da ta System, the pictures w ere available for mete-orological use within one hour of the time t hey we re taken. The mosaic of Figure 6 includesonly six triplets; a normal se t taken f ro m sout h Pole t o North P ole on the daylight portion ofthe orbit includes 33 triple ts taken at 91-second intervals. With 800 lines on the TV raster,the ground resolu tion at the subsatel lite point is better than one-half mile. Th e wealth of detail


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THE NIMBUS I FLIGHT 5.-N I LE V A LLE YA N D E AS TE RNM E D I TE R R A N E A N

ORBIT 279-SEPT. 16

Figure 5-A mosaic strip from the advanced vidiconcamera system.

1 1120 90 60 30'0

Figure 6-One-day coverage of a representative APTground station.

in the AVCS pictures, detail in clouds, detail inice are as and sand areas, detail in geologicformations, of snow cover in mountain areas,all provide a challenging and rewarding re se r-voir of data which will be of undoubted value t ospecialized studies y e t to be performed.

The single TV ca me ra of the A P T systemis s im i l a r to the AVCS camera, with the ex-ception of a wide-angle le ns (108") and a poly-styrene layer which provides stora ge of thepicture image while the vidicon is scanned inthe readout cycle. Fou r line per second read-out of the 800-line ra st er occupies 200 seconds

of the 208 seconds between pictures . The APTS transmission s from the spa cecra ft in the 136-megacycle band are received by a sim ple ground station and fac sim ile machine.

The APTS system, announced to the meteorological community in 1962, was developedesp eciall y for the Nimbus pr ogram by the Aeronomy and Meteorology Division of GSFC, undercon tra cts with the Radio Corporation of Ame rica , and Fairchild Stratos, Inc. A first flight testwas provided by the TIR06 VI11 satellite in 1963. Over 60 ground stations have reported re-ception of APTS data fr om Nimbus I . Thes e stations, operated by the USWB, he militaryweather ser vic es, various European weather services, and by private users , were uniformityenthu sias tic about the quality of the local cloud cover d ata received, and its ready applicabilityto local foreca st requirements. The camera w a s designed for coverage of an a r e a 1050X 1050nautical mil es fr om the design altitude, with a 300-mile over lap in the north-south directio n.At one station pic tur es were normally received from the overhead pass , and from the previousand the following passes. The coverage obtained in one day at one station is shown in Figure 7.


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6 WILBUR B. HUSTON AND HARRY PRESS

25N

20N

15N

1ON

SEPT 4 19522

30N

25N

20N

15N

1ON130W 125W 120W

SEPT 3 19162

30N

25N

20N

15N

, 10N120W lliW ll0W

Figure 7-The development of a typi ca l storm as recorded at anAPT station 1000 miles from the storm center.

Nine complete and one partialpicture can provide very nearlycontinental coverage. Di rec t photosof the Caribbean hurrica ne zonewere available in the Washingtona r e a at noon each day during thesatellite lifetime. Consideringthe average speed of weather sys-tems as they move during theirlifecycle , the APTS coverage of1500 mile s ra dius provides an ex-ceedingly valuable tool for thelocal forecaster in even the are asof dense weather stat ions , as inthe East ern United States. Moreisolated foreca sters reported thatthe pictu res provided informationon the location of weather frontsand s to rm s ystem s not availablein any other way.

Unlike the AVCS pictures, the APTS pic tur es req ui re manual gridding by the us er . Thegridding process is both quick and stra ight forward. When use is made of the a ids developedfor this purpose by the A i r Weather Service, precomputed gri ds a r e provided for a range ofaltitudes at 50-kilometer i nterv als about the design orbital altitude, and for nominal lati tudesat 10-degree int ervals from the equator to the poles. These g rids a r e adjusted to the 10 inchX 10 inch fram e s ize of the facs imil e machine in the APT ground station. Thei r use in connec-tion with the spacecraft ephemeris data provided through the meteorological teletype net per-mits a choice of g ri ds applicable to the time of the start of tra nsm iss ion of each receivedpicture. An example given in Figure 7 illustrates the development and movement of tropicalst or m Tillie, west of Baja, Cal ifornia on September 3 and 4 . These pictures were recorded atthe NASA A P T station at Pt. Mugu, California. This station, located at 34%, 119W was ap-proximately 1000 miles f rom the s to rm c enter on September 3 . Based on the demonstrat edsuccess on Nimbus I, the automatic picture tra nsm iss ion sy ste m has been adopted as one ofthe basic came ra sys tem s for the TIROS operational sy ste m (TOS) s er i e s of sate llit es developedby NASA for the U. S . Weather Bureau to meet the interim national requirements f or an opera-tional meteorological sat ellite s ystem .

High Resolution Infra red Radiometer (HRlR]The key element of the HRIR system, described in detail in a subsequent paper in this

volume, is a scanning radiometer which senses infrared radiation at a wavelength of about4 microns. The radiometer scan s a st ri p perpendicular to the orbi tal path. The width of 1 lescan strip is about 5 nautical miles at the subsatel lite point and incre ases at the horizon, asillustrated in Figure 8. The radiometer signal, recorded on a spacecraft tape reco rder, is


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T H E NIMBUS I FUGHT

N

7

S

Figure 8-The earth path of the fie ld of the Nimbus hig h resolution infra red radiometer.

read out upon ground command at the CDA stations. Like AVCS, it is relay ed to Washingtonwhere the radiometer signal is reconstituted in a facsimile recorder and a picture is formed.The pictorial st rip rec ords, which cover a region from the North to the South Po les on thenighttime side of th e earth, are gridded automatically, an example of which is shown on theleft in Figure 8.

The HRIR for the first time provided accur ate, instantaneous pictorial t emper ature map-ping of the earth and of cloud tops. Some initial studies demonstra ting the data ar e report edsubsequently in this volume as well as in Reference 2.

The HRIR experimen t with its slow-response sensor and the repetitive s can pattern de-pends heavily on the space craft attitude control system. This three-axis ear th stabilizationsy st em provides the stable platform essentia l for the automatic reconstitution of the sca ns topermit a high resolution picto rial presentation of the sense d information.

PERFORMANCE SUMMARYOn September 23, 1964, after 26 days of operation, one of the few electromechanical sub-

systems on the spacecraft, the solar array drive, jammed and the paddle drive system which


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8 WILBUR B. HUSTON AND HARRY PRESSkeeps the sol ar array oriented to the sun failed. Since the so lar array stuck in the satellitemorning sunri se position, som e sola r power w a s received, but not enough to supply the space-cra ft with adequate power to ope rate even the essentia l space craft sy ste ms (altitude control,telemetry and command). After seven orbits, b atter y discharge reached a point where attitudecont rol and tele metry failed.

During the 26 days of operation there we re a tot al of 379 orbi ts , 125 of which we re blindin that spacecra ft command w a s not feasible at eit her CDA site. With the design orb it only,two passes a day would have been blind, thus 73 of the blind orbits (18%)are chargeable to theeccentric orbit. Of the 254 orb its on which dat a could theoretically have been obtained, so mewer e missed because of sho rt acquisition times , oth ers because of a conservative doctrine

Table IPerformance Summary of the Nimbus I Spacecraft.

(August 28 - September 23, 1964)

TotalBlindAVCS DataHRIR Da t aAPT Data

Orbits379125190178171

Pictures--

1210060001930

Table I1Data Form ats Available fo r the

Nimbus Sensory Systems.AVCS & HRIR

Daily Log and Coverage MapsNegative FilmPositive Contact PrintsSelected 8 x 10 Prints

HRIRDigital Magnetic Tape (7090 Compatible)Analog Magnetic Tape

APTDaily Coverage MapsMagnetic TapesSelected 8 x 8 Facsimile Photos

FUTURE PLANS

which dictated the turn-off of sen sory sys tem swhen spacecr aft operational problems wer eencountered. The extent of the data obtainedand presently on file at GSFC is shown inTable I.

AVCS data wer e rece ived on 190 orb its .The tota l of 12,100 pic tur es which resultedfro m these 190 orb its indicates that completecoverage (100 pictures per orbit) w a s notobtained; again a consequence of the ell ipt ica lorbit. The count of APT pictu res repre sen tsthose made at stations under NASA control,and for which tapes suitable for reproductiona r e available. A total count of pictures in-cluding all cooperating stations is not available.

Catalogs of the Nimbus I s ens ory dat aare being prepar ed in orde r to make the datamore readily available to the scientific com-munity (References 5 and 6). The sc ope ofthe cataloging effor t, and the f orm ats in whichdata can be made available a r e summarizedin Table 11. Daily log and coverage maps ofthe AVCS and HRIR data designed for rea dyrefer ence , a r e contained in these catalogs.Sample catalog pages a r e shown in Fig ure s 9and 10 and in Table 111.

Future Nimbus spac ecra ft provide the opportunity to fly fu rth er, more advanced meteorolog-ical experiments. The broadest possible participation by the gene ral scient ific community issought in the development of such expe rimen ts. The second spac ecr aft of the se ri es, designated


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J THE NIMBUS I FLIGHT 9

180' 140'W 100W 6OoW 2OoW 0 20'E 60E 100E 140'E 180'Figure 9-Sample daily coverage map from the AVCS data catalog-equatorial projection.

DATA ORBIT234180

0DATA ORBIT233

0

E

?80

DATA ORB11235180

13 SEPTEMBER 1964NORTH POLAR REGIONS

DATA ORBIT234180

DATA ORBIT2340

SOUTH POLAR REGIONSDATA ORBIT235

0

DATA ORBIT2371An

20 E

90E

O E

0DATA ORBIT236

0

180

Figure 1O-Sample da il y coverage map from the AVCS data catalog-polar projection.


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10 WILBUR B. HUSTON AND HARRY PRESSr

,vTable I11

Sample Log Sheet and Index of A V C S Pictures.

T;-.:::;E INDEX ' INDEX 'YIN F R W ' iicnr ' SUN ' GEOGRAPHIC FEATURESFRAME TI* l u l l ASC NOM I K M l ANGLE~

1-21240583, 1 .I7 51 45, +21.9 , 48 1 12 , I2 ARCTIC ICE1 . , ,2-21160578, 2 ,I7 47 12, +17.4 , 529 , 29 , 1

2 ' 2. * .2 33 23 3

3-21160578 3 17 45 41. l l 5 . 9 , 54 9 , 35 , I~~I .

4-21160578, 4 I7 44 10. + !4 4 , 569 , 4 1 , L Sasksfchewan, Last Yountaln Lake2 SBP*sfcher.n, Ls9t Younfal" Lake~ 4 3 Laker Winnlpe 0115 , Manitoba I Dauphin.~ ~ . 4 .5.21160578. 5 '1 7 2 3 9 i i y i 590 : 47 1 1 Yellowtone n f w ?5 2 Garrison Reservoir.~

. * .2 33 23 3

3-21160578 3 17 45 41. l l 5 . 9 , 54 9 , 35 , I~~I .

4-21160578, 4 I7 44 10. + !4 4 , 569 , 4 1 , L Sasksfchewan, Last Yountaln Lake2 SBP*sfcher.n, Ls9t Younfal" Lake~ 4 3 Laker Winnlpe 0115 , Manitoba I Dauphin.~ ~ . 4 .5.21160578. 5 '1 7 2 3 9 i i y i 590 : 47 1 1 Yellowtone n f w ?5 2 Garrison Reservoir.~5 1 ' 3. -6-21160578 6 17 41 08 +11.3 61 2 , 52 , 1

, -. -P11-$V578. a .I7 38 06 + 8.3 I 65 8 + 63 , 1239-211-60578 9 117 36 35 + 6 . 8 ! 68 2 69 . 1

~~ . 8 . 1 . 19 , 1 ::lQ-21+!2578. .17 35 04 + 5.3 1 706 74 I I

~7 10 I : , . 2~~p 10 3 YWltP" Pen.-L-21140578' 1 1 ,I7 33 33. + 3.7 : 730 , 79 , 1I 1 2 ouste.s1* - El_ s.1vpdor cst .3 Honduras I W I C ~ T ~ P Y P' m' 1 1 'lZ5Z11-5P578 I 12 ,I7 32 02, + 2 . 2 , 754 ~ 83 , I

W T L Y cum

FRONTAL Bums

HURRICWE DOM - N. EDGEFrontal B u v i rCu I Ch Over Gulf State*

cumB o a Ch

Nimbus C, is presently under construction, and is scheduled for flight in 1966. Its sensorycomplement is the same as Nimbus I, with the addition of a Medium Resolution Radiometer forstud ies of emitted and refle cted radiation in fiv e sp ec tr al regions . Succeeding launches, at ap-proximately 18 month inter val s, wi l l for the first time, employ a number of experiments tomeasure the atmospheric structure from sate llite attitudes. It is anticipated that the capabilityof these spacecraft for global meas urem ents of atmospheric s tru ctu re, w i l l make the stabilizedorbiting meteorological observation platform an important element in fu ture effor ts of lar gescal e, long range numeric al weather anal ysis and prediction. It may also be safely assumedthat out of the meteorological experiments and spacec raf t technology developments of the Nim-bus series w i l l come the next phase of the national operationa l satelli te syste m.

REFERENCES

1. Bandeen, W. ., "Eart h Oblateness and Relative Sun Motion Cons ideration in the Determina-tion of an Ideal Orbi t fo r the Nimbus Meteorological Sate llite," NASA Tech nical NoteD-1045, July, 1961.

2. Stampfl, R. A . , and Press, H., "Nimbus Spac ecra ft System," Aerospace Engineer ing21(7): 17-28, July 1962.


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'L.'THE NIMBUS I FLIGHTA '

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3 . Fujita, T., nd Bandeen, W., "Resolution of the Nimbus High Resolution Radiometer," SMRTResearch Paper, No. 40,February, 1965.

4 . Stampfl, R. A . , The Nimbus spacecraft and its communication system, Goddard Space FlightCenter Document X-650-65-75, March, 1965.

5. Staff Members, "Nimbus I Users' Catalog: AVCS and APT," Goddard Space Flight Center,March, 1965.

6. Staff Members, "Nimbus I High Resolution Radiation Data Catalog and Users' Manual,"Goddard Space Flight Center, January, 1965.


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THE HIGH RESOLUTION INFRAREDRADIOMETER [HRIR] EXPERIMENT

L. L. Foshee, I . L. Goldbergand C. E. Catoe

INTRODUCTIONThe Nimbus High Resolution Infrared. Radiometer (HRLR) as designed to perform two

major functions: first, to map the earth's cloud cover at night to complement the telev isioncoverage during the daytime portion of the orbit. Second, to me asur e the temp era tur es of cloudtops and ter ra in featu res. Nimbus is an earth-oriented satellite which trave ls in a high-noon,near-polar orbit. The HRTR is mounted at the bottom of the spac ecraft so that it has an unob-structed view of the earth . During the 25 days of Nimbus I operation, data for about 50 percentof all nighttime orbit s were obtained, each one covering a region fro m horizon to horizon inwidth and ranging fr om North Pole to South Pole. Because the radio meter opera tes in the 3.4to 4.2-micron region, measurements taken during the daytime do not reveal t ru e su rface tem-peratures: This is because of reflected so la r radiation which is added to the emitted surfaceradiation. However, reflected sunlight in this spec tral region does not saturate the radiomete routput and usable, though poor er quality, pictur es can be made.

THE HIGH RESOLUTION INFRARED RADIOMETERTh e single-channel scanning radiometer, built by ITT Indus trial Laboratorie s, is shown in

Figure 1 . It contains a lead selen ide (PbSe) photoconductive cell which is radiation cooled to-75C and op erates int he 3.4- to4.2-micron"window" region. The white co llars shownin the figure are sunshields to prevent solarradiation fr om entering the radiometer dur-ing space craft sun ris e and sunset. The scanmi r ror is located between the sunshields.The cylindrical projection at one end containsthe motor which driv es both the scan mir ro rand the chopper. The front of the rectangularpyramid which is part of the radiative cool-ing assem bly can be seen. The electronicsare located around this pyramid. The ra -diometer, which weighs 1 1 . 3 pounds and con-su me s 4 watts of power, can measure Figure ] -N imbus I high resolution infrared radiometer.

13


18/93

14 L. L. FOSHEE, I. L. GOLDBERG AND C. E. CATOE

OETECTOR

Figure 2-Radiative cooling system for the HRlRlead selenide detector.

SECONDARYIN-FLIGHT CALIBRATIONd,R,LAtA:):;v TARGET/ PRlVRY

/U 'CHOPPERFigure 3-Schematic of the HRlR optical system.

radiance temperatures between 210Kand 330K with a noise-equivalent tem-perature difference of 1C for a 250Kbackground. The radiative cooling sy st emis shown in Figure 2. Cooling is accom-plished by means of a black cooling patch atthe bottom of a highly reflective gold-coatedpyramid. The pyramid is oriented to viewcold sp ace during the e ntire o rbit and thepatch is suspended by thin wires to reduceheat conduction fr om the housing. The de-tector is connected to the cooling patch by ahigh thermal conductance transfer bar.

The radiometer has an instantaneousfield of view of about 1/2 degree, which atan altitude of 925 kilometers correspo nds toa subsatellite ground resolution of 8 kilom-eters. Figure 3 llus trate s the HRIR opticalsystem. The scan mir ror is inclined 45 de-grees to the axis of ro tation which is coinci-dent with the spac ecra ft velocity vec tor.The optical s can path thus lies in a planeperpendicular to the orbital motion. The ra-diation reflected from the scan mi rro r ischopped at the focus of a 4-inch f / l modifiedCassegrainian telescope. It is then re -focused at the detector by means of a reflec-tive rel ay which contains the 3.4 to 4.2-

micron filter. The scan rate of 44.7 revolut ions per minute was chosen so that contiguousscann ing would occur in the neighborhood of the subsate llit e trac k, with increa sing overl aptoward the horizon. However, because of the unintentional ec cen tri c orbi t of Nimbus I, gapsof about 5 kilometers occurred along the subsatellite tr ack at perigee. Toward apogee the gapsdecreased to zero.

In contrast to television, no image is formed within the rad iome ter; the HRIR se nso rmerely t ran sfo rms the received radiation into an elec tric al (voltage) output with an informationbandwidth of 280 cycles per second. A trace of a portion of an actua l Nimbus I analog reco rdis shown i n Figure 4. The radi omet er sca n mi r ro r continuously rota tes the field of view ofthe detector through 360 degrees in a plane norm al to the space craf t velocity vector. The de-tector views the in-flight blackbody calibration target (which is a part of the radiometerhousing), outerspace, earth, outerspa ce, and r et ur ns again to inter cept the in-flight blackbodytar get . The space and housing-viewed p ar ts of th e scan , which can be identified without diffi-culty, serve as part of the in-flight check of calibrati on. hf or ma ti on on housing temp era tur es ,which are monitored by thermistors, are telemetere d to the ground sta tions and for calibration


19/93

THE HIGH RESOLUTION INFRARED RADIOMETER (HRIR) EXPERIMENT 15

EAST OF PHILIPPINES

Figure 4-Portion of an analog record showing nearly tw o HRIR scan cycles.

purposes are constantly compared with the temperatures obtained from the radiometer housingscan. The space scan serves as the z ero refer ence point. During the space sweep a permanentmagnet on the scan mirror gear triggers an electronic gate and a multivibrator so that theseven pulses shown in Figure 4 are generated. These pulses are used to synchronize auto-matic display equipment on the ground.

HRIR SUBSYSTEMA simplified block diag ram of the HRIR subsystem is shown in Figure 5. The radiometer

video output (0 to -6.4 v dc) is fed to an FM modulator which converts the radiometer voltageto frequency modulated s igna ls (8.25 o 10 kcps) that are recorded on one track of a 4-tracktape rec ord er. Simultaneously, 10- kilocycle AM timing sig nals fr om the Nimbus space craftclock a r e placed on a second tr ac k on the tape. When the se two t racks are completely re-corded, the direction of the tape travel is automatically reversed and the recording continueson the other two track s. The recorder accepts data at a tape spee d of 3.75 inches per secondand upon command plays back all four channels simultaneously at 30 inches per second. The 8to 1 playback speedup causes a directly proportional increase in the FM signal frequencies (66to 80 kc). The four tape rec order outputs (two video and two time code) a r e fed into the fourchannels of the HRIR mult iplexer where each channel is translated to its assigned position inthe HRIR subsystem base band (i.e., 55 to 69.5 kc and 127.5 to 141.5 kc for the two video channels).The four channels are combined in an adder circuit to produce a composite HRIR sub systembase band which is fed to the Advanced Vidicon Camera Sy stem (AVCS) mult iplexer. In theAVCS multiplexer, the HRIR subsystem base band is combined with the AVCS base band and thecomposite signal is fed, o the Nimbus S-band transmitter for transmission to the CommandData Acquisition (CDA) stat ion.

At the CDA station, the HRIR information is demultiplexed and recorded on magnetic tape.It is then transmitted to the Goddard Space Flight Center, where the FM signal is demodulated,synchronized and displayed by a Westrex photofacsimile rec ord er. The facsimile rec ord erconve rts the radiomet er output electric al signals into a continuous strip picture, line by line,on 70 mm film. Blanking cir cui ts i n the Westre x recor der rej ect unwanted sec tions of eachline scan ; only the e art h sc an and, for calibration purposes, ve ry small portions of the s pace


20/93

16 L. L. FOSHEE, I. L. GOLDBERG AND C. E. CATOE

I I 'COMPUTER I 'TIMINGDEMOD

TAPEREC DEMUX RCVR-

TiminqI &, pq

DEMOD

Figure 5-Simplif ied block diagram of the HRlR subsystem.

DAF

scan and the radiometer housing sca n a r e recorded on the film stri p. The processing time be-tween spacecraft playback command and display for one f u l l nighttime orbit is about 30 minutes.

Although the s ignal level resolution in the pictor ial display is an or de r of magnitude poorerthan in the corresponding analog record, the picture is invaluable for recognition and qualita-tive analysis of the meteorological information. Only 10 signal levels can be displayed in thegreyshades of the picture while about 1 0 0 levels are contained in the original signal. An illus-tration is given in Figure 6a, which is a reproduction of a sel ected portion of the photofacsimiledisplay of an orbital st rip covered by the HRIR near midnight on September 20, 1964 rangingfr om about the equator at the bottom to the Gulf of Ala ska at the top. The blackness of eachpicture element vari es directl y with the intensity of the radiation sensed by the radi omete r.The warm wat ers of the tropical Pacific Ocean can be recognized in the very da rk ar e a nearthe bottom in cont ras t to the somewhat coole r temperatures of the North Pac ific Ocean shown bythe da rk patches ju st north and south of 40%. The "medium gray" around 30% is a large massof very low altitude str atu s clouds or fog. The stri ng of smal l, very bright (cold) spot s near137 W and 38" to 40% indicate isolated, very high altitude cumulus clouds relating to thunder-st or m activity.

A much more quantitative picture results when the original analog signals are digitizedwith full fidelity and the digital data a r e pro cessed by an IBM 7094 computer where c ali-bration and geographic re ferencing is applied automatically. Ext rac ts of such automatic,


21/93

17I TH E HIGH RESOLUTION INFRARED RADIOMETER (HRIR) E XPER IMEN T

IO k n

EQUATOR

Figure 6-A positive print of nighttime HRlR data: (a)Pictor ial presentation of cloud and water temperaturesover the North Paci fic measured by the HRlR at midnighton 20 September 1964. (Dark shades are warm, whiteshades are cold.) (b) Automatically produced digitalmap of cloud surface temperatures.

numeric presentations are shown in Figure6b. The very highest cloud in Figure 6a,located in the Intertropical ConvergenceZone (11.5' -12% and 145-146W), is char-act eriz ed by extremely low radia tion intensi-ties corresponding o blackbody tempera turesof 190K ne ar the center of the cloud top(Figure 6b).

The absolute and relative geographicallocations of each picture el ement scanned by

II IIFigure 7-An example of automatic gridded HRIRdata.

the radiometer strongly depends on the stability of the spacecraft. The Nimbus controls sys-tem has a demonstrated pointing accuracy of about 1degree for the spacecraft axis. A pointinger ro r of 1 degree corresponds to a subsatell ite er ro r of 16 km in the location of a picture ele-ment for an alti tude of 925 km. On a global bas is, th is is an acceptable error for meteorologicalanalysis. Automatic gridding of the data is done by the same equipment used to grid the tele -vision pic ture s. For pictori al presentation grid points a r e computed with a CDC 924 computerand added to the HRIR data in analog for m. Each grid point is converted from coordina tes oflatitude and longitude to coordinates compatible with the electr ica l signa ls, Le., line numberand m i r ro r ang1.e. An example of automat ic gridding is shown in Figure 7. Because of the


22/93

L. L. FOSHEE, I. L. GOLDBERG AN D C. E. CATOE8la rg e distortions and poorer data ne ar the horizon, only the cen tral 107-degree portion (asmeasured at the satellite) is gridded.

Th e line along the ri'ght edge of Figure 7 indicates the horizon seen by the radi omete r.There is a cle ar indication of fluctuation in that line aft er the spac ecraf t had passe d over thehigh tropical clouds near the equator. This indicates the disturbances which the high altitude,cold clouds presented to the infrar ed horizon scanne r of the Nimbus control s sys tem causingthe spac ecraft to roll slightly as it passed over the equator.

CALIBRATIONIn discussing the calibration , thre e fundamental quantities must be defined: the effective

spectral response, , he effective radiance, N, and the equivalent blackbody tem per atu re, T,, .E f f ec t ive S pect ra l ResponseThe radiation re ceived by the radiometer is reflected by five front-surface aluminized

mirrors and transmitted through a filter before reaching the PbSe detector. The effectivespectral response, + A , is defined as

where RA is the combined spectral reflectivity of all five front-surface mirrors, f ,spec tral transm ittance of the f ilter and AX is the sp ect ral absorptivity of the detector

Table I In the actual computation ofNimbus I HRIR Optics.

Type: Multilayer wide band-pass in terf ere nceSubstrate: GermaniumTransmission: 0.76 (calculated)

Scan MirrorType: Evaporated S i0 ove r hard-coated aluminuSubstrate: AluminumReflectivity: 0.96 (estimated)

Filter

Cassegrainian telescope (primary and secondary)Type: Front-surface A 1 with Si 0 protectiveSubstrate: Glass

coating

Relay OpticsType: Front-surface gold coated with Si 0 pro-Substrate: GlassReflectivity of the 4 mir ror surfaces (Casse-

grainian telescope and relay optics): 0.92(estimated)

tective coatings

is the

the spec-tral reflectivities, RA , of al l mirrored sur-face s were assu med to be constant over thepa ss band of the filter and hence were nor-malized to unity. The resultant equation usedi n computing 4A , herefore, w a s

The m ateria ls used in the optics a r e givenin Table I. The function fA w a s taken fromInternational Telephone and Telegraph Lab-orator ies measurements of the f ilt er used inHRIR Unit F-2, the instrument flown onNimbus I and the function AA w a s t a k e n fromse ve ra l typical labo rator y curves of PbSedetectors. The effective spect ral responseis given in Figure 8.


23/93

TH E HIGH RESOLUTION INFRARED RADIOMETER (HRIR) EXPERIME NT 198Ef fect ive RadianceBecause of its narrow field of view, the

HRIR essentially measures beam radiationor radiance toward the satell ite along theoptical axis. In the preflight laborato ry cal-ibra tion , the field of view of the rad iom ete rwas filled by a blackbody tar ge t whose tem-per atu re could be v aried and accuratelymeasured over a range of 190K to 340K.From the temperature of the blackbody tar-get, TBB, he spe ctr al radiance of the tar getis determined by the Planck function B x. Th eintegrat ion of t his function ove r the effectivespectral response, 4 , yields that portion ofthe radiance of the target to which the ra-diometer responds, the "effective radiance,N," given by-

Equivalent Blackbody TemperatureThe effective radiance to which the

orbiting rad iome ter responds may be ex-pressed by

where Nx is the spectral radiance in the di-rection of the sa tellit e fr om the earth andits atmosphere. It is convenient to ex pr essthe measurem ent fr om o rbit in te rm s of anequivalent tem per atu re of a blackbody fillingthe field of view which would cause the same

WAVE LENGTH, A (MICRONS)

Figure 8-The ef fect ive spectral response of theHRlR versus wavelength.

HRlR CALIBRATIONLABORATORY IN ORBIT

Figure 9-Schematic illus tra tion of the relationshipsbetween laboratory calibration and T measurementsmade i n orbit.

respons e from the radiometer. From Equations 3 and 4 it is seen that this "equivalent black-body temperatur e" correspo nds to the target temperature, T B B , of the blackbody used in thelaboratory calibration. This relationship is expressed schemati cally in Figure 9. Therefore,the radiome ter measurem ents can be expressed either as value s of effective radiance, N, or asequivalent blackbody temp eratu res, TBB . The N versus T,, function from Equation 3 is given inFigure 10 .


24/93

20 L. L. FOSHEE, I. L. GOLDBERG AND C. E. C A T O E

EQUIVALENT 0 uC KI OD Y TEMPERATURE, T K )-Figure 10-N versusTBB.

180 m 220 20 la 180 m 320 MOEQUIVALENT 0U CK 8OD Y TEMPERATURE, Tn O (K)

Figure 11-HRIR calibration.

The only independent varying par ame terof consequence in the calib ration wa s thecooled PbSe cell tem per atu re. All othe rtemp eratu re and voltage varia tions wereautomat ically compensated for and/o r regu-lated within very narrow limi ts. Therefore,separate calibrations were made for severaldifferent cell temperat ures . The blackbodytarget temperature, T,, ,w a s vari ed between190K and 340K n 10% st eps and the ra-diometer output voltages were recorded fora given cell temp erature. Then the PbSecell was stabilized at a new temp erat ure andthe targe t tempe rat ure was cycled againfrom 190% to 340K.

After the radiometer was integrated inthe spac ecra ft and subjected to vacuum-therm al environmental testing, a check ofcalibration was performed. These data wereobtained approximately eight months afterthe original laboratory calib ration of the ra-diometer. Essentially the same techniqueswere employed in the calibration check aswere used in the original laboratory calibra-tion. The blackbody targe t temp eratu re wasvaried from 190% to 340K in 20K st ep s andthe sub carri er frequencies from the FMmodulator were recorded on the spacecraftHRIR tape recorder (cf. Figure 5). After onefull target tem perat ure cycle, the data wereplayed back through the entire system andtransmitted via the S-band tran smitt er in thespacecraft. The data were received at the

ground station, demultiplexed, demodulated and displayed in the for m of an analog tra ce on anoscillographic reco rd, thus simulating as fully as possible, on a sys tem basis, the orbital con-ditions. Close examination of both set s of ca libra tion data revea led no significant changes overthe eight months period during which the r adio mete r underwent te sting and integration with thespacecraft; therefore, w e have high confidence in the validity of the ca libration. The definitiv ecalibration data fo r the HRIR F-2 unit which w a s flown on Nimbus I a r e shown in Figure 11.Par ame tri c curves for different values of the PbSe cell temp era tur e a r e given. Values Of T,,can be converted to N from Figure 10. In orbit the calibration was checked at one targ et tern-perature during each scan by means of a blackbody t arge t of known tem per atu re mounted onthe upper side of the radiometer (Figure 3).


25/93

,

T H E HIGH RESOLUTION INFRARED RADIOMETER (HRIR) EXPERIMENT 21aFLIGHT PERFORMANCE OF THE RADIOMETER

One of the unique features of this experiment was the radia tive cooling technique. The cellcooled to its desired temperature of -75Cwithin less than 3 hours af ter launch and remainedat a satisfact ory tempera ture therea fter. The re were no indications of degradation in the gold-coated cell-cooling horn nor was any difficulty experienced in the cell-patch suspension sys-tem. The suspension sys tem had been extensively tested pr io r to launch, having withstood'satisfacto rily the various types of vibration tests to which th e radiometer had been subjectedin the test program for som e 95 minutes.

The radiom eter housing stabilized at its anticipated temperature of 11C and remained the rewithin 2.5"C hroughout its lifetime. The temperatu re stability of the entire rad iome ter con-tributed in large pa rt to the corresponding stability of th e PbSe cell temperature. The tempera-tu re of the motor end of the cast ing stayed wel l above -5"C,which is the lower limit for thesuccess ful operation of the motor.

The calibrat ion of the radiomet er remained stable throughout its lifetime and there was nodiscernible degrada tion of th e characteristic s of the detector cell, m irr or s o r the interferencefilter.

Results have shown that radiance leve ls corresponding to the equivalent blackbody temp er-atu re in the range fro m 210% to 320K were resolved with an accuracy of 2% or better. Somesmoothing of t he analog tr ac es of the radiometer signal is required to achieve this accuracy.An arb itra ry point measurement taken from the analog tr ac e without smoothing can deviate anadditional 2K due to periodic noise i nterference induced electroni cally into the ra diom eteramplifiers and superimposed on the signal, making the overall uncertainty of a point measure -ment about *4%.

Results from aircraft flights at night with a modified HRIR during Januar y and F ebr uar y1964 fr om an altitude of 12 kilometers indicate that stratocymulus and altocumulus cloudequivalent blackbody tem per atu res we re about 2C cooler than actual cloud temperatures, whileocean equivalent blackbody temperatures were approximately 5"C cooler than measured seasurfa ce temperat ures. The overwater flights were made near the Bahamas where the measuredwater temperature varied between 23C and 25C. Da t a fro m Nimbus I and calculat ions byKunde (se e subsequent article) show approximately the sa me r esul ts.

CONCLUSIONSThe Nimbus I High Resolution Infr ared Radiometer expe riment has extensively mapped

in fr ared radiation emitted during nighttime f rom the tops of clouds and fro m land, s e a and icesurfa ces ove r the entire world. With a resolution approaching that of TIROS television pic-tur es and with a far simpler scanning geometry than the scanning radiometers previously flownon TIROS, the Nimbus HRIR significantly increased the ability of meteorological satellites tomake detailed and accura tely placed measureme nts of emitted radiation and thereby, to infe rcharacteristics of the horizontal and vert ical s truct ure of weather syst ems. In interpreting thephotofacsimile data one must initially exe rt som e effort to asso ciat e the visible con trast s with


26/93


27/93

THEORETICAL RELATIONSHIP BETWEEN EQUIVALENTBLACKBODY TEMPERATURES AND SURFACE TEMPERATURES

MEASURED BY THE NIMBUS HIGH RESOLUTION INFRARED RADIOMETERVirgil G . Kunde

INTRODUCTIONIn general, radio metri c exp eriments fr om meteorological satellites have been designed for

thr ee basic types of mea sure ment s (Reference 1):1. To map daytime and nighttime surfa ce features and clouds from meas ureme nts 01

radiation emitted by these surfa ces through an atmospheric "window . Iv2 . To determine atmospheric stru cture from measurements made in a spe ctr al region of

high atm osph eric absorption.3. To dete rmine the planetary heat balance of the eart h from meas ureme nts of the in-

coming sol ar radiation and the total outgoing long-wave radiation.Each type of measurement has its associat ed problams in determining the appropria te

meteorological para mete r f rom the radio metri c measurement of reflected sol ar radiationand/or emitted te rre st ri al radiation. Here w e a r e concerned only with the first type ofmeasurement.

Th e Nimbus I High Resolution Inf rared Radiometer (HRIR) mapped the cloud cover andte rr es tr ia l feat ures by measuring their radiating temperatures at night-when there is no solarinte rfer ence-thro ugh an atmospheri c window between the 4.26 micron absorption band ofcarbon dioxide and the 3.17 micron band of water vapor. Because thi s is a window region, ala rge fracti on of th e radiation mea sured by the radiometer o rigin ates fro m underlying cloudor ground surfaces. For a per fec tly clean window region-where th er e is no atmosphericabsorption-all of the measured radiation originates from the surfac e, and the surfa ce radiatingtemperature is the ref ore measured directly. At the other extreme, in a highly absorbing region,all of the surf ac e radiation is absorbed by the atmosphere and, in this ca se, sate llite measure-ment s would not give any indication of the surf ace radiating tem per atu re. In the ran ge betweena per fec tly clea n window region and a highly absorbing region, the HRIR window reg ion fallsvery n ear the pe rfec tly clean window extreme.

The gene ral purpo se of this a rti cle is to show quantitatively that the overlying atmospherehas little effect on the outgoing radiation observed by the HRIR and that the HRIR measurements,therefore, give a good esti mate of the surfa ce temperature. The only atmospheric effect consideredhe re will be atmospheric absorption. The effec t of surface emissivit y on the observed radiationwill a ls o be illus trate d sinc e the radiation emitted by the surfac e depends not only on the tru e

23


28/93

24 VIRGIL G. K UNDEsurfa ce temperature but also on the sp ectra l emissivity of the surface. Therefore, one of theproblems associated with determining the tru e surfa ce tempera ture fr om the radiating tem-perature of the surfac e is that the su rfa ce spec tral emissivity must be known. With the com-putations presented in this article, an attempt w i l l be made to as se ss the ac curac y and validityof

a) HRIR measuremen t of cloud-top rad iating temp era tures, wh ere, if the temperatu re dis-tribution with height is known, the height of the cloud-top can be determined, providedthe cloud-top can be adequately defined and,

b) HRIR me asurement of ground or ocean radiating temperatures over cloud-free areas.

THE FOUR MICR ON "WINDOW" REGIONBefore going on to a more detailed disc ussion of the calculations, a qualitative description

of the four micron "window" would see m to be in or de r. For convenience, we ref er to th is"window" a s the four micron "window" even though it actually extends f rom about 3.4 to 4.0microns. The four micron window is shown in Figure 1 where the lower solid line representsan infrared prismatic so lar spectrum-a transmission spectrum of the earth's atm ospherewith the sun as a so urce of radiation-taken by Shaw (Reference 2). The upper solid linerepresents the solar irradiance at the top of the atmo sphere assuming the sun radi ate s as ablackbody at 6,000"K. Because the location of the continuum is not known exactly, the positioningof the solar irradiance curve relative to the solar spectrum is somewhat arbitrary. The darke rshaded area represents the solar energy loss, due mainly to molecular absorption, in the earth'satmosphere. The dashed curve of Figure 1 repre sents the HRIR effective spectral responsecurve with half power points at 3.55 and 4.10 micr ons . Qualitatively, Figure 1 shows that th eHRIR measureme nts a r e made through a fairl y clean atmospheric window as the solar spectrumindicates that the transmission through the atmosphere is largest in the region where the HRIRis spectrally sensitive. A slightly cl eaner window would r esul t i f the sho rt wavelength cutoffof the effective spectral response w a s sharper, and i f the entire effective spec tral responsecurve was shifted slightly to the s hort wavelength sid e.

The dominant molecular absorption bands in the four micron region a r e the overtoneparallel 2 u , vibration-rotation band of H,O a t 3.17 microns and the para llel v 3 fundamentalvibration-rotation band of CO at 4.26 microns. The approximat e regions of absorpti on fo rthese two bands a r e indicated by the horizontal ar ro ws in Figure 1. Absorption by the isotopicspecies of carbon dioxide and wate r vapor al so occ ur s in this region , with the most prominentisotopic band being the u 1 fundamental of HDO at 3.67 microns . The absorption due to theQ-branch of this band is very evident in the so la r spec tru m of Figure 1.

Additional molecular absorption in the four micro n window oc cu rs because of the minorconstituents such as N,O and CH,. The wavelength and upper st at e of the vibrational tr ansi -tions of the absorption bands due to these minor cons tituents a r e als o indicated in Figur e 1(Reference 3). These bands have been identified in the solar spectrum, however, not all ofthem a r e visible in this particular sol ar sp ect rum because the prismatic spect ral resolution


29/93

I

TH E O R E TIC A L R E L A T IO N S H IP B E TW E E N E Q U IV A L E N T B L A C K B O D Y TE MP E R A TU R E S A N D S U R FA C E TE MP E R A TU R E S ME A S U R E D 25B Y T H E H R IR

1\\ \ I

1 I I I I2400 2600 2800 3000 3200

WAVE NUMBER (crn- I )I I I I I I I I I I I I d

4.2 4.0 3.8 3.6 3.4 3.2WAVELENGTH ( p )

Figure 1-The four micron "window" region. The upper solid curve represents the solar irradiance of a 6,000"Kblackbody a t the top of the atmosphere and the lower solid curve an in frared prismatic solar spectrum (Reference2) . The darker shaded area represents the solar energy loss, due mainly to molecular absorption, in the earth'satmosphere. The horizontal lines ind ica te approximately the regions of absorption due to the major absorbinggases. Thedashed curve i s the effe ctive spectral response of the HRIR.

The ver ti ca l lines in dicate the center of absorption bands due to major constituents (Reference 3) .

is not gre at enough to resolve all the bands. In addition, some of the bands are overshadowed bythe s tro ng absorption of H,O and CO,.

All of t he se tra nsi tion s originate in the ground state with the exception of the N, 0 band at3 . 8 8 microns which originates in the V, level. The CO, band, at 4.12 microns, is also an upperstate band with its lower level being 2 u , . Generally, the minor constituents a r e considered tobe distrib uted uniformly throughout the atmosphere while H,O is concentrated mainly at loweraltitude s. Thus, while it may be des ira ble to consider the absorption of the minor constituentsat high altit udes, thei r eff ect can usually be neglected at low al titudes due to the overshadowingstro ng absorption by H , and CO,


30/93

VIRGIL G. KUNDE

I I I I I I A more quantitative presentation of thefour micron window is given in Figure 2.The solid curve rep resen ts the radiometereffective sp ectra l response and the dashedcurves give representative absorptions forCO, and H,O. The absorption is from theground to the top of the atm osp here for atypical mid-latitude a tmosphere (1 precipi-table c m of wa ter ) and a zenith angle, 0".The absorption curves wer e obtained fr omtrans miss ion tables calculated by Stull, Wya t tand Plass (References 4 and 5). In their cal-culations, the quasi-random model (Reference6) w a s used t o approximate the mole cularabsorption of CO, and H,O. Absorption linesof eight major iso topic specie s of CO, andfour major isotopic species of H,O were in-cluded in their calculations. In Figure 2, itcan be seen that the absorption by H ,O de-creases as we go further into the wing of theP-bran ch of the 3.17-micron band, then in-creases slightly at 3.67 microns due to the

'Figure 2-The Nimbus I HRlR "window." The solid curve mo isotoDe. Again. the absorDtion de-,represents the effective spectral response of the HR IR .The dashed curves represent the absorption of carbondioxide and water vapor from the ground to the top of

to a minimum at about 3.95 micronsand then starts to increase due to

the atmosphere for a typical mid-latitude atmosphereand a zenith angle of 0". the R-branch of the 6.3-micron H ,O band.The 4.26-micron CO, band has a R-branch

band head at 4.17 microns (Reference 7),therefore, the absorption for wavelengths less than 4.17 microns is due to the wings ofstrong lines in the R-branch.

In their CO, trans miss ion calculation, Stull, Wyatt and Plass (Reference 5) have used theBenedict modification of the Lorentz line shape (Reference 8) to repr esen t the line shape in thewings at distances gre ate r than 2.5 wavenumbers fr o m the line cent er. For comparison pur-poses, they have also made calculations at one temperature (300%) and one pressure (1 atm)for a Lorentz line shape. The comparison of the trans miss ion values for the two different lineShapes shows that the Benedict modification gives a substantially higher transmission for longpath lengths in window reg ion s. The Benedict modification used by Stul l, Wya t t and Plass wasdetermined fro m experimentaldata for self-broadened CO, . Although no detailed ca lculationsexist, t here a re indications that f or foreign-broadened CO,, which is the condition found in theea rth 's atmosphere , the form of the Benedict modification may still apply but the cor recti onfactors W i l l be substan tially diffe rent (Reference 8). Thus, it is not known with any certaintywhich line shape is applicable in the ea rth 's atmo sphe re. Young (Referenc e 9) and Drayson(Reference lo), in way of con tra st to Stull, Wyatt and plass, have used the Lorentz line shapefor CO, , deciding the re was no justification fo r using the Benedic t modification until ex perimental


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THEORETICAL RELATIONSHIP BET WEEN EQUIVALENT BLACKBODY TEMPERATURES AND S URF A CE TE M P E R A TU R E S MEASURED 27. BY T H E H R IRdata representativ e of the earth's atmosphere was available. This problem be ars directly onthe calculations of the outgoing radiance at the top of the atmo sph ere in the HRIR window region,since it is in window regions where the tran smission diff erences a r e mo st significant.

A s transmissio n tables for CO,, for theLorentz line shape, a r e available for only onetemperatur e and one pressu re, it is not pos-sible to compare directly an absorption curvefo r the Lorentz line shape with the absorp tioncurve of Fig ure 2which r epr ese nts the Bene-dic t modified lin e shape. However, a com-parisor? of the absorp tion cur ves for theLorentz and Benedict modified line shapescan be made for a temperature of 300%, apr es su re of one atmosph ere, and a C O , pti-cal path length of 200 cm-atm. This com-parisonfor the four micron window region isshown in Figure 3 , where it can be seen thatthere are ver y significant differences betweenth e two curv es throughout the enti re spe ctralregion of inte rest. The lar ges t difference isabout 0.27 at 4 microns . Thus, the variationof transparency of the HRIR window due to thetwo different line shap es should have a verynoticeable effect on the outgoing radiance.

0 3 . 4 3 . 6 3.8 4 . 0 4 . 2. 2WAVELENGTH ( m icr o n s)

Figure 3-Absorption curves, based on the quasi-randomspectral model, for the Lorentz line shape and for theBenedict mo di fic at ion of the Lorentz line shape. Thecomparison i s for a pressure of one atmosphere, a tem-perature of 300K and an optical path length of 200cm-atm, for C 0 2 .

The calculations of the outgoing radiance, which is discussed in the following section, weremade using the Benedict modified line shape transmis sion values. Comparison calculationsusing the Lor entz line s hape transm issi on values could not be made because of the unavailabil-i t y of thes e tran smis sion values as a function of temperatu re, pre ssu re, and gas concentration.If these tables we re available and if one could eliminate other a r e a s of uncertain ty which occurin the det erminati on of the outgoing radian ce, than it may be possible t o observationally verifyone of the line shapes f ro m the HRIR measurements. Because we a r e working in a fairly cleanwindow, the t a s k of eliminating or at least minimizing the other ar e a s of unce rtainty is notnecessarily insurmountable.

RADIATIVE TRANSFER EQUATION SOLUTIONTh e outgoing effective radiance which the HRIR should me asu re has been calculated theoret-

ically for sev era l model atmospheres using t h e radiative transfer equation in the form


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28 VIRGIL G. KUNDEwhere R (4 0 ) is the outgoing effect ive radia nce at the top of the atmosph ere and rep res ent sthe outgoing radiation measured by the HRIR, +A is the eff ectiv e spe ctr al response of the HRIR,

is the spectral emis sivity which is considered to be independent of viewing angle, B is thePlanck function, r x is the total beam transmission from a given level in the atmosphere to thetop of the atmosphere , and is taken equal to the produc t of the tra nsm iss ion s of the individualabsorbing gases (CO, and H,O), r x is the reflectivity for a perfectly diffuse ref lecto r, T F ~s thedownward monochromatic radiant emittance, A is wavelength, T is temperature, P is pressure, p iis the density of the i tof the atmosphere in any downward direction. The sub scrip t s rep res ent s the lower boundarysu rfa ce which may be the ground o r a cloud.

absorbing gas, and 4 s geometrical path length measur ed f rom the top

The firs t te rm on the right-hand side of Equation 1 represents the contribution of the under-lying radiating su rf ace to the outgoing effective radiance, where it is assumed that the surfac eradia tes with a spe ctr al em issivity which may not be unity. Thus, at a given temperature, theemit ted radiation from the non-blackbody su rface is le ss than that from a blackbody surface.The surface spectral emissivity is very important in determining the true surface temperaturefrom the surface radiating temp eratu re, as we shall se e in the resul ts. Because it has beenassumed that the surface is a non-blackbody, not all of the radiation incident on the sur fac e wi l lbe absorbed-par t of it wi l l be reflected upward, back into the atmosphere. Th is component ofthe outgoing effective radiance is rep resent ed by the second term on the right-hand sid e ofEquation 1. The third ter m re pre sen ts the contribution of the atmospheric emitting gas es tothe radiation observed by the HRIR. Because we a r e in a window region, most of the outgoingradiation is due to the radiating sur face, while the atmospheric emission contribution is verysmall. To anticipate the res ult s somewhat, the surf ace cont ributes about 95% and the atmos-phere contributes about 5% of the observed radiation. These numbers indicate that the absorp-tion and subsequent re-em ission of radiation in the at mosphere is very small, and thus thecomponent of outgoing radiation due to the downward radiation being reflected upward is neglectedin the calculations.

Neglecting the second term, the radiative transfer equation (equation 1) akes the form

The model atmospheres chosen for the calculations give the t emp era tur e and density of theabsorbing gases a s a function of altitude, h . Given the tem per atu re profile and assuming asur fac e pressure of 1013.25 mb, the pres sur e as a function of height is obtained for each modelfrom the hydrostatic equation. If 0, the zenith angle at the ground is specified, the geometry fo ra concentrically stratified atmosphere gives a relationship between 4 and h. By using the modelatmospheres and the relation between 8 and h , Equation 2 can be expressed as

N(4 = 0 , s )


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THEORETICAL RELATIONSHIP BETWEEN EQUIVALENT BLACKBODY TEMPERATURES AND SURFACE TEMPERATURES MEASUREDB Y T H E H R IR

299

Defining

Equation 3 becomes

The quantity $(h, 0 ) rep resent s the therm al emission contribution of each atmospheric layerto the observed outgoing radiation. The outgoing, effective radiance at the top of the atmosphere,N(.e = 0 , s), w a s then determined for various an gles 8, or s evera l model atmosphe res fromEquation 5. The res ult s of the se calculations are expressed in te rm s of equivalent blackbodytemperature (TBB) rather than effective radiance, through the expression

The reduction f rom the non-homogeneous to the homogeneous a tmosp here w a s accomplishedby scalin g the half-width pr es su re and temperatu re dependence of the transmi ss ion functioninto the optical path length, thus allowing a standard transmiss ion function at normal tempera-ture and pressure to be used (Reference 11). The generalized absorption coefficients, whichwer e c onsidered to be tem peratu re independent, were derived from the transm issi on tables ofWya t t , Stull and Plass (References 4 , 5) .

The reduced optical path length ( u* ) is expressed ask 2

u* = (+yl+) u ,where u is the optical path length, Po is standard pressure, To is standard temperature, and k,and k, are cons tant s. The exponent k,, varies f r om 1.0 for non-overlapping lin es to 0.5 fo rheavily overlapping lines. It has been illust rated by Moller (Reference 12), that k, is de-pendent on wavelength, pressure, and on the amount of absorbing gas. For thi s investigation,k, w a s derived from the t ransmi ssi on data of Wyatt, Stull and Plass (References 4 and 5 ) for atemperature of 300% and for p res sur es of 0.5 and 1.0 atmospheres. The value of k, determinedfor CO, w a s 1.0, independent of optical path length. For H,O, a mean value of k, = 0.8 w a sadopted, as k w a s found to be ext remely dependent on the H, 0 optical path length. The ex-ponent k, , was taken as 0.5 for both H,O and CO,. It should be pointed out that the predictedvalue of the outgoing effective radiance is not very sensitive to the values of k, and k, because


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30 VIRGIL G. KUNDEmost of the radiative transfer occurs near the ground, where the (P/Po) and ( T o / T ) rat ios ar every close to unity.

MODEL ATMOSPHERESThe important prop ertie s which distinguish one model atmosphere from another in these

calculations a r e the surface temperature and th e tota l amount of water vapor. Because we a r ein a window region, the surfa ce tem pera ture is the most influential part of the temperatureprofile as it essentially produces the outgoing radiation, whereas, the remaining pa rt of thetemperature profile has little influence on the outgoing radiation due to the small amount ofatmospheric absorption and re-emission. Again, because we a r e in a window region, it is thetotal amount of absorbing gas, not the density distribution, which has the lar ges t effect on theoutgoing radiation. CO, w a s assumed to be uniform at 0.0310/0 y volume in each model andthus, the t o t a l amount of CO, w a s essentially the sam e in each model (Table I) . Therefore,for the atmospheric absorbing gase s, it is only the va riat ion of the tot al amount of H,O be-tween models that has a varying effect on the outgoing radiation. The model atm osp her eswere chosen to reflect a representa tive range of surfa ce temper atur e and total amount of H,O.

These models a r e shown in Figure 4. The left-hand side of F igu re 4 presents the tempera-ture as a function of altitude and the right-hand sid e, the vert ica l distribution of the mixing

7c

6C

5c

2 tvI

c-

(3I 3c

2c

1(

(

I ' I 1 ' I ' 1 ' 1

I 1 I I I A?200 22 0 240 260 280 301TEMPERATURE ( O K )

i /0 320

WATER VAPOR MIXING RATIO ( g / k g )Figure 4-Model atmospheres. The temperature profiles are indicated in the left-hand side of the figure and the

water vapor mixing ratio as a function of altitude on the right-hand side.


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~

Surface Carbon Dioxide Wat er VaporAtmosphere Temperature u(cm-atm) u(pr-cm)(OK)Tropic a1 305. 0 247.4 4.70ARDC Std (1959) 288.0 247.3 1.16

THEORETICAL RELATIONSHIP BETWEEN EQUIVALENT BLACKBODY TEMPERATURES AN D S U R F A C E T E M P E R A T U R E S MEASURED 31e B Y T H E H RI R

High Latitude W inter-Co ld

Table I

246.5 247.8 0.14

ra tio of H,O. The tota l amount of H,O in a vertical column, indicated in Table I, can be ob-tained by in tegrating the H,O mixing rati o profile over altitude. The High Latitude WinterModel (References 13 and 14 ) represents a case with a low sur fac e tem per atu re of 246.5% anda low total H,O content of 0.14 precipitable cent imeters ( pr- cm); the Tropi cal Model (Refer-ence 15) a cas e with a high su rfac e temperature of 305.0% and a high to ta l H,O content of4.70 pr -cm and the ARDC Standard Model (1959) (Reference 15) an in termedia te case with asur fac e tem perat ure of 288.0% and a tot al H,O content of 1.16 pr- cm.

RESULTSThe r es ul ts of the calcu lation of th e outgoing effective radia nce fo r the model atmosph ere s

of Figure 4 and for seve ral values of the surface emissivity ar e discussed in this section. It isto be emphasized that these results are for clear sky conditions only and, theref ore, a r e notstr ict ly applicable where p artial cloudy conditions exist. In addition, only molecular absorp-tion in the atm osphe re has been considered. The effects of high thin c ir ru s clouds (Reference16) and/or of extinction and emission by aer oso ls( References16 thru 20) have not been taken intoaccount. The difficulties encountered in inferring surface temp erat ure fro m satellite radiancemeasurem ents have been discussed by Wark (Reference 21). A good over-all review of atmos-pheric radiative proces ses in. he infrared (2 to 5 micron s) has been made by the BoeingCompany (Referen ce 22).

In Figure 5, the theoretical sur fac e temperatu re minus equivalent blackbody t empe ratu redifferences a r e given as a function of the zenith angle at the ground for the model atmospheresof Figure 4 . The graybody surface emissivity has been taken as unity. The theoretical tem-tent and a low surfa ce temperature, is about 0.3"K for zenith angle s le ss than about 75". Thus,for clear sky conditions and a rea l atmosphere similar to the High Latitude Winter Modelwhere atm ospheric absorption is the only important so ur ce of attenuation, the HRIR shouldmeasure about 0.3% lower than the surfa ce radiating tempera ture. Because the surfac e emis-sivity has been a ssumed to be unity, the surf ace radiating temp erat ure and the actual sur facetemperature are equivalent. For the Tropica l Model, which had the high tot al wat er vapor con-tent and a high surface tem perature, the theoretical temperature difference is about 2.3% forlow zenith angl es, and inc rea ses to about 8.0% at a zenith angle of 75". Because of the larg er

pe ra tu re d iffe rence for the High Latitude Winter Model, which ha da low total water vapor con-


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32 VIRGIL G. K UNDE02 - " DRY " H I GLAT. WINTE(T, = 246OK

II-

h 4 - ARDC STD (1959)v (T, = 288K)2 6 -

" WET " TROPICAL(T, = 3 0 5 " K )121 I I I I I I I I0 IO 20 30 40 50 60 70 80

ZENITH ANGLE ( degrees )Figure 5-Theoretical surface temperature minus HRIRequivalent blackbody temperature difference as a func-tion of zenith angle for the model atmospheres of Figure4. A graybody surface emissivity of unity and clear skyconditions have been assumed.

16

14

12

10EYL

X9WI 6

4

2

0

ARDC STD ATMOSPHEREZENITH ANGLE = 0'CLEAR SK Y C O N D I T I O N S

. = 1

FRACTION OF ATMOSPHERIC. CONTRIBUTION TO O U T G O I N GEFFECTIVE RADIANCE = 0.05

.01 .02FRACTIONAL OU TG OI NG EFFECTIVE RADIANCE ( k m )

Figure 6-The fractional contribution to the total out-going effective radiance from each atmospheric layer.The assumed conditions are noted in the upper le ft -hand corner.

water vapor content, the Tropical Modelshows a grea te r degree of limb-darkeningthan does the High Latitude Winter Model. Thus,the theore tical calculations predic t the HRIR"window" is a very clean "window." Th is isindicated by the results which show that theobserved equivalent blackbody temperatureshould be within about 2K of the sur fac e tem -per atu re for low zenith angles. How well theobserved temp erature differences comparewith the theoretical temperature differenceswill be discussed in the following papers.

The vert ical distribution of the atmos-pheric emission is shown in Figure 6 for theARDC Standard Model (1959) atmosphere, azenith angle of 0 degrees and a graybody sur-face emis sivi ty of unity. The ab sc is sa , as deter-mined by the ratio $(h , H = O)/& , representsthe fraction of the tota l observed rad iationfrom an atmospheric layer. For example,the therma l emi ssion of a layer one kilom-ete r thick at a height of one kilometer isabout 2% of th e total rad iation observed bythe HRIR, hile a layer one kilometer thickat 5 kilometers contributes only about 0.1%.The total observed radiation f rom the atmos-phere is proportional to the ar e a under thecurv e. In this ca se, the fraction of the atmos-pher ic contribution to the outgoing effectiveradiance is about 5% . Thus, most of the ra-diation observed by the HRIR, about 95%,originates from the ground for the verticalviewing case. A s the zenith angle increasesto 75", the ground contribution decreases toabout 87% due to the increased atmo sphe ricabsorption. The ground contribution for theHigh Latitude Winter and Tropi ca l Model atlow zenith angles is about 98% and 91% of thetotal observed radiation respectively.

Fro m Figure 6, it can also be seen that for radiating surfaces such as thick opaque cloudsat leve ls above 8 to 10 kilomete rs, t he HRIR equivalent blackbody t em pe ra tu re gives the cloudradiating temperature directly, as there is no atmo sphe ric absorption above these levels. Thi stype of situation, along with supporting mete orological data on the cloud- top tem per atu re,


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.THEO RETI CAL R ELATIONSH IP BETWEEN EQU IVALE NT BLACKBODY TEMPERATURES AND SURFACE TEMPERATURES MEASURED

B Y T H E H R IR3 3*

affords a good opportunity for obtaining an effec tive graybody em iss ivi ty of the cloud in theHRIR spectral region.

The surf ace minus equivalent blackbody tempera ture d ifference depends not only on atmos-pheri c absorption but al so on the emissivi ty of the radiating sur face . The effect of surf aceemissivity on the theoretical temperat ure difference is shown in Figure 7 for the High LatitudeWinter and Tropi cal Model. The solid line curv es repres ent a graybody surface emissivity ofunity and ar e identical to the corresponding curves of Figure 5. The dashed line curves re pre-sent a graybody sur face emiss ivity of 0.9. The resu lts, again, apply only for c le ar sky condi-tions. In general, decre asing the surf ace emissivity to 0.9 inc rea ses the temperat ure differ-enc es by about 2.0% for low zenith ang les and less then 2.0% for very high zenith angles. Thetemperature difference for the High Latitude Winter Model increases from 0.3% to 2.0% andthe Tropical Model from 2 . 3 % to 5.0% at low zenith angles . A t very high zenith angles, theeffect of surfa ce emissivity on the tempe rature difference de cre ase s as the surface contribu-tion to the outgoing radiation becomes sm al le r. The resu lts of F igure 7 indicate that i f theemissivity of the surfac e is less then unity, which is tru e to some degree, then assuming theemissivity is unity ca use s the tru e surface temperature to be underestimated. Thus the sur -face emissivity must be known in order to obtain the true surfa ce temper ature f rom the surfac eradiating temperature.

Satellite radiation mea sur eme nts have been obtained previously in the atmospheric windowat 8 to 12 microns from the TIROS meteorological satellite. A comparison of the TIROS VI18 to 12-micron window with the Nimbus window is presented in Figure 8. The upper curverepresents the Nimbus HRIR window and the lower curve the TIROS VI1 8 to 12-micron window.The se resu lt s a r e fo r the ARDC Standard Model (1959) and for a sur fac e emissivity of unity.The te mpe rat ure dif fer enc es a r e about 1.5 and 4.8% for the Nimbus window and about 9.2 and14.9% for the TIROS VI1window for zenith angles of 0" and 7 5 " respective ly. Thus, the NimbusHRIR window shows considerable improvement over the 8 to 12 micron window used in the

0E S = 1.0

\12b I b :o 310 'io i o do :o 'i o do '

ZENITH ANGLE (degrees)

O l4 - NIMBUS I 3.5 - 4.1

c s = II24 I I I I I I I I I I0 10 20 30 40 50 60 70 80 90ZENITH ANGLE (degrees)

Figure 7-Surface emissivity ef fect on the theoretical Figure 8-A comparison of the theoretical temperaturetemperature differences as a func tion of zenith angle. differenceasa functi on of zenith angle for the Nimbus IClear sky conditions and a graybody surface emissivity HRIR 4 micron "window" and the TIROS VI1 8- to 12-are assumed. micron "window." The results are for clear sky condi-

tions, the ARDC Std. (1959) Atmosphere and a graybodysurface emissivity of unity.


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34 V I RG I L G. KUNDETIROS ser ies . Actual resu lt s from the Nimbus I HRIR measurem ents indica te that the re isexcellent agreement between the known surface temperatures and the radiometrically measuredtemperatures i f the above mentioned theoreti cal calculations ar e applied and i f completelyclear sky conditions can be ascertained.

CONCLUSIONSTh e outgoing radiation which should be observed by the Nimbus I HFUR w a s calculated for

sev era l model atmospheres. The results, considering only atmospheric absorption, indicatethe equivalent blackbody temperature measured by the HRIR should be within 2K of the sur-face radiating tempe rature fo r cl ea r sky conditions and low zenith angles which ag re es withthe actual HRIR observations. Surface radiation contributes about 95% and the atmosphereabout 5% of the observed outgoing radiation for the ARDC Standard Model Atmosphere (1959)for clear sky conditions and low zenith angles. Decreasing the su rface emissivi ty from 1.0 to0.9 increases the difference between the true surface temperature and the equivalent black-body temperature which the HRIR should measure, by about 2%.

The Nimbus HFUR 4 micron window re presen ts considerable improvement over the TIROS8 to 12-micron window in measuring the surface radiating temperature as the correction foratmospheric absorption is much sma lle r. Because the correction is so small, in practice, astandard cor recti on can be made ranging from about 0.3% for high latitude regions to about2% for equatorial regions. In principle, of course, the TIROS 8 to 12-micron res ult s can als obe corrected for atmospheric absorption. However, the correction is fairly large and rela-tively more sensit ive to changes in the atmospheric model so that a standard corre ction wouldnot be a satisfactory solution. While the Nimbus HRIR 4-micron window does re presen t an im-provement in obtaining the su rfa ce radiating temperatu re, it elevates in importance theproblems of sur fac e emiss ivity and sou rces of attenuation other than molecular absorpt ion,which must be solved in order to obtain true surface temperatures from a satellite measure-ment. The preceding discussion pertains only to the determination of surfac e temp era ture sfrom HRIR measurements. For pure mapping purposes, such as the mapping of nighttime cloudcover and qualitative terrestrial features the corrections discussed in this article may beneglected.

1.

2.

3 .4.

REFERENCESNordberg, W., "Research with Tiros Radiation Measurements," Astronaut. and AerospaceEng. 1(3):76-83, April 1963.Shaw, J. H., Chapman, R. N., Howard, J. N., and Okholm, M. L., "A Grating Map of theSolar Spectrum from 3.0 to 5.2 Microns," Astrophys. J. 113(2):268-298, March 1951.Goody, R. M., "Atmospheric Radiation," Oxford: Clarendon Press, 1964.Wyatt, P. J., Stull, V. R., and Plass, G. N., "The Infrar ed Tran smitta nce of Water Vapor,"Newport Beach, California: Aeronutronic Div. Ford Motor Co., Report SSD-TDR-62- 127,V01.2, September 1962. Also, Appl. Optics 3(2):229-241, February 1964.


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L

355. Stull, V. R., Wyatt , P. J., and Plass, G. N., "The Infrared Transmittance of Carbon

Dioxide," Newport Beach, California: Aeronutronic Div. Ford Motor Co., Repor tSSD-TDR-62-127,Vol. 3, January 1963. Also Appl. optics 3(2):243-254, February 1964.

6. Wyatt, P. J., Stull, V. R., and Plass, G. N., "Quasi-Random Model of Band Absorption,"J. Opt. SOC . Amer. 52(11):1209-1217,November 1962.

7. Plass, G. N., "Spectral Emissivity of Carbon Dioxide from 1800-2500 cm- ' , I t J. Opt.Soc. Amer. 49(8):821-828, August 1959.

8. Winters, B. H., Silverman, S., and Benedict, W. S., "Line Shape in the Wing Beyond theBand Head of the 4.3,~and of COz," J. Quant. Spectrosc. Radiat. Trans fer 4(4):527-537,July -August 1964.

9. Young, Charles, "A Study of the Influence of Carbon Dioxide on Radiative Transfer in theStr atosph ere and Mesosphere," Ann Arbor, Michigan: Universi ty of Michigan, 1964(Dissertation).

10. Drayson, S. R., "Atmospheric Slant Path Transmission in the 15pCO, Band," Ann Arbor,Michigan: University of Michigan, Technical Repor t 05863-6-T, N65-20944, November1964. Also, published as a NASA Technical Note D-2744,April 1965.

11. Elsasser, W. M., and Culbertson, M. F., "Atmospheric Radiation Tables," Meteorol.Monographs,4(23): 1-43,August 1960.

12. Moller, F., and Raschke, E., "Evaluation of TIROS 111 Radiation Data," Munchen, Germany:Ludwig-Maximilians-Universit'gt,Meteorologisches Institut, Interim Report No. 1, July1963.

13. Cole, A. E.,Kantor, A. J., and Cour t, A., "Supplemental Atmospheres, " in: Interim Noteson Atmospheric Properties-32 (R ev,), Bedford, Massachusetts: A i r Force CambridgeRese arch Laboratories, Office of Aerospace Research, June 1963.

14. Manabe, S. and Moller, F., 'Qn the Radioactive Eauilibrium and Heat Balance of theAtmosphere," Monthly Weather Rev . 89(12):503 -532, December 1961.

15. Hanel, R. A., Bandeen, W. R., and Conrath, B. J., "Infrared Horizon of the Planet Earth,"J. Atmos. Sci. 20(2):73-86, March 1963.

16. Zdunkowski, W., Henderson, D., and Hales, J. V., "The Influence of Haze on InfraredRadiation Measu rement s Detected by Space Vehicles," Salt Lake City, Utah:Weather, Inc., AD 605 688, July 1964.

Intermountain

17. Deirmendjian, D., Atmospheric Extinction of Infrared Radiation," Quart. J. Roy. Meteor.SOC. 86(369):371-381, July 1960.

18. Curcio, J. A., Knestrick, G. L., and Cosden, T. H., "Atmospheric Scat tering in the Visibleand Infrared," Washington, D. C.: U. S. Naval Research Laboratory, NRL Report 5567,January 1961.


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.I0

3619.

20.

21.

22.

Diermendj an, D., "Scattering and Polarization Proper tie s of Polydispersed Suspensionswith Part ial Absorption," Santa Monica, Califronia: The Rand Corporat ion, RM-3228-PR,July 1962.Penndorf. R . B., "Scattering and Extinction Coefficients for Sma ll Absorbing and Non-absorbing Aerosols,'' J. Opt . SOC .Amer. 52(8):896-904, August 1962.Wark, D. Q., Yamamoto, G . , and Lienesch, J. H., "Methods of Estimating Infrared Fluxand Surface Tempe ratur e from Meteorological Satellites," J . Atmos. S c i . 19(5):369-384,September 1962.McDonald, R. K., and Bell, J., et. al., "Infrared Satellite Backgrounds. Part I. Atmos-pheric Radiative Processes, " Seattle, Washington: Boeing Company, Repor t D2-90054;AFCRL 1069; AD 273 099, September 30, 1961.


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na 2.v TERRESTRIAL FEATURES OBSERVED

BY THEHIGH RESOLUTION INFRARED RADIOMETERbb'William Nordberg and R. E. Samuelson

INTRODUCTIONIt is possible to obtain an excellent nighttime representation of te rr es tr ia l cloud and te r-

rain fea tures by means of sa telli te infrared radiometry. In addition, a quantitative asses smentof the effective blackbody temperature corresponding to each feature can be obtained. TheNimbus HRIR has proven to be we l l suited for these purposes.

MA PPIN G BLACKBODY TEMPERATURES WIT H HIGH RESO

Documents

Observations From the Nimbus I Meteorological Satellite