THE UNIVERSITY OFMI CHIGAN COLLEGE OF ENGINEERING Department of Aerospace Engineering High Altitude Engineering Laboratory Technical Report THE CALIBRATION OF AND INTERPRETATION OF DATA FROM AND 0.2-4.0 MICRON CHANNELS OF THE F-1 AND F-4 THE 0.55-0.85 MICRON MRIR RADIOMETERS F. L. Bartman Approved by: L. M. Jones, Director High Altitude Engineering Laboratory ORA Project 05863 under contract with: NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CONTRACT NO. NASr-54(03) WASHINGTON, D.C. administered through: OFFICE OF RESEARCH ADMINISTRATION ANN ARBOR February, 1966

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TABLE OF CONTENTS Page LIST OF TABLES v LIST OF FIGURES vii LIST OF MATHEMATICAL SYMBOLS xi ABSTRACT xv 1. INTRODUCTION1 2, CHARACTERISTICS OF THE VISIBLE CHANNELS OF THE F-l AND F-4 MRIR RADIOMETERS 2 35 METHOD OF CALIBRATION 7 4. INTERPRETATION OF THE MRIR DATA 10 4. 1 Percent Reflectance 10 4.o2. The Possibility of Error in the Interpretation 12 4, 21o Weighted Average Reflectance 12 4.2.2, Directional Effects 13 5. DESCRIPTION OF CALIBRATION SOURCES 17 5.1, Source Characteristics 17 5.~2. The University of Michigan Calibration Sources 17 5. 2, 1. Introduction 17 542.2o, Carbon Arc-MgO Reflector 18 5, 23, Tungsten Filament Lamp-MgO Reflector 21 5 2,4. Hemisphere Source 23 5 3o Santa Barbara Calibration Source 29 6* IN-FLIGHT CALIBRATIONS 32 6.1, The Direct Sun Signal Calibration Technique 32 6o2., Surface Calibration of the Direct Sun Signal Optics 33 70 CALIBRATIONS OF THE F-1 MRIR 37 7T.1 Santa Barbara Calibration Data 37 7T2, The University of Michigan Calibration Data 37 73o, Direct Sun Signal Calibrations 59 iii

TABLE OF CONTENTS (Concluded) 8. CALIBRATIONS OF THE F-4 MRIR 8olo Santa Barbara Calibration Data 8,.2. University of Michigan Calibration Data 85 3 Direct Sun Signal Calibrations 9. ERROR ANALYSIS 9ol Experimental Errors in Calibration 9,2, Corrected Calibration Curve for F-1 MRIR 0,2-4 Micron Channel 95 3~ Errors in Interpretation, Examples 10o CONCLUSIONS AND RECOMMENDATIONS l1o ACKNOWLEDGMENTS 12. REFERENCES Page 62 62 67 72 76 76 78 78 83 85 86 iv

LIST OF TABLES Table Page Io Nominal Characteristics of F-4 MRIR (0,2-4~0 micron channel) 2 IIo Relative Spectral Radiance of Carbon Arc and MgO Reflector Calibration Source 21 III. Relative Spectral Radiance of 2000-Watt Tungsten Lamp and MgO Reflector Calibration Source 24 IV. Relative Spectral Radiance of Hemisphere Calibration Source 28 V. Calculation of Correction Factors for Direct Sun Signal Calibration of F-4 MRIR 35 VI. Calculation of Effective Spectral Radiance of Sun and Calibration Sources for 0.55-0.85 Micron Channel of F-l MRIR 45 VIIo Calculation of Effective Spectral Radiance of Sun and Calibration Sources for 0o.2-40 Micron Channel of F-1 MRIR 46 VIIIo University of Michigan F-1 Calibration Data (Carbon arc-MgO reflector source-February, 1963) 55 IX, University of Michigan Fl Calibration Data (Lamp-MgO reflector source —August, 1963), 56 X. University of Michigan F-1 Calibration Data (Hemisphere source-May, 1964) 57 XI1 Calculation of Effective Spectral Radiance of Sun and Calibration Sources for 0o2-440 Micron Channel of F-4. MRIR 70 XIIo University of Michigan F-4 MRIR Calibration Data 71 XIII. Direct Sun Signal Calibrations of F-4 MRIR 74 XIVo Summary of Errors in the Calibration and Use of the MRIR 77 XV. Reflectances Measured by MRIR Compared with True Average Reflectances 82 v

LIST OF FIGURES Figure Page 1o Optical arrangement of NIMBUS five-channel radiometer (MRIR), 3 2, Relative characteristic response, 49, of 0,55-0.85 micron channel of F-1 MRIRo 4 35 Relative characteristic response, ot, of 0,2-4o0 micron channel of F-l and F-4 MRIR instruments. 5 4o Field of view contours of the 0,2-4 0 micron channel of the F-4 VMRIRo 6 5~ Weighting function, HS 4 for 0o55-0.85 micron channel of F-1 radiometero 14 6. Weighting function, Hs K. for 0.2-4.0 micron channel of F-4 radiometero 15 7o Bi-directional reflectance. 16 8, Carbon arc-4MgO reflector calibration source, 19 9. Relative spectral radiance of carbon arc-MgO reflector calibration source, 20 10. Tungsten filament lamp-MgO reflector calibration source, 22 11. Relative spectral radiance of 2000 watt lamp-MgO reflector calibration source, 25 12o Hemisphere source. 26 153 Relative spectral radiance of hemisphere calibration source, 27 14. SBRC calibration source, 30 15o Spectral radiance of SBRC calibration source. 31 16, Solar spectral irradiance curves at sea level with varying optical air masses, 34 vii

LIST OF FIGURES (Continued) Figure Page 17. Correction factors for F-4 MRIR direct solar calibrate signals. 36 18. F-1 MRIR calibration data-SBRC data, November 1962, 0.55 -0o85 micron channel. 38 19o F-1 MRIR calibration data-SBRC data, June 1964, 0.55-0.85 micron channel. 39 20. F-l MRIR calibration data-SBRC data, 0.55-0.85 micron channel, scanner temperature = 25 C, electronic temperature = 250C. 40 21. F-1 MRIR calibration data-SBRC data, November 1962, 0.2 -4.0 micron channel. 41 22. F-1 MRIR calibration data —SBRC data, June 1964, 0.2-4.0 micron channel. 42 235 F-l MRIR calibration data-SBRC data, 0.2-4.0 micron channel, scanner temperature = 25~C, electronic temperature 25~C. 43 24. Effective spectral irradiance of sun, Hsii for 0.55-0.85 micron channel of F-1 MRIRo 47 25, IncL for carbon arc —MgO reflector and 0.55-0.85 micron channel of Fil MRIR. 48 260 kn4fi i for lamp-MgO reflector and 0.55-0.85 micron channel of F-l MRIR, 49 27.o. ci for hemisphere source and 0,55-0.85 micron channel of F-il MRIRo 50 28. Effective spectral irradiance of sun, H i for 0.2-4.0 micron channel of F-l MRIR. 51 29~, fi9i for carbon arc-MgO reflector and 0.2-4.0 micron channel of F-I MRIR 52 30, ki*tci for lamp-MgO reflector and 0.2-4.0 micron channel of F-1 MRIR. 53 viii

LIST OF FIGURES (Concluded) Figure Page 31o i f* for hemisphere source and O 2-4.0 micron channel of F-1 MRIRo 32~ University of Michigan F-1 MRIR calibration data, 0o55 -O085 micron channelO 335 University of Michigan F-l MRIR calibration data, 0o2-4.0 micron channel, 34. SBRC F-4 MRIR calibration data, 0.,2-4.0 micron channel, October 1964. 355 SBRC F-4 MRIR calibration check 0 0 ron cannel April 1965. 54 60 63 36~ SBRC F-4 MRIR calibration data, 0,2i4o0 micron channel, December 1965. 37~ SBRC F-4 MRIR calibration, 0.2-4.0 micron channel, (25,25) data for November 1964, April 1965, and December 1965. 38. Effective spectral irradiance of sun, Hsl. for Os2-4.0 micron channel of F-4 MIR. 5390 lci for hemisphere source and 0 2-4.0 micron channel of F-4 MRIRo 40~ University of Michigan F-4 MRIR calibration data, 0o2-4o0 micron channel. 41L Observed solar spectrum and black-body intensities for temperatures of 6000~K and 5700~K [after F. S. Johnson, J. Meteorol. 11, 4-351 1954. 42. University of Michigan F-l MRIR calibration data, 0,2-4.0 micron channel, corrected for error in thermopile reading. 435 Spectral reflectance curves for "middle layer clouds" and a "green leaf," 65 66 68 69 73 77 79 80 ix

LIST OF MATHEMATICAL SYMBOLS 2 Ar = the effective area of the radiometer aperture, cm2 F = a correction factor for the atmospheric attenuation of the direct beam of the sun. 2 Hs = the solar constant = 0O1395 watts o cm-2 H = spectral irradiance of MgO reflector by calibration lamp watts cm - HsX = solar spectral irradiance, watts cm'2 AX-lo Hsi = same as above (i used as a summation index), kn = constantequal to ratio: spectral radiance of a source/relative spectral radiance of a source. kr constant, equal to ratio: response of radiometer channel/relative response of radiometer channel. m = optical path length, atmosphereso 2 -1 Nc = radiance of calibrating source, watts o cm-2 steradianlo N' effective radiance of a calibrating source, watts o cm-2o steradian"l, NcX = spectral radiance of calibrating source, watts. cm-2. steradian"1. Ali Ns = radiance of a surface reflecting solar radiation, watts cmn2o steradian"l. N1 effective radiance of a surface reflecting solar radiation, watts o cm-2o steradian-. Ns'(100) = effective radiance of a surface reflecting solar radiation. when surface has 100% diffuse reflectance. r(8e$l) = directional reflectance of a surface. R = distance between source and MgO reflectance plateo xi

LIST OF MATHEMATICAL SYMBOLS (Continued) RI = responsivity of a channel of the radiometer, averaged over the entire field of view. Tm = atmospheric transmission of wavelength X over optical path m. Vc = output voltage of radiometer when viewing calibrating source. V = output voltage of radiometer when viewing a surface which is reflecting solar radiation, Vsc = output voltage of radiometer when viewing sun directly through solar calibrate optics. Wc = radiant emittance of calibrating source, watts * cm2. W = effective radiant emittance of calibrating source, watts o cm-2 W = spectral radiant emittance of calibrating source, watts o cm-2o A- o z = solar zenith angle. 0 - angle in general sense. S = zenith angle of incidence of a ray, $2 = zenith angle of reflectance of a ray. l = azimuth angle of incidence of a ray. 2 = azimuth angle of a reflectance of a ray. y = characteristic response of a channel of the radiometer. - = response of direct solar calibrate optics. <^ = relative characteristic response of a channel of the radiometer, = same as above (i used as a summation index), X = wavelength microns AX = wavelength interval, microns, xii

LIST OF MATHEMATICAL SYMBOLS (Concluded) = microns. - 30 14159. p% = spectral bi-directional reflectance of a surface. p = average reflectance of a surface for solar radiation, P = effective average reflectance of a surface (as measured by radiometer). p(OI.01~ 022S ) = bi-directional reflectance of a surface. T1 = extinction optical thickness. 4c\ = relative spectral radiance of calibrating source, ci = same as above (i used as a summation index) Q = solid angle, steradians. xiii

ABSTRACT A program of calibrations of the 0. 55-0.85 micron and 0. 24o0 micron channels of the F-1 and F-4 MRIR radiometers has been carried out and a hemisphere calibration source has been developed. The characteristics of the calibration source, the method of calibration and interpretation of data in terms of percent reflectance, and the system of direct solar calibrations for use on satellites are presented and discussed, Laboratory calibration data are presented and are shown to agree with Santa Barbara Research Center Data. The direct solar calibration data are presented and the limitation of this in-flight calibration system is discussed. An error analysis indicates a precision of ~2% and possible systematic errors of 5-8% in the calibrations. The possibility of large errors in the interpretation of reflectance data obtained with the MRIR is discussed and shown by example. xv

1o INTRODUCTION The NIMBUS 6 satellite which will be launched early in 1966 will carry a Medium Resolution Infrared Radiometer (MRIR) designed-to measure the thermal radiation emitted, and the solar radiation reflected and scattered back out into space by the earth, clouds and the atmosphere, Two flight models of the MRIR (models F-1 and F-4) have been tested and calibrated in the laboratory and on high altitude balloon flights by members of the High Altitude Engineering Laboratory of The University of Michigan, working under contract NASr-54(03) with NASA Goddard Space Flight Center. The purpose of this report is to describe the method of calibration of the visible channels of the MRIR, to evaluate the accuracy of the calibration and to discuss the problem of interpretation of the data obtained with this type of instrument. 1

2. CHARACTERISTICS OF THE VISIBLE CHANNELS OF THE F-1 AND F-4 MRIR RADIOMETERS The NIMBUS MRIR is a five-channel scanning radiometer designed to measure the flux of thermal radiation and the reflected and scattered solar radiation from the earth and its atmosphere.l The optical design of the radiometer is illustrated schematically in Figure 1. Incoming radiation is reflected by the scanning mirror into the five Cassegranian telescopes and then passes through the chopper into the collecting optics, filter, and bolometer detector. The associated electronics consists of preamplifiers, mixer circuits, power amplifiers, synchonous detectors, output filters, and power supply. The relative characteristic response o of the two visible channels of the F-l (flight model 1) radiometer are shown in Figures 2 and 3. The response of the one visible channel of the F-4 MRIR is also shown in Figure 3. This data was supplied with the radiometers by the Santa Barbara Research Center. Other characteristics of the visible channels are illustrated by those of the F-4 MRIR visible channel, given in Table I.2 The field of view contours of the 0.2-4.0 micron channel of the F-4 MRIRare shown in Figure 4. TABLE I NOMINAL CHARACTERISTICS OF F-4 MRIR (0.2-4.0 micron channel) Spectral region (5% points) Field of view (-6 db points) Optical entrance aperture Effective system f/no. System input noise System gain Responsivity NEPD Time constant Target range Output impedance Output voltage range 0.2-4.8 microns 2.8~ 1.72 in. diam 0.93 0.59 tvolts rms 3.9'103 (rms to dc) 1.6-105 volts/watt/cm2 1.52-10-8 watts/cm2 0.02 sec 0 to 80% albedo 5.'80K 0 to -6.4 volts - 2

Sun Rays During In-Flight Check of Calibration of 0. 55-0. 85 Micron and 0. 2 -4.0 Micron Channel Sun Optics Calibration I m Immersed Detector Scan Mirror Chopper I I I Unimmersed I I I I IDetector Figure 1. Optical arrangement of NIMBUS five-channel radiometer (MRIR).

100l 1962 80 70 June, 1962 <6,% I I I I I I I I 30- \ I \ 20 I 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Wavelength,, Microns Figure 2. Relative characteristic response, 4, of 0.55-0.85 micron channel of F-1 MRIR.

100 I — ' I I 'I 40 20 1 0 1 2 3 4 Wavelength,, Microns Figure 3. Relative characteristic response, f, of 0.2-4.0 micron channel of F-1 and F-4 MRIR instruments.

0.2-4.0 micron channel Data 7-31-63 vertical field of view - - - - horizontal field of view - _ _ ____ -2 -1 0 +1 +2 Angle from Horizontal Axis (degrees) Figure 4. Field of view contours of the 0.2-4.0 micron channel of the F-4 MRIR. 6

35 METHOD OF CALIBRATION The method used for the calibration of the MRIR visible channels is referred to in the literature3 as the "near-extended source method." A diffuse source larger-than the radiometer aperture, which will completely fill the radiometer field of view, is placed a short distance from the radiometer, so that the transmissivity of the medium between the source and the instrument is essentially unity. The radiant emittance of the source,assumed to be uniform over the radiometer field of view, is varied over the range of values desired. Corresponding values of source radiant emittance and radiometer output voltage are recorded. These data provide the basis for calibration curves which must be interpreted as indicated in the following discussion. The characteristics of a perfectly diffuse calibration source are given by~ o 1/~~ p-2 Nc = Nc% d. watts cm2 steradian O Wc = WCe dX watts * cm2 0 and the relations Nc = 9 Nc =L where NcX is the spectral radiance, watts o cm2o steradian-l micron1o Nc is the spectral radiant emittance, watts - cm"2 micronrlo Nc is the radiance, watts - cm'2. steradian-lo Wc is the radiant emittance, watts ~ cm-2. The equation which describes the output voltage of a visible channel of the radiometer when its entire field of view is uniformly illuminated by the calibration source is: Vc = R'oArooNc (1) or 7

Vc = Rt'Aro 2o where R' is the responsivity of the channel of the radiometer averaged over the entire field of view, volts/watt, Ar is the effective area of the radiometer aperture, cm2 Q is the solid angle viewed by the radiometer, steradians. N1 is the "effective" radiance of the diffuse calibration source, watts cm 2 steradianQ! W1 is the "effective" radiant emittance of the diffuse calibration source, watts o cm"-2 NC, the effective radiance, is the quantity which actually effects the radiometero Its value is given by the integral: NC NC dX (2) o where 0k is the effective spectral response of the radiometer channel. X is the wavelength in micronso In the case of the perfectly diffuse source that we assumed, we also have the relation: 00 WI - ntN = W -oodX To ci t0e dh To simplify the discussion in the remainder of this report we will consider only the.values of radiance Nc or N5o Values of radiant emittance Wc or WI can be calculated if desired and if warranted. To summarize, the calibration procedure consists of the following steps, (a).Vary Nc of the diffuse calibration source for which the distribution of spectral radiance is known. (b) Record corresponding values of the radiometer output voltage V< and the source radiance Nco (c) For a given value of Nc, calculate a corresponding value of Nc, using Equation (2). Values of NcX and 0k must be known, 8

(d) Plot the calibration curves of Vc vs. N0 This calibration procedure should be carried out at several instrument temperatures covering the range of temperatures over which the instrument may operate, 9

4. INTERPRETATION OF THE MRIR DATA 4. 1o PERCENT REFLECTANCE The visible channels of the MRIR radiometer are designed to obtain a measurement of the fraction of incident solar radiation which is reflected or scattered back out into space from the surface of the earth, the clouds and the atmosphere. Thus the attempt is made to interpret the radiometer readings in terms of an equivalent average "reflectivity" or "albedo" of that portion of the earth and its atmosphere lying within the radiometer field of view. The interpretation is made as follows. Recall.that the solar constant is defined to be the amount of solar radiation received at normal incidence (i.e., on a plane surface normal to the sun's rays) outside of the at mosphere at the mean earth-sun distance, The value used is 2.0 langleys min. (2 cal. cm2 minmel or 0.1595 watts cm'2) The solar constant can be expressed as: Hs = Hs dX (3) 0 where Hs is the solar constant (irradiance), 0.1395 watts cm-2. HsA is the solar spectral irradiance in watts ~ cm-"2 micron-l If one solar constant of radiation at normal incidence is reflected from a perfectly diffuse reflecting surface with spectral reflectance pA, the radiance of the reflecting surface will be: 00 Ns = l/ P Hs% dX and the reflectance of the surface, averaged over all wavelengths will be; _ Hsx dX p o (4) co 1 Hs,% dX 0 10

This average reflectance is a weighted average with weighting function taken to be.the spectral distribution of the solar radiationo Where the surface.is received by the visible channel of the MRIR radiometer, the radiometer voltage will be: V. Rt' A ooKN' s r s where Ns is the effective radiance of the reflected solar constant, iLe,, S-, A 0 Ns, JX For pa = 1 (i.e, 100% reflectance), then we would have: N7(100L ) = J H dX - o For pA 7 1, we may calculate an "effective" average reflectance of the surface-: co P, s. 0 0 The average reflectance p' thus obtained is a weighted average with weighting function taken to be the product of spectral distribution of the solar radiation multiplied by the instrument response function0 The calibration curve of the visible channel of the MRIR may be given in terms of percent reflectance by taking that value of N equal to Ns(l00%) as representing 100% reflectance, for radiation normally incident upon the surface viewed. - Further a radiometer voltage reading Vc corresponding to a value of NC = 0~5 N(100%l ) represents a reflectance of 50% for normally incident radiation. Similarly, in general, we define the percent reflectance of solar radiation, as measured by the visible channel of the MRTIR radiometer by: r~ Xc:NO P' o N(l00%) 1= d H- % oC 0 11

Finally, to correct for angles of incidence other than 90~, it is necessary to divide the value of reflectance obtained by cos Z, where Z is the zenith angle of the incident radiation. To summarize then, the interpretation of radiometer readings as percent reflectance is made by changing the radiometer calibration curves to the form of NI/N(lO%) vs. Vc. For normal incidence, a given radiometer reading corresponds to a reflectance equal to the value of Nc/Ns(lOO) obtained from this curve. For other angles of incidence of the solar rays it is necessary to divide by cos Z, where Z is the solar zenith angle. Thus: Nc N(100O%).cos z NcX GE dX 0o:(7) cos Z - HlOXj dX o One final correction to be made is a correction for variation in values of Hsko Values of Hs/ used in drawing calibration curves are values for the sun at its mean distance from the earth. Variations of values of Hs during a year are as great at ~+3,3 and should be corrected for, 4,2. THE POSSIBILITY OF ERROR IN THE INTERPRETATION 4 2.1. Weighted Average Reflectance The values of percent reflectance measured by the visible channel of the MRIR differ from the actual reflectance-which we desire to measure as follows: For normal incidence we want to measure: 00 Ef sPA Hs5 dX P = ---- (5) 1 / Hs% ds OWe actually measure We actually measure. 12

I P Hs dX J NCi % d% O O ~p = -------..... -- (7) - s" HS a Sd % H did O O 0 0 In addition to calibration errors, which relate to the substitution of 00 Nc% ~? dX O 0 for 0 " P PK HsX ~k dk, o we have the error in interpretation due to the fact that we have measured a weighted average reflectance using the weighting function H5s *% instead of the function Hs^o Weighting functions for the visible channels of the F-1 and F-4 MRIR radiometers are shown in Figures 5 and 6. The weighting function Ns, 4 for the narrow band channel of the F-l MRIR is limited to the range 0.55 to 0,85 micron and emphasizes the range of 0,58 to 0.7 micron, The weighting function Hd 4>) for the wide band channel of the F-4 MRIR places less emphasis on the wavelength region below 1 micron than the HAs weighting function would, Since the response of the wide band channel is essentially the same for both the F-1 and F-4 radiometers, only one curve has been shown in Figure 60 It is obvious that the reflectances measured with the 0f55-0.75 micron channel of the radiometer cannot be interpreted as 0.2-4 micron reflectances, except possibly in very special cases. Also the difference in emphasis of the O,2-4.0 micron HS 40% weighting function from the desired Has weighting function may lead to significant errors in certain cases. Examples will be given in Section 8.2 of this report, 44.2 2 Directional Effects The quantity p', measured by the radiometer is the bi-directional reflectance illustrated in Figure 7, In this figure, ds is an element of the reflecting surface (e1, j) are the zenith and azimuth angles of the incident ray, and (02,02) are zenith and azimuth angles of a reflected ray, 13

0 0.120 ~ ' H ^ ^ \1PI~~~~~~~~~sk kH^ (Nov. 1962 Data) -. 0....._ _._ _ _ _ _ _ _ "u c~ 0. 05 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2.0 2.5 3 Wavelength (Microns) Figure 5. Weighting function, HsXhq, for 0.55-0.85 micron channel of F-1 radiometer..0

0 0.20 L) ~ 0. 15 ~R S I I. / [ III I~ \ I \. Wavelength (Microns) CI) Cd cW 0. 05 'I 0. 2 0. 3 0. 4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2.0 2.5 Wavelength (Microns) Figure 6. Weighting function, Hsx,$xy for 0.2-4.0 micron channel of F-4 radiometer. 3. 0

z I - 1 Y /360-q2 Figure 7. Bi-directional reflectance. The quantity P(e1,01,G02,2) is called the bi-directional reflectance. The total (hemispherical) reflectance of the surface for radiation incident from the direction (01,l1) is called the directional reflectance: 2jt it/2 r(e1,4l) = p / plO(e l,4,e2,42) sin e2 cos e2 de2 da2 (8) 2=0 2=0 If the surface is a perfectly diffuse reflector then P(01,1,e2,2):=.p, a constant, and r(9Ei) 1 = p If the reflecting surface is not diffuse then, it is necessary to measure p(e1,01,e92,2) over the hemisphere (i.e., all angles e2,Q2) for a large number of incident angles, in order to make a correct interpretation. The characteristics of the reflectance and scattering of solar radiation by the earth and its atmosphere is of the non-diffuse type and therefore the assumption of diffuse reflectance may result in significant error. Errors in interpretation of NIMBUS MRIR data due to neglect of directional reflectance characteristics will be discussed more completely in another report.

5, DESCRIPTION OF CALIBRATION SOURCES 5.14 SOURCE CHARACTERISTICS The calibration provides values of the integral 0 corresponding to radiometer voltage readingso Thus it is necessary to know accurately the values of NK% over the spectral range of the channel (Oa,2_4 microns)o In addition the source should have uniform radiance Nhc over the entire area within the radiometer field of view, The intensity of the source should be variable in magnitude so that values of the integral covering the range of 0 to 100% reflectance (or greater) can be obtained. At any intensity setting,, the source should be stable for a length of time suitable for the calibration. 5~2. THE UNIVERSITY OF MICHIGAN CALIBRATION SOURCES 50 2o1, Introduction Three sources have been used at The University of Michigan for the calibration of the visible channels of the Fal and F-4 MRIRso. The basic configurations are similar for all three of these sources, Radiation from a lamp of a suitable type is allowed to reflect diffusely from a specially prepared surface, In generals the characteristics of the reflected radiation are determined as follows0 The relative spectral distribution of the reflected light tc is determined by comparison with the known spectral distribution of a secondary standard lamp calibrated by the National Bureau of Standardso The total radiance of the target: NcX. dX is determined by an Eppley thermopile which has been calibrated by comparison to a secondary standard thermopile The absolute values of the spectral radiance NcXm are then calculated from 17

NcX = kn Ac where k is obtained from: 00 00 kn / c \ dk -= Nc dX (9) o o The diffuse nature of the reflection of radiation from the reflecting surface is established by measurements made at various reflection angles, The characteristics must be established at all levels of intensity which are used in the calibration run. 52o.2, Carbon Arc-MgO Reflector The first source for the MRIR calibration consisted of a Strong Engineering Company UHII carbon arc with MgO coated flat reflector plate. The arrangement is shown schematically in Figure 8, The radiation from the carbon arc is reflected by an elliptical mirror, passes through an opening in the baffle system, is incident upon the MgO reflector plate and reflected into the MRIR. The diffuse nature of the reflectance of an MgO reflector has been well established,4 Because of the poor stability of the carbon arc, the intensity of the reflected radiation is continuously monitored during the calibration by the Eppley thermopile. The baffles are used to prevent radiation from reaching the MRIR or Eppley thermopile by reflection from objects other than the MgO plate. The intensity of the radiation falling on the MRIR is varied by changing the angle e and thus changing the angle of incidence of the radiation on the flat plate. The relative spectral radiance of this source is given in Table II and Figure 9o Although it was possible to use this source for calibration of the MRIR, several characteristics, ie,, its basic instability and very high power dissipation made it difficult to use, Although the carbon arc system is built with a rough servo control to maintain the position of the rods and thus the arc as carbon burns off of the end of the rod., the arc itself moves around slightly with resulting intensity and spectral changes of the radiation being used for calibration. The very high power dissipation (10 kilowatts) of the carbon arc resuited in a general heating of the apparatus and indeed of the room itself, This gradual change of thermal radiation environment with time made it difficult to maintain accuracy in the calibration procedure.

Flat Plate Coated With MgO 2 ~~ Light and Heat Baffles H \O Carbon Arc Eppley Thermopile MRIR Being Calibrated Figure 8. Carbon arc —MgO reflector calibration source.

30. 0 25.0 20.0 15.0 0-\ c\ C\ 10. 0 5. 0 0 1.0 2.0 2.8 Wavelength, Microns Figure 9. Relative spectral radiance of carbon arc-MgO reflector calibration source. source. 20

TABLE II RELATIVE SPECTRAL RADIANCE OF CARBON ARC AND MgO REFLECTOR CALIBRATION SOURCE Relative Relative Relative Wavelength, Spectral Wavelength Spectral avelength Spectral Wavelength, Emittance micron Emittance Emittance, Emittance, micron micron micron 0.24 0 0.76 16.8 1.40 4.4 0.28 0 0.78 15.8 1.44 4. 0.32 1.1 0.8o 14.7 1.48 4.2 0.36 7.6 0.82 13.7 1.52 3.9 0.40 28.8 0.84 12.9 1.563.6 o.44 26.2 0.86 12.3 1.60 3.4 o.48 27.3 0.88 11.8 1.64 3.1 1.68 2.8 0.50 26.3 0.92 10.8 1.72 2.6 0.52 23.8 0.96 9.9 1.76 2.4 0.54 22.9 1.00 9.0 1.80 2.3 o.56 22.3 1.04 8.6 1.84 2.1 o.58 21.5 1.08 8.3 1.88 2.0 o.60 21.0 1.12 7.9 1.92 1.8 0.62 20.6 1.16 7.4 1.96 1.6 o.64 20.2 1.20 6.7 2.00 1.5 0.66 19.9 1.24 6.1 o.68 19.5 1.28 5.5 2.20 0.9 0.70 19.2 1.32 5.1 2.40 0.4 0.72 18.6 1.36 4.6 2.60 0 0.74 17.8 5.2.3. Tungsten Filament Lamp-MgO Reflector The arrangement of the second source used for MRIR calibration at The University of Michigan is shown schematically in Figure 10. Radiation from a 2000 watt tungsten filament lamp is allowed to reflect from the flat MgO reflector plate. The intensity of the calibration source is varied by changing the distance R between the lamp filament and the reflector plate surface. The spectral radiance of this source was determined as noted above in Section 5.2.1 and then checked as follows. The spectral distribution of the irradiance of the lamp at R = 1 meter was determined by comparison with several working standards of radiance. Data on the spectral reflectivity of a MeO coated plate were obtained from the literature and extended by measurements in our laboratory. 21

Plate Coated With MgO Watt Filament Lamp r) r\D MRIR Radiometer Figure 10. Tungsten filament lamp-MgO reflector calibration source.

The spectral radiance of the source was then calculated from the relation NX H 2:po~Cos 45. watts 4 m'2 steradian-l (10) R2 where Nk s the spectral radiance of the MgO reflector, IH = the spectral irradiance of the MgO reflector by the lamp at 1 meter distance. Pp = the bi-directional reflectance of the MgO reflectoro R = the distance of the lamp from the MgO reflector in meters. The factor cos 45~ enters into the equation because of the 45~ angle between the normal to the MgO reflector and the direction of incident radiation from the lamp, Results obtained by this second technique agree with results obtained by the first technique within about 5%o The relative spectral radiance of this source as determined by the first technique are given in Table III and Figure 11l Excess power dissipation and associated heating was still a problem with this source, and maximum albedo values obtained were lower than desired. In addition the MgO coating on the flat plate was extremely delicate. In spite of extreme care, the coating would not last any great length of time0 Accordingly a new source was designed to eliminate these difficulties0 50 2.44 Hemisphere Source The third source built for calibration of the MRIR is shown schematically in Figure 12o Radiation from ten lamps located on the circumference of the hemisphere undergoes multiple reflections in the hemisphere before passing through the exit holeo All portions of the inside surface are brush coated with Burch sphere paint No0 2210 over Burch No0 2201-S undercoato The lamps mounted around the circumference of the hemisphere are 150 watts each, General Electric No, 1958 incandescent quartz-iodineo The intensity of radiation emitted by the hemisphere is varied by turning on the number of lamps in units of two (diametrically located)0 Usually 5 steps of 2, 4, 6, 8, 10 lamps are usedo The relative spectral radiance of this source is given in Table IV and Figure 135 23

TABLE III RELATIVE SPECTRAL RADIANCE AND MgO REFLECTOR OF 2000-WATT TUNGSTEN LAMP CALIBRATION SOURCE X Relative Relative Relative Spectral Spectral Spectral WavWavelen, Wavelengthtra Spectral micron Emittance, Emittance, m Emittance, H\xp HEpX HxpX...%.4... 0.20 0.24 0.28 0.32 0.36 0.40 0.44 0.48 0.52 0.56 0.60 0.64 0.68 0.72 0.76 0.80 o.84 0.88 0.92 0.96 1.00 1.04 1.08 1.12 1.16 1.20 1.24 1.28 1.32 1.36 1. 4o 1.44 1.48 1.52 0 0.10 0.40 1.10 1.8 4.6 7.3 10.6 15.6 22.0 50.6 42.1 53.2 62.3 70.9 78.1 84.4 89.6 93.9 96.7 99.1 100.4 100.3 99.6 98.7 97.3 95.8 93.4 90.7 87.9 84.7 81.5 78.2 74.9 1.56 1.60 1.64 1.68 1.72 1.76 1.80 1.84 1.88 1.92 1.96 2.00 2.04 2.08 2.12 2.16 2.20 2.24 2.28 2.32 2.36 2.40 2.44 2.48 2.52 2.56 2.60 2.64 2.68 2.72 2.76 2.80 2.84 2.88 71.3 67.8 64.5 61.6 58.7 55.6 52.7 49.6 46.9 44.5 42.3 38.5 38.3 36.6 35.1 33.3 31.9 30.2 28.8 27.4 26.3 25.1 23.6 22.1 20.5 19.4 17.9 16.7 15.4 14.3 13.1 12.1 11.1 10.1 2.92 2.96 3.00 3.04 3.08 3.12 3.16 3.20 3.24 3.28 3.32 3.36 3.40 3.44 3.48 3.52 3.56 3.60 35.64 3.68 3.72 3.76 3.80 3.84 3.88 3.92 3.96 4.00 4.04 4.08 4.12 4.16 4.20 9.4 8.5 7.8 6.9 6.3 5.9 5.3 5.0 4.5 4.1 3.9 3.7 3.4 3.2 3.0 2.8 2.6 2.3 2.1 2.0 1.9 1.8 1.7 1.7 1.5 1.3 1.1 1.0 O 080 0.60 0.40 0.20 0

100 Q. 80 I\ 60 C.) 60 10 3. Wavelength, Microns 40 -ib 0 — 20 0 1.0 2.0 3.0 4 Wavelength, Microns Figure 11. Relative spectral radiance of 2000 watt lamp-Mg0 reflector calibration source..0

Coated With Burch Sphere Paint Light Baffles Lamp. \ [ GE #1958 Quartz Iodine Lamp (150 Watts) aR0 'Li ~ ~10 Lamps Equally Spaced Around ~i_ 1 Circumference of Hemisphere MRIR Radiometer Figure 12. Hemisphere source.

100 80 60 Lo I 0 30, 60 - D I 1.0 2.0 3.0 Wavelength, Microns Figure 13. Relative spectral radiance of hemisphere calibration source. 4. 0

TABLE IV RELATIVE SPECTRAL RADIANCE OF HEMISPHERE CALIBRATION SOURCE Relative Relative Relative Wavelength Spectral Wavelength Spectral Wavelength Spectral mi Emitcron Emittance, micron Emittance,.._micron E.mittanr.cne. m..,cicron, c *C% *Cj% *C%~~~mcro ~ck~ Ic c 0.38 0.69 0.94 92.5 1.48 28.9 0.40 1.39 0.96 92.2 1.50 29.6 0,42 2.17 0.98 90.3 1.52 29.0 0.44 3.65 1.00 89.6 1.54 29.0 o.46 4.97 1.02 88.4 1.56 28.5 0.48 6.93 1.04 87.1 1.58 28.4 0.50 8.99 1.06 85.0 1.60 27.7 0.52 12.50 1.08 83.3 1.62 26.5 0. 54 1570 1.10 82.3 1.64 25.3 o.56 20,30 1.12 78.2 1.66 22.1 0.58 25.10 1.14 75.7 1.68 19.6 0.60 34.90 1.16 70.4 1.70 17.6 0.62 46.10 1.18 67.7 1.72 16.7 o.64 53.0 1.20 65.5 1.74 16.7 0.66 63.0 1.22 65.7 1.76 15.8 0.68 67.5 1.24 65.8 1.78 15.4 0.70 75.0 1.26 62.9 1.80 14.5 0.72 86.4 1.28 62.2 1.82 13.7 0 74 86.3 1.30 60.0 1.84 11.7 0.76 90.8 1.32 61.1 1.86 11.7 0.78 92.5 1.34 54.2 1.88 9.14 0.80 94.9 1.36 49.2 1.90 7.40 0.82 96.9 1 38 51.3 1.92 6.46 0.84 99.0 1.40 33.7 1 94 5.93 o.86 100.0 1.42 34.5 1.96 5.78.88 97.6 1.44 28.6 1.98 3.00 0.90 97. 3 1.46 26.9 2.00 0 0.92 95.4 28

This source is relatively easy to use for calibration of the MRIRo Maximum power dissipation is only 1500 watts and this power is only used a fraction of the time~ 5 35 SANTA BARBARA CALIBRATION SOURCE The source used at the Santa Barbara Research Corporation is shown in Figure 14o The size of the source is significantly smaller than those used at The University of Michigan, Indeed it is made small enouth to be used inside of a vacuum system along with black body sources for calibration of the thermal channels of the MRIRP The General Electric Company Noo 212 photo enlarger lamp is the basic source of radiation, It is used with an additional diffuser. lens and plastic filters as shown in Figure 14. In addition several mirrors, not shown in the schematic diagram are used to direct the light to the MRIR under calibrationo, The intensity (and spectral distribution) of the lamp are varied by changing the voltage applied to the lamp, Curves of this calibration source radiance for several lamp voltages are shown in Figure 15. Note that the relative spectral distribution changes with lamp voltage4 29

Plastic Filter I I A,_- - Plastic Filter Condenser Lens Diffuser G.E. PhotoEnlarger Lamp #212 L i Figure 14. SBRC calibration source. 30

0.04 0.03 + / 75 Volts Volts ^o U..a) C) CO, 0 I p. fl co 0) 0. 0 2.0 0 Figure 15. Spectral Wavelength, Microns radiance of SBRC calibration source. 31

6. IN-FLIGHT CALIBRATIONS 6,1,o THE DIRECT SUN SIGNAL CALIBRATION TECHNIQUE Because of the pronounced deterioration of the TIROS 5-channel radiometer which was noticed on satellite flights of this instrument,5 provisions have been made for in-flight checks of calibration of the MRIR radiometer~ The arrangement for check of calibration of the 0.2-4 micron channel is shown schematically in Figure 1. The NIMBUS satellite is flown with such orientation that the axis of the scan mirror lies in the orbital plane which is lined up with the sun, During a certain portion of the orbit when the axis of the MRIR is aligned within 6~ of the sun, solar radiation will enter the small opening located in the MRIR housing and be reflected down into the scan mirror cavity. When the scan mirror looks vertically upward into the housing this "direct" solar signal is reflected from the scan mirror into the optics of the 0.2-4 micron channel. The output of the radiometer for this solar calibrate signal can be written as: Vsc Rt HsX ~X d. (11) where sc isafco where Sc is a factor which represents the attenuation of the "direct solar signalv" in the optical path from the opening of the housing to the scan mirror, Changes in sensitivity due to change in angle within the 6~ acceptance angle for solar radiation must be included in this factor. This direct solar signal check of calibration can be of considerable help in monitoring deterioration of the 0,2-4 micron channel over a period of time provided: (1) The orbital plane of the satellite does contain the sun. (2) The orientation of the MRIR direct solar calibrate port is accurately known since the calibrate signal is a function of angle within the field of view. A problem arises, however, in monitoring any possible change in the 0,2 -4 micron channel in the time between the last check before launch until the first direct solar calibrate signal in orbit. To predict accurately what the first solar calibrate signal will be requires an accurate prelaunch calibration of the direct solar calibrate optics, 32

6.2. SURFACE CALIBRATION OF THE DIRECT SUN SIGNAL OPTICS For this calibration the instrument is taken outside on a clear day and allowed to view the sun. The amplitude of this direct sun signal is recorded. A correction factor for the attenuation of the direct solar radiation is then calculated and applied to the recorded sun signal. In the direct sun signal calibrations at SBRC a single reading is taken with the sun at a high angle. The correction factor is then calculated from the data of solar spectral irradiance at sea level for varying optical air masses taken from the hand book of Geophysics. This data is reproduced in Figure 16. If HsX is the solar irradiance at the top of the atmosphere and Hmx is the solar irradiance at sea level, then the correction factor is taken to be: Z HSX ~ AX F = Z (12) Values of Hm% were obtained from the curves of Figure 16 by interpolation. In the direct sun signal calibrations of the F-4 MRIR at Michigan, data was taken on several clear days for many different sun angles. Correction factors were prepared from solar irradiance data taken from the handbook of Geophysics with atmospheric attenuation calculated for Elterman's clear standard atmosphere.6 The correction factor was calculated from HsX X AX F = E X A m where T is the atmospheric transmission through the atmosphere with optical path length m. From Elterman's report: T = exp [-T'.m] (15) wnere the values of extinction optical thickness TX were ootained from Elterman's tables. Values of F were calculated for the F-4 MR1R for optical path lengths of 1, 1.5, 2, 53 4, and 5 (zenith angles of 0, 48.3~, 60~, 70.6~, 75I5~0 and 78.70, respectively). Details of the calculations are shown in Table V. The values of F were plotted against m (see Figure 17). This graph can be used to obtain correction factors for solar zenith angles of 0 to 78~ (according to Elterman's clear standard atmosphere). The results of direct sun signal calibrations made on the surface at SBRC and at The University of Michigan, and on a balloon flight are discussed in Section 8.3. 55

10. 6. 0 O o 4..,-I *r4.r-4 ~ 2. P. cq I 1. (Y s o -. - -- 0. 0 If] I P = 760 MM Pressure W = 2. 0 PR. CM Water Vapor* D = 300 Dust Particles/CM3 0. 28 ATMO-CM Ozone* 2 QX (m=0)(Solar Constant) = 1322 Watts/M *per unit of optical air mass, I 0 1 0 1 6 I v F 1! / 0 6 4 2 210 L L - - - - - - - _ l.A,\\I 1.0 0. 6 0. 4 0. 2 0. 0. - -0.30 0. 50 0. 70 0. 90 1. 10L! Optical Air Mass m=1 2 3 4 5 =Degrees 0 60.0 70. 5 75.5 78.5 I I RIM / '11 \ I I I IIITI I I7 I I _ I _ I _ I _ I I1 L II I -- _ I _,,illY. T S / 0.06 0.04 0.02 0.01 0.1 1.20 1.40 1.60 1.80 2.00O Figure 16. air masses. Solar spectral irradiance curves at sea level with varying optical 34

TABLE V CALCULATION OF CORRECTION FACTORS FOR DIRECT SUN SIGNAL CALIBRATION OF F-4 MRIR X Hsxo^ TA HsT1AX T%-5 HX hATH5.AX HsAT A H^ HXTAX H STA s AT*^AX 0.2 0.86 0.25 8.76 0.30 27.03 0.25 6.8 0.125 3.9 1.7 0.4 0.1 0.35 35.00 o.48 16.8 0.331 11.6 8.1 3.9 1.9 0.9.-40 52.21 0.59 30.8 o.454 23.7 18.2 10.7 6.3 3.7 o.45 55.06 o.67 36.9 0.549 30.3 24.7 16.5 11.1 7.4 o.50 48.83 0.715 34.8 0.604 29.5 24.9 17.8 12.7 9.1 0.55 49.58 0,725 35.9 0.616 30.6 26.0 18.8 13.6 9.9 0.60 45.84 0.77 35.2 0.675 30.9 27.1 20.9 16.1 12.4 o.65 42.54 0.79 33.6 0.704 30.0 26.6 21.0 16.6 13.1 0.70 37.64 0.815 30.6 0.734 27.6 24.9 20.53 16.5 13.5 0.75 27.97 0.825 23.1 0.75 21.0 19.1 15.8 13.0 10.8 0.80 24.93 0.835 20.8 0.763 19.0 17.4 14.5 12.1 10.1 0.85 25.68 o.84 21.7 0.768 19.7 18.2 15.3 12.9 10.8 0.90 26.83 o.85 22.8 0.786 21.1 19.4 16.5 14.0 11.9 ~095 27.00 0.855 23.1 0.793 21.4 19.7 16.8 14.4 12.53 1.00 50.69 o.86 43.6 0.800 40.5 37.4 32.2 27.7 23.8 1.10 44.98 0.86 38.7 0.80 35.9 33.3 28.6 24.6 21.2 1.20 38.48 0.862 33.1 0.802 28.4 28.5 24.6 21.2 18.3 1.50 32.01 o.865 27.7 o.807 25.8 24.0 20.8 18.0 15.6 1.40 26.57 0.87 23.1 0.814 21.6 20.1 17.5 15.2 13.2 1.50 22.42 0.875 19.6 0.820 18.4 17.2 15.1 13.2 11.6 1.60 18.70 0.88 16.4 0.828 15.5 14.4 12.7 11.2 9.8 1.70 15.40 0.885 13.6 0.835 12.9 12.0 10.6 9.4 8.3 1.80 12.37 o.89 11.0 0.842 10.4 9.8 8.7 7.7 6.9 1.90 10.54 0.89 9.4 0.842 8.8 8.4 7.4 6.7 5.9 2.00 8.94 0.895 8.0 0.848 7.6 7.2 6.5 5.8 5.2 2-10 7.71 0.895 6.9 0.848 6.6 6.2 5.6 5.0 4.5 2-20 6.68 0.896 6.0 0.849 5.7 5.4 4.8 4.3 3.9 2.50 5.77 0.898 5.2 0.849 4.9 4.7 4.2 3.8 3.4 2.40 4.90 0.90 4.4 o0.855 4.2 4.o 3.6 3.2 2.9 2.50 4.28 0.901 3.9 0.856 3.6 3.5 3.2 2.9 2.6 2.60 35.82 0.902 3.4 0.857 3.3 3.1 2.8 2.5 2.3 2.70 35.539 0.903 3.1 0.859 2.9 2.8 2.5 2.3 2.0 2.80 5.01 0.904 2.7 0.861 2.6 2.4 2.2 2.0 1.8 2.90 2.63 0.905 2.5 0.862 2.3 2.3 2.1 1.9 1.7 5.00 2.34 0.906 2.1 o.864 2.0 1.9 1.7 1.5 1.4 35.10____ 861.4 657.3 584.2 524.6 426.7 351.4 292.2 \ii \kq Z HSXOAk 861.4 F1 =- - 6H5A = 1.28 Z1 = HST1l-A 57.3 2 = 584.2 = 1.47 F5 = 861,.4 = 1.64 53 24.6 861.4 F4 = = 2,02 426.7

F. Correction Factor 2.. 1. 0 30~ 45~ I I I 600 Sun's Zenith Angle 70~ 75~ 78~ m, Optical Path Length 0 1 2 3 4 5 Figure 17. Correction factors for F-4 MRIR direct solar calibrate signals.

7o CALIBRATIONS OF THE F-1 MRIR 7.1 SANTA BARBARA CALIBRATION DATA Calibrations of the F-l MRIR were made at SBRC in November, 1962, and in June, 1964. These data are summarized in Reference 7. Both calibrations involved 2 sets of measurements made at the different combinations of scanner and electronic module temperatures; ioe.:: -- Set 1 Set 2 Scanner Electronic Scanner Electronic Temperature, Temperature, Temperature, Temperature, C 0C 0C C~C 50 25 50 50 25 25 0 0 0 25.'For the.055-0.~85 micron channel, the calibration changed between November, 1962, and June, 1964o Further tests of the instrument at SBRC indicated that this change was due to a decrease in transmission of the 0.55-0 85 micron filters which had suffered an apparent polymerization of the balsam cement which bonds two elements of the filter.~ The data are shown in Figures 18 and 19. The change in calibration is shown by comparison of the (25,25) curves for the two calibrations in Figure 20, The 0,2-4 micron channel showed no change in calibration over this time period. The data are shown in Figures 21 and 22. The two (25,25) curves are shown for comparison in Figure 23. It can be seen that they agree within experimental error, 7.2 THE UNIVERSITY OF MICHIGACN CALIBRATION DATA Calibrations of the F-l MRIR were performed at The University of Michigan as follows: (a) Carbon Arc and MgO Reflector-February, 19635. (b) Lamp and MgO ReflectortAugust 9 1963. (c) Hemisphere Source-April and May, 1964.

-12.0 -10. 0 -8. 0 -6. 0 -4. 0 -2. 0 (25, (50, 50.. ~~~~~~~. 10 0) Output Voltage "% Reflectance" 90 Figure 18. F-1 MRIR calibration data-SBRC data, November 1962, 0.55-0.85 micron channel.

-12. 0 -10.0 Output Voltage -8. 0 -6. 0 -4.0 0 -2.0 "% r / I I I I 0 10 20 30 40 Figure 19. F-1 MRIR calibration d channel. 25) 50) Reflectance" 50 60 70 80 90.ata-SBRC data, June 1964, 0.55-0.85 micron

-12. 0 ~~~~~~~~~~~-12.~~~~~ ~~~0 Nov., 1962 June, 1964 -10. 0 Output Voltage -8.0 -6. 0 -4. 0 -2. 0 "% Reflectance" 0 20 40 60 80 Figure 20. F-l MRIR calibration data-SBRC data, 0.55-0.85 micron channel, scanner temperature - 25~C, electronic temperature = 25~C.

-12. Output Voltage JH -6. 0 -4. 0 -2. 0 "% Reflectance" 0 10 20 30 40 50 60 80 90 100 Figure 21. F-1 micron channel. MRIR calibration data-SBRC data, November 1962, 0.2-4.0

-12. 0 50) -10. 0 -8. 0 -6. 0 -4. 0 -2. 0 Output Voltage "%o Reflectance" 0 0 10 20 30 40 50 60 70 80 90 Figure 22. channel. F-1 MRIR calibration data-SBRC data, June 1964, 0.2-4.0 micron

-12. Nov., 1962 June, 1964 -10. 0 Output Voltage -8. 0 -6. 0 -4. 0 -2. 0 "% Reflectance" 0 0 10 20 30 40 50 60 70 80 90 Figure 23. F-1 MRIR calibration data-SBRC data, 0.2-4.0 micron channel, scanner temperature = 25~C, electronic temperature = 25~C.

The response of a channel of the MRIR is given by $i = kr ~ values of &{, the relative response of the 0.55-0.85 micron and 042-4 micron channels of the F-1 MRIR are shown in Figures 2 and 3. The constants kr are not known for these two channels, however it will be seen that they are not required for calibrations in terms of percent reflectance. Recall that the spectral radiance of a source is given by: Nci = kn $ci and Nc kn ci Aki = kn ci Ai and that the effective radiance of a source is written as: N = k n i cl I i (14) i The spectral irradiance of the sun is Hsi (see Figure 5) and thus the effective radiance of a target with 100Q diffuse reflectance is krZ ^S^ H^ (15) Ns = i i si Values of 49 Hsi c and O!N and O1i are given for the sun and all three sources for the 0.55-0.85 micron channel in Table VI. Similar data for the 0.2-4 micron channel is given in Table VII. Curves of 'Hsi and.i!ci are given in Figures 24 through 31. The calibration data taken in February, 1963, with the carbon arc-MgO Reflector Source are shown in Table VIII. Percent reflectance is computed from: kr kn E Xi ci AXi kn.f Ici AXi -.i i kr y A tH 1 r H p t _ _ -., _. I ii si i si kn is computed from: N NC kn = z NCi0Ak ci i 44

TABLE VI CALCULATION OF EFFECTIVE SPECTRAL RADIANCE OF SUN AND CALIBRATION SOURCES FOR 0.55-0.85 MICRON CHANNEL OF F-1 MRIR Carbon Arc and Lamp and Hemisphere with A\,,t__Sun Mgo Reflector MgO Reflector Burch Paint microns 1i. Hsi 'ci %i i,watt i knci ci ci __watts/m win _ 0.51 0.52 0.53 o.54 0.55 0.56 0.57 o.58 0.59 o.60 0.61 0.62 0.63 0.64 0.65 o.66 0.67 o.68 0.69 0.70 0.71 0.72 0.73 0.74 0.75 0.76 0.77 0.78 0.79 0.80 0.81 0.82 0.85 0.84 0.85 o.86 0.005 0.010 0.030 o.o85 0.210 0.385 0.625 0.760 0.865 0.940 o.980 1.000 0.99 0.955 0.910 0.838 0.800 0.760 0.700 0.655 0.575 0.485 o.415 0.345 0.280 0.215 0.180 0.140 0.115 o.o80 0.045 0.030 0.020 0.013 o.oo8 0.005 19.45 19.25 19.55 19.80 19.49 19.10 19.07 19.09 19.00 18.36 17.83 17.48 17.11 16.75 16.50 16.32 16.03 15.60 15.17 15.o6 14.73 14.30 14.05 13.69 15.53 13.28 12.75 12.45 12.23 11.99 11.74 11.52 11.23 10.86 10.61 10.36 0.097 0.193 0.586 1.685 4.093 7.354 11.919 14.508 16.455 17.258 17.473 17.480 16.939 15.996 15.015 13.676 12.824 11.856 10.619 9.825 8.470 6.936 5.831 4.725 3.788 2.855 2.295 1.743 1.406 0.959 0.528 0.346 0.225 0.154 o.o85 0.052 256.225 23.8 22.9 22.3 21.5 21.0 20.6 20.2 19.9 19.5 19.2 18.6 17.8 16.8 15.8 14.7 13.7 12.9 12.3 0.258 17.12 1.947 20.21 8.586 23.07 16.340 27.35 19.740 32.10 20.600 38.05 19.291 44.23 16.676 49.94 14.820 55.88 12.576 60.64 9.021 65.40 6.141 70.15 3.612 74.43 2.212 78.47 1.176 82.04 0.411 85.61 0.168 88.70 0.171 12.5 1.718 15.7 8.882 20.3 0.125 1.335 20.786 25.1 19.076 30.174 34.9 32.806 38.05 46.1 46.1 42.246 53.0 50.615 41.850 63.0 52.794 42.469 67.5 51.3 39.719 75.0 49.125 31.719 86.4 41.904 24.202 86.3 29.774 16.002 90.8 19.522 10.986 92.5 12.950 6.563 94.9 7.592 2.568 96.9 1.153 99.0 2.847 1.287 0.062 91.55 0.458 100.0 0.500 153.617 359.710 427.468 45

TABLE VII CALCULATION OF EFFECTIVE SPECTRAL RADIANCE OF SUN AND CALIBRATION SOURCES FOR 0.2-4.0 MICRON CHANNEL OF F-1 MRIR Sun - Carbon Arc and Lamp and' Hemisphere with X9 ______, MgO Reflector MgO Reflector Burch Paint microns H Hi H si i l _microns Hwj atts/m2 iHsi ci ci kntci knoifci c i. ici 0.3 0.41 62.79 25.74 0.55 0.23 0.75 0.31 0.4 0.76 156.34 118.82 28.80 21.89 4.60 3.50 1.39 1.06 0.5 0.67 203.54 136.37 26.30 17.62 13.10 8.78 8.99 6.02 o.6 0.64 181.84 116.38 21.00 13.44 30.60 19.58 4.90 22.34 0.7 0.59 148.80 88.38 19.20 11.33 57.75 34.07 75.00 44.25 0.8 0.53 120.21 63.71 14.70 7.79 78.10 41.39 94.90 50.30 0.9 0.59 95.08 56.10 11.0 6.67 91.75 54.13 97.0 57.41 1.0 0.825 74.78 61.69 9.00 7.43 99.10 81.76 89.60 73.92 1.1 0.89 58.09 51.70 8.10 7.21 99.95 88.96 82.50 73.25 1.2 0.94 47.60 44.74 6.70 6.30 97.30 91.46 65.50 61.57 1.3 0.957 38.18 36.54 5.30 5.07 92.05 88.09 60.00 57.42 1.4 0.97 30.87 29.94 4.40 4.27 84.70 82.16 33.70 32.69 1.5 0.977 25.17 24.59 4.05 3.96 76.55 74.79 29.60 28.92 1.6 0.99 20.68 20.47 5.40 3.37 67.80 67.12 27.70 27.42 1.7 0.993 17.13 17.01 2.70 2.68 60.15 59.73 17.60 17.48 1.8 0.96 14.29 13.72 2.30 2.21 52.70 50.59 14.50 13.92 1.9 0.968 12.00 11.62 1.90 1.84 45.70 44.24 7.40 7.16 2.0 0.977 10.15 9.92 1.50 1.46 38.50 37.61 2.1 0.975 8.63 8.41 1.20 1.17 35.85 34.95 2.2 0.983 7.39 7.26 0.90 0.88 31.90 31.36 2.3 0.98 6.36 6.23 0.65 0.64 28.00 27.44 2.4 0.974 5.50 5.36. 40 0.39 25.10 24.45 2.5 0.905 4.78 4.33 0.20 0.18 21.30 19.28 2.6 0.945 4.18 3.95 -- - 17.90 16.92 2.7 0.625 3.66 2.29 14.85 9.28 -2.8 0.85 3.22 2.74 12.10 10.29 2.9 0.90 2.84 2.56 9.75 8.78 3.0 0.96 2.54 2.44 7.80 7.49 3.1 o. 94 2.24 2. 11 6.10 5.73 3.2 0.95 1.99 1.89 5.00 4.75 3.3 0.957 1.81 1.73 4.00 3.83 3.4 0.969 1.62 1.57 3 5.40 3.29 3.5 0.908 1.46 1.33 2.90 2.63 3.6 0.895 1.31 1.17 2.30 2.06 3.7 0.700 1.18 0.83 1.95 1.37 3.8 0.675 1.07 0.72 1.70 1.15 3.9 o0.645 0.97 o.63 1.40 0.90 4.o 0.482 0.90 o.453 1.00 o.48 4.1 0.500 o.84 0.42 0.50 0.25 4.2 0.195 0.77 0.15 --- --- 4.3 0.152 0.71 0.11 4.4 0.177 o.65 0.12 4.5 o0.191 o.58 0.11 4.6 0.175 0.50 0.09 4.7 o.o60 o.44 0.03 4.8 --- 0.43 -- 4.9 --- 0.42 -- 5.0 -

18, 0r 16.0 - 14. 0 - 12. 0 10. 0 8. 0 H.1 si i I 1 Wavelength, k i Microns 6. 0 4. 0 2. 0 0. 5 0. 6 0. 7 0. 8 Figure 24. channel of Effective F-1 MRIR. spectral irradiance of sun, Hgsii for 0.55-0.85 micron 47

80 70 60 50 40 30 r ci 1 20 10 Wavelength, k i Microns 0.5 0.6 0.7 0.8 Figure 25. fitcI for carbon arc-MgO reflector and 0.55-0.85 micron channel of F-l MRIR. 48

80 70 60 50 40 ci i 2( 11 Wavelength, Xi, Microns 0.5 0.6 0.7 Figure 26. kn{irci for lamp-MgO reflector and F-1 MRIR. 0. 8 0.55-0.85 micron channel of

80.0 70. 0 60.0 50, 0 40.0 30. 0 20. 0 10. 0 0. 5 Figure 27. MRIR. 6i ci Wavelength, ki, Microns 0. 6 0.7 0.8 0.55-0.85 micron channel of F-1 bi!ci for hemisphere source and 50

140 120 100 80 \J H 60 Wavelength, XN. 0 1.0 2.0 3.0 Effective spectral irradiance of sun, Hsiji for 0.2-4.0 micron F-1 MRIR. 4. 0 Figure 28. channel of

20 ci \j1 10 Wavelength, Xi., Microns O1 0 1.0 2.0 3.0 4. Figure 29. yItci for carbon arc-MgO reflector and 0.2-4.0 micron channel of F-1 MRIR. 0

k i *ci n 1 ci 80 60 40 20 Wavelength, ki, Microns 0 1.0 2.0 3.0 kn4lici for lamp-MgO reflector and 0.2-4.0 micron channel of Figure 30. F-1 MRIR.

140F 120 'ci 60 40 20 Wavelength, Xi, Microns 1.0 2.0 3.0 4. 0 Figure 31. MRIR. fIrci for hemisphere source and 0.2-4.0 micron channel of F-1

TABLE VIII UNIVERSITY OF MICHIGAN F-1 CALIBRATION DATA (Carbon arc-MgO reflector source-February, 1963) ^W ~ Nc o.0.55-0.85 Micron Channel 0.2-4.0 Micron Channel Watts/m2 Watts/m2 sr MRIR, p, MRIR, Volts % Reflectance Volts % Reflectance 208 66.21 1.5 14.7 1.8 15.9 398 126.69 3.2 28.1 3.6 30.4 597 190.03 4.9 42.1 5.3 45.5 787 250.51 6.6 55.5 6.9 60.0 977 310.99 8.4 68.9 8.6 74.5 1177 374.65 10.1 83.0 10.3 84.8 1376 437.99 11.8 97.0 11.9 105.0 1575 501.34 12.8 111.0 The necessary numerical values for the 0.55-0.85 micron channel are: Z.. AX. i 11 1 C i i si Z t: c. AX. i 1 Cl 1 1Fl~ = 17.004 = 81.56 3.072 Thus: 3.072 Nc 17.004 81.56 17.004 81.56 or 3 07.2 17.004 856 Nc = 0.2215 Nc 17.004. 81.56 The necessary numerical values for the 0.2-4 micron channel are: 1 t H. t i i si 7 4>' t c AX. i i cl 1 = 314.00 = 12.80 55

Thus: -_, 12.80 Nc 17.004 314 P(%) 12.80 = 0125804- Nc = 0.2397 Nc 17.004 ~ 314 The calibration data taken in August, 1963, with the lamp-Mg0 Reflector Source are shown in Table IX. TABLE IX UNIVERSITY OF MICHIGAN F-l CALIBRATION DATA (Lamp-MgO reflector source-August, 1963) 0.55-0.85 Micron Channel 0.2-4.0 Micron Channel R MRIR, p' MRIR, _Volts % Reflectance Volts % Reflectance 1.00 -- 2.0 0.8 8.2 0.79 0.45 3.2 1.29 13.1 0.65 0.75 4.7 2.03 19.4 o.56 1.10 6.3 2.82 26.2 0. 150 7.9 5.75 52.8 0.456 1.90 9.5 4.80 59.5 0.425 2.50 11.1 5.75 45.9 0.400 2.50 12.4 6.40 51.5 0.570 2.90 14.5 7.60 59.9 The radiance of the MgO plate, with irradiance from the lamp at 45~0 is: Nc - 0.7072 1 kn fci A\i watts/m2-steradian and the reflectance is computed from and the reflectance is computed from: 56

kr * 707 1 kn ci i Aki r t TI R2 i n p = --—. kr Z 4i H5 i kn *ci ii A1i.2250 i IR2 ' iZ; H I i i si For the 0.55-0.85 micron band L kn Pj nk 1 i p'(o) = ci Ai = 7.194 719.4 ~ 0.225 1.985 81.56. R2 R2 For the 0.2-4.0 micron band Z kn i.i *ei AXi = 114.5 -,( = 11450 - 0.225 1 _ 8.205 p314.00 R2 - The calibration data taken with the Hemisphere Source in April and May, 1964 are shown in Table X. TABLE X UNIVERSITY OF MICHIGAN F-l CALIBRATION DATA (Hemisphere source-May, 1964) No. of wc Nc.55-0.85 Micron Channel 0.2-4.0 Micron Channel Lm CpsRR MRRIR, P' Lamps Watts/m2 Watts/m2 sr MRIRP _Volts % Reflectance Volts % Reflectance 1.45 -2 226.29 72.03 15 10.2 2.03 17.8 1.61 4 5445.5.29- 24.8 -4 443.52 141.18 342 20.0 41 34.9 5.42 4.10 6 651.71 207.45 4.9- 29.4.84- 51 5.07 6.00 6.31-4 7.68 -8 859.89 273.71 36.- 58.7 7.- 67.7 61046 1047.91 10 1049.97 334.21 770- 47.3 9.52- 82.7 7.97 9.83 57

The % reflectance is computed from kr kn i % ci A-i 1 PT = kn i ' 4 ci AXi Z H. Jt i 1 S1 kr e H. t i 1 S kn is computed from: n =~ c.N.i E ic AXi i The necessary numerical values for the 0,55-0.85 micron channel are: Z "ci Axi i 1 z! H. J i I Si Z *ci Ai I = 74.038 = 81.56 = 8.549 Thus: -, = 8.549 74.038. Nc 81.56 or 854.9 Nc 74.038. 81.56 =.14157 Nc The necessary numerical values for the 0.2-4 micron channel are: 1 i si I Aci1 Z ci AX. i = 314. oo = 57.51 Thus: = 57.51 74.038 Nc 314.00 or P' -= 57 512 N 74.038 * 314.00 c = 0.2474 Nc 58

The calibration curves using the data of Tables VIII to X are plotted in Figures 32 and 35. The voltage indicated is the MRIR voltage output into a 56800 ohm load(The University of Michigan calibrations which were taken with a 52600 ohm load are corrected to a 56800 ohm load). For the 0,2-4 micron channel, the curves of Figure 33 show excellent agreement between the February, 1963, and May, 1964 data. The August, 1963 departs from the other two sets in a strange fashion, that suggests that the 1/R2 law variation used for irradiance of the reflector plate by the lamp is not valid. Figure 32 indicates complete disagreement between three University of Michigan calibrations for the 0,55-0o85 micron channelO The change of transmission of the filter of this channel due to polymerization of the balsam cement used as a bonding agent was demonstrated by SBRCo It is possible that the transmission of the channel may have been changing wildly during the time of the measurements. No other explanation for the disagreement is apparent. Again the bend in the calibration curve for the August, 19635 data indicates that the 1/R2 law used in this calibration was not valid. SBRC data taken at the same scanner and electronic temperatures have been shown for comparison. Disagreement with University of Michigan data is indicated. Probable causes of the disagreement are discussed and corrected calibration curves for the 0.2-4 micron channel are shown in Section 9.2 of this report, 7. 3 DIRECT SUN SIGNAL CALIBRATIONS The direct sun signal calibration system was installed on the F-l MRIR, but was not adjusted or ground calibrated for use, Direct sun signals were obtained on the June 26, 1965, balloon flight, however it is not possible to judge the performance of the system because of the lack of surface calibrations. 59

-12.0 -10. 0 -8.0 -6.0 SBRC, Nov., 1962 U/SBRC, June, 19 U.M. May, 1964/ \ J / _ U. M. Feb.., 1963 - - --- I Output Voltage ON -4. -2. Scanner Temp. = 25~C ' Electronic Temp. = 25~C..~~ Fe. l6 U.M. Aug., 196 o Feb., 1963 A Aug., 1963 * May, 1964 "% Reflectance'. 0 10 20 Figure 32. University of micron channel. 40 50 60 70 80 Michigan F-l MRIR calibration data, 0.55-0.85 90 100

-12. 0,1962 SBRC Data 10. 0 -8. 0 LOutput Voltage HO k-J -6. 0 -4. 0 -2. 0 0 0.20-4.0 Micron Channel Scanner Temp. = 25~C Electronic Temp. = 25~C o Feb., 1963 Aug., 1963 * May, 1964 "% Reflectance" Figure 335 channel. University of Michigan F-l MRIR calibration data, 0.2-4.0 micron

8, CALIBRATIONS OF THE F-4 MRIR 8. o SANTA BARBARA CALIBRATION DATA Calibrations of the F-4 MRIR were made at SBRC in November, 1964, on April 8, 1965, and in December, 1965, The November, 1964, and December, 1965, calibrations were complete sets made for 10 combinations (2 sets of 5 each) of scanner and electronic module temperatures, i.e,: Set 1 Set 2 Scanner Electronic Scanner Electronic Temperature, Temperature, Temperature, Temperature, ~C 0C 0C ~C 50 25 50 50 40 25 40 40 25 25 25 25 10 25 10 10 O 25 0 0 The calibration on April 8, 1964, was made after a faulty chopper motor drive amplifier was replaced, This was merely a spot check of calibration with both scanner and electronic modules at 25~Co It should be noticed that the (25,25) calibration in set 1 and (25,25) calibration in set 2 above provide an opportunity to evaluate the precision of the calibrations. The three sets of calibration data are shown in Figures 34, 55, and 56. In Figure 34 a comparison of the two (25,25) curves, which are almost identical indicates a very high precision for the October, 1964, calibrations. The same comparison in Figure 36, indicates a significantly degraded precision in the December, 1965, calibration, ie,, at 75% reflectance, there is a difference of 0.35 volts, Note also the much greater effects of electronic module temperature variations in the December, 1965, calibrations, Figure 37 shows the (25,25) data for all of the SBRC calibrations, At first glance it appears that the calibration curve has changed between each calibration, ioe., it seems that the curve has moved downward between October, 1964, and April, 1965, and then back upward by December, 1965, However, it should be noticed that the apparent error established by comparison of the 62

0,, 50):0, 40)4,5, 25):0, 25 50, 25) 0, 25) 0, 25) 25) -6. 0 -5. 0 -4. 0 -3. 0 -2. 0 -1. 0 Scanner Voltage Electronics I Temperature ON "% Reflectance" 0 10 20 30 40 50 60 70 80 Figure 34. SBRC F-4 MRIR calibration data, 0.2-4.0 micron channel, October 1964. 90

25) -6. 0 -5.0 -4. 0 Voltage -3. 0 ON 4:: -2. 0 -1.0 0 "% Reflectance" Figure 35. 1965. SBRC F-4 MRIR calibration checky 0.2-4,0 micron channel, April

(50, 50: (40, 40) (25, 25) (40, 25) (25, 25) (10, 25) (0, 25) (10, 10) ((0, 0) -6. 0 -5. 0 -4. 0 (50, Voltage \n -3. 0 -2.0 -1. 0 0 "% Reflectance" 0 10 20 30 40 50 60 70 90 Figure 36. 1965. SBRC F-4 MRIR calibration data, 0.2-4.0 micron channel, December

/12/64, -6.0 0 -o -5. 0 Output Voltage -4. 0 -3.0 -2. 0 -1. 0 "% Reflectance" 10 20 30 40 50 60 70 80 Figure 37. SBRC F-4 MRIR calibration, 0.2-4.0 micron channel, (25,25) data for November 1964, Apri-l1965, and December 1965. 90

December 20, 1965, and December 29, 1965, measurements is of approximately the same magnitude as the apparent calibration shift. Thus it cannot be concluded that the change in calibration curves indicated on this graph indicates a shift in instrument characteristics but is merely evidence of the limit of repeatibility of the calibration data, 8 2. UNIVERSITY OF MICHIGAN CALIBRATION DATA The calibrations of the F-4 MRIR were made with the hemisphere source4 The response of the radiometer is given by: 9i = 0,94 where 0f are values of the relative response average over small wavelength intervals see Figure 35 The spectral radiance and radiance of the Burch hemisphere with n lamps turned on are given by: Nci = kn ci Nc = kn ci Axi i The effective radiance of the Burch hemisphere is written as: N' 0.94 k Z 4' %fr Ax N'c - 9 n 1 i i I The spectral irradiance of the sun is Hsi (see Figure 5), and thus the effective radiance of a target with 100l diffuse reflectance is: 5. 1; 51 Nt 0 94 L 1 Values of H and the products H and h t:c are given in Table XI Curves of 1 Hs and 1 ji are given in Figures 38 and 39, The calibration data taken in 1965 and early 'n 1966 are shown in Table XII. The percent reflectance is computed from 0.94 kn.{ ~ci Ai jr ^ i si o i, i - 67

100 80 60 40 20 Wavel 0 Figure 38. channel of 4 3 1 Effective spectral irradiance of sun, Hsili, for 0.2-4.0 micron F-4 IfRI.

!i rii 1 11 a0 40 20 Wavelength, Xi, Microns 1 2 3 ieCi for hemisphere source and 0.2-4.0 micron channel of F-4 4 Figure 39. MRIR,

TABLE XI CALCULATION OF EFFECTIVE SPECTRAL RADIANCE OF SUN AND CALIBRATION SOURCES FOR 0.2-4.0 MICRON CHANNEL OF F-4 MRIR., H5i/ 2 m1s ciici Sr Hi watt s/m i i microns ~ watts/r _bx CL nL icrons, watts/m2 0.3 0.90 62.79 25.12 --- 2.7 0.983 3.66 3.60 0.4 0.565 156.34 38.35 1.39 0.78 2.8 0.989 3.22 3.18 0.5 0.504 203.54 102.53 8.99 4.53 2.9 0.995 2.84 2.85 0.6 0.525 181.84 95.47 34.90 18.32 3.0 0.973 2.54 2.47 0.7 0.538 149.80 80.59 75.00 40.35 3.1 0.962 2.24 2.15 0.8 0.452 120.21 51.95 94.90 41.00 3.2 0.952 1.99 1.89 0.9 0.580 95.08 55.15 97.30 56.43 3.3 0.927 1.81 1.68 1.0 0.710 74.78 53.09 89.60 63.62 3.4 0.988 1.62 1.60 1.1 0.815 58.09 47.54 82.30 67.07 3.5 0.937 1.46 1.37 1.2 0.888 47.60 42.27 65.50 58.16 3.6 0.987 1.31 1.29 1.3 0.92 358.13 35.43 60.00 55.68 3.7 0.850 1.18 1.00 1.4 0.955 30.87 29.42 33.70 32.12 3.8 0.705 1.07 0.75 1.5 0.963 25.17 24.36 29.60 28.65 3.9 0.661 0.97 0.64 1.6 0.973 20.63 20.12 27.70 26.95 4.o 0.620 0.90 0.56 1.7 0.991 17.13 16.93 17.60 17.44 4.1 0.520 0.84 0.44 1.8 0.948 14.29 13555 14.50 13.75 4.2 0.360 0.77 0.28 1.9 0.961 12,00 11.55 7.40 7.11 4.3 0.180 0.71 0.13 2.0 0.953 10.15 9.72 --- -- 4.4 0.138 0.65 0.09 2.1 0.973 8.63 8.40 4.5 0.180 0.58 0.10 2.2 0.974 7.39 7.20 4.6 0.202 0.50 0.10 2.5 0.978 6.36 6.22 4.7 0.100 0.44 0.04 2.4 0.980 5.50 5.39 4.8 0.015 0.43 0.01 2.5 0.952 4.78 4.55 4.9 --- 0.42 - 2.6 0.970 4.18 4.05 5.0.-.jci9Ai = 74.038 i -0.9 Z~Hsi = 258.8 J i o.94 ZicijAXi = 50.00 i kn is computed from: kn = N T nci AXi i The necessary numerical values are: Z tci AXi = 74.038 0.94 z,, t i i 1 ~ Hi = 258.8 70

TABLE XII UNIVERSITY OF MICHIGAN F-4 MRIR CALIBRATION DATA Epply No. 4609 with CaF2 filter and special aperture 0,15325 MIV/watt/m2 Thermopile Thermopile used: sensitivity: Date 1/16/65 - -~-U ----- No. of Lamps 2 4 6 8 10 2 4 6 8 10 2 4 6 8 10 MRIR Volts.-I 6/3-16/65 1.37-1.43 2.75-2.95 4.20-4030 5.45-5.63 1.22-1.32 2.63-2.81 3.98-4.23 5.36-5.61 6.74-7.02 1.22-1.28 2.57-2.66 3.92-4.04 5.28-5.40 6.56-6.61 Thermopile, PLV 27.0 56.2 84.0 111.0 26.0 54.0 82.0 108.0 134.0 25.0 52.0 80.0 105.0 151.0 Wc Watts/m2 Nc Watts/m2 sr........W.... _ _ 203.8 64.9 424.2 135.0 634.0 201.8 837.7 266.6 196.2 62.5 407.5 129.7 618.9 197.0 815.1 259.4 1011.3 321.9 188.7 60,1 392.5 124.9 603.8 192.2 792.5 252.3 988.7 314.7 16.9 35.2 52.6 69.6 16.3 33.8 51.4 67.7 84.0 15.7 33.6 50.1 65.8 82.1 -P % Reflectance D - -------- -- -- - -- -- -- -- 1/14/66

0.94 E 1 Ai 5 = 50.00 Thus,. = 5000 O Nc 74.038 258 8 or vwlb *0 ^ W oNC 0.2609 N, 744038 2588 c 2609 N The calibration curves using the data of Table XII are plotted in Figure 40o It is interesting to note that the output of The University of Michigan calibration source has decreased by about 6% in the year January, 1965, to January, 1966, The (25^25) calibration curve of the F-4 MRIR has not changed significantly in this period according to The University of Michigan calibrations. It was indicated in Section 8.1 of this report that although at first glance it might indicate that there was a change in calibration of this channel between October, 1964, and December, 1965, this shift:appeared to lie within the repeatability of the SBRC calibrations. Indeed an average of the 4 SBRC calibrations cannot be distinguished from the average of The University of Michigan calibrations. 8,3. DIRECT SUN SIGNAL CALIBRATIONS Results of the direct sun signal calibrations are shown in Table XIII, Note first that the sun signal received on the March 10, 1965 balloon flight was 300 volts with the balloon at 54 mb, pressure altitude, A correction factor of lo05 for atmospheric absorption yields a predicted orbital direct sun signal of 3o15 volts. The SBRC sun signal calibrations showed predicted orbital sun signal calibrations of 3,6 volts (November 19, 1966) and 5316 volts (January 6, 1966), the latter value is in remarkably good agreement with the balloon flight resulto Surface calibrations at The University of Michigan gave a wider range of predicted orbital sun signal calibrations than the SBRC data did. It is apparent that this method of calibration of the direct sun signal is not very reliable, The excellent agreement between the balloon flight data 72

June, 1965 (X 1 96b(A) -6. 0 - Jan. 1965 (0) -5. 0 -4. 0 Output Voltage -3.0 -2. 0 Scanner Temp. = 25~C Electronics Temp. = 25~C -1. 0 "% Reflectance" 0 10 20 30 40 50 60 70 80 90 Figure 40. University of Michigan F-4 MRIR calibration data, 0.2-4.0 micron channel.

TABLE XIII DIRECT SUN SIGNAL CALIBRATIONS OF F-4 MRIR m = optical path length Vs = MRIR sun signal F = F.V, correction factor predicted orbital direct sun signal Sun Date Elevation m Vs F FV Notes Angle 11/19/64 38.7 2.4 1.5 3.6 At SBRC 3/10/65 5/19/65 5/20/65 5/21/65 1/6/66 15.4 66.4 65.6 53.7 64.7 67.9 63.2 56.7 45.3 36.0 30.0 37.5 45.4 54.3 60.8 67.1 64.4 60.2 53.9 47.2 37.5 33.0 0.252 5 1.05 5.1 Balloon Flight Data, 54 mb pressure altitude 1.09 1.095 1.24 1.1053 1.081 1.118 1.193 1.404 1.70 2.00 1.64 1.40 1.227 1.142 1.o89 1.101 1.148 1.23 1.36 1.64 1.84 1.77 1.77 1.77 1.87 1.80 1.87 1.70 1.48 1.38 1.62 1.38 1.87 1.89 1.88 1.70 1.55 1.46 1.46 1.29 1.31 1.31 1.37 1.32 1.35 1.33 1.34 1.43 1.53 1.65 1.51 1.42 1.37 1.33 1.31 1.32 1.33 1.37 1.42 1.51 2.41 2.32 2.42 2.54 2.45 2.39 2.51 2.43 2.26 2.28 2.45 1.96 2.56 2.51 2.46 2.24 2.06 2.00 2.07 1.95 At U. of Mich. At U. of Mich. At U. of Mich. 1.83 2.0 1.58 3.16 At SBRC 2/7/66 30.0 1.625 2.0 1.65 2.68 At U. of Mich. (hazy) 2/11/66 2/12/66 33.6 33.8 1.76 2.01 1.55 3.12 At U. of Mich. (blue sky between clouds) 1.79 2.32 1.56 3.62 At U. of Mich. (clear) 74

and the January 6, 1966, SBRC data and the February 11, 1966, University of Michigan data is apparently fortuitous, There are several possible sources of error in the technique used. (a) First, the assumption that the Handbook of Geophysics data on Elterman's clear standard atmosphere can be used to correct for the attenuation of the direct solar beam may easily lead to errorr It is apparent that Elters man's model does not fit the Michigan atmosphere in May under clear sky conditions, The data of May 21, 1965, especially indicate this in that the morning and afternoon data for the same solar elevation differ by as much as 2O0, If calibrations are made which depend on corrections for atmospheric attenuation, much more accurate information on moisture and aerosol content must be available. (b) Second, the calculation of the correction factor F uses the response function of the MRIR without consideration of the spectral response characteristic of the direct sun signal optics, (c) Finally, the direct sun signal calibration is sensitive to angle variations within the field of view of its optics4 This sensitivity can easily result in a reading smaller than the maximum value* 75

9. ERROR ANALYSIS 9,. 1 EXPERIMENTAL ERRORS IN CALIBRATION The equations involved in the calibrations of the visible channels of the MRIR are Vc = R' Ar (krl)kn ci) AXi (16) L (kr)kn(ci) Aki i The radiometer vo e The radiometer voltage is proportional to the "actual" effective target radiance, In the calibration this voltage is plotted against p', the ratio of a calculated effective target radiance divided by a calculated effective radiance for 100% diffusely reflected solar radiance. Errors in Vc are measurment errors, estimated to be less than 0.1 volt. Errors in p' arise from: (a) Errors in measured values of 0io (b) Errors in measured values of kn *cio (c) Errors in values of Hsi used. If all work is done carefully and accurately maximum errors for kr %i should be less than %1o The maximum error in measurement of kn Vci should be less than 8% at 0~25 micron and less than 3% at 2,6 microns (the maximum uncertainty quoted by the National Bureau of Standards8 for standards of spectral radiance)~ In addition values of kn *i have a error in precision (repeatability) of about 1%. Errors in values of Hsi used should be less than 1%, if corrections are made for variations in values of earth-sun distance when interpretations of measured reflectances are made, If 5700~K to 6000~K black body radiation data are used instead of actual values of Hsig errors of up to 5% may result, especially in the O.55-0o85 micron channel. This can be easily seen by reference to Figure 41, which compares the spectrum of solar irradiance with black body intensities for 6000~K and 5700K,. 76

zi ca C c4 o. u: Wavelength (u) Figure 41. Observed solar spectrum and black body intensities for temperatures of 6000~K and 5700~K [after F. S. Johnson, J. Meteorol. 11, 431, 1954]. A summary of errors in the calibration and use of the visible channels of the MRIR is given in Table XIV. From this table it can be seen that the TABLE XIV SUMMARY OF ERRORS IN THE CALIBRATION AND USE OF THE MRIR Quantity Magnitude Comments I I I t Vc 1% Precision (repeatability) kr i kn ci r 1% [less than 1% less than 3-8% less than 1% Precision Systematic error Systematic error Systematic error Correct for earth-sun distance! Do not use black body data! Errors in interpretation: Measured p' is a weighted average reflectance with weighting function Hs k<. whereas: Desired p is weighted average reflectance with weighting function Hsx. Measured p' is bi-directional reflectance-not a total reflectance. 77

precision of the calibration should be better than 2%, the systematic error should be less than 3-8%o In addition, significant errors may be made in the interpretation of the data. This error of interpretation can easily be 10% or larger (see Section 953). 9.2,o CORRECTED CALIBRATION CURVE FOR F-1 MRIR 0.2-4 MICRON CHANNEL During the calibrations of the F-4 MRIR, it was noticed that an error could easily be introduced into readings of source radiance if too much time was taken in this processo The cause of the error was the increase of thermal radiation of the source with time. Since the thermopile with CaF2 filter has a response out to and beyond 10 microns, it responds to thermal radiation as well as visible radiation, Comparison of readings of the same thermopile with CaF2 and quartz filters indicated an error as great as 5~.6% could occur. A discussion of the experimental technique used on the F-l MRIR calibration lead to the conclusion that this error probably did exist in the calibration of the F~l MRIR. Indeed if a correction is applied to the (University of Michigan) data, excellent agreement is obtained between University of Michigan and SBRC calibrations. The modified University of Michigan calibration is shown and compared with the SBRC curves in Figure 42. 9o 3 ERRORS IN INTERPRETATION, EXAMPLES Two examples of the measurement of reflectance with the visible channels of the NIMBUS MRIR are considered, the reflectance of a "green leaf"lO and of "middle layer clouds" 11 The spectral reflectance curves considered are shown in Figure 435 For each curve the quantities T and "' have been calculated for each of the visible channels of the MRIR, i e,, the "true" average reflectances for solar radiationo 0.85 Z P Hsk A% 0.55 P (18), 55-0.85 = o,85 Z Hs, AX oo055 78

-12. 0 SBRC Data 1962 1 964 Data -10.0 -8. 0 -6. 0 -4. 0 -2. 0 0 Output Voltage Scanner Temp. = 25~C Electronic Temp. = 25 C "% Reflectance" 0 10 20 30 40 50 60 70 80 90 Figure 42. un~vul-oj,... __ -- "T-R calibration data, channel, corrected for error in thermopile reactin,. 0.2-4.0 micron

100 80 L P Spectral Reflectance 60 Middle Layer Clouds Co 0 40 Green Leaf 20 1. 0 2. 0 3. 0 4. 0 Figure 43. leaf." Spectral reflectance curves for "middle layer clouds" and a "green

P, 2-4.0 4.0 Z PX HsX Ax 0.2 4.0 HsX AX 0.2 (19) and the reflectances measured by the MRIR 0.85 = 0-55 0.85 0.55 PA HsX ~X AX (20) HskX ~ Ax 0.2-4.0 4.0 0.2 4.0 0.2 Pk Hs A (21) HsX k AX where the values of eter. by used are the appropriate values for the F-l MRIR radiom The results of these calculations are shown in Table XV. The true average reflectance of the "middle layer clouds" is almost the same for the 0.55-0.85 and 0.2-4.0 micron regions. Both channels of the MRIR would give excellent measurements of these average reflectances. For the "green leaf" the situation is different and errors of interpretation could result. Although the spectral reflectance varies widely as a function of wavelength, the true average reflectances P0.55-0.85 and p0.2-4.0 are about the same, 0.26. The measured average reflectance 0o.2-4.0 gives an excellent result 0.27, however the measured value PIo is much too low, 0.16.

REFLECTANCES MEASURED BY MRIR Substance o.2-4.6 Middle layer clouds 0.659 Green leaf 0.264 TABLE XV COMPARED WITH TRUE Po.55-o.85 0.710 0.261 AVERAGE REFLECTANCES Po).2-4.6 P. 55-0.85 0.645 0.714 0.275 0.156, - Use of -55 8 for earth albedo would require a correction factor of 1.6 in the case of the "green leaf 1.6 in the case of the "green leaf." 82

10. CONCLUSIONS AND RECOMMENDATIONS The following conclusions are drawn from the calibration program de-, scribed above, (1) The program of calibrations of the 0o55-085 micron and 02-4,0 micron channels of the MRIR's has developed to the point at which reliable calibrations can be made. (2), The hemisphere source which has been developed as a part of this program can be used with confidence in making this type of calibration, It provides a suitable range of intensities, 0-85% reflectance, of known spectral distribution; is stable and relatively efficient in its use of electrical power. As a calibration source it is far superior to the "carbon-arc-MgO reflector plate" or "tungsten lamp-MgO reflector plate" sources, (5) The calibrations of the F-l and F-4 MRIRs have been carried out satisfactorily, i e,, (a) Calibrations of the 0.2-4,0 micron channel of the F-4 MRIR are repeatable and agree with SBRC calibrations within the estimated precision and systematic error range of this type of measurement, (b) Calibrations of the 0,2-4,0 micron channel of the F-l MRIR carried out with the "carbon-arc-MgO reflector plate" and hemisphere sources are consistant and agree with SBRC calibrations, although a correction factor derived from F-4 calibration work. was appliedo (c) Calibrations of the 0.55-0.85 micron channel of the F-l MRIR show large variations as a function of time, This variation is explained by the deterioration of the filter used in this channel, (4) The method of calibration and interpretation of the data in terms of percent reflectance can lead to large errors in interpretation for signals of some spectral distributions. An example is given in Section 9.35 Additional error results from the assumption that the bidirectional reflectance measured is a diffuse reflectance, (5) The direct solar calibrate system is limited in its use by the method of calibration, in which the sun is viewed from the earth's surface and corrections are made for atmospheric transmission, The accuracy of this type of correction is poor and thus for NIMBUS satellite applications, the deterioration of the radiometer during the satellite launch phase may not be accurately measured. 83

Recommendations for future MRIR calibrations are: (1) Reports on MRIR calibrations should include all calibration details, including (a) Spectral distribution and intensities of calibrating source, (b) Secondary standards to which radiation measurements are referred, (c) Values of solar irradiance used for the calculation of percent reflectance, so that comparisons between the work of various calibrating groups may be better compared. (2) The characteristics of the calibrating sources should be monitored and checked at regular intervals so that changes can be noted~ (3) The characteristics of the complete optical system used in the did rect sun signal calibrations should be determined. A method of calibration which doeno depend on estimated corrections for atmospheric transmission should be developed. 84

11, ACKNOWLEDGMENTS This calibration work has been carried out as a part of a program of atmospheric radiation measurements under Contract NASr-54(05) with NASA Goddard Space Flight Center, The many helpful discussions of the subject matter of this report with Dr. W, A. Nordberg, Mr. W. R. Bandeen, and Mr. A. McCullough of Goddard Space Flight Center, and Mr. FP R. Malinowski of Santa Barbara Research Center are gratefully acknowledged. Members of the High Altitude Engineering Laboratory who have contributed to the calibration work are Messrs. M. T. Surh, -Wan Yo Lee, L. W. Chaney, Gunar Liepins, L, W. Carls and M. G. Whybra,* Thanks are also due to Professor E. So Epstein helpful comments and suggestions on this report. and L:. Mo Jones for their *Now with the Radio Astronomy Laboratory of The University of Michigano 85

12, REFERENCES 1. Hummer, R. F. and F. R. Malinowski, "NIMBUS Five-Channel Scanning Radiometer," paper presented at Ninth National Infrared Symposium (IRIS), 6-8 May, 1963, Dallas, Texas, 2. "Data Book for NIMBUS Medium Resolution Infrared Radiometer (F-4)," Santa Barbara Research Center, Goleta, California, 30 November, 1964. 3o Holter, KM R., S. Nudelman, G. HL Suits, W. L. Wolff, Go J. Zissis, "Fundamentals of Infrared Technology," P. 54, The McMillan Co., 1962. 4, Sanders, Co Lo and E E.,K Middleton, "The Absolute Spectral Diffuse Reflectance of Magnesium Oxide.in. the Near Infrared," J. Opt, Soc. Amero, *43_, 58, 1953'. 5o Bandeen, Wo: R., ML Haley, 'jI. Strange, "A Radiation Climatology in the Visible and Infrared from the TIROS Meteorological Satellites," Report No. X-651-64-218, Goddard Space Flight Center, Greenbelt, Maryland, August 1964, 6. Elterman, L, '"Atmospheric Attenuation Model, 1964, in the Ultraviolet9 Visible and Infrared Regions for Altitudes to 50 km,," Environmental Research Paper No. 46, AFCRL, Office of Aerospace Research, USAF-, Lo Go Hanscom Field, Massachusetts, September, 1964, 7. Malinowski., F. R*, "Investigation of F-l MRIR —Final Report," Report on Contract NAS 5-757, Santa Barbara Research Center, Goleta California, 2 November, 1964. 8. Stair, Ro, R. G, Johnston and E. W, Halbach, "Standard of Spectral Radiance for the Region of 0.25 to 2.6 Microns," Journo Reso NoBoSo,, 64A, 291-296, July-August, 1960. 9. Johnson, F. S., "The Solar Constant," J. Meteorol, Vol. 1 po 431 -439, 1954, 10o Gates, D, Mo, H, J. Keegan, J. C. Schleter, and V. R. Weidner, "Spectral Properties of Plants," Applied Optics, 4,No, 1, January 1965 11, Novosel'tsev..Ye. P., "Spectral Reflectivity of Clouds," NASA TT-F5328, National Aeronautics and Space Administration, Washington D.Co, April.1965, (Translation of article from Trudy Glavnoy Geofizicheskoy Observatorii imemi A. I. Voycykova, No. 152, 1964..) 86

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