Calibration and comparative measurements of pyrheliometric instruments using natural sunligh

Drummond, A. J.

Laue, E. G.

English

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_.
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
Technical Memorandum No .^33-312
Pyrheliometric Comparisons at the
1 JPL 47able Wountain AFacility -
A. J. Drummond
i The Eppley Laboratory
Newport, Rhode Island -3
and
E. G. Laue
The Jet Propulsion Laboratory
Pasadena, California
Approved by:
George Cooranson, Manager
Space Simulators and
Facility Engineering Section
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
PASADENA. CALIFORNIA
! December 1 S, 1966
	 ^.^
Copyright © 1966
Jet Propulsion laboratory
California Institute of Technology
Prepared Under Contract No.WAS 7-100
National Aeronautics & Space Administration
JPL TECHNICAL MEMORANDUM NO. 33-312
FOREWORD
The results described herein are based on the work performed jointly
by the Eppley Laboratory, Newport, Rhode Island, and the jet
Propulsion Laboratory, California Institute of Technology, Pasadena,
California, and was sponsored by the National Aeronautics and Space
Administration under Contract NAS 7-100.
P RECEDING PAGE BLANK NOT AWED.
JPL TECHNICAL MEMORANDUM NO. 33-312
CONTENTS
I. Introduction	 . .	 . .	 .	 . .	 . . .	 .	 . . .	 . . .	 .	 .	 . . 1
11. Primary Pyrheliometric Facilities . • . • • • . • • • • • • 1
III. Standard Measurement Series • • • • • • • • • . • • . • 2
IV. Tests of Surface and Flight Radiometers • • • • • • • . • • 3
V. Concurrent Tests • • • • • • • • • • • • • • • • • . • • 6
VI. Summary . . . . . . . . . . . . . . . . . . . . . . . 7
TABLE
1. Solar radiation data from tested pyrheliometers . . . . . . . . . . 2
FIGURES
1. JPL TM-2 overall solar irradiance test setup . . . . . . . . . . . . 3
2. Primary working standard pyrheliometers . . . . . . . . . . . . 4
3. SCME signal readout equipment . . . . . . . . . . . . . . . . 5
4. JPL Angstrom measurement equipment . . . . . . . . . . . . . 6
S. Spectrolab test arrangement . . . . . . . . . . . . . . . . . 7
JPL TECHNICAL MEMORANDUM NO. 33.312
ABSTRACT
This study concerns a series of comparative measurements of various
pyrheliometric instruments which were made using natural sunlight
at the JPL Table Mountain Facility. The details shown in this report
introduce instrument calibration techniques for flight and surface
radiometers. Methods of observation are also described. Responsi-
bility for instrumental comparisons has been left to the participating
organizations.
I. INTRODUCTION
An essential aspect of the JPL-Eppley Solar Constant
Measurement Experiment' is the introduction of instru-
ment calibration techniques for the Flight and Surface
Radiometers developed for this project and their evalua-
tion. During 1965 and early in 1966 such calibrations
were performed at Newport, Rhode Island, and at
Pasadena, California, with the aid of artificial sources
of radiant energy. These included high power tungsteii-
filament lamps and xenon and carbon arcs. It was con-
sidered that a logical extension of the laboratory-based
studies represented a comparison under conditions of
natural sunlight, of the new multi-channel radiometers
with precision pyrheliometers, at a high-level location.
Therefore, it was decided to undertake a calibration
program of this type at the JPL Table Mountain facility,2
employing several Xngstrom electrical-compensation pyr-
'See JPL Techni^al Memorandum No. 33-298.
'Previously the site of the principal solar-constant observing station
of the Smithsonian Institution.
heliometers as references. These instruments constitute
part of the Eppley group of primary pyrheliometers
maintained, in collaboration with Weather Bureau-ESSA,
as the United States standards for reproduction of the
International Pyrheliometric Scale (IPS). The period
April 7-9, 1966 was chosen since operational programs
with the radiometers were scheduled for the summer of
this year.
Since this was the first such occasion on the West
Coast, and after the Smithsonian Institution had ceased
to provide such standards (about 1960), that primary
pyrheliometers would be available at the Table Mountain
site, it was further decided to invite other West Coast
organizations concerned with maintaining pyrheliometric
standards to participate in the planned comparisons. Ad-
vantage was also taken of this opportunity to verify the
instrument constants of the two JPL standard Xngstrom
pyrheliometers.
11. PRIMARY PYRHELIOMETRIC FACILITIES
The building and related data are described as follows:
1. Location. Building TM-2, the JPL Solar Test Facil-
ity, at Table Mountain, California, is located at lati-
tude 34.4 0
 N, longitude 117.7 • W, and at a 7550-ft
altitude.
2. Primary references. The Eppley standard Xngstr5m
electrical compensation pyrheliometer No. 2028 was
Necked, on site, against second Eppley standard
gstr5m pyrheliometer No. 518-agreement to
within ±0.217v. On return to the Eppley Laboratory,
these two pyrhehometers were' compared with a
JPL TECHNICAL MEMORANDUM NO. 33-312
third working standard-Xngstrom pyrheliometer
No. 162, the principal instrument of the primary
group-and the agreement was also good to with-
in 0.25%.
3. Readout instrumentation. The Eppley control unit
No. 2311 was used; it incorporates a potentiomet-
rically referenced precision milliameter, which is
checked frequently to better than 0.1 9o'.
4. Auxiliary reference pyrheliometers. The JPL
Angstrom pyrheliometers (Eppley model) 65% and
6551, with Eppley control unit modified to read
electrical current with a digital voltmeter refer-
enced potentiomeu ically, were used.
5. Observers. The o'% ­rimers were A. J. Drummond of
Eppley Laboratory and J. Gatewood of Jet
Propulsion Laboratory.
III. STANDARD MEASUREMENT SERIES
Table 1 records comparative data covering solar radiation intensities, using Xngstrom compensating pyrheliometers.
Table 1. Solar radiation data from tested pyrheliometers
Solar radiation liftn	 ios'
Dab, Tinto,
1966 PST
°Cal an' aBe-' mw an' w 14,
April
 1100-1121 1.490 103.9 96.5
1205-1217 1.408 98.1 91.2
April
 0821-0838 1.379 96.1 89.3
0850-0910 1.405 97.9 91.0
0923-0935 1.411 98.3 91.4
0940-1000 1.418 98.8 91.8 JPL
1010-1030 1.432 99.8 92.7
1055-1114 1.414 98.5 91.6
1125-1146 1.412 98.4 91.4
1200-1220 1.392 97.0 90.1 JPL
1320-1335 1.424 99.3 92.2
1345-1400 1.418 98.8 91.8
1425-1440 1.319 91.9 85.4
1448-1501 1.156 80.6 74.9
April
 0940-0955 1.443 100.6 93.4
1000-1020 1.457 101.5 94.3
1020-1030 1.464 102.0 94.8
1038-1053 1.465 102.1 94.9
1103-1120 1.455 1.4 94.2
1130-1140 1.447 "YM 93.3
1148-1158 1.438 100.2 93.1
1205-1215 1.418 98.8 91.8
-1228 1.416
Can
1p2^2e3
h
98.7 91.7
and J►l 1.904.n
. C^o^ino•m	 d eo,. ►yr^ilenat^Mo 6550	 oen^igod^to	 .
►lode mootwemenb were in Col an-3 min-. and od w valor computed withthe following amVenknct
i col(W) em-2 min-, - 69.7 mw an4
 - 64.75 w If-^
JPL TECHNICAL MEMORANDUM NO. 33-312
IV. TESTS OF SURFACE (E. 3171) AND FLIGHT (E. 3172) RADIOMETERS
The rad6rmcters %N-cre mounted, side by side, oil
Eppley solar tracking unit. Signal readout (sunlight
exposure all(] screening—thermal zero) was obtained,
employing it Leeds and Nortluup do thcrmocouplc am-
plifier and strip-chart millivolt potentiometric recorder
assembly. This system was checked with ill
precision voltage reference (accuracy 0.1'/(). The accu-
racy of the radiometer reference (total flux) channels
was established to ..thin 0.5`4, IPS as standard. Rela-
tively, mutual cOIISIStellcy, in gen,_rl, of all 12 channels
of each radiometer. \cas indicated to be of the order of
1',', as iudgc-d by the Table 'Mountain and the Newport
basic tungsten-optical bench examinations. Figure I
shows the overall test setup at the TM-2 (SCME' radi-
ometers are oil extreme right). Figure 2 is a closeup
of the prinlary Angstrom compensation pyncchometers
and associated instrumentation. Figure 3 shows readout
instrumentation for the SCME.
'Solar Constant \teasuncinent Experiment.
Fig. 1. JPL TM-2 overall solar irradiance test setup
JPL TECHNICAL MEMORANDUM NO. 33-312
i	 %f
Fig. 2. Primary working standard pyrheliometers
JPL TECHNICAL MEMORANDUM NO. 33-312
Fig. 3. SCME signal readout equipment
AOIOW
V_
t^
JPL TECHNICAL MEMORANDUM NO. 33-312
V. CONCURRENT TESTS
0
In addition to the two primary J PL Angstrom pyrheli-
ometers' mentioned above, there were three Eppley nor-
mal incidence pyTheliometers (No. 5183, 5185, and 5196)
recording at the Solar Test Facilities.
At the Industrial Users' Site, which is approximately
1 .!-mi -west and at a 300-ft lower elevation, the following
gyrheliometric measuring equipment was operated:
1. Aerospace Controls Corporation
Eppley MKV 7132
Hyeal 14718
Hycal 17597
'The JPL .*ingstrom instrument is shown in Fig. 4.
Hycal 17 599
Hycai 17600
Hycal 17601
Hycal 17602
2. Hoffman Electronics Corporation
Eppley 180° pyrheliometer (pyranometer) 2957
3. Spectrolab, Division of Textron Industries (Fig. 5)
Heliotek normal incidence pyrheliometer E.3907
Spectrolab normal incidence pyrheliometer E.4897
Spectrolab TRW Mod DR-2 212
Spectrolab TRW Mod DR-2 252
Boeing	 TRW Mod DR-2 253
Boeing	 TRW Mod DR-2 152
0
Fig. 4. JPL Angstrom measurement equipment
1	 ^
1	 \
Fig. 5. Arrangement
JPL TECHNICAL MEMORANDUM NO. 33-312
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' - •_,r.`	 1	
""aff'i'x_+=..^_	 f .	 rM,^	 :..
	
o	 _	 _
VI. SUMMARY
Responsibility for redetermination of pyrheliometer
calibration was left with relevant organizations. The
JPL—Eppley facility was intended solely as a source of
absolute radiometric reference.
In the interpretation of the standard values tabulated
above, it should be emphasized that the effective aper-
ture ante (i e., total field of view) of the Eppley model
of the ngstrom electrical compensation pyrheliometer
is 5.0 deg. In pyrheliometer comparisons with the Sun as
the source, where the highest accuracy is desired, con-
sideration must be given to the circumsolar sky radia-
tion'. In attempting to correct the measurement of the
direct intensity for standard instrument aperture con-
dition, not only must the aperture angle he taken into
account, but also atmospheric turbidity conditions (i.e.,
molecular and aerosol scattering).
'Angstrom, A., and lihode, B., " Pyrheliometric measurements with
special regard to the circumsolar sky radiation," Tellus, 18, 1966,
pp. 25-,33.
JPL TECHNICAL MEMORANDUM NO. 33-312
ACKNOWLEDGMENTS
The cooperation of Earle Ivie, JPL Table Mountain manager, and
Charles Powell, manager of the Solar Test Facility, aided greatly in
accomplishing this calibration study.


Pyrheliometric comparisons at the JPL Table Mountain Facility

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