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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
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NASA NTRS1978

Accurate solar 'constant' determinations by cavity pyrheliometers

Total solar irradiance was observed outside the earth's atmosphere by three types of absolute cavity pyrheliometer in a June 1976 sounding-rocket experiment. The 1367-W/sq m average solar 'constant' result is uncertain by less than + or - 0.5%, the most accurate determination to date. Nearly simultaneous observations by the Nimbus 6 earth-radiation-budget total-irradiance detector of 1389-W/sq m exceeded the rocket result by 1.6%. These recent results are discussed in the context of a summary of solar-constant determinations made above the troposphere by cavity pyrheliometers.

Willson, R. C.↗
NASA NTRS1981

Low-cost solar array project: Four absolute cavity radiometer (pyrheliometer) intercomparisons at New River, Arizona: Radiometer standards

Four detailed intercomparisons were made for a number of models of cavity-type self-calibrating radiometers (pyrheliometers). Each intercomparison consisted of simultaneous readings of pyrheliometers at 30-second intervals in runs of 10 minutes, with at least 15 runs per intercomparison. Twenty-seven instruments were in at least one intercomparison, and five were in all four. Summarized results and all raw data are provided from the intercomparisons.

Estey, R. S.↗
NASA NTRS1977

The self-calibrating sensor of the Eclectic Satellite Pyrheliometer /ESP/ program

A self-calibrating thermopile sensor having a cavity type receiver is described. Its incorporation into pyrheliometric instrumentation, including the electronics and control, is also discussed. Completed devices are capable of solar irradiance measurements with an accuracy to within 0.5% and can detect solar variability to a level approaching 0.001% in space.

Hickey, J. R.↗
NASA NTRS1983

Pyrheliometer With Improved Accuracy

Instrument maintains components at constant temperature and allows tenfold improvement in instrumental accuracy. Two units make up improved pyreheliometer. Radiometer absorbs radiant energy and generates electrical signal. Control unit, which incorporates improvements, provides amplifiers, controls, and calibration circuits. Instrument 10 times more accurate than previous sion.

Estey, R. S.↗
NASA NTRS1984

Special Pyrheliometer Shroud Development

To insure that the insolation values accurately represent the input power to a power conversion unit the field of view (FOV) of the concentrator aperture and the insolation radiometer must be the same. The calculations, implementation, and results of this approach are covered. Three instruments were used to measure the insolation: an Eppley Normal Incidence Radiometer (NIP) and two versions of the kendall cavity radiometer. The shrouds used to limit the FOV of the radiometers were designed to simulate the FOV of the PDC-1 concentrater with the cold water cavity calorimeter. This technique of matching the FOV of an insolation radiometer to the FOV of a specific concentrater and receiver aperture appears to be both practical and effective. The efficiency of a power conversion unit will be too low if the insolation is measured with a radiometer which has a FOV which is larger than the FOV of the concentrator.

Dennison, E. W.↗
NASA NTRS2003

Sunlight Transmission through Desert Dust and Marine Aerosols: Diffuse Light Corrections to Sun Photometry and Pyrheliometry

Desert dust and marine aerosols are receiving increased scientific attention because of their prevalence on intercontinental scales and their potentially large effects on Earth radiation and climate, as well as on other aerosols, clouds, and precipitation. The relatively large size of desert dust and marine aerosols produces scattering phase functions that are strongly forward- peaked. Hence, Sun photometry and pyrheliometry of these aerosols are more subject to diffuse-light errors than is the case for smaller aerosols. Here we quantify these diffuse-light effects for common Sun photometer and pyrheliometer fields of view (FOV), using a data base on dust and marine aerosols derived from (1) AERONET measurements of sky radiance and solar beam transmission and (2) in situ measurements of aerosol layer size distribution and chemical composition. Accounting for particle non-sphericity is important when deriving dust size distribution from both AERONET and in situ aerodynamic measurements. We express our results in terms of correction factors that can be applied to Sun photometer and pyrheliometer measurements of aerosol optical depth (AOD). We find that the corrections are negligible (less than approximately 1% of AOD) for Sun photometers with narrow FOV (half-angle eta less than degree), but that they can be as large as 10% of AOD at 354 nm wavelength for Sun photometers with eta = 1.85 degrees. For pyrheliometers (which can have eta up to approximately 2.8 degrees), corrections can be as large as 16% at 354 nm. We find that AOD correction factors are well correlated with AOD wavelength dependence (hence Angstrom exponent). We provide best-fit equations for determining correction factors from Angstrom exponents of uncorrected AOD spectra, and we demonstrate their application to vertical profiles of multiwavelength AOD.

Russell, P. B.↗
NASA NTRS1978

Status of knowledge of the extraterrestrial solar 'constant' and spectral distribution

Current knowledge of the total solar flux (the solar constant) and its spectral distribution at the top of the earth's atmosphere is reviewed. The development of pyrheliometers and radiation scales is traced, noting that active cavity pyrheliometers have been developed to reduce measurement uncertainty to within 0.1/%. The weighted mean solar constant from ground-based, aircraft, balloon, spacecraft and sounding rocket observations has been calculated to be 1369 W/sq m. Future observations are planned in order to reduce uncertainty to the 0.1% level required for climatological studies. A comparison of the measurement and models of solar spectral irradiance obtained by Thekaekara (1969, 1974), Arvesen (1969) and Labs and Neckel (1968, 1975) shows that the spectrum of Labs and Neckel is most accurate in the 0.4 to 1.25 micron range, that of Arvesen is best in the 0.3 to 0.4 abd 1.25 to 2.5 micron ranges and the model of Labs and Neckel is best at wavelengths greater than 2.5 microns.

Willson, R. C.↗
NASA NTRS1980

Direct measurement of solar luminosity variation

Two rocket flights of an absolute pyrheliometer, separated by 30 months, indicate an increase in solar luminosity (solar constant) of 0.4 percent. The significance of this result is considered in light of the instrument performance during the rocket flights and of pre- and postflight intercomparisons with independently maintained pyrheliometers. There is a high probability that the measured difference is real. Additional observations are required to determine whether the difference results from random fluctuations in solar luminosity, a nonrandom change of short duration, or a sustained change that has climatological significance.

Willson, R. C.↗
NASA NTRS1981

Solar total irradiance observations by active cavity radiometers

Investigations have been conducted concerning the development of new cavity pyrheliometer for an absolute measurement of solar irradiance both in preflight testing of spacecraft and in flight experiments to measure the total solar flux. A series of instruments developed as part of this program are electrically self-calibrated cavity pyrheliometers whose mode of operation is characterized by the name Active Cavity Radiometer (ACR). The temperatures of their cavity sensors are servocontrolled, actively maintained at constant temperatures relative to their heat sinks by electrical heating. The solar irradiance is related to the International System of Units by measuring the difference in amount of electrical heating required with the cavity sensor alternately shaded and exposed to the sun. Studies conducted with the ACR are discussed. While variations of a few tenths of a percent in solar total irradiance lasting no more than a week or two have been detected, no long term trends were identified.

Willson, R. C.↗
NASA NTRS1988

Intercomparison of solar total irradiance data from recent spacecraft measurements

Earth Radiation Budget Experiment solar total irradiance data obtained by the solar monitors of the NOAA 9 and 10 spacecraft are presently compared with those obtained by the Solar Maximum Mission's Active Cavity Irradiance Monitor 1, as well as sounding rockets and the Nimbus 7 Earth Radiation Budget channel 10C. A disparity is noted among total irradiance values obtained in near-simultaneous measurements with absolute pyrheliometers of similar design and operational modes which (1) indicates the uncertainty associated with currently available instruments, and (2) recommends intercalibration of electrically self-calibrating pyrheliometers from spacecraft.

Mecherikunnel, A. T.↗
NASA NTRS1988

Active Cavity Radiometer (ACR)

The objective of the Active Cavity Radiometer (ACR) experiment on the Atmospheric Laboratory for Applications and Science (ATLAS) mission is the measurement of the total solar irradiance with state-of-the-art accuracy and precision. This experiment is part of an ongoing program of space flight observations to study short- and long-term variations in the total solar output of optical energy. Precise observations of solar total irradiance provide information on the solar cycle and other long-term trends in solar output that are of climatological significance as well as short-term solar physics phenomena such as radiation anisotropy, active region structure, missing flux due to sunsports, bolometry of solar flares, global oscillations, coronal holes, and large-scale convective flows. The principal role of the ATLAS ACR observations will be in support of extended solar irradiance experiments on free-flying satellites. Annual in-flight comparison of observations by both ATLAS and free-flying experiments is an important part of sustaining the long-term precision of the climatological solar irradiance data base at the required + or - 0.1 percent level. Another role for ATLAS solar irradiance measurements will be establishment of the radiation scale at the solar total flux level in the International System of Units (SI). Two types of pyrheliometers, the ACR and SOLCON, will be directly intercompared during the ATLAS 1 mission. Addition of other sensors is planned for future reflights. Comparisons of solar observations by different pyrheliometers in the shuttle space environment will provide the most definitive experiment for determining their accuracy in defining the radiation scale at the solar total flux level.

Willson, R. C.↗
NASA NTRS1990

Solar Constant Data from Earth Radiation Budget Measurements

At present, solar total irradiance measurements are made from four satellites using electrically self calibrating pyrheliometers, as a part of the earth radiation budget measurement programs. The Earth Radiation Budget (ERB) mission onboard Nimbus-7 spacecraft (Nimbus/ERB) started solar total irradiance measurements in November 1978, and is still obtaining irradiance data on every orbit, daily. The Earth Radiation Budget Experiment (ERBE) solar monitors onboard Earth Radiation Budget Satellite (ERBS), NOAA-9 and NOAA-10 started solar total irradiance measurements in October 1984, January 1985, and October 1986, respectively. Our knowledge of solar total irradiance and its variability has grown remarkably during the past few years, as a result of the above measurements, and the high precision data obtained from Solar Maximum Mission/Active Cavity Radiometer Irradiance Monitor-1 (SMM/ACRIM-1). The results from a comparative study of the solar constant data available from the above missions are presented. The solar constant value derived from the sensors agree within the uncertainty associated with absolute pyrheliometers available at present. An attempt will be made to correlate the solar irradiance variability with other solar parameters. The measurements from Nimbus-7/ERB started November 1978, as the solar cycle 21 was increasing in activity. The solar luminosity reached a maximum in the spring of 1979. The irradiance then decreased slowly to a minimum which lasted from 1984 through 1986. The irradiance is presently increasing towards a new maximum. It appears that the solar constant value follow an eleven year cycle.

Mecherikunnel, A. T.↗
NASA NTRS1971

International comparison of radiometers - Techniques and results.

Description of an international comparison of radiometers made on May 10 through 14, 1971, with 24 instruments, each one representing the calibration standard of a different country or a major laboratory. All were pyranometers, viewing the sun and sky, thus bypassing the conventional method of transferring the standard from Angstrom pyrheliometers. This is the first time that pyranometers from all around the globe have been brought together in order to verify the scale on which to report solar radiation data. A highly accurate data collection system measured simultaneously the output of each instrument and that of a reference standard at 20-sec intervals. Over 300,000 radiometric signals were recorded under varying atmospheric conditions, as well as data on turbidity, temperature, and other relevant parameters. Computer programs were developed for overnight analysis of the data and distribution to participants on the following day. Among major findings are that instruments of different types and independent calibration such as those of Eppley, U. S. A., World Radiation Center, Switzerland, and Melbourne, Australia, show better agreement than anticipated, that certain national standards have to be revised by as much as 16% and that the cosine factor is highly dependent on the type of instrument.

Thekaekara, M. P.↗
NASA NTRS1972

Circumsolar radiation and the International Pyrheliometric Scale.

Issue is taken with the ascription of the 2% discrepancy in values of solar irradiance at the surface of the earth, found between the measurements performed by the absolute radiometers of Kendall and Wilson and those obtained by Eppley-Angstrom pyrheliometers, to the possibility that the two types of instruments might collect different amounts of circumsolar radiation. It is shown that this explanation is very unlikely.

Geist, J.↗
NASA NTRS1972

Experimental comparisons of the international pyrheliometric scale with the absolute radiation scale.

Solar irradiance measurements by instruments (Eppley Angstrom Pyrheliometers) reproducing the International Pyrheliometric Scale (IPS) have been compared with measurements made simultaneously by a number of JPL Active Cavity Radiometers (ACR) and the Practical Absolute Cavity Radiometer (PACRAD). The results of these tests demonstrate a systematic difference between the absolute radiation scale as defined by the ACR and PACRAD measurements and the IPS.

Willson, R. C.↗
NASA NTRS1973

Active cavity radiometer.

The active cavity radiometer (ACR) is a pyrheliometer that accurately defines the absolute radiation scale. The physics of the pyrheliometric method and the ACR approach to this method are presented in detail. A mathematical abstraction of the method is generated through a quasi-equilibrium analysis of the power balance of the ACR's cavity detector. An error analysis is carried out on the quasi-equilibrium equation to determine the uncertainties of ACR measurements relative to the absolute radiation scale. The uncertainty of ACR measurements as a function of irradiance level is presented in graphical form.

Willson, R. C.↗
NASA NTRS1975

Sunfall Monitor calibration plan

The initial on-site, and subsequent periodic calibration and adjustments are described for the pyrheliometer, pyranometer, equatorial mount, and the data management system.

Lollar, R. B.↗