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Sources of Differences in On-Orbit Total Solar Irradiance Measurements



James J. Butler, R Barnes, B. Carol Johnson, Joseph P. Rice, Eric L. Shirley


There is a 5 W/m2 difference between current on-orbit Total Solar Irradiance (TSI) measurements. On 18-20 July 2005, a workshop was held at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland that focused on understanding possible reasons for this difference, through an examination of the instrument designs, calibration approaches, and appropriate measurement equations. The instruments studied in that workshop included ACRIM III on ACRIMSat, TIM on SORCE, VIRGO on SoHO, and ERBE on ERBS. Presentations for each instrument included descriptions of its design, its measurement equation, and its uncertainty budget. The workshop also included a session on satellite- and ground-based instrument comparisons and a session on laboratory-based comparisons and the application of new laboratory comparison techniques. The workshop has led to investigations of the effects of diffraction and of aperture area measurements on the differences between instruments. In addition, a laboratory-based instrument that uses optical power measurements (with lasers that underfill the apertures of the TSI instruments), and irradiance measurements (with lasers that overfill the apertures of the TSI instrument), and a cryogenic electrical substitution radiometer as a standard for comparing the instruments is discussed. A summary of the workshop and an overview of the ongoing research efforts are presented here.
Journal of Research (NIST JRES) -
No. 4


on-orbit measurements, radiometry, total solar irradiance, uncertainty workshop


Butler, J. , Barnes, R. , Johnson, B. , Rice, J. and Shirley, E. (2009), Sources of Differences in On-Orbit Total Solar Irradiance Measurements, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed February 23, 2024)
Created July 12, 2009, Updated October 12, 2021