Exo-atmospheric solar irradiance measurements made by the solar irradiance community since 1978 incorporate limiting apertures with diameters measured by a number of metrology laboratories using a variety of techniques. Knowledge of the aperture area is a critical component in the conversion of radiant flux measurements to solar irradiance. A National Aeronautics and Space Administration(NASA) Earth Observing System (EOS)-sponsored international comparison of aperture area measurements of limiting apertures provided by solar irradiance researchers was performed, the effort being executed by the National Institute of Standards and Technology (NIST) in coordination with the EOS Project Science Office. Apertures that had institutional heritage with historical solar irradiance measurements were measured using the absolute aperture measurement facility at NIST. The measurement technique employed non-contact video microscopy using high-accuracy translation stages. We have quantified the differences between the participating institution's aperture area measurements and find no evidence to support the hypothesis that pre-flight aperture area measurements were the root cause of discrepancies in the long-term TSI satellite measurements. Another result is the assessment of the uncertainties assigned to the methods used by the participants. We assess that the uncertainties assigned to the participants values may be underestimated.
Citation: Applied Optics
Pub Type: Journals
aperture area measurements, aperture area uncertainty, on-orbit TSI differences, total solar irradiance (TSI)