Accuracy and Precision Actually Achieved for Large-aperture Integrator Sources for Aircraft and Space Investigations

Abstract

Large-aperture integrator sources are required to calibrate a number of the satellite instruments which will be used for documenting and understanding global change in the US Earth Observing System (EOS) satellite program. In this paper the salient characteristics of large-aperture sources as used at the NASA/Goddard Space Flight Center are identified. These characteristics are source spatial uniformity, calibration accuracy and output stability. One use of these sources in maintaining the satellite sensor performance in orbit through vicarious calibrations is reviewed.A transportable hemispherical source is shown to have spatial uniformity of better than 0,3%, have stability of near 1%, and to be capable of being calibrated with an uncertainty near 1,6%. Vicarious calibrations of the AVHRR with a transfer spectrometer on board a NASA ER-2 platform flown at 21 km are shown to be capable of individual calibrations with an uncertainty of 3,5%. Annual trends in the satellite responsivity can be corrected to an uncertainty of 1,5% with this approach.