Long-term monitoring of radiometer sensitivity for radiometric comparisons among optical laboratories

The SIMBIOS (Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies) Program was conceived as a result of a NASA management review of the agency's strategy for monitoring the bio-optical properties of the global ocean through space-based ocean color remote sensing. SIMBIOS Radiometric Intercomparisons (SIMRICs) were carried out in 2001 and 2002. The purpose of the SIMRICs was to ensure a common radiometric scale among the calibration facilities that are engaged in calibrating in-situ radiometers used for ocean color related research and to document the calibration procedures and protocols. The SeaWiFS Transfer Radiometer (SXR-II) measured the calibration radiances at six wavelengths from 411nm to 777nm in the participating laboratories. The measured radiances were compared with the radiances expected by the laboratories. NIST calibrations of the SXR-II were obtained in December 2000, December 2001 and January 2003. Two independent light sources (SQMs, SeaWiFS Quality Monitors) were used to monitor changes in the SXR-II responsivity between the NIST calibrations and after, with monthly measurements until the end of 2003, and less frequent measurements thereafter. This paper discusses the calibration and trending history of the SXR-II from December 2000 to June 2008.