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OTD News Measuring Global Carbon Concentrations In 2002, a team of physicists from PL's Optical Technology Division completed characterization of (and resolved calibration issues concerning) the Marine Optical Buoy (MOBY). MOBY, a NOAA instrument, is the centerpiece of the primary ocean measurement site for vicarious calibration of satellite ocean color sensors. MOBY is deployed in the Pacific Ocean off of the coast of Lanai, Hawaii. It is an essential instrument in the science of quantifying the global carbon cycle for the U.S. Global Change Research Program, concerned primarily with chlorophyll concentrations in the oceans. 
Deployment of MOBY in the Pacific Ocean. Photo courtesy of the
NASA SeaWiFS Project webpage.
Since late 1996 the time series of normalized water-leaving radiances, determined from the array of radiometric sensors attached to MOBY, are the primary basis for the on-orbit vicarious calibrations of the American Sea-viewing Wide Field-of-view Sensor (SeaWiFS), the Japanese Ocean Color and Temperature Sensor (OCTS), the French Polarization Detection environmental Radiometer (POLDER), the German Modular Optoelectronic Scanner on the Indian Research Satellite (IRS1-MOS), and the American Moderate Resolution Imaging Spectrometer (MODIS). The MOBY calibration reference is a key element in the international effort to develop a global, multi-year time series of consistently calibrated ocean color products using data from a wide variety of independent satellite sensors. The Marine Optical System (MOS) system is used as a down-welling irradiance and up-welling in-water radiance profiler in two configurations: as the sensor for the Marine Optical Buoy (MOBY) and as a mobile, shipboard-deployable sensor. NIST physicists performed detailed radiometric characterization of the MOS and MOBY configurations. In this work, a rigorous study of the MOS Profiler was performed using broadly tunable, narrow-band lasers at the NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS). These measurements enabled an accurate determination of the stray light contribution to the measured signals from MOS. A stray light correction algorithm was developed and assessed using MOS measurements of a laboratory color-filtered source of spectral radiance. In a second phase of the study, a mobile tunable laser system was developed and deployed at the MOBY field support site at the University of Hawaii Marine Center in Honolulu, Hawaii, and was used to characterize the stray light performance of MOBY. As a result of this characterization, a correction algorithm was applied to MOBY system responses, measurements of the color-filtered source, and subset of the in-water data sets. These preliminary results were used to revise the calibration of SeaWiFS and MODIS. 
Diagram of MOBY. Diagram courtesy of the
NASA SeaWiFS Project webpage.
Final analysis of the NIST radiometric calibration and characterization work performed in 2002 will result in improved values for all MOBY data sets. These results impact the calibrations of all ocean color satellites that use MOBY and MOS data sets and will provide more accurate values of the retrieved spectral radiance attenuation coefficients and bio-optical products, i.e. pigment concentrations.
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