NIST characterizes NOAA instrument for measuring global chlorophyll concentrations. (General Developments).

In 2002, a team of physicists from NIST completed characterization of, and resolved calibration issues concerning, the Marine Optical Buoy (MOBY). MOBY, a NOAA instrument developed with the support of NASA, 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 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 as measured remotely by radiometric techniques.

As a calibration reference site, MOBY is a key element in the international effort to develop a global, multiyear time series of consistently calibrated oceancolor products using data from a wide variety of independent satellite sensors. Since late 1996, 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 American Moderate Resolution Imaging Spectrometer (MODIS), the Japanese Ocean Color and Temperature Sensor (OCTS), the French Polarization Detection Environmental Radiometer (POLDER), and the German Modular Optoelectronic Scanner on the Indian Research Satellite (IRS1-MOS).

NIST physicists performed detailed radiometric characterizations of the MOBY sensor system, both in the laboratory in Gaithersburg and at the MOBY Operations Facility in Honolulu. The studies were made possible by the NIST facility for spectral irradiance and radiance responsivity calibrations using uniform sources (SIRCUS). SIRCUS uses broadly tunable, narrow-band lasers to produce wide-area illumination. It provides the unique ability to measure spectral channel cross talk due to stray-light effects. To complete the work on MOBY in Honolulu, it was necessary to develop and implement a portable version of SIRCUS.

As a result of the NIST work, a correction algorithm was applied to MOBY system responses. These results were used to revise the calibration of SeaWiFS and MODIS, and will result in more accurate values and additional constraints on the atmospheric corrections for all ocean color satellites that use MOBY data. Refinements and improvements to the correction algorithm are in progress. Ultimately, the data will affect the models used to interpret ocean-color measurements in terms of biological factors (e.g., chlorophyll concentrations). The duration and magnitude of phytoplankton primary productivity impacts carbon fixation, carbon export, and air-sea carbon flux, so this work results in more accurate estimates of the oceanic net carbon biological uptake.

CONTACT: Carol Johnson, [phone]; [e-mail] or Steve Brown, [phone]; [e-mail]. MOBY Principal Investigator, Dennis Clark, [phone]; [e-mail].