Ocean color radiometry provides essential information about phytoplankton concentration and dissolved organic matter, and hence about primary productivity, global carbon cycling, and ultimately the effects of both on the planet's climate.
The primary reference instrument for the United States and most international ocean-color sensors is the Marine Optical Buoy (MOBY), an automated radiometric system operated by NOAA and deployed about 20 km offshore from Lanai, Hawaii, where the atmospheric and water conditions support utilizing this area as a surface radiometric reference standard. The water is clear (low chlorophyll) and the spatial uniformity is good and representative of most of the world's oceans; likewise the atmosphere is clear (low aerosols) so atmospheric correction is more certain. Clouds can prevent satellite observations of the MOBY site, but the system is optimally located for local conditions and the retrieval rate from cloud contamination is similar to typical cloud coverage fraction in ocean scenes.
MOBY uses optical fiber-coupled collectors mounted on three long arms that stand off its central axial mast to detect both downwelling irradiance and upwelling radiance at depths of 1 m, 5 m, and 9 m, and at wavelengths from 340 nm to 950 nm. The optical fibers mate to two spectrographs (for long and short optical wavelengths) in a 2 meter-long canister at the base of the axial mast. The system also records total irradiance just above the sea surface, and upwelling radiance from directly beneath the axis.
MOBY has been in operation since 1997, and is now operated for NOAA by principal investigators Ken Voss of the University of Miami, and Mark Yarbrough of Moss Landing Marine Laboratory (part of the California State Universities). The instrument's data are used to determine on-orbit calibration factors for various satellite sensors including the long-running MODIS and recent SeaWIFS instruments. NASA's Earth Observing System (EOS) supported the development of MOBY under the direction of Dennis Clark (NOAA, deceased). Currently, teams of researchers are using MOBY to vicariously calibrate the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard NASA's Suomi National Polar-orbiting Partnership (Suomi NPP) spacecraft and the VIIRS aboard the NOAA-20 weather spacecraft.