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 representative of most of the world’s oceans -- clear, has low chlorophyll and good spatial uniformity; 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.
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 downwelling spectral irradiance just above the sea surface, and upwelling radiance from directly beneath the axis.
MOBY has been in operation since 1997. It is operated for NOAA by principal investigator Ken Voss of the University of Miami (UM), with co-investigators Art Gleason (UM) 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 on NASA’s Terra and Aqua satellites, and the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard NASA's Suomi National Polar-orbiting Partnership (Suomi NPP) spacecraft and the NOAA-20 weather spacecraft. The MOBY team is implementing MOBY-Refresh for NOAA that will replace the existing system after a crossover interval. At the same time, in a complimentary effort, the MOBY team is working on MarONet for NASA’s upcoming Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission where the instrument design is identical to MOBY-Refresh but the planned deployment site will be offshore of western Australia.