For the past decade, the Marine Optical Buoy (MOBY), a radiometric buoy stationed in the waters off Lanai, Hawaii,
has been the primary in-water oceanic observatory for the vicarious calibration of U. S. satellite ocean color sensors,
including the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectrometers
(MODIS) instruments on the National Aeronautics and Space Administration's (NASA's) Terra and Aqua satellites.
The MOBY vicarious calibration of these sensors supports international effort to develop a global, multi-year time series
of consistently calibrated ocean color data products. A critical component of the MOBY program is establishing
radiometric traceability to the International System of Units (SI) through standards provided by the U. S. National
Institute of Standards and Technology (NIST). A detailed uncertainty budget is a core component of traceable
metrology. We present the MOBY uncertainty budget for up-welling radiance and discuss additional considerations
related to the water-leaving radiance uncertainty budget. Finally, we discuss approaches in new instrumentation to
reduce the uncertainties in in situ water-leaving radiance measurements.
Determination of the water-leaving spectral radiance using in-water instrumentation requires measurements of the upwelling
spectral radiance (L<sub>u</sub>) at several depths. If these measurements are separated in time, changes in the
measurement conditions result in increased variance in the results. A prototype simultaneous multi-track system was
developed to assess the potential reduction in the Type A uncertainty in single set, normalized water-leaving radiance
achievable if the data were acquired simultaneously. The prototype system employed a spectrograph and multi-track
fiber-coupled CCD-detector; in situ in-water tests were performed with the prototype system fiber-coupled to a small
buoy. The experiments demonstrate the utility of multi-channel simultaneous data acquisition for in-water measurement
applications. An example of the potential impact for tracking abrupt responsivity changes in satellite ocean color
sensors using these types of instruments as well as for the satellite vicarious calibration is given.
The water-leaving spectral radiance is a basic ocean color remote sensing parameters required for the vicarious
calibration. Determination of water-leaving spectral radiance using in-water radiometry requires measurements of the
upwelling spectral radiance at several depths. The Marine Optical System (MOS) Remotely Operated Vehicle (ROV) is
a portable, fiber-coupled, high-resolution spectroradiometer system with spectral coverage from 340 nm to 960 nm.
MOS was developed at the same time as the Marine Optical Buoy (MOBY) spectrometer system and is optically
identical except that it is configured as a profiling instrument. Concerns with instrument self-shadowing because of the
large exterior dimensions of the MOS underwater housing led to adapting MOS and ROV technology. This system
provides for measurement of the near-surface upwelled spectral radiance while minimizing the effects of shadowing. A
major advantage of this configuration is that the ROV provides the capability to acquire measurements 5 cm to 10 cm
below the water surface and is capable of very accurate depth control (1 cm) allowing for high vertical resolution
observations within the very near-surface. We describe the integrated system and its characterization and calibration.
Initial measurements and results from observations of coral reefs in Kaneohe Bay, Oahu, extremely turbid waters in the
Chesapeake Bay, Maryland, and in Case 1 waters off Southern Oahu, Hawaii are presented.
The Marine Optical Buoy (MOBY) provides values of water- leaving radiance for the calibration and validation of satellite ocean color instruments. Located in clear, deep ocean waters near the Hawaiian Island of Lanai, MOBY measures the upwelling radiance and downwelling irradiance at three levels below the ocean surface plus the incident solar irradiance just above the surface. The radiance standards for MOBY are two integrating spheres with calibrations based on standards traceable to the National Institute of Standards and Technology (NIST). For irradiance, the MOBY project uses standard lamps that are routinely calibrated at NIST. Wavelength calibrations are conducted with a series of emission lines observed from a set of low pressure lamps. Each MOBY instrument views these standards before and after its deployment to provide system responses (calibration coefficients). During each deployment, the stability of the MOBY spectrographs and internal optics are monitored using three internal reference sources. In addition, the collection optics for the instrument are cleaned and checked on a monthly basis while the buoy is deployed. Divers place lamps over the optics before and after each cleaning to monitor changes at the system level. As a hyperspectral instrument, MOBY uses absorption lines in the solar spectrum to monitor its wavelength stability. When logistically feasible during each deployment, coincident measurements are made with the predecessor buoy before that buoy's recovery. Measurements of the underwater light fields from the deployment vessel are compared with those from the buoy. Based on this set of absolute calibrations and the suite of stability reference measurements, a calibration history is created for each buoy. These calibration histories link the measurement time series from the set of MOBY buoys. In general, the differences between the pre- and post-deployment radiance calibrations of the buoys range from +1% to -6% with a definitive bias to a negative difference for the post- deployment values. This trend is to be expected after a deployment of 3 months. To date, only the pre-deployment calibration measurements have been used to adjust the system responses for the MOBY time series. Based on these results, the estimated radiometric uncertainty for MOBY in-water ocean color measurements is estimated to be about 4% to 8% (kequals1). As part of a collaboration with NIST, annual radiometric comparisons are made at the MOBY calibration facility. NIST personnel use transfer radiometers and integrating spheres to validate (verify) the accuracy of the MOBY calibration sources. Recently, we began a study of the stray light contribution to the radiometric uncertainty in the MOBY systems. A complete reprocessing of the MOBY data set, including the changes within each MOBY deployment, will commence upon the completion of the stray light characterization, which is scheduled for the fall of 2001. It is anticipated that this reprocessing will reduce the overall radiometric uncertainty to less than 5% (kequals1).