fully characterized and supply values that are SI-traceable. The more robust this procedure, the more reliable is the final product. The MOBY project has gone to great effort to meet these objectives by incorporation of check standards, repeat calibrations, close linkage with NIST, expert and dedicated staff, good instrument design, and so forth. The evidence of the degree of the stability and precision of the MOBY products and the atmospheric correction procedures is demonstrated in Franz et al. (2007).

The MOBY site was selected to represent the majority of the observed natural sources—the open oceans. Coastal regions exhibit variations in Lw and the other terms in the measurement equation on many different temporal and spatial time scales compared to the MOBY site off Lanai, Hawaii. The sensor measures Lt, the proper interpretation of these data depend on thorough understanding of the sensor characterization (linearity, polarization, spectral out of band, etc.) and response to this top of the atmosphere radiant flux. A case could be made for additional sites, including coastal sites, with a MOBY-like site producing the basic calibration and the other equally robust sites, serving to explore the intricacies of the atmospheric correction methods, dark pixel assumptions, and the satellite sensor characterization functions themselves.

In conclusion, study of the measurement equation and robust experimental design establishes that the MOBY approach and its uncertainty values are necessary for productive ocean color research. An examination of the uncertainty in upwelling spectral radiance for MOBY is given in Brown et al. (2007), where the Type A random uncertainty was estimated to be 1 percent. Franz et al. (2007) state that without the vicarious calibration provided by MOBY, the bias in Lwn resulting from the errors in the pre-flight calibration for SeaWiFS would have been 25 percent at 490 nm and 75 percent at 412 nm and the mean Ca retrieval would be biased low by 25 percent.



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