minimize sources of bias and improve the data return (e.g., consideration of cloud cover probabilities).
The MOBY site has been used to calibrate ocean color sensors in the post-Coastal Zone Color Scanner (CZCS) era, and the requirements for ocean color vicarious calibration have been well documented (NASA, 2003). Here, we present the concept of random and systematic uncertainty and the measurement equation to support the role of a MOBY-like facility in ocean color research. First, in order to have acceptable values for the random components of uncertainty one can either have a broad distribution, or a narrow, well-defined distribution. The latter requires fewer samples to achieve the same uncertainty value. The well-defined, narrow distribution is found at the MOBY facility, with its stable marine atmosphere, central Pacific location, uniform oligotrophic waters, and robust instrument design that results in good measurement precision. An example of a broader distribution would be the BOUSSOLE or Aeronet-OC (Bailey et al., 2008). It can be argued that the best technical approach for vicariously calibrating a new satellite sensor such as VIIRS would be to have a precise, stable dataset so that the number of observations required to produce the asymptotic value of the random uncertainty component is as small as possible. The Ocean Biology Processing Group (OBPG) studied the effect of sample size on the uncertainty in the vicarious gain coefficient using MOBY for SeaWiFS; it concluded 45 to 60 samples would be required for reliable vicarious calibration of a stable sensor such as SeaWiFS (see Figure B.1).
Second, in order to have acceptable values for the systematic components of uncertainty, one needs a robust, well-characterized, high-quality assured dataset. Spectral biases cannot be tolerated at any level, because the bio-optical algorithms rely on band ratios. Unidentified or difficult to quantify bias introduced by the “atmospheric correction” (e.g., the process of estimating all the terms in the measurement equation except Lw) are mitigated by selecting sites where the atmospheric conditions and the marine environment are as simple as possible, the necessary ancillary data are available, and the models can be verified and improved upon. Likewise, the in situ instrument must be
FIGURE B.1 Mean vicarious gains, g, derived for SeaWiFS bands at 443, 555, and 765 nm based on calibration samples between September 1997 and March 2006. Individual gains from the mission-long set of calibration match-ups were randomly sampled; growing the sample set one case at a time and averaging to show the effect of increasing sample size on g. Vertical error bars show the standard error on the mean at each sample size.
SOURCE: Franz et al., 2007; used with permission from the Optical Society.