| Popis: |
The NASA Ocean Color calibration team continued to reanalyze and improve on their approach to the on-orbit calibration of the Visible Infrared Imaging Radiometer Suite (VIIRS), aboard the Suomi National Polar-orbiting Partnership (NPP) satellite, now five years into its Earth Observation mission. As the calibration was adjusted for changes in ocean band responsitivity with time, the team also observed the variance and autocorrelation properties of calibration trend fit residuals, which appeared to have a standard deviation within a few tenths of a percent. Autocorrelation was observed to be different between bands at the blue end of the spectrum and bands at the red/NIR end, which are affected by significant changes in responsitivity stemming from mirror contamination. This residual information offered insight into the effect of small calibration biases, which can cause significant trend uncertainties in regional time series of surface reflectance and derived products. This work involves modeling spurious trends that are inherent to the calibration over time and that also arise between reprocessing efforts because of extrapolation of the time-dependent calibration table. Uncertainty in calibration trends was estimated using models of instrument and calibration system trend artifacts and correlated noise models using Monte Carlo techniques. Combined table reprocessing and extrapolation biases are presented for the first time. Calibration trend uncertainty is then propagated through to ocean color remote sensing reflectance and chlorophyll-a concentration time series. The results quantify the smallest trend observable in these oceanic parameters. This quantification furthers our understanding of uncertainty in measuring regional and global biospheric trends in the ocean using VIIRS, and better defines the roles of records in climate research. |
