On-orbit calibration of SeaWiFS: revised temperature and gain corrections

Abstract

The NASA Ocean Biology Processing Group's Calibration and Validation (Cal/Val) Team has used SeaWiFS onorbit lunar and gain calibration data, in conjunction with mission-long trends of global ocean color data products, to diagnose and correct recently emergent residual drifts in the radiometric response of the instrument. An anomaly analysis of the time series of global mean normalized water-leaving radiances, the atmospheric correction parameter <i>&isin;</i>, and chlorophyll show significant departures from the mission-long trends beginning in January 2006. The lunar time series trends for the near infrared (NIR) bands (765 nm and 865 nm) show significant periodic departures from mission-long trends beginning at the same time. <i>&isin;</i> is dependent on the ratio of these two bands; trends in this parameter would propagate through the atmospheric correction algorithm to the retrieved water-leaving radiances. An analysis of fit residuals from the lunar time series shows that the focal plane temperature dependencies of the radiometric response of the detectors for these two bands have changed over the 9+ year mission. The Cal/Val Team has used these residuals to compute a revised set of temperature corrections for data collected starting 1 January 2006. The lunar calibration data and a mission-long ocean color test data set have been reprocessed with the revised temperature corrections. The reprocessed data show that the trends in the NIR bands have been minimized and that the departures of the water-leaving radiances, <i>&isin;</i>, and chlorophyll from the mission-long trends have been greatly reduced. The anomaly analysis of the water-leaving radiances in the 510 nm band still shows a residual drift of -2.9% over the mission. The anomaly analysis of the &isin; time series shows a residual drift of +2.8% over the mission. A corresponding drift is not observed in the lunar calibration time series for the NIR bands. The lunar calibration data are obtained at a different set of instrument gains than are the ocean data. An analysis of the mission-long time series of on-orbit gain calibration data shows that the gain ratios for the NIR bands change -0.76% (765 nm) and +0.56% (865 nm) over the mission, corresponding to a -1.3% drift in the band ratio. The lunar calibration time series for the NIR bands have been corrected for this gain drift, and the change in radiometric response over time has been recomputed for each band. The mission-long ocean color test data set has been reprocessed with these revised corrections for the NIR bands. The anomaly analysis of the reprocessed water-leaving radiances at 510 nm shows the drift to have been essentially eliminated, while the anomaly analysis of epsilon shows a reduced drift of +2.0%. These analyses show the sensitivity of ocean color data to small drifts in instrument calibration and demonstrate the use of time series of global mean geophysical parameters to monitor the long-term stability of the instrument calibration on orbit. The two updates to SeaWiFS radiometric calibration have been incorporated into the recent reprocessing of the SeaWiFS mission-long ocean data set.