Sensor-independent approach to the vicarious calibration of satellite ocean color radiometry

Abstract

The retrieval of ocean color radiometry from space-based sensors requires on-orbit vicarious calibration to achieve the level of accuracy desired for quantitative oceanographic applications. The approach developed by the NASA Ocean Biology Processing Group (OBPG) adjusts the integrated instrument and atmospheric correction system to retrieve normalized water-leaving radiances that are in agreement with ground truth measurements. The method is independent of the satellite sensor or the source of the ground truth data, but it is specific to the atmospheric correction algorithm. The OBPG vicarious calibration approach is described in detail, and results are presented for the operational calibration of SeaWiFS using data from the Marine Optical Buoy (MOBY) and observations of clear-water sites in the South Pacific and southern Indian Ocean. It is shown that the vicarious calibration allows SeaWiFS to reproduce the MOBY radiances and achieve good agreement with radiometric and chlorophyll a measurements from independent in situ sources. We also find that the derived vicarious gains show no significant temporal or geometric dependencies, and that the mission-average calibration reaches stability after approximately 20-40 high-quality calibration samples. Finally, we demonstrate that the performance of the vicariously calibrated retrieval system is relatively insensitive to the assumptions inherent in our approach.

A 50 % increase in the mass of terrestrial particles delivered by the Mackenzie River into the Beaufort Sea (Canadian Arctic Ocean) over the last 10 years

“…Such an increase of more than 50 %, observed over the 11-year period, is much higher than the uncertainty in the SPM concentration estimated from MODIS satellite data using our algorithm (±25 %, according to Doxaran et al, 2009Doxaran et al, , 2012; this is an uncertainty which is significantly minimized when producing monthly averages of SPM concentrations (assuming there is no correlation between SPM and clouds). Moreover, due to regular vicarious calibrations of the MODIS-Aqua sensor (Franz et al, 2007;Meister et al, 2012), a temporal degradation in the satellite data quality cannot alone explain the observed variations. This confirms the consistency of the result presented here.…”

Assessment of uncertainty in the ocean reflectance determined by three satellite ocean color sensors (MERIS, SeaWiFS and MODIS‐A) at an offshore site in the Mediterranean Sea (BOUSSOLE project)

“…This is extremely challenging considering the present technology, and probably will remain an elusive goal. It is also worth noting that what is referred to as a ''vicarious calibration'' in the field of ocean color remote sensing is not an absolute calibration of the sensor only, but an adjustment of the overall response of the sensor plus the atmospheric correction algorithm [Gordon, 1997[Gordon, , 1998Franz et al, 2007]. The goal is to absorb unavoidable residual uncertainties in order to get the correct answer in terms of the normalized water-leaving radiance, in particular for the blue bands.…”

Joint retrieval of aerosol and water-leaving radiance from multispectral, multiangular and polarimetric measurements over ocean

“…The selected aerosol model is then extrapolated to shorter-wavelength visible bands and applied to the measured TOA radiances to retrieve normalized waterleaving radiance (nLw) (Gordon and Wang, 1994;Gordon, 1997). To reduce errors caused by this atmospheric correction procedure and instrumental radiometric uncertainties, empirical gain factors are derived by forcing agreement between retrieved nLw values and in situ measurements obtained at the Marine Optical Buoy (MOBY) site in Lanai, Hawaii (Franz et al, 2007).…”