Match-up analyses of the water-leaving radiances as
well as for the Chl a concentrations have been performed
for the MERIS, SeaWiFS, and MODIS-A data products.
The analyses all follow the same procedure, wherein a
5×5 pixel box is extracted from the level-2 product, and directly compared to the same quantity as
derived from the data collected in situ from the ship or
from the buoy. For the SeaWiFS and MODIS-A sensors,
the criteria to be respected for a given comparison point to be kept in the final analysis are as described in Bailey
et al. 2000. They are essentially the same for the MERIS
sensor, except that the flags are not identical.
The combination of data availability, orbit characteristics
and the quality criteria yields 68 matchups for MERIS
(data from September 2003 to December 2005), 89 matchups
for SeaWiFS (data from September 2003 to December 2004)
and 65 matchups for MODIS-A (data from September 2003
to May 2005)
The data used are a function of the satellite mission. For MERIS, level -2 1km reduced resolution data processed
by the MERIS prototype version 7.4.1 (called ”MEGS7.4.1”)
are used. These data provide the water-leaving radiance reflectance, ρw, defined as pi time the ratio of the normalized water-leaving radiance to the downwelling irradiance just aboce the sea surface. For SeaWiFS, level -2 GAC data from reprocessing 5
(completed March 18, 2005) are used, and for MODIS-A,
level -2 GAC data from reprocessing 1 (completed February
2005) are used. These data provide the fully normalized
water-leaving radiance, [Lw(λ)]N.
The transformations needed to derive fully compatible
quantities between the satellite and the in situ measurements
(i.e., accounting for the bidirectional nature of the
light field) are not detailed here.
Concerning the water-leaving radiances or reflectances,
results are displayed as scatter plots of the satellite-derived
versus in situ measurements for MERIS, SeaWiFS, and MODIS-A (all wavelengths presented together, see here).
The associated statistics are provided separately for each
wavelength, as well as for the full data set, in Tables.