2 Satellite Water Colour Observations in African Seas
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radiance added by scattering in the atmosphere and the radiance lost due to absorption,
that is, atmospheric correction has to be applied. This is conventionally referred to
as deriving “Level 2” data (atmospherically corrected radiances, reflectances and
derived products), from “Level 1” (radiances measured by the satellite). Level 1
to Level 2 conversion also tries to correct for sun-glint, sky reflection and light
transmission at the water surface. “Level 0” refers to the raw data packets transmitted
from the satellite, whose redundancies and encodings make them less useful for
applications. “Level 3” refers to image composites covering particular time periods
(often 8-day, or monthly), or larger, often global, areas. “Level 4” is sometimes used
to refer to products derived from satellite data, but with addition of information from
other sources. A subdivision of Level 1 to Levels 1A and 1B is used to denote pixel
location (latitude and longitude) information added in 1B.
The major component of atmospheric correction is due to Rayleigh scattering by
atmospheric gases, which can be computed knowing the viewing geometry of the
scene, the sun and the sensor. Precise calculation requires exact calibration of the
sensor, including detailed knowledge of its polarization sensitivity, and also accurate
atmospheric pressure at the ocean surface. The much more variable component due
to aerosol scattering requires added measurements, which over water can be provided
by the same sensor using wavelengths longer than 700 nm, where signal from the
water is usually negligible. These measured radiances therefore show the strength
and spectral properties (Angstrom coefficient, if a simple power-law) of the aerosol
component, which can then be extrapolated to shorter wavelengths at which waterleaving radiances are significant. Measurements at the shortest wavelengths are the
most problematic, causing problems for estimates of CDOM.
2.2 Chlorophyll Fluorescence Measurements from Satellites
We also present here results from measurements of chlorophyll fluorescence which
can be made by more recent sensors such as MODIS and MERIS (but not VIIRS) using bands near 683 nm (Gower and Borstad 1990; Abbott and Letelier 1999; Gower
and King 2007a). Measurements are made in the wavelength range where fluorescence is emitted (675–695 nm), and also at nearby wavelengths where fluorescence
is absent or much reduced, to give background reference radiances. Oxygen absorption at wavelengths longer than 687 nm needs to be avoided, so MODIS measures
fluorescence at 673 nm, and MERIS at 681 nm, shorter wavelengths than otherwise preferred. In both cases, fluorescence is measured as Fluorescence Line Height
(FLH), computed as radiance at a central wavelength, above a reference radiance linearly interpolated from two adjacent bands, one at a shorter wavelength and one at a
longer. For MODIS, measurements are made at 665, 673 and 748 nm and for MERIS
at 665, 681 and 709 nm. For large-area studies, FLH is computed from Level 2 data,
but the atmospheric correction for FLH is relatively small, and FLH values are often
computed from Level 1 radiances. This has the advantage of avoiding problems with
atmospheric correction, which are usually more severe in coastal areas, but requires a
method for rejecting data affected by cloud or strong sun glint. For MERIS, a typical
33
radiance added by scattering in the atmosphere and the radiance lost due to absorption,
that is, atmospheric correction has to be applied. This is conventionally referred to
as deriving “Level 2” data (atmospherically corrected radiances, reflectances and
derived products), from “Level 1” (radiances measured by the satellite). Level 1
to Level 2 conversion also tries to correct for sun-glint, sky reflection and light
transmission at the water surface. “Level 0” refers to the raw data packets transmitted
from the satellite, whose redundancies and encodings make them less useful for
applications. “Level 3” refers to image composites covering particular time periods
(often 8-day, or monthly), or larger, often global, areas. “Level 4” is sometimes used
to refer to products derived from satellite data, but with addition of information from
other sources. A subdivision of Level 1 to Levels 1A and 1B is used to denote pixel
location (latitude and longitude) information added in 1B.
The major component of atmospheric correction is due to Rayleigh scattering by
atmospheric gases, which can be computed knowing the viewing geometry of the
scene, the sun and the sensor. Precise calculation requires exact calibration of the
sensor, including detailed knowledge of its polarization sensitivity, and also accurate
atmospheric pressure at the ocean surface. The much more variable component due
to aerosol scattering requires added measurements, which over water can be provided
by the same sensor using wavelengths longer than 700 nm, where signal from the
water is usually negligible. These measured radiances therefore show the strength
and spectral properties (Angstrom coefficient, if a simple power-law) of the aerosol
component, which can then be extrapolated to shorter wavelengths at which waterleaving radiances are significant. Measurements at the shortest wavelengths are the
most problematic, causing problems for estimates of CDOM.
2.2 Chlorophyll Fluorescence Measurements from Satellites
We also present here results from measurements of chlorophyll fluorescence which
can be made by more recent sensors such as MODIS and MERIS (but not VIIRS) using bands near 683 nm (Gower and Borstad 1990; Abbott and Letelier 1999; Gower
and King 2007a). Measurements are made in the wavelength range where fluorescence is emitted (675–695 nm), and also at nearby wavelengths where fluorescence
is absent or much reduced, to give background reference radiances. Oxygen absorption at wavelengths longer than 687 nm needs to be avoided, so MODIS measures
fluorescence at 673 nm, and MERIS at 681 nm, shorter wavelengths than otherwise preferred. In both cases, fluorescence is measured as Fluorescence Line Height
(FLH), computed as radiance at a central wavelength, above a reference radiance linearly interpolated from two adjacent bands, one at a shorter wavelength and one at a
longer. For MODIS, measurements are made at 665, 673 and 748 nm and for MERIS
at 665, 681 and 709 nm. For large-area studies, FLH is computed from Level 2 data,
but the atmospheric correction for FLH is relatively small, and FLH values are often
computed from Level 1 radiances. This has the advantage of avoiding problems with
atmospheric correction, which are usually more severe in coastal areas, but requires a
method for rejecting data affected by cloud or strong sun glint. For MERIS, a typical