SATELLITE MEASUREMENTS
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the signal, producing noise which requires correction. The greatest
contribution comes from light scattered by the atmosphere into the field of
view of the sensor, which may make up over 90% of the measured radiance,
including skylight reflected by the sea surface into the sensor. The
atmospheric correction procedure must account for this in order to estimate
the water leaving radiance for each of the spectral bands recorded by the
sensor.
Scattering by the air gas molecules themselves can be directly calculated
for each pixel in the field of view of an imaging sensor, but scattering by
larger particles of aerosols such as water vapour or dust particles cannot be
calculated because their distribution in the atmosphere is unknown and
impossible to predict. Instead this part of the atmospheric correction uses
the radiance measured in two spectral bands from the near infra-red part of
the spectrum. Because the sea absorbs almost all incident solar near-infrared
radiation, any measured at the top of the atmosphere must have been
scattered by the atmosphere. This is then used to estimate how much aerosol
scattering has occurred in the visible channels where the water leaving
radiance is not zero, and so the correction is accomplished.
3.2.2
Estimating water content from its colour
When the atmospheric correction has been successfully applied to
satellite ocean colour data, the result is an estimate of the water-leaving
radiance in each spectral channel in the visible waveband, normalised to
reduce dependence on the sun’s elevation and the viewing incidence angle.
Effectively the normalised water leaving radiance should represent what a
sensor would measure if looking straight down from an orbit that takes it just
above the sea surface at the bottom of the atmosphere. This is what our eyes
would detect as the colour and brightness of the sea, ignoring any light
reflected from the surface. The primary challenge of ocean colour remote
sensing is to derive quantitative estimates of the type and concentration of
those materials in the water which affect its apparent colour.
Photons of visible wavelength e.m energy from the sun that enter the sea
will eventually interact with molecules of something in the sea. The
outcome will be either that the photon is scattered, in which case it may
change its direction with a chance of leaving the sea and contributing to
what the sensor sees, or it will be absorbed. The probability of scattering or
absorption depends on the wavelength of the light and the material which it
encounters. The molecules within sea water tend to preferentially scatter
shorter wavelengths of light (the blue part of the spectrum) and
preferentially absorb longer wavelengths (the red end). This is why pure sea
water with little other content appears blue.
The pigment chlorophyll-a which is found in phytoplankton has a strong
and fairly broad absorption peak centred at 440 nm in the blue, but not in the
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the signal, producing noise which requires correction. The greatest
contribution comes from light scattered by the atmosphere into the field of
view of the sensor, which may make up over 90% of the measured radiance,
including skylight reflected by the sea surface into the sensor. The
atmospheric correction procedure must account for this in order to estimate
the water leaving radiance for each of the spectral bands recorded by the
sensor.
Scattering by the air gas molecules themselves can be directly calculated
for each pixel in the field of view of an imaging sensor, but scattering by
larger particles of aerosols such as water vapour or dust particles cannot be
calculated because their distribution in the atmosphere is unknown and
impossible to predict. Instead this part of the atmospheric correction uses
the radiance measured in two spectral bands from the near infra-red part of
the spectrum. Because the sea absorbs almost all incident solar near-infrared
radiation, any measured at the top of the atmosphere must have been
scattered by the atmosphere. This is then used to estimate how much aerosol
scattering has occurred in the visible channels where the water leaving
radiance is not zero, and so the correction is accomplished.
3.2.2
Estimating water content from its colour
When the atmospheric correction has been successfully applied to
satellite ocean colour data, the result is an estimate of the water-leaving
radiance in each spectral channel in the visible waveband, normalised to
reduce dependence on the sun’s elevation and the viewing incidence angle.
Effectively the normalised water leaving radiance should represent what a
sensor would measure if looking straight down from an orbit that takes it just
above the sea surface at the bottom of the atmosphere. This is what our eyes
would detect as the colour and brightness of the sea, ignoring any light
reflected from the surface. The primary challenge of ocean colour remote
sensing is to derive quantitative estimates of the type and concentration of
those materials in the water which affect its apparent colour.
Photons of visible wavelength e.m energy from the sun that enter the sea
will eventually interact with molecules of something in the sea. The
outcome will be either that the photon is scattered, in which case it may
change its direction with a chance of leaving the sea and contributing to
what the sensor sees, or it will be absorbed. The probability of scattering or
absorption depends on the wavelength of the light and the material which it
encounters. The molecules within sea water tend to preferentially scatter
shorter wavelengths of light (the blue part of the spectrum) and
preferentially absorb longer wavelengths (the red end). This is why pure sea
water with little other content appears blue.
The pigment chlorophyll-a which is found in phytoplankton has a strong
and fairly broad absorption peak centred at 440 nm in the blue, but not in the